Plastics Extrusion

Advanced Manufacturing Techniques

Module-I

Plastics Extrusion Shri Sant Gajanan Maharaj College of Engineering, Shegaon

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PLASTICS EXTRUSION

Cross-section of a plastic extruder to show the screw Plastics extrusion is a high volume manufacturing process in which raw plastic material is melted and formed into a continuous profile. Extrusion produces items such as pipe/tubing, weather stripping, window frames, adhesive tape and wire insulation.

PROCESS In the extrusion of plastics, raw thermoplastic material in the form of small beads (often called resin in the industry) is gravity fed from a top mounted hopper into the barrel of the extruder. Additives such as colorants and UV inhibitors (in either liquid or pellet form) are often used and can be mixed into the resin prior to arriving at the hopper. The material enters through the feed throat (an opening near the rear of the barrel) and comes into contact with the screw. The rotating screw (normally turning at up to 120 rpm) forces the plastic beads forward into the barrel which is heated to the desired melt temperature of the molten plastic (which can range from 200 °C (392 °F) to 275 °C (527 °F) depending on the polymer). In most processes, a heating profile is set for the barrel in which three or more independent PID controlled heater zones gradually increase the temperature of the barrel from the rear (where the plastic enters) to the front. This allows the plastic beads to melt gradually as they are pushed through the barrel and lowers the risk of overheating which may cause degradation in the polymer. Extra heat is contributed by the intense pressure and friction taking place inside the barrel. In fact, if an extrusion line is running a certain material fast enough, the heaters can be shut off and the melt temperature maintained by pressure and friction alone inside the barrel. In most extruders, cooling fans are present to keep the temperature below a set value if too much heat is generated. If forced air cooling proves insufficient then cast-in heater jackets are employed, and they generally use a closed loop of distilled water in heat exchange with tower or city water.

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Plastic extruder cut in half to show the components At the front of the barrel, the molten plastic leaves the screw and travels through a screen pack to remove any contaminants in the melt. The screens are reinforced by a breaker plate (a thick metal puck with many holes drilled through it) since the pressure at this point can exceed 5000 psi (34 MPa). MPa). The screen pack/breaker plate assembly also serves to create back pressure in the barrel. Back pressure is required for uniform melting and proper mixing of the polymer, and how much pressure is generated can be 'tweaked' by varying screen pack composition (the number of screens, their wire weave size, and other parameters). This breaker plate and screen pack combination also does the function of converting "rotational memory" of the molten plastic into "longitudinal memory". After passing through the breaker plate molten plastic enters the die. The die is what gives the final product its profile and must be designed so that the molten plastic evenly flows from a cylindrical profile, to the product's profile shape. Uneven flow at this stage would produce a product with unwanted stresses at certain points in the profile. These stresses can cause warping upon cooling. Almost any shape imaginable can be created so long as it is a continuous profile. The product must now be cooled and this is usually achieved by pulling the extrudate through a water bath. Plastics are very good thermal insulators and are therefore difficult to cool quickly. Compared with steel, plastic conducts its heat away 2000 times more slowly. In a tube or pipe extrusion line, a sealed water bath is acted upon by a carefully controlled vacuum to keep the newly formed and still molten tube or pipe from collapsing. For products such as plastic sheeting, the cooling is achieved by pulling through a set of cooling rolls. Sometimes on the same line a secondary process may occur before the product has finished its run. In the manufacture of adhesive tape, a second extruder melts adhesive and applies this to the plastic sheet while it’s still hot. Once the product has cooled, it can be spooled, or cut into lengths for later use.

TYPICAL WORKPIECE MATERIALS Typical materials that are used in extrusion molding include but are not limited to: acetal, acrylic, nylon, polystyrene and acrylonitrile butadiene styrene (ABS) and polycarbonate.

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TYPES SHEET/FILM EXTRUSION For products such as plastic sheet or film, the cooling is achieved by pulling through a set of cooling rolls (calender or "chill" rolls), usually 3 or 4 in number. Running too fast creates an undesirable condition called "nerve"- basically, inadequate contact time is allowed to dissipate the heat present in the extruded plastic. In sheet extrusion, these rolls not only deliver the necessary cooling but also determine sheet thickness and surface texture (in case of structured rolls; i.e. smooth, levant, haircell, etc.). Often co-extrusion is used to apply one or more layers to obtain specific properties such as UVabsorption, soft touch or "grip", matte surface, or energy reflection. A common post-extrusion process for plastic sheet stock is thermoforming, where the sheet is heated until soft (plastic), and formed via a mold into a new shape. When vacuum is used, this is often described as vacuum forming. Orientation (i.e. ability/ available density of the sheet to be drawn to the mold which can vary in depths from 1 to 36 inches typically) is highly important and greatly affects forming cycle times. Thermoforming can go from line bended pieces (e.g. displays) to complex shapes (computer housings), which often look like they've been injection moulded, thanks to the various possibilities in thermoforming, such as inserts, undercuts, divided moulds. Plastic extrusion onto paper is the basis of the liquid packaging industry (juice cartons, wine boxes...); boxes...); usually an aluminum layer is present as well. In food packaging plastic film is sometimes metallised , see metallised film.

BLOWN FILM EXTRUSION The manufacture of plastic film for products such as shopping bags is achieved using a blown film line. This process is the same as a regular extrusion process up until the die. The die is an upright cylinder with a circular opening similar to a pipe die. The diameter can be a few centimetres to more than three metres across. The molten plastic is pulled upwards from the die by a pair of nip rolls high above the die (4 metres to 20 metres or more depending on the amount of cooling required). Changing the speed of these nip rollers will change the gauge (wall thickness) of the film. Around the die sits an air-ring. The air-ring cools the film as it travels upwards. In the centre of the die is an air outlet from which compressed air can be forced into the centre of the extruded circular profile, creating a bubble.This expands the extruded circular cross section by some ratio (a multiple of the die diameter). This ratio, called the “blow-up ratio” can be just a few percent to more than 200 percent of the original diameter. The nip rolls flatten the bubble into a double layer of film whose width (called the “layflat”) is equal to ½ the circumference of the bubble. This film can then be spooled or printed on, cut into shapes, and heat sealed into bags or other items.

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An advantage of blown film extrusion over traditional film extrusion is that in the latter there are edges where there can be quality (thickness) variations.

OVERJACKETING EXTRUSION In a wire coating process, bare wire (or bundles of jacketed wires, filaments, etc) is pulled through the center of a die similar to a tubing die. Many different materials are used for this purpose depending on the application. Essentially, an insulated wire is a thin walled tube which has been formed around a bare wire. There are two different types of extrusion tooling used for coating over a wire. They are referred to as either "pressure" or "jacketing" tooling. The selection criteria for choosing which type of tooling to use is based on whether the particular application requires intimate contact or adhesion of the polymer to the wire or not. If intimate contact or adhesion is required, pressure tooling is used. If it is not desired, jacketing tooling is chosen. The main difference in jacketing and pressure tooling is the position of the pin with respect to the die. For jacketing tooling, the pin will extend all the way flush with the die. When the bare wire is fed through the pin, it does not come in direct contact with the molten polymer until it leaves the die. For pressure tooling, the end of the pin is retracted inside the crosshead, where it comes in contact with the polymer at a much higher pressure.

TUBING EXTRUSION Extruded tubing process, such as drinking straws and medical tubing, is manufactured the same as a regular extrusion process up until the die. Hollow sections are usually extruded by placing a pin or mandrel inside of the die, and in most cases positive pressure is applied to the internal cavities through the pin. Tubing with multiple lumens (holes) must be made for specialty applications. For these applications, the tooling is made by placing more than one pin in the center of the die, to produce the number of lumens necessary. In most cases, these pins are supplied with air pressure from different sources. In this way, the individual lumen sizes can be adjusted by adjusting the pressure to the individual pins.

COEXTRUSION Coextrusion is the extrusion of multiple layers of material simultaneously. This type of extrusion utilizes two or more extruders to melt and deliver a steady volumetric throughput of different viscous plastics to a single extrusion head (die) which will extrude the materials in the desired form. This technology is used on any of the processes described above (blown film, overjacketing, tubing, sheet). The layer thicknesses are controlled by the relative speeds and sizes of the individual extruders delivering the materials. There are a variety of reasons a manufacturer may choose coextrusion over single layer extrusion. One example is in the vinyl fencing industry, where coextrusion is used to tailor the layers based 5|Page


on whether they are exposed to the weather or not. Usually a thin layer of compound that contains expensive weather resistant additives are extruded on the outside while the inside has an additive package that is more suited for impact resistance and structural performance.

