plastic injection After material injected by nozzle, runner systems will convey the molten material from the sprue to the gate. basic runner system consist of sprue, main runner, sub runner, cold slug, and gate. since the design of the runner system has large effect on molding quality, molding efficiency, cycle time, cooling time and other factor, it is important key when design of mold. Runner System Construction Sprue sprue will contact first time with melt plastic, at the injection process, sprue always contact with nozzle from the injection machine, so consider when design the sprue consider the ball radius of nozzle, make sure it isn't make undercut on sprue.The dimensions of the sprue depend primarily on the dimensions of the molded part in particular the wall thickness, too little sprue will make sprue cold faster and make insufficient flow.
Main Runner First time when design runner system, determine number of cavities, shape and gate types, then arrange the cavity layout,at the multi cavity of mold balancing of runner is important, balancing ensures virtually equal flow of plastic trough each gate. To achieve balancing runner layout must be though, Sub Runner this part will continues flow from main runner until the gate, the dimension of sub runner and how much sub runner system will be, is very depending on how much cavity of the mold, and how the layout, what the layout use herring bone, H type, or star type. at the picture above use H type runner layout.
Cold Slug when runner layout bent, clod slug may be required to continue plastic flow of better quality to the cavities.The cold slug well helps the flow of material through the runner system by stopping colder, higher viscosity material moving at the forefront of the molten mass entering into the cavity.The length of the well is usually equal to or greater about 1,5-2 time than the runner diameter. Gate Basic purpose of the gate is to limit the flow of the melted plastic, but it must allow that melted plastic flow and fill the cavity easily. considering the fluidity of the material, gate should be made near the center of gravity of molding or the line to make flow uniform and melted material even, then make the gate thick but still considering melted plastic material, becouse thin part will cooled easily and faster. the gate type is very various depend on part shape, mold layout, mold system, etc, here some explanation about design those gate type:
1. Direct gate What is Gate ? The entrance through which molten plastic flows into the cavity of the mold from the runner. This is where the flow or melting of molten plastic is controlled by control type of gate or gate dimension. It is positioned at the thickest area of a part. other function of gate is the gate makes it easier for the runner, which will be unnecessary after molding, to be separated from the part.is recommended to consider gating options early in part design. Direct Gate direct gate or sprue gate we can define as connecting directly end sprue to the molding and used as a gate, this type of gate is commonly used for single cavity molds, where sprue feeds the material to cavity molds directly rapidly with minimum pressure drop.
this type of gate is applied to molding of boxes or cups like bathtub, plastic bucket, and others, particularly large sized products. like at the picture above, shown cups product with direct gate before cutting. Dimensions and Construction from picture below we can concluded that direct gate is very easy to design,
from picture above the construction of direct gate is one parts with the product, after injection process, the sprue will cut manually. parting line between cavity and core can directly placed at top surface of product or more lower from top surface product, it very depending of product shape and characteristics.
starting sprue diameter is controlled by nozzle of injection
- then, make sure the diameter of sprue orifices must be larger than nozzle exit diameter, choose from 1-2 mm or about 1/32 inch. - give taper at the sprue from orifice diameter to product about 1-3 degree, it is will made the product demolded from cavity side more easy. - in general the diameter of the gate much greater than the thickness of the product. Advantages - Low cost of design. - The injection pressure was reduced due to direct cavity filling. - Simple way to design gate and runner system. - easy design and maintenance. Disadvantages some disadvantage of direct sprue are: - since excessive stress is left easily around the gate, cracking often starts around the gate. - when using non standard sprue tapers, will more expensive with little gain. - gate mark left on part surface after runner of sprue trimmed of. - This gate type can only produce one part per shot. - The gate has to be cut manually by a cutter. - Not applicable in little size product.
2.side gate, edge dimension design edge gate is the simplest type of gate and is used wherever the product can or must be gated from parting line. it's designed commonly for injection machine attended by operator, because the product must separated from the runner system by hand with the aid of side cutter.
