Summary of mold gate position design

Time:2020-12-24 16:06:20 / Popularity: / Source:

First, requirements for gate location

1. Appearance requirements (gate marks, weld lines)
2. Product functional requirements
3. Mold processing requirements
4. Warpage of product
5. Is gate easy to remove
Design of gate 

Second, impact on production and function

1. Flow Length determines injection pressure, clamping force, and whether product is filled or not. Shortening flow length can reduce injection pressure and clamping force.
2. Location of gate will affect holding pressure, size of holding pressure, and whether holding pressure is balanced. Keep gate away from unstressed position of product (such as bearing) to avoid residual stress. Exhaust gas must be considered at gate location to avoid wind accumulation. Do not place gate at a weak or embedded part of product to avoid core shaft.

Third, skill of choosing gate location

1. Place gate at the thickest part of product, gating from the thickest part can provide better filling and pressure holding effect. If holding pressure is insufficient, thinner area will solidify faster than thicker area. Avoid placing gate where thickness changes suddenly to avoid hysteresis or short shots.
2. If possible, gating from center of product. Placing gate in the center of product can provide equal length flow. Size of flow length will affect required injection pressure. Central gating makes holding pressure in all directions uniform and avoids uneven volume shrinkage.
3. Gate : Gate is a short groove with a small cross-sectional area to connect runner and mold cavity. Cross-sectional area is therefore small, in order to obtain following effects:
1) Soon after cavity is injected, gate will be cold
2) Easy to remove gate
3) After gate is removed, leaving only a few traces
4) Make filling of multiple mold cavities easier to control
5) Reduce excessive packing
There are no hard and fast rules for method of designing gates. Most of them are based on experience, but there are two basic elements that must be compromised:
1. The larger cross-sectional area of gate, the better, and the shorter length of channel, the better, so as to reduce pressure loss when plastic passes.
2. Gate must be narrow to facilitate cold formation and prevent excessive plastic from flowing back. Therefore, gate is in the center of the runner, its cross-section should be as round as possible. However, gate switch is usually determined by switch of module.
3. Gate size:
Size of gate can be determined by cross-sectional area and length of gate. Following factors can determine the best size of gate:
1) Rubber flow characteristics
2) Thickness of molded parts
3) Amount of rubber injected into cavity
4) Melting temperature
5) Mold temperature
When determining gate location, following principles should be adhered to:
1. Rubber injected into each part of cavity should be as even as possible.
2. Rubber material injected into mold should maintain a uniform and stable flow frontline at all stages of injection process.
3. Possible occurrence of weld marks, bubbles, cavities, vacant positions, insufficient injection and spraying should be considered.
4. Operation of dewatering port should be made as easy as possible, preferably automatic operation.
5. Location of gate should match all aspects.

Fourth, balance of gate

If a balanced runner system cannot be obtained, following gate balance method can be used to achieve goal of uniform injection molding. This method is suitable for molds with a large number of cavities. There are two ways to balance gate: changing length of gate channel and changing cross-sectional area of gate. In another case, that is, when mold cavity has a different projected area, gate also needs to be balanced. At this time, to determine size of gate, one of gate sizes must be determined first, it is required to find ratio of its corresponding cavity volume, this ratio is applied to comparison of its gate with each corresponding cavity, and each gate can be obtained one after another. After actual trial injection, balance operation of gate can be completed.

Fifth, position of gate in the runner

When plastic flows into runner, plastic is the first to reduce heat (cooling) and solidify when it approaches mold surface. When plastic flows forward, only solidified plastic layer flows through. And because plastic is a low heat transfer substance. Solid plastic forms green insulating layer and retaining layer can still flow. Therefore, under ideal circumstances, gate should be set at position of cross runner layer to achieve the best plastic flow effect. This situation is most common in circular and hexagonal cross runners. However, trapezoidal cross flow channel cannot achieve this effect, because gate cannot be set in the middle of runner.

Sixth, direct gate or sprue gate

Gate directly supplies plastic to finished product. Gate adheres to finished product. In two-plate mold. Sprue gate is usually one cavity, but in design of three-plate mold or hot runner mold, usually multi-cavity.
Disadvantages: Formation of nozzle marks on the surface of finished product will affect appearance of finished product. Size of nozzle mark lies in small diameter hole of nozzle.

Seventh, ejection angle of nozzle and length of nozzle

Therefore, large nozzle mark can be reduced, as long as delay of above-mentioned nozzle is reduced. However, diameter of nozzle is affected by diameter of furnace nozzle, and nozzle must be easy to be ejected from mold, ejection angle cannot be less than 3 degrees. So only length of nozzle can be shortened