EXTRUSION COATING Extrusion coating is using a blown or cast film process to coat an additional layer onto an existing rollstock of paper, foil or film. For example, this process can be used to improve the characteristics of paper by coating it with polyethylene to make it more resistant to water. The extruded layer can also be used as an adhesive to bring two other materials together. A famous product that uses this technology is tetrapak.

COMPOUND EXTRUSIONS A compound extrusion is an extrusion of composite of composite materials. Usually twin-screw extruders are used because they give better conveyance characteristics and production rates. They also give a more uniform extrudate. When reinforcing fibers are mixed in a twin screw reduces screw wear because the fibers can be introduced later into the melt. Single-screw extruders are used for simple extrusions that have little variance in material formulation and viscosity.

INJECTION MOLDING Injection molding (British English: moulding) is a manufacturing process for producing parts from both thermoplastic and thermosetting plastic materials. Material is fed into a heated barrel, mixed, and forced into a mold cavity where it cools and hardens to the configuration of the mold cavity. After a product is designed, usually by an industrial designer or an engineer, molds are made by a moldmaker (or toolmaker) from metal, usually either steel or aluminium, and precision-machined to form the features of the desired part. Injection molding is widely used for manufacturing a variety of parts, from the smallest component to entire body panels of cars. of cars.

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PROCESS CHARACTERISTICS Utilizes a ram or screw-type plunger to force molten plastic material into a mold cavity Produces a solid or open-ended shape which has conformed to the contour of the mold Uses thermoplastic or thermoset materials Produces a parting line, sprue, and gate marks Ejector pin marks are usually present

APPLICATIONS Injection molding is used to create many things such as wire spools, packaging, bottle caps, automotive dashboards, pocket combs, and most other plastic products available today. Injection molding is the most common method of part manufacturing. It is ideal for producing high volumes of the same object. Some advantages of injection molding are high production rates, repeatable high tolerances, the ability to use a wide range of materials, low labour cost, minimal scrap losses, and little need to finish parts after molding. Some disadvantages of this process are expensive equipment investment, potentially high running costs, and the need to design moldable parts. parts.[6]

EXAMPLES OF POLYMERS BEST SUITED FOR THE PROCESS Most polymers may be used, including all thermoplastics, some thermosets, and some elastomers. elastomers.[7] In 1995 there were approximately 18,000 different materials available for injection molding and that number was increasing at an average rate of 750 per year. The available materials are alloys or blends of previously developed materials meaning that product designers can choose from a vast selection of materials, one that has exactly the right properties. Materials are chosen based on the strength and function required for the final part but also each material has different parameters for molding that must be taken into account. Common polymers like Epoxy and phenolic are examples of thermosetting of thermosetting plastics while nylon, polyethylene, and polystyrene are thermoplastic.

DESIGN CONSIDERATIONS FOR THE PROCESS It is important when designing products for injection molding that you consider how they will be formed in the machine, how they will be taken out of the machine, and the structure of the final product. Some important guidelines are: Use approximately uniform wall thicknesses throughout your designs. Keep walls thin - typically between 1/32" and 1/10". This allows for proper cooling and reduces cost by minimizing use of material. Thin walls also reduce problems with material shrinkage. Although some unevenness will occur due to shrinkage, walls as thick as 1/5" can be used. Keep wall thickness at least wall length / 50. Keep 90 deg walls under 0.25" high. Keep thickness of ejection pin surface wall at least .07".

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To strengthen parts, instead of using thicker walls, use additional structures such as ribs. Use fillets at the base of ribs. When using a rib make it about half the main wall thickness. Round corners and edges wherever possible. For easy release of the part from the mold, add a slight taper to the sides (typically ~ 2 deg) - especially for textured walls and walls higher than 0.25". Avoid undercuts that are impossible to remove from the mold. Lighter colors hide flow patterns better than dark colors. Where walls meet at a 90 angle, round inside and outside to at least .05" radius - sharper outside corners can create molding problems and sharper inside corners will increase tooling cost. Keep holes at least .015" from edges. edges.[10]

EQUIPMENT

Paper clip mold opened in molding machine; the nozzle is visible at right Injection molding machines consist of a material hopper, an injection ram or screw-type plunger, and a heating unit. They are also known as presses, they hold the molds in which the components are shaped. Presses are rated by tonnage, which expresses the amount of clamping force that the machine can exert. This force keeps the mold closed during the injection process. Tonnage can vary from less than 5 tons to 6000 tons, with the higher figures used in comparatively few manufacturing operations. The total clamp force needed is determined by the projected area of the part being molded. This projected area is multiplied by a clamp force of from 2 to 8 tons for each square inch of the projected areas. As a rule of thumb, 4 or 5 tons/in 2 can be used for most products. If the plastic material is very stiff, it will require more injection pressure to fill the mold, thus more clamp tonnage to hold the mold closed. The required force can also be determined by the material used and the size of the part, larger parts require higher clamping force.

MOLD Mold or die are the common terms used to describe the tooling used to produce plastic parts in molding.

Traditionally, molds have been expensive to manufacture. They were usually only used in mass production where thousands of parts were being produced. Molds are typically constructed from hardened steel, pre-hardened steel, aluminium, and/or beryllium-copper alloy. The choice of material to build a mold from is primarily one of economics, steel molds generally cost more to construct, but their longer lifespan will offset the higher initial cost over a higher number of parts 8|Page


made before wearing out. Pre-hardened steel molds are less wear resistant and are used for lower volume requirements or larger components. The steel hardness is typically 38-45 on the Rockwell-C scale. Hardened steel molds are heat treated after machining. These are by far the superior in terms of wear resistance and lifespan. Typical hardness ranges between 50 and 60 Rockwell-C (HRC). Aluminium molds can cost substantially less, and when designed and machined with modern computerized equipment, can be economical for molding tens or even hundreds of thousands of parts. Beryllium copper is used in areas of the mold which require fast heat removal or areas that see the most shear heat generated. The molds can be manufactured by either CNC machining or by using Electrical Discharge Machining processes

MOLD DESIGN

Standard two plates tooling – core and cavity are inserts in a mold base – "Family mold" of 5 different parts The mold consists of two primary components, the injection mold (A plate) and the ejector mold (B plate). Plastic resin enters the mold through a sprue in the injection mold, the sprue bushing is to seal tightly against the nozzle of the injection barrel of the molding machine and to allow molten plastic to flow from the barrel into the mold, also known as cavity The sprue bushing directs the molten plastic to the cavity images through channels that are machined into the faces of the A and B plates. These channels allow plastic to run along them, so they are referred to as runners. The molten plastic flows through the runner and enters one or more specialized gates and into the cavity geometry to form the desired part. The amount of resin required to fill the sprue, runner and cavities of a mold is a shot. Trapped air in the mold can escape through air vents that are ground into the parting line of the mold. If the trapped air is not allowed to escape, it is compressed by the pressure of the incoming material and is squeezed into the corners of the cavity, where it prevents filling and causes other defects as well. The air can become so compressed that it ignites and burns the surrounding plastic material. To allow for removal of the molded part from the mold, the mold features must not overhang one another in the direction that the mold opens, unless parts of the mold are designed to move from between such overhangs when the mold opens (utilizing components called Lifters). Sides of the part that appear parallel with the direction of draw (The axis of the cored position (hole) or insert is parallel to the up and down movement of the mold as it opens and closes) closes) [19] are typically angled slightly with (draft) to ease release of the part from the mold. Insufficient draft can cause deformation or damage. The draft required for mold release is primarily dependent on the depth of the cavity: the deeper the cavity, the more draft necessary. Shrinkage must also be taken into account when determining the draft required. If the skin is too thin, then the molded 9|Page