Dimension Picture below shown ideal configuration and practical gate, suggested dimension for general application of edge or side gate are
Runner width and Height for side gate or edge gate is preferred that height is about 0,70,8 from width of runner. the width of runner very depending on product size and weight, it from 3 mm until 7 mm in width ( product mass until 200 gram in maximum). Gate width and height is about W=3h,with W is width and h is height (look at Herbert Rees, Mold Engineering, pg 159) but this rule can't applied for all part or product form, specially for thick product that have large in width such us panel and cover, when design the dimension of gate take the rule that height of gate is must about 75 % from the parts thickness, Suggested take 0,6 until 0, 7 from parts thickness, but specially for elastomer material take more large about 80-90 % from parts thickness. T is gate width, for product mass until 200 gr, suggested take 1 until 5 mm in width , for the very large product some times have width until 12 mm. C is Land of gate from runner, the length is should be no more than 1,5 mm. also preferred make taper before gate land, in that picture taper are change with R in both side, make R, taper or not is depend on side gate size and form.
Picture 1.A
Picture 1.B both those picture shown edge gate or side gate with different design, application, parting line, taper and R system. taper from bottom runner to gate land have various dimension, the taper in picture 1A is enough large, about 30-45 degree, in picture 1B taper is about 10-20 degree. beside those type of side gate, they are still various type of side gate or edge gate, will upload in other post. Side gate, edge gate design as we know Gate In injection molding, is the channel through which the molten resin flows from the runner into the cavity. Generally, it is small and solidifies first. The restricted opening through which the plastic enters the mold cavity. Standard Gate this type of gate have various name, the common name are standard gate, edge gate and side gate, this is the most typical gate in molds, which is applied to almost all types of plastics. look at picture below, it's single side gate from sprue until gate and product (click the picture to enlarge)
how about the parting line? the are about 3 methods that we can choose when design parting line, 1. core side process,this type of parting line only core side that's must be machined , picture below shown side gate design with flat parting line in core side process.
this type parting line is useful applied in product without taper in side of product, but when product have taper in side (like picture below) it will undercut and need slider to form the side of product.
2. cavity side process, this type of gate design parting line is one method to hide the marks of gate after injection process. at this type runner channel machined in cavity side, then a little gate machined at core side. this type of gate parting line is more good and more cheap than number one when product have taper like picture below.
3. cavity and core side parting line, this type parting line for gate and runner is not common to use, when use this type parting line runner and gate, you must use cylindrical type of runner of semi cylindrical type of runner.
3. Tab gate
general construction of tab can see at picture below
basically melted material is fed to this gate from a separate primary gate (after runner), the function of the gate typically is employed for flat and thin parts to reduce the shear stress in the cavity, Tab gate able to used in various material plastic such us ABS, PS, acrylic, PP, PVC, Polycarbonate. SAN and other which has relatively low fluidity. at this type of gate, the high shear stress generated around the gate that trimmed after molded. this gate sometimes also called the collision gate. Function in Product and General Rule The thickness of tab gate should be the same as the part wall, yo can also make more thin, but when designed parts for cosmetic parts if preferred that the thickness is same with the product thickness. rule number two is, make the gate as center as possible in the product side, but you must consider the product size and shape. basically minimum tab width is about 6,5 mm and the tab thickness minimum is 75 % from the thickness product for more picture and drawing view,please look at picture below
(Top View of Tab Gate)
(Front view of Tab Gate)
(Side View of Tab Gate)
4. film gate Film gate also called flash gate is very thin compare to another gate, this thin gate like film which have parallel runner before the gate, this type of gate used for straight edges. picture below shown film gate with main parts.
in generally this gate used in thin and flat requirement, like flat mobile phone cap, ipod cap and others. but sometimes this gate is not likely going to have a flat flow front, the make flow material more stable and flat, make temperature molds more higher than usual, but cycle time will also higher. here is film gate from top view
Dimension recommended dimension is approximately 0.2 mm to 0.6 mm in thick, the land area (parallel runner) is also be kept small approximately 0.6 mm - 1 mm depend the large and weight of product. Other Design picture below shown other design of film gate, with more reliable and easy in process.
look the different model of film gate, and different design of parallel runner. picture below shown simulation of plastic flow from those design
and the last picture shown time that needed to cooling the melt plastic in mold
from the picture above we can choose the right place to make cooling pipe, the red color marks mean need more time to cool the product.
