Plastic Injection Molds Training Objective After watching the program and reviewing this printed mater ial, the viewer will become familiar with the variety, design, and productive use of plastic injection molds. Mold components are identified and explained The various mold types are featured Aspects of mold maintenance and storage are detailed
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The Injection Molding Process Injection molding is the most widely used method of producing parts out of thermoplastic material. The molten thermoplastic material is injected into the mold at high pressure. Once this material cools and solidifies, the mold opens and the part is ejected. During the injection molding cycle, the mold serves several purposes, including: Determining the finished shape of the part Venting trapped air or gas during injection Acting as a heat exchanger to draw heat from the part to aid in solidification Providing a means of ejecting the part from the mold
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Mold Components All molds contain a number of common design features. These feat ures include: • • • • • • • • •
Mold base Mold cavity Mold core Sprue bushing Runner system Gates Vents Cooling system Ejector system, plus other components
The mold base is an arrangement of steel blocks manufactured to specific dimensions. Mold bases may be purchased from commercial mold base manufacturers or produced in-house by moldmakers. The basic mold base consists of two halves. The 'A' half, which is also referred to as the stationary half, or the injection half, and the 'B' half, which is also referred to as the moving half, or the ejector half. The mold cavity which creates the outer image or surface of the part is usually mounted on the 'A' half of the mold, while the mold core which reproduces the inner image of the part is typically mounted on the 'B' half of the mold. Collectively the cavity and core halves are known as the 'cavity set'. Mold bases, cavity and cores are commonly made from special mold steels or from other materials such as beryllium copper, stainless steel, aluminum, brass, kirksite, and epoxy. The softer mold materials are generally used for prototype molds and limited production runs. All plastics have their own shri nk factor, meaning they shrink at a certain rate as they cool and solidify. Depending on the type of material to be injection molded, moldmakers must take it's shrink factor into consideration when producing the cavity set. For example, if a material shrinkage is calculated to be one-
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Plastic Injection Molds hundredth of an inch for a part six inches in length, a total of six-hundredths of an inch must be added to the mold design to compensate for shrinkage. Additionally, draft angles or tapers are machined into t he side walls of the cavity set t o facilitate part removal from the mold. These tapers typically range from 1° to 2° per side. Once completed, cavity sets may be heat treated to protect them from the harsh injection molding environment. Molds may also be coated or plated with wear resistant surfacing material, such as nickel and hard chrome. The interfacing plane between mold halves is called the parting line. Depending on the complexity of the part, there may be several such parting lines. Proper alignment of the mold halves is accomplished by using leader pins and bushings. The mold halves are mounted on platens which are components of the injection machine. Most injection machines have three platens: • •
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The stationary platen, which holds the 'A' half of the mold. The movable platen, which holds the 'B' half of the mold and moves back and forth on the injection machine’s four tie bars. The rear stationary platen, which holds the other end of the tie bars, thus anchoring the entire system.
A locating ring on the mol d centers to a hole on the stat ionary platen. This then allows the nozzle of the heating cylinder to seat firmly against the sprue bushing on the 'A' half of the mold. The sprue bushing directs the molten material from the heating cylinder nozzle into the mold's runner system. A mold's runner system is the channel or network of channels through w hich the material flows to reach the cavity set. Surface runners are the most common runner design, and are half-round channels machined into the surfaces of each of the mold halves. Once the molten thermoplastic flows through the runner system it reaches the cavity set through an interface called the gate. The mold gate restricts and controls the flow of plastic into the mold. Passage through the gate causes a frictional rise in material temperature, extending the materials flow into the mold. Common types of gates include: • • • • •
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edge gate, which is usually located on the parting line, and is the most common gate type. submarine gate, which brings material under the parting line to fill the cavity from below. tab gate, which redirects material flow into the mold. ring gate, which is used in molding round or cylindrical parts. fan gate, which is used to spread material quickly over a large area.
To remove trapped air and process gases during injection, a mold venting system is needed. The number and size of the vents are determined by part geometry, material type, viscosity, and the rate of injection. These vents are ground on the parting line of the mold. The hot thermoplastic remains in the mold under pressure until it cools. This cooling is usually achieved by water circulating in channels machined into the mold. Proper cooling contributes to controlled part shrinkage, part strength and quality. Overall, the speed of the injection molding cycle is controlled by the efficiency of the cooling system. Once the parts are sufficiently cooled and solidified, the mold opens and an ejector system, usually in the form of knockout pins, is used to aid in part ejection. Ejector systems are mounted on the ejection side of the mold and are typically activated by pneumatic or hydraulic cylinders. In addition to knockout pins, other ejector methods include stripper plates, stripper rings, and air pressure ejection. Sometimes a sprue puller is used to remove molded plastic from the sprue bushing as the part is ejected.
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Plastic Injection Molds Injection Mold Types Typical injection molds designed to meet specific production requirements include: The cold-runner two-plate mold, which consists of two plates with the cavity and core mounted in them. The sprue, runners, and gates, along with the part are molded simultaneously, and then separated after ejection. The cold-runner three-plate mold, which includes a stripper plate that automatically separates the sprue, runners and gates from the parts during ejection. The hot-runner mold, which uses an electrically heated manifold that maintains material temperature in the runners at the same level of the material in the injection cylinder. The runner system is contained in a plate of its own and does not open during ejection of the part. This is also known as 'runnerless' molding and can decrease cycle time by 25 percent or more. The insulated-runner mold, which uses large diameter runners and no heaters. During injection the outer layers of plastic in the runner solidify and insulate the inner material, keeping it at molding temperatures. •
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Mold Maintenance and Storage Over time and repetitive use, mold components and surfaces will degrade. The use of inserts and laminated construction for mold surfaces subjected to high wear is recommended. Rust is also a major factor. Cleaning and lubrication are critical measures between manufacturing cycles and for short-term and long-term storage.
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Plastic Injection Molds Review Questions 1.
Injection mold bases, cavities, and cores are most commonly made from: a. special aluminums b. epoxies c. beryllium copper d. special mold steels
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The usual range of mold draft angles is: a. 5° to 10° per side b. 1° to 4° per side c. 1° to 2° per side d. 0.5° to 1° per side
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The sprue bushing is located on the: a. 'A' half of the mold b. 'B' half of the mold c. movable platen d. rear stationary platen
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The purpose of a fan gate is to: a. speed solidification b. control injection speed c. spread material over a large area d. redirect the flow of material
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Edge gates are usually located on: a. the 'A' half of the mold b. the sprue c. the parting line d. below the parting line
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Mold vents are found: a. around the whole cavity set b. on the parting line c. near the gates d. within the 'B' half of the mold
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The major factor in injection cycle speed is: a. the type of thermoplastic b. the temperature of the plastic c. injection speed and pressure d. efficiency of the cooling system
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A cold-runner three-plate mold has: a. no sprue bushing b. a stripper plate c. integrated sprue pickers d. a larger gate system
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The mold design known as 'runnerless' is the: a. the hot-runner mold b. the two-plate cold-runner mold c. the insulated-runner mold d. none of the above
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Plastic Injection Molds Answer Key 1. 2. 3. 4. 5. 6. 7. 8. 9.
d c a c c b d b a
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