part will tend to shrink onto the cores that form them while cooling, and cling to those cores or part may warp, twist, blister or crack when the cavity is pulled away. The mold is usually designed so that the molded part reliably remains on the ejector (B) side of the mold when it opens, and draws the runner and the sprue out of the (A) side along with the parts. The part then falls freely when ejected from the (B) side. Tunnel gates, also known as submarine or mold gate, is located below the parting line or mold surface. The opening is machined into the surface of the mold on the parting line. The molded part is cut (by the mold) from the runner system on ejection from the mold. Ejector pins, also known as knockout pin, is a circular pin placed in either half of the mold (usually the ejector half) which pushes the finished molded product, or runner system out of a mold. The standard method of cooling is passing a coolant (usually water) through a series of holes drilled through the mold plates and connected by hoses to form a continueous pathway. The coolant absorbs heat from the mold (which has absorbed heat from the hot plastic) and keeps the mold at a proper temperature to solidify the plastic at the most efficient rate. To ease maintenance and venting, cavities and cores are divided into pieces, called inserts, and sub-assemblies, also called inserts, blocks, or chase blocks. By substituting interchangeable inserts, one mold may make several variations of the same part. More complex parts are formed using more complex molds. These may have sections called slides, that move into a cavity perpendicular to the draw direction, to form overhanging part features. When the mold is opened, the slides are pulled away from the plastic part by using stationary “angle pins” on the stationary mold half. These pins enter a slot in the slides and cause the slides to move backward when the moving half of the mold opens. The part is then ejected and the mold closes. The closing action of the mold causes the slides to move forward along the angle pins. Some molds allow previously molded parts to be reinserted to allow a new plastic layer to form around the first part. This is often referred to as overmolding. This system can allow for production of one-piece tires and wheels. 2-shot or multi-shot molds are designed to "overmold" within a single molding cycle and must be processed on specialized injection molding machines with two or more injection units. This process is actually an injection molding process performed twice. In the first step, the base color material is molded into a basic shape. Then the second material is injection-molded into the remaining open spaces. That space is then filled during the second injection step with a material of a different color. A mold can produce several copies of the same parts in a single "shot". The number of "impressions" in the mold of that part is often incorrectly referred to as cavitation. A tool with one impression will often be called a single impression(cavity) mold. A mold with 2 or more cavities of the same parts will likely be referred to as multiple impression (cavity) mold. Some extremely high production volume molds (like those for bottle caps) can have over 128 cavities.

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In some cases multiple cavity tooling will mold a series of different parts in the same tool. Some toolmakers call these molds family molds as all the parts are related.

EFFECTS ON THE MATERIAL PROPERTIES The mechanical properties of a part are usually little affected. Some parts can have internal stresses in them. This is one of the reasons why it's good to have uniform wall thickness when molding. One of the physical property changes is shrinkage. A permanent chemical property change is the material thermoset, which can't be remelted to be injected again. again.[30]

TOOL MATERIALS Tool steel or beryllium-copper are often used. Mild steel, aluminum, nickel or epoxy are only suitable for prototype or very short production runs. Modern hard aluminum (7075 and 2024 alloys) with proper mold design, can easily make molds capable of 100,000 or more part life.

GEOMETRICAL POSSIBILITIES The most commonly used plastic molding process, injection molding, is used to create a large variety of products with different shapes and sizes. Most importantly, they can create products with complex geometry that many other processes cannot. There are a few precautions when designing something that will be made using this process to reduce the risk of weak spots. First, streamline your product or keep the thickness relatively uniform. Second, try and keep your product between 2 to 20 inches. The size of a part will depend on a number of factors (material, wall thickness, shape,process etc). The initial raw material required may be measured in the form of granules, pellets or powders. Here are some ranges of the sizes.

Method

Raw Materials

Maximum Size Minimum Size

Injection Molding (thermo-plastic) (thermo-plasti c) Granules, Pellets, Powders 700 oz.

Less than 1 oz.

Injection Molding (thermo-setting) Granules, Pellets, Powders 200 oz.

Less Than 1 oz.

MACHINING Molds are built through two main methods: standard machining and EDM. Standard Machining, in its conventional form, has historically been the method of building injection molds. With technological development, CNC machining became the predominant means of making more complex molds with more accurate mold details in less time than traditional methods.

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The electrical discharge machining (EDM) or spark erosion process has become widely used in mold making. As well as allowing the formation of shapes which are difficult to machine, the process allows pre-hardened molds to be shaped so that no heat treatment is required. Changes to a hardened mold by conventional drilling and milling normally require annealing to soften the mold, followed by heat treatment to harden it again. EDM is a simple process in which a shaped electrode, usually made of copper or graphite, is very slowly lowered onto the mold surface (over a period of many hours), which is immersed in paraffin oil. A voltage applied between tool and mold causes spark erosion of the mold surface in the inverse shape of the electrode.

COST The cost of manufacturing molds depends on a very large set of factors ranging from number of cavities, size of the parts (and therefore the mold), complexity of the pieces, expected tool longevity, surface finishes and many others. The initial cost is great, however the piece part cost is low, so with greater quantities the overall price decreases.

INJECTION PROCESS

Small injection molder showing hopper, nozzle and die area With Injection Molding, granular plastic is fed by gravity from a hopper into a heated barrel. As the granules are slowly moved forward by a screw-type plunger, the plastic is forced into a heated chamber, where it is melted. As the plunger advances, the melted plastic is forced through a nozzle that rests against the mold, allowing it to enter the mold cavity through a gate and runner system. The mold remains cold so the plastic solidifies almost as soon as the mold is filled.

INJECTION MOLDING CYCLE The sequence of events during the injection mold of a plastic part is called the injection molding cycle. The cycle begins when the mold closes, followed by the injection of the polymer into the mold cavity. Once the cavity is filled, a holding pressure is maintained to compensate for material shrinkage. In the next step, the screw turns, feeding the next shot to the front screw.This causes the screw to retract as the next shot is prepared. Once the part is sufficiently cool, the mold opens and the part is ejected.

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TIME FUNCTION The time it takes to make a product using injection molding can be calculated by adding: Twice the Mold Open/Close Time (2M) + Injection Time (T) + Cooling Time (C) + Ejection Time (E) Where T is found by dividing: Mold Size (S) / Flow Rate (F)

Total time = 2M + T + C + E T = V/R V = Mold cavity size (in3) R = Material flow rate (in3/min) [37] The total cycle time can be calculated using tcycle = tclosing + tcooling + tejection ejection

The closing and ejection times, can last from a fraction of a second to a few seconds, depending on the size of the mold and machine. The cooling times, which dominate the process, depend on the maximum thickness of the part.

DIFFERENT TYPES OF INJECTION MOLDING PROCESSES Although most injection molding processes are covered by the conventional process description above, there are several important molding variations including: Co-injection (sandwich) molding Fusible (lost, soluble) core injection molding Gas-assisted injection molding In-mold decoration and in mold lamination Injection-compression molding Insert and outsert molding Lamellar (microlayer) injection molding Low-pressure injection molding Metal injection molding Microinjection molding Microcellular molding Multicomponent injection molding (overmolding) Multiple live-feed injection molding 13 | P a g e


Powder injection molding Push-Pull injection molding Reaction injection molding Resin transfer molding Rheomolding Structural foam injection molding Structural reaction injection molding Thin-wall molding Vibration gas injection molding Water assisted injection molding Rubber injection Injection molding of liquid silicone rubber For more details about the different types injection processes.

PROCESS TROUBLESHOOTING Optimal process settings are critical to influencing the cost, quality, and productivity of plastic injection molding. The main trouble in injection molding is to have a box of good plastics parts contaminated with scrap. For that reason process optimization studies have to be done and process monitoring has to take place. To have a constant filling rate in the cavity the switch over from injection phase to the holding phase can be made based on a cavity pressure level. Having a stable production window the following issues are worth to investigate: The Metering phase can be optimized by varying screw turns per minute and backpressure. Variation of time needed to reload the screw gives an indication of the stability of this phase. Injection speed can be optimized by pressure drop studies between pressure measured in the Nozzle (alternatively hydraulic pressure) and pressure measured in the cavity. Melted material with a lower viscosity has less pressure loss from nozzle to cavity than material with a higher viscosity. Varying the Injection speed changes the sheer rate. Higher speed = higher sheer rate = lower viscosity. Pay attention increasing the mold and melt temperature lowers the viscosity but lowers the sheer rate too. Gate seal or gate freeze / sink mark / weight and geometry studies have the approach to prevent sink marks and geometrical faults. Optimizing the high and duration of applied holding pressure based on cavity pressure curves is the appropriate way to go. The thicker the part the longer the holding pressure applied. The thinner the part the shorter the holding pressure applied. Cooling time starts once the injection phase is finished. The hotter the melted plastics the longer the cooling time the thicker the part produced the longer the cooling time.