5. fan gate How to design fan gate in thermoplastic injection mold fan gate, is a gate spreading toward in to cavity, basically this gate is thin similarly with previous gate (film gate) and is used in same purpose with film gate. with the gate construction it permits rapid filling of large parts or fragile section mold area with large entry area. it is used to create uniform flow into wide parts. Purpose 1. thereby keeping the cross sectional area constant. 2. This ensures a constant and more even plastic flow into the cavity into a wide area at a slow injection speed, minimizing backfilling and reducing imperfections and stresses in the part.
3. used for thick-sectioned moldings and enables slow injection without freeze-off. 4. favored for low stress moldings or where warpage and dimensional stability are main concerns. 5.The melt velocity will be constant. 6.The entire width is being used for the flow. 7. The pressure is the same across the entire width Construction and Design
side View
The gate should taper in both width and thickness, to maintain a constant cross sectional area, although at picture below thickness is doesn't tapered.
Dimension the land thickness can be vary thin relative to the part thickness because the gate is very wide.the maximum thickness should be no more than 80% of the part thickness. if the parts very thin like 0,8 mm, you can also use 0.7 mm fan gate thickness, but when rubber is used as material parts, use 100 % of the thickness parts is more easy to control flow and product quality. The gate width varies typically from 6 mm up to 25% of the cavity length.in large parts is often use fan gate as wide as the parts it self. Simulation Picture all picture below based on injection simulation process, using material PS ASAHIPS 693 : Asahi Chemical other data are ==== Machine setup ==== Melt temperature = 503.15 K Mold temperature = 323.15 K Fill time = 0.08 s Packing/Holding time = 4.24 s Cooling time = 7.72 s Packing/Holding pressure = 12627300.00 Pa ==== Machine specification ==== Required clamp force = 27647.70 N Required shot size (GPS) = 0.01 kg Simulation results mold time filling in seconds
time to cool after injection from below picture we can learn where is the best position to make cooling layout, after you make cooling layout, enter the cooling simulation process to your parts. if the distribution is same in all shape of part (unless runner and gate) your design is good to realize.
Pressure distribution
all picture is enabled to enlarge, click the picture to enlarge.
6. Disc gate Diaphragm Gate Design in Thermoplastic Mold Actually this gate is another variation of side gate, but this gate have circumferential shape like fan gate,some time it also named as disc gate. basic modeling of diaphragms gate is like the picture below
this type applied when at the center of product has a trough hole, plastic flow when injections is similar to direct gate, but flow plastic material more constant and controlled becouse this gate have circular flat thickness. this gate punched out after ejection. Design and Size the following picture below shown basic part and size of diaphragm gate.
The gate height (t) is preferred about 0.1mm -0.17 mm The circular runner height (T) is recommended about 5-10 mm depend on your size of product. Land size (D-d) is about 1mm-5 mm each side. Runner geometry they are various geometry that we can use in runner, such us 1. full round 2. half round 3. trapezoid use full round and trapezoid shape is recommended in various mold. Half round runners are not recommended because of their low volume to surface ratio here some area calculation of cross section of runner (source : http://www.dsm.com)
Runner dimensions The diameter of a runner depends on its length in addition to the part volume, part flow length, injection machine capacity, and gate size, Generally runner must never be smaller than the largest wall thickness of the product, it's usually make from 3 mm until 15 mm. picture below shown relation between runner diameter and maximum runner length (source : http://www.dsm.com)
The runner should be large enough to minimize pressure loss, yet small enough to maintain satisfactory cycle time, Large runners are not economical because of the amount of energy that goes into forming, and then regrinding the material that solidifies within them. Runner layout In general they are three type of runner layout 1. "H" bridge (branching) runner system 2. Standard (herringbone) runner system (conventional runner) 3. Radial (star) runner system look at picture below shown standard runner and H bridge type runner.
( and below is star layout runner system (both bottom picture)
Balancing the runner system ensures that all mold cavities fill at the same rate and pressure, if the product have not similar product geometry, it is recommended to use mold analysis software like mold flow or C-Mold.