MOLDING TRIAL When filling a new or unfamiliar mold for the first time, where shot size for that mold is unknown, a technician/tool setter usually starts with a small shot weight and fills gradually until the mold is 95 to 99% full. Once this is achieved a small amount of holding pressure will be applied and holding time increased until gate freeze off (solidification time) has occurred. Gate solidification time is an important as it determines cycle time, which itself is an important issue in 14 | P a g e


the economics of the production process. Holding pressure is increased until the parts are free of sinks and part weight has been achieved. Once the parts are good enough and have passed any specific criteria, a setting sheet is produced for people to follow in the future. The method to setup an unknown mold the first time can be supported by installing cavity pressure sensors. To see how much the cavities are filled the pressure in the cavity gives a good indication for. Once the mold is set up the first time modern monitoring systems can save a reference curve of the cavity pressure. With that it is possible to reproduce the same part quality on another molding machine within a short setup time.

MOLDING DEFECTS Injection molding is a complex technology with possible production problems. They can either be caused by defects in the molds or more often by part processing (molding)

Molding Defects

Alternative name

Descriptions

Blister

Blistering

Raised or layered Tool or material is too hot, often caused by a zone on surface of lack of cooling around the tool or a faulty the part heater

Burn marks

Black or brown burnt areas on the Air Burn/ Gas part located at Tool lacks venting, injection speed is too Burn/ furthest points from high Dieseling gate or where air is trapped

Causes

Color streaks Colour (US) streaks (UK)

Masterbatch isn't mixing properly, or the material has run out and it's starting to come Localized change of through as natural only. Previous colored color/colour material "dragging" in nozzle or check valve.

Delamination

Contamination of the material e.g. PP mixed with ABS, very dangerous if the part is Thin mica like layers being used for a safety critical application as formed in part wall the material has very little strength when delaminated as the materials cannot bond

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Flash

Embedded contaminates

Flow marks

Burrs

Mold is over packed or parting line on the Excess material in tool is damaged, too much injection thin layer exceeding speed/material injected, clamping force too normal part low. Can also be caused by dirt and geometry contaminants around tooling surfaces.

Embedded particulates

Foreign particle (burnt material or other) embedded in the part

Flow lines

Injection speeds too slow (the plastic has Directionally "off cooled down too much during injection, tone" wavy lines or injection speeds must be set as fast as you patterns can get away with at all times)

Deformed part turbulent flow material

Jetting

Knit Lines

Weld lines

by Poor tool design, gate position or runner. of Injection speed set too high.

Small lines on the backside of core pins or windows in parts that look like just lines.

Caused by the melt-front flowing around an object standing proud in a plastic part as well as at the end of fill where the melt-front comes together again. Can be minimized or eliminated with a mold-flow study when the mold is in design phase. Once the mold is made and the gate is placed one can only minimize this flaw by changing the melt and the mold temperature.

polymer breakdown Excess water in the granules, excessive from hydrolysis, temperatures in barrel oxidation etc

Polymer degradation

Sink marks

Particles on the tool surface, contaminated material or foreign debris in the barrel, or too much shear heat burning the material prior to injection

[sinks]

Holding time/pressure too low, cooling time Localized depression too short, with sprueless hot runners this can (In thicker zones) also be caused by the gate temperature being set too high. Excessive material or thick wall

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thickness.

Short shot

Non-fill Short mold

Splay marks

Moisture in the material, usually when Circular pattern hygroscopic resins are dried improperly. Splash mark / around gate caused Trapping of gas in "rib" areas due to Silver streaks by hot gas excessive injection velocity in these areas. Material too hot.

Stringiness

Stringing

Warping

Partial part

Lack of material, injection speed or pressure too low, mold too cold

String like remain Nozzle temperature too high. Gate hasn't from previous shot frozen off transfer in new shot

Lack of holding pressure (holding pressure is used to pack out the part during the Empty space within holding time). Also mold may be out of part (Air pocket) registration (when the two halves don't center properly and part walls are not the same thickness).

Voids

Weld line

/

Knit line / Discolored Meld line / where two Transfer line fronts meet

Twisting

Distorted part

Mold/material temperatures set too low (the line material is cold when they meet, so they flow don't bond). Point between injection and transfer (to packing and holding) too early.

Cooling is too short, material is too hot, lack of cooling around the tool, incorrect water temperatures (the parts bow inwards towards the hot side of the tool)

TOLERANCES AND SURFACES Molding tolerance is a specified allowance on the deviation in parameters such as dimensions, weights, shapes, or angles, etc. To maximize control in setting tolerances there is usually a minimum and maximum limit on thickness, based on the process used. used.[41] Injection molding typically is capable of tolerances equivalent to an IT Grade of about 9– 14. 14. The possible tolerance

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of a thermoplastic or a thermoset is ±0.008 to ±0.002 inches. Surface finishes of two to four microinches or better are can be obtained. Rough or pebbled surfaces are also possible.

Molding Type Typical Possible

Thermoplastic ±0.008 ±0.002

Thermoset

±0.008 ±0.002

LUBRICATION AND COOLING Obviously, the mold must be cooled in order for the production to take place. Because of the heat capacity, inexpensiveness, and availability of water, water is used as the primary cooling agent. To cool the mold, water can be channeled through the mold to account for quick cooling times. Usually a colder mold is more efficient because this allows for faster cycle times. However, this is not always true because crystalline materials require the opposite of a warmer mold and lengthier cycle time.

POWER REQUIREMENTS The power required for this process of injection molding depends on many things and varies between materials used. Manufacturing Processes Reference Guide states that the power requirements depend on "a material's specific gravity, melting point, thermal conductivity, part size, and molding rate." Below is a table from page 243 of the same reference as previously mentioned which best illustrates the characteristics relevant to the power required for the most commonly used materials.

Material

Specific Gravity Melting Point (°F)

Epoxy

1.12 to 1.24

248

Phenolic

1.34 to 1.95

248

Nylon

1.01 to 1.15

381 to 509

Polyethylene 0.91 to 0.965

230 to 243

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INSERTS Metal inserts can be also be injection molded into the workpiece. For large volume parts the inserts are placed in the mold using automated machinery. An advantage of using automated components is that the smaller size of parts allows a mobile inspection system that can be used to examine multiple parts in a decreased amount of time. In addition to mounting inspection systems on automated components, multiple axial robots are also capable of removing parts from the mold and place them in latter systems that can be used to ensure quality of multiple parameters. The ability of automated components to decrease the cycle time of the processes allows for a greater output of quality parts. Specific instances of this increased efficiency include the removal of parts from the mold immediately after the parts are created and use in conjunction with vision systems. The removal of parts is achieved by using robots to grip the part once it has become free from the mold after in ejector pins have been raised. The robot then moves these parts into either a holding location or directly onto an inspection system, depending on the type of product and the general layout of the rest of the manufacturer's production facility. Visions systems mounted on robots are also an advancement that has greatly changed the way that quality control is performed in insert molded parts. A mobile robot is able to more precisely determine the accuracy of the metal component and inspect more locations in the same amount of time as a human inspector.

TRANSFER MOLDING Transfer molding, like compression molding, is a process where the amount of molding material (usually a thermoset plastic) is measured and inserted before the molding takes place. The molding material is preheated and loaded into a chamber known as the pot . A plunger is then used to force the material from the pot through channels known as a sprue and runner system into the mold cavities. The mold remains closed as the material is inserted and is opened to release the part from the sprue and runner. The mold walls are heated to a temperature above the melting point of the mold material; this allows a faster flow of material through the cavities. Transfer Molding. This is an automated operation that combines compression-, molding, and

transfer-molding processes. This combination has the good surface finish, dimensional stability, and mechanical properties obtained in compression molding and the high-automation capability and low cost of injection molding and transfer molding. Transfer Molding is having a "piston and cylinder"-like device built into the mold so that the rubber is squirted into the cavity through small holes. A piece of uncured rubber is placed into a portion of the transfer mold called the "pot." The mold is closed and under hydraulic pressure the rubber or plastic is forced through a small hole (the "gate") into the cavity. The mold is held closed while the plastic or rubber cures. The plunger is raised up and the "transfer pad" material may be removed and thrown away. The transfer mold is opened and the part can be removed. The flash and the gate may need to be trimmed. Another key point is that a premeasured amount of thermosetting plastic in powder, preform, and even granular form can be placed into the heating chamber. The molds in both compression and transfer molding remain closed until the curing reaction within the material is complete. Ejector pins are usually incorporated into the design of the 19 | P a g e


molding tool and are used to push the part from the mold once it has hardened. These types of molding are ideal for high production runs as they have short production cycles. Transfer molding, unlike compression molding uses a closed mold, so smaller tolerances and more intricate parts can be achieved. The fixed cost of the tooling in transfer molding is greater than in compression molding and as both methods produce waste material, whether it be flash or the material remaining in the sprue and runners, transfer molding is the more expensive process. Transfer molding (TM) (or resin transfer molding, RTM) differs from compression molding in that in TM the resin is inserted into the mold (or tool) which contains the layers of fibres or a preform, whereas in compression molding prepregs or molding compounds are in the mold which is then heated and pressure is applied. No further pressure is applied in TM. In RTM the resin is injected or drawn into a mold, which contains the fibres, from a homogeniser under low pressure. The mold can be made from composites for low production cycles or with aluminium or steel for larger production. The differences between the two types being that metal has better heat transfer, hence quicker cycle times; metal lasts longer and deforms less, but at a higher cost. The main problem with this production route is that air can be trapped in mold and hence a method must be incorporated for allowing this air to escape. A number of solutions to the problem exist including extending one level of reinforcement beyond the cavity (with a 25% resin loss), appropriate vents and creating a vacuum in the mold (which also improves quality). Larger structures, better properties (less movement of fibres), increased flexibility of design and lower cost are some of the advantage this process has over compression molding due mainly to the low pressure injection. Other benefits include rapid manufacture, not labour intensive, ability to vary reinforcements easily or include cores such as foam and produce low and high quality products. In the semiconductor industry, package encapsulation is usually done with transfer molding due to the high accuracy of transfer molding tooling and low cycle time of the process. However, the drive to introduce "Green" manufacturing is becoming a mandatory process in most semicon assembly operations. New transfer mold designs integrated with suitable surface treatments like CrN, MiCC and H Cr plating are becoming more popular in the industry. Some common products are utensil handles, electric appliance parts, electronic component, and connectors. Transfer molding is widely used to enclose or encapsulate items such as coils, integrated circuits, plugs, connectors, and other components.

BLOW MOLDING Blow molding, also known as blow forming, is a manufacturing process by which hollow plastic parts are formed. It is a process used to produce hollow objects from thermoplastic.

In general, there are three main types of blow molding: extrusion blow molding , injection blow molding, and stretch blow molding .

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The blow molding process begins with melting down the plastic and forming it into a parison or preform. The parison is a tube-like piece of plastic with a hole in one end in which compressed air can pass through. The basic process has two fundamental phases. First, a preform (or parison) of hot plastic resin in a somewhat tubular shape is created. Second, a pressurized gas, usually air, is used to expand the hot preform and press it against a mold cavity. The pressure is held until the plastic cools. This action identifies another common feature of blow molded articles. Part dimensional detail is better controlled on the outside than on the inside, where material wall thickness can alter the internal shape. Once the plastic has cooled and hardened the mold opens up and the part is ejected.

PRODUCTS A blow molding renaissance is occurring in which engineers and designers are discovering and promoting blow molding for a wide variety of industrial or technical application. Toy wheels, automobile seat back, ductwork, surf boards, bellows, fuel tanks, flower pots, automobile bumpers, double- walled tool cases, and cabinet panels are just a few examples of the many creative design being developed.

EXTRUSION BLOW MOLDING In extrusion blow molding (EBM), plastic is melted and extruded into a hollow tube (a parison). Blow molding is the forming of a hollow object by “blowing” a thermoplastic molten t ube called a parison in the shape of a mold cavity. Extrusion blow molding is the most widely used of many blow molding methods. This parison is then captured by closing it into a cooled metal mold. Air is then blown into the parison, inflating it into the shape of the hollow bottle, container or part. After the plastic has cooled sufficiently, the mold is opened and the part is ejected. There are two extrusion blow processes: continuous and intermittent.

CONTINUOUS EXTRUSION BLOW MOLDING Continuous and Intermittent are two variations of Extrusion Blow Molding. In Continuous Extrusion Blow Molding the parison is extruded continuously and the individual parts are cut off by a suitable knife. EBM processes may be either continuous (constant extrusion of the parison) or intermittent. Types of EBM equipment may be categorized as follows:

Continuous extrusion equipment

rotary wheel blow molding systems shuttle machinery Examples of parts made by the EBM process include dairy containers, shampoo bottles, hoses/pipes, and hollow industrial parts such as drums.

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INTERMITTENT BLOW MOLDING Intermittent extrusion may be also called shot extrusion. Parison shot extrusion is accomplished by means of a reciprocating screw almost identical to those used in injection molding machines. [8]

In Intermittent blow molding there are two processes: straight intermittent is similar to injection molding whereby the screw turns, then stops and pushes the melt out. With the accumulator method, an accumulator gathers melted plastic and when the previous mold has cooled and enough plastic has accumulated, a rod pushes the melted plastic and forms the parison. In this case the screw may turn continuously or intermittently. Intermittent extrusion machinery

reciprocating screw machinery accumulator head machinery

CONTROLLING WALL DISTRIBUTION Control of wall distribution is the heart of blow molding. There are two primary techniques in extrusion blow molding for controlling wall distribution: Programming and die shaping. Programming is the control of the wall thickness, from top to bottom, of the parsion as it emerges from the die head tooling during extrusion. In die shaping, sectors of the die bushing or mandrel are machined to thicken the parison longitudinally in those areas where the part being formed requires greater thickness. The diameter of the die tooling is very important, for it determines the parison diameter. Too small a parison will rupture r upture or “blow out” because of too much stretch. Too large a parison will result in too much flash, and cause trimming problems. problems.[10]

INJECTION BLOW MOLDING The injection molding phase consists of injection molding a thermoplastic material into a hollow, tube-shaped article called a preform. The preform is transferred on a metal shank, called the core rod, into a blow mold. The process of injection blow molding (IBM) is used for the production of hollow glass and plastic objects in large quantities. In the IBM process, the polymer is injection molded onto a core pin; then the core pin is rotated to a blow molding station to be inflated and cooled. This is the least-used of the three blow molding processes, and is typically used to make small medical and single serve bottles. The process is divided into three steps: injection, blowing and ejection. The injection blow molding machine is based on an extruder barrel and screw assembly which melts the polymer. The molten polymer is fed into a manifold where it is injected through nozzles into a hollow, heated preform mold. The preform mold forms the external shape and is clamped around a mandrel (the core rod) which forms the internal shape of the preform. The preform consists of a fully formed bottle/jar neck with a thick tube of polymer attached, which will form the body. 22 | P a g e


The preform mold opens and the core rod is rotated and clamped into the hollow, chilled blow mold. The core rod opens and allows compressed air into the preform, which inflates it to the finished article shape. After a cooling period the blow mold opens and the core rod is rotated to the ejection position. The finished article is stripped off the core rod and leak-tested prior to packing. The preform and blow mold can have many cavities, typically three to sixteen depending on the article size and the required output. There are three sets of core rods, which allow concurrent preform injection, blow molding and ejection. Each thermoplastic resin has its own set of tooling design parameters. Hot melt density, shrink factors, stretch ratios, blow pressure, venting criteria, and surface area of the tooling must all be known prior to designing any tooling.

STRETCH BLOW MOLDING With the commercial introduction of containers, stretch blow molding became a common term in the blow molding industry. Stretch blow molding is the method of producing a plastic container from a preform or parison that is stretched in both the hoop direction and the axial direction when the preform is blown into its desired container shape. Stretch blow molding is possible for various thermoplastic materials such as acrylnitrile(AN), polystyrene (PS), Polyvinyl chloride (PVC), polyamide (PA), polycarbonate (PC), Polysulfone, acetal, polyarlyate, polypropylene (PP), surlyn, and polyethylene terephthalate (PET). Amphorous materials such as PET. Etc. In the stretch blow molding (SBM) process, the plastic is first molded into a "preform" using the injection molding process. These preforms are produced with the necks of the bottles, including threads (the "finish") on one end. These preforms are packaged, and fed later (after cooling) into a reheat stretch blow molding machine. In the SBM process, the preforms are heated (typically using infrared heaters) above their glass transition temperature, then blown using high pressure air into bottles using metal blow molds. Usually the preform is stretched with a core rod as part of the process. In the single-stage process both preform manufacture and bottle blowing are performed in the same machine. The stretching of some polymers, such as PET (polyethylene terephthalate) results in strain hardening of the resin, allowing the bottles to resist deforming under the pressures formed by carbonated beverages, which typically approach 60 psi.The main applications are bottles, jars and other containers. Advantages of blow molding include: low tool and die cost; fast production rates; ability to mold complex part; produces recyclable parts. Increase the material’s tensile strength, barrier properties, clarity. Reduce weight stretch blow molding produces a container from less raw material and with improved economics and bottle properties. Disadvantages of blow molding include: limited to hollow parts, wall thickness is hard to control.

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NEW APPLICATIONS OF BLOW MOLDING TECHNOLOGY COEXTRUSION BLOW MOLDING OF LARGE PARTS Coextrusion is used extensively in small bottles and containers, but increasingly also for large blow molded parts (18.92 liters or 5 gallons and larger). Its growth, however, is very healthy and will accelerate in the coming years because of the need for recycling of plastic and the application of this technology to manufacture of products requiring improved barrier properties.

THREE-DIMENSIONAL BLOW MOLDING The three-dimensional blow molding concept was developed several years ago in Japan. The most successful of these technologies is the Placo X-Y machine which moves the mold under the head. There are many advantages to three-dimensional blow molding, including minimal flash, seamless parts, and sequential extrusion. Many complex shapes can be easily produced using the threedimensional blow molding process. At this time, process options for three-dimensional molding: X-Y process, suction blow molding, and curved blow molding are offered by Krupp.

HARD-SOFT-HARD AND SOFT-HARD-SOFT TECHNOLOGY It has been shown that a combination of various materials imparts specific properties to a part in extrusion blow molding. Here, the extruded parison is built up in a radial direction by several overlaying layers (traditional or radial coextrusion). New in practical application, however, is the use of various materials in the axial direction, that is, specific article sections may be provided with specific properties by choosing corresponding materials.

LONG-GLASS-FIBER-REINFORCED BLOW MOLDING The use of short-glass-fiber-reinforced thermoplastics (mainly PA 6, PA 6.6, and PP) with medium fiber lengths between 0.2 and 0.4 mm (0.007 in. and 0.0156 in.) has been known for some time. Unfortunately it was impossible to realize improvements in tensile strength, modulus of elasticity, and deflection temperature under load in the order hoped for by using glass fibers. The reason was the insufficient length of the fibers, which was reduced substantially from its original length as a consequence of the shearing action in the extruder.

BLOW MOLDING FOAM TECHNOLOGY Krupp Kautex, Borealis, and OBG Design introduced foam technology, based on a patent held by leading equipment manufacture Krupp Kautex (Patent Number US 4.874.649). A special-recipe mastermix from Borealis, a major polylefin producer, allows highly reliable processing and consistent results.

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POLYMER CHARACTERISTICS RELEVANT FOR BLOW MOLDING High-Density Polyethylene (HDPE) High impact strength Low temperature toughness, Excellent resistance to chemicals, Good electrical insulating properties, and Poor ultraviolet resistance.

Acrylonitrile-butadiene-styr Acrylonitrile-butadiene-styrene ene (ABS) ABS is a hard, tough material; Good impact resistance; Good electrical insulation properties; and Versatile additive, filler, and reinforcing agents acceptance;

Polycarbonate Polycarbonate (PC) Excellent resistance to heat, Hard, tough material, Good impact resistance, and Excellent transparency.

Polypropylene Polypropylene (PP) Good impact strength (poor at cold temperatures), Good chemical resistance, High abrasion resistance, High melt strength.

Polyphenylene Polyphenylene Oxide (PPO) Good flame retardancy, Good chemical resistance, Good impact resistance, and Retains mechanical properties in high heat environments.

Polyethylene Polyethylene Terephthalate (PET) Used in injection blow molding, produces clear amorphous preforms. Since there is less orientation, the impact strength is reduced.

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EQUIPMENTS OF BLOW MOLDING There are six kinds of PET Blow Molding Machineries: Single Stage Blow Molding Machine Two Stage Blow Molding Machine Integrated Two Stage Blow Molding Machine PET Blow Molding Machines Automatic PET blow molding machine Semi Automatic PET blow molding machine

SINGLE STAGE BLOW MOLDING MACHINE All one-stage injection stretch blow molding machines derived from this original Stretch Blow design are referred to as classic one-stage machines, as the concept has long since been extended into other PET developments. The classic one-stage machines design is extremely versatile in that the same basic machine design can be used to make a wide variety of bottles and jars in all shapes and sizes, which was the standard single-stage machine in the early years, had eight cavities for 1.5 liter bottles.

TWO STAGE BLOW MOLDING MACHINE In the early developments, preforms were made by continuously extruding a PET Blow Molding tube. To make these preforms, a preform manufacturing machine that took a continuously extruded PET tube, heated and closed one end, and then heated the other and formed a thread finish by blow molding. This process had a faster output rate, at 12000 preforms per hour, than the early injection molding routes of 8 and 16 cavity moulds. Being extruded, the preforms could be multilayered with barrier materials. The system was overtaken by injection molded preforms as the cavitations increased to 32 and beyond. The quality of the injection molded (IM) neck, adding for example vent slots, made the IM finish preferable. Moreover, IM technology is available from more than one company, giving customers greater technical and commercial choice. Two-stage technology machine with six blow moulds operating at around 4000 bottles per hour. Subsequent mould and cooling development increased the output to 6000 bottles per hour. Two stage PET processing includes: Making preforms by Injection Molding Blowing bottles by Stretch Blow Molding Because it is more flexible than one step processing, it is widely accepted in Plastic packaging market.

INTEGRATED TWO STAGE BLOW MOLDING MACHINE In Integrated Two Stage approach the preforms were made by more conventional injection molding routes (with the number of cavities optimized to match the required output) and then, while still hot, were carried to a separate blowing machine with the optimized number of blow moulds to suit the required output. This was the first integrated approach to PET bottle making. 26 | P a g e


Here the equipment had the same number of injection cavities as blowing molds. This was a more compact approach and proved ideal for small batch output with excellent glossy surfaces.

PET BLOW MOLDING MACHINERY PET Blow Molding Machineries are suitable for small and medium sized bottle makers and drinks manufacturers worldwide to produce their own bottle. The machine starts with the preform, heat it, and then stretch-blow the heated preform in the blow mold into oriented PET container. According to different requirement, PET Blow Molding Machine has the ability to produce PET bottle from narrow neck finishes to wide mouth finishes in the size from 5 ml to 20,000 ml. The wide applicability for both narrow and wide neck, reliable capability make PET Blow Molding Machine become popular and attractive to medium and small size bottle and drink manufacturer all over the world. PET bottles and containers are widely utilized in food and drink industry. PET has the advantages of virtually unbreakable, easy to handle and transport, low cost and easy to manufacture and recycle. There are two type of Pet Blow Molding Machine: Automatic Pet Blow Molding Machine Semi-Automatic Pet Blow Molding Machine

AUTOMATIC PET BLOW MOLDING MACHINE Automatic PET Blow Molding Machine is a new generation of fully automatic and versatile machine for mass production of PET bottles and jars. Ideally suitable for large scale factories and for applications where stringent quality requirements have to be met. Substantial reduction in operating cost is achieved by saving of manpower. There are two types of Automatic PET Blow Molding Machine One Stage Automatic PET Blow Molding Machine Two Stage Automatic PET Blow Molding Machine

Full-automatic PET stretch blow molding machine is the most stable two-step automatic stretch blow molding machine. It has one to four cavities. It can blow bottles in shapes such as carbonated, mineral, pesticide, cosmetics, wide-mouth, hot filling, and other packing containers, which is made of plastic of crystalline type such as PET and PP etc. Features (Automatic PET Blow Molding Machine)

Runs stably, whether voltage is 200V or 240V, or whether room temperature is 0C or 400C. Conveying preforms automatically with conveyor. Strong penetrability and good and swift distribution of the heat by letting the bottles rotate by itself and revolute in the rails simultaneously in the infrared preheater.

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Minimum parameter settings through touch LCD, normally only temperature settings should be regulated, no need to play with time settings; Multi-language interfaces Easy installation and starting Satisfaction with different atmospheric pressure for blowing and mechanical action by dividing the blowing and action into three parts in the air pressure diagram of the machine. More mechanical & synchronized design to reduce fault to minimum level. Low rejection rate Air recycling unit to save compressed air 20~25%, Reduced heating power, Driving is mostly done by motor directly, instead of compressed air. 200% more efficient than traditional one. Adopts Aluminum mold, lighter to be installed or unloaded; 10 molding data memories to make mold changeover quicker. High safeties with security automatic-locking apparatus in each mechanical action, which will make the procedures turn into a state of safety in case of a breakdown in certain procedure. Safe, reliable, and unique design of the position of valve to make the air pressure diagram of the machine easier to understand.

SEMI AUTOMATIC PET BLOW MOLDING MACHINE SEMI-AUTO PET blow molding machines are the most cost-effective solution for mass production of jars and bottles for small and medium scale factories. The machines are also designed for the production of big jars and containers. There are two types of Semi Automatic PET Blow Molding Machine One Stage Semi Automatic PET Blow Molding Machine Two Stage Semi Automatic PET Blow Molding Machine

Two Stage Semi Automatic PET Blow Molding Machines are fit to blow carbonated beverage bottles, mineral water bottles, cosmetics bottles and hot-filling bottles. With microcomputer controlling system, it controls various technical parameter needed more accurately and more steadily. It can operate easily without any special training and more safely. Features

Simple installation & startup adjustment. Two Stage Semi Automatic PET Blow Molding Machine Ensures extremely uniform heating for PET preforms, superior molding performance and minimum product defects. Offers two functions in a single machine by simply changing the mold from single to double cavity mold. Heating zones can be adjusted to meet production variations. Equipped with special designed air storage unit. Heating tubes are adjustable in any direction such as up or down, forward or backward to suit various preform sizes and shapes. 28 | P a g e


Full-time stabilized heating system. Heating temperature and speed are adjustable according to sizes and features of preforms. Independent Temperature Control for different heating lamp to get ideal preform heating results. Steady operation, negligible trouble and easy maintenance. Operated easily without special training. Requires little base space, suitable for small to medium size factories. Negligible waste rate of finished bottle. Blow Molding Applications

Milk bottles Pharmaceutical bottles Antifreeze bottles Polypropylene bottles Coliseum seats One-piece chair Ice chests and coolers Double-wall player case Garbage cans Drums Fuel tanks

DECORATION OF BLOW MOLDED PRODUCTS There are several ways to decorate blow molded products, they include: Labels- both post-molding and in-molding Decals Screen printing Hot stamping Pad printing Paint

FINISHING Finishing of a blow molded part should be considered in product design, mold engineering, and process planning stages.

EXTRUSION

Extruded aluminium with several hollow cavities; slots allow bars to be joined with special connectors.

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Extrusion is a process used to create objects of a fixed cross-sectional profile. A material is pushed or drawn through a die of the desired cross-section. The two main advantages of this process over other manufacturing processes is its ability to create very complex cross-sections and work materials that are brittle, because the material only encounters compressive and shear stresses. It also forms finished parts with an excellent surface finish. finish.[1]

Extrusion may be continuous (theoretically producing indefinitely long material) or semicontinuous (producing many pieces). The extrusion process can be done with the material hot or cold. Commonly extruded materials include metals, polymers, ceramics, concrete and foodstuffs. Hollow cavities within extruded material cannot be produced using a simple flat extrusion die, because there would be no way to support the center barrier of the die. Instead, the die assumes the shape of a block with depth, beginning first with a shape profile that supports the center section. The die shape then internally morphs along its length into the final shape, with the suspended center pieces supported from the back of the die.

PROCESS

Extrusion of a round blank through a die.

The process begins by heating the stock material. It is then loaded into the container in the press. A dummy block is placed behind it where the ram then presses on the material to push it out of the die. Afterward the extrusion is stretched in order to straighten it. If better properties are required then it may be heat treated or cold worked.

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The extrusion ratio is defined as the starting cross-sectional area divided by the cross-sectional area of the final extrusion. One of the main advantages of the extrusion process is that this ratio can be very large while still producing quality parts.

HOT EXTRUSION Hot extrusion is done at an elevated temperature to keep the material from work hardening and to make it easier to push the material through the die. Most hot extrusions are done on horizontal hydraulic presses that range from 250 to 12,000 tons. Pressures range from 30 to 700 MPa (4,400 to 102,000 psi), therefore lubrication is i s required, which can be b e oil or graphite for lower temperature extrusions, or glass powder for higher temperature extrusions. The biggest disadvantage of this process is its cost for machinery and its upkeep.

Hot extrusion temperature for various metals metal s[1]

Material

Temperature [°C (°F)]

Magnesium

350-450 (650-850)

Aluminium

350-500 (650-900)

Copper

600-1100 (1200-2000)

Steel

1200-1300 (2200-2400)

Titanium

700-1200 (1300-2100)

Nickel

1000-1200 (1900-2200)

Refractory alloys

up to 2000 (4000)

The extrusion process is generally economical when producing between several kilograms (pounds) and many tons, depending on the material being extruded. There is a crossover point where rolling becomes more economical. For instance, some steels become more economical to roll if producing more than 20,000 kg (50,000 lb).

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COLD EXTRUSION Cold extrusion is done at room temperature or near room temperature. The advantages of this over hot extrusion are the lack of oxidation, higher strength due to cold working, closer tolerances, good surface finish, and fast extrusion speeds if the material is subject to hot shortness. Materials that are commonly cold extruded include: lead, tin, aluminum, copper, zirconium, titanium, molybdenum, beryllium, vanadium, niobium, and steel. Examples of products produced by this process are: collapsible tubes, fire extinguisher cases, shock absorber cylinders, automotive pistons, and gear blanks.

WARM EXTRUSION Warm extrusion is done above room temperature, but below the recrystallization temperature of the material the temperatures ranges from 800 to 1800 °F (424 to 975 °C). It is usually used to achieve the proper balance of required forces, ductility and final extrusion properties. properties.[3]

EQUIPMENT There are many different variations of extrusion equipment. They vary by four major characteristics: characteristics:[1] 1. Movement of the extrusion with relation to the ram. If the die is held stationary and the ram moves towards it then its called "direct extrusion". If the ram is held stationary and the die moves towards the ram its called "indirect extrusion". 2. The position of the press, either vertical o r horizontal. 3. The type of drive, either hydraulic or mechanical. 4. The type of load applied, either conventional (variable) or hydrostatic.

A single or twin screw auger, powered by an electric motor, or a ram, driven by hydraulic pressure (often used for steel and titanium alloys), oil pressure (for aluminum), or in other specialized processes such as rollers inside a perforated drum for the production of many simultaneous streams of material. Typical extrusion presses cost more than $100,000, whereas dies can cost up to $2000. [edit] Forming [edit] Forming internal cavities

There are several methods for forming internal cavities in extrusions. One way is to use a hollow billet and then use a fixed or floating mandrel. A fixed mandrel, also known as a German type, means it is integrated into the dummy block and stem. A floating mandrel, also known as a French type, floats in slots in the dummy block and aligns itself in the die when extruding. If a solid billet is used as the feed material then it must first be pierced by the mandrel before extruding through the die. A special press is used in order to control the mandrel independently from the ram. The solid billet could also be used with a spider die, porthole die or bridge die. All of these types of

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dies incorporate the mandrel in the die and have "legs" that hold the mandrel in place. During extrusion the metal divides and flows around the legs, leaving weld lines in the final product. Direct extrusion

Plot of forces required by various extrusion processes.

Direct extrusion, also known as forward extrusion, is the most common extrusion process. It works by placing the billet in a heavy walled container. The billet is pushed through the die by a ram or screw. There is a reusable dummy block between the ram and the billet to keep them separated. The major disadvantage of this process is that the force required to extrude the billet is greater than that need in the indirect extrusion process because of the frictional forces introduced by the need for the billet to travel the entire length of the container. Because of this the greatest force required is at the beginning of process and slowly decreases as the billet is used up. At the end of the billet the force greatly increases because the billet is thin and the material must flow radially to exit the die. The end of the billet, called the butt end, is not used for this reason. Indirect extrusion

In indirect extrusion, also known as backwards extrusion, the billet and container move together while the die is stationary. The die is held in place by a "stem" which has to be longer than the container length. The maximum length of the extrusion is ultimately dictated by the column strength of the stem. Because the billet moves with the container the frictional forces are eliminated. This leads to the following advantages: A 25 to 30% reduction of friction, which allows for extruding larger billets, increasing speed, and an increased ability to extrude smaller cross-sections There is less of a tendency for extrusions to crack because there is no heat formed from friction The container liner will last longer l onger due to less wear The billet is used more uniformly so extrusion defects and coarse grained peripherals zones are less likely.

The disadvantages are: Impurities and defects on the surface of the billet affect the surface of the extrusion. These defects ruin the piece if it needs to be anodized or the aesthetics are important. In order to get around this the billets may be wire brushed, machined or chemically cleaned before being used. 33 | P a g e

Advanced Manufacturing Techniques This process isn't as versatile as direct extrusions because the cross-sectional area is limited by the maximum size of the stem.

Hydrostatic extrusion

In the hydrostatic extrusion process the billet is completely surrounded by a pressurized liquid, except where the billet contacts the die. This process can be done hot, warm, or cold, however the temperature is limited by the stability of the fluid used. The process must be carried out in a sealed cylinder to contain the hydrostatic medium. The fluid can be pressurized two ways: ways:[6] 1. Constant-rate extrusion: A ram or plunger is used to pressurize the fluid inside the container. 2. Constant-pressure extrusion: A pump is used, possibly with a pressure intensifier, to pressurize the fluid, which is then pumped to the container.

The advantages of this process include: No friction between the container and the billet reduces force requirements. This ultimately allows for faster speeds, higher reduction ratios, and lower billet temperatures. Usually the ductility of the material increases when high pressures are applied. An even flow of material. Large billets and large cross-sections can be extruded. No billet residue is left on the container walls.

The disadvantages are: The billets must be prepared by tapering one end to match the die entry angle. This is needed to form a seal at the beginning of the cycle. Usually the entire billet needs to be machined to remove any surface defects. Containing the fluid under high pressures can be difficult.

Drives

Most modern direct or indirect extrusion presses are hydraulically driven, but there are some small mechanical presses still used. Of the hydraulic presses there are two types: direct-drive oil presses and accumulator water drives. Direct-drive oil presses are the most common because they are reliable and robust. They can deliver over 35 MPa (5000 psi). They supply a constant pressure throughout the whole billet. The disadvantage is that they are slow, between 50 and 200 mm/s (2– 8 ips). Accumulator water drives are more expensive and larger than direct-drive oil presses, plus they lose about 10% of their pressure over the stroke, but they are much faster, up to 380 mm/s (15 ips). Because of this they are used when extruding steel. They are also used on materials that must be heated to very hot temperatures for safety reasons. Hydrostatic extrusion presses usually use castor oil at pressure up to 1400 MPa (200 ksi). Castor oil is used because it has good lubricity and high pressure properties. 34 | P a g e


EXTRUSION DEFECTS Surface cracking - When the surface of an extrusion splits. This is often caused by the extrusion temperature, friction, or speed being too high. It can also happen at lower temperatures if the extruded product temporarily sticks to the die. Pipe - A flow pattern that draws the surface oxides and impurities to the center of the product. Such a pattern is often caused by high friction or cooling of the outer regions of the billet. Internal cracking - When the center of the extrusion develops cracks or voids. These cracks are attributed to a state of hydrostatic tensile stress at the centerline in the deformation zone in the die. (A similar situation to the necked region in a tensile stress specimen)

MATERIALS METAL Metals that are commonly extruded include: Aluminium is the most commonly extruded material. Aluminium can be hot o r cold extruded. If it is hot extruded it is heated to 575 to 1100 °F (300 to 600 °C). Examples of products include profiles for tracks, frames, rails, mullions, and heat sinks.

Copper (1100 to 1825 °F (600 to 1000 °C)) pipe, wire, rods, bars, tubes, and welding electrodes. Often more than 100 ksi (690 MPa) is required to extrude copper.

Lead and tin (maximum 575 °F (300 °C)) pipes, wire, tubes, and cable sheathing. Molten lead may also be used in place of billets on vertical extrusion presses.

Magnesium (575 to 1100 °F (300 to 600 °C)) aircraft parts and nuclear industry parts. Magnesium is about as extrudable as aluminum.

Zinc (400 to 650 °F (200 to 350 °C)) rods, bar, tubes, hardware components, fitting, and handrails. Steel (1825 to 2375 °F (1000 to 1300 °C)) rods and tracks. Usually plain carbon steel is extruded, but alloy steel and stainless steel can also be extruded.

Titanium (1100 to 1825 °F (600 to 1000 °C)) aircraft components including seat tracks, engine rings, and other structural parts.

Tungsten carbide this is one of the most commonly used extruded metals due to its extreme [citation needed ]

toughness and ability to withhold its own form.

Magnesium and aluminium alloys usually have a 0.75 μm (30 μin). RMS or better surface finish. Titanium and steel can achieve a 3 μm (125 μin). RMS. In 1950, Ugine Séjournet, of France, invented a process which uses glass as a lubricant for extruding steel. steel.[10] The Ugine-Sejournet, or Sejournet, process is now used for other materials that have melting temperatures higher than steel or that require a narrow range of temperatures to extrude. The process starts by heating the materials to the extruding temperature and then rolling it in glass powder. The glass melts and forms a thin film, 20 to 30 mils (0.5 to 0.75 mm), in order to separate it from chamber walls and allow it to act as a lubricant. A thick solid glass ring that is 0.25 to 0.75 in (6 to 18 mm) thick is placed in the chamber on the die to lubricate the extrusion as it is forced through the die. A second advantage of this glass ring is its ability to insulate the heat of the billet from the die. The extrusion will have a 1 mil thick layer of glass, which can be easily removed once it cools. 35 | P a g e


Another breakthrough in lubrication is the use of phosphate coatings. With this process, in conjunction with glass lubrication, steel can be cold extruded. The phosphate coat absorbs the liquid glass to offer even better lubricating properties.

PLASTIC Plastic extrusion commonly uses plastic chips or pellets, which are usually dried in a hopper before going to the feed screw. The polymer resin is heated to molten state by a combination of heating elements and shear heating from the extrusion screw. The screw forces the resin through a die, forming the resin into the desired shape. The extrudate is cooled and solidified as it is pulled through the die or water tank. In some cases (such as fibre-reinforced tubes) the extrudate is pulled through a very long die, in a process called pultrusion. A multitude of polymers are used in the production of plastic tubing, pipes, rods, rails, seals, and sheets or films.

CERAMIC Ceramic can also be formed into shapes via extrusion. Terracotta extrusion is used to produce pipes. Many modern bricks are also manufactured using a brick extrusion process.

FOOD

Macaroni is an extruded hollow pasta.

Extrusion has found great application in food processing. Products such as pastas, breakfast cereals, Fig Newtons, cookie dough, Sevai, Idiappam, jalebi, french fries, baby food, dry pet food and ready-to-eat snacks are mostly manufactured by extrusion. In the extrusion process, raw materials are first ground to the correct particle size (usually the consistency of coarse flour). The dry mix is passed through a pre-conditioner, where other ingredients are added (liquid sugar, fats, dyes, meats and water depending on the product being made), steam is also injected to start the cooking process. The preconditioned mix is then passed through an extruder, and then forced through a die where it is cut to the desired length. The cooking process takes place within the extruder where the product produces its own friction and heat due to the pressure generated (10– 20 bar). The cooking process utilizes a process known as starch gelatinization. Extruders using this process have a capacity from 1– 25 25 tonnes per hour depending on design. Use of the extrusion cooking process gives the following food benefits:

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Advanced Manufacturing Techniques Starch gelatinization Protein denaturation Inactivation of raw food enzymes Destruction of naturally occurring toxins Diminishing of microorganisms of microorganisms in the final product

DRUG CARRIERS Extrusion through nano-porous, polymeric filters is being used to manufacture suspensions of lipid vesicles liposomes or Transfersomes for use in pharmaceutical products. The anti-cancer drug Doxorubicin in liposome delivery system is formulated by extrusion, for example.

DESIGN The design of an extrusion profile has a large impact on how readily it can be extruded. The maximum size for an extrusion is determined by finding the smallest circle that will fit around the cross-section, this is called the circumscribing circle. This diameter, in turn, controls the size of the die required, which ultimately determines if the part will fit in a given press. For example, a larger press can handle 60 cm (24 in) diameter circumscribing circles for aluminium and 55 cm (22 in). diameter circles for steel and titanium. Thicker sections generally need an increased section size. In order for the material to flow properly legs should not be more than ten times longer than their thickness. If the cross-section is asymmetrical, adjacent sections should be as close to the same size as possible. Sharp corners should be avoided; avoi ded; for aluminium alumin ium and magnesium the minimum radius should be 0.4 0 .4 mm (1/64 in) and for steel corners should be 0.75 mm (0.030 in) and fillets should be 3 mm (0.12 in). The following table lists the minimum cross-section and thickness for various materials.

Material

Minimum cross-section [cm² (sq. in.)] Minimum thickness [mm (in.)]

Carbon steels 2.5 (0.40)

3.00 (0.120)

Stainless steel 3.0-4.5 (0.45-0.70) (0.45-0.70)

3.00-4.75 (0.120-0.187)

Titanium

3.0 (0.50)

3.80 (0.150)

Aluminium

<2.5 (0.40)

1.00 (0.040)

Magnesium

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Plastics Extrusion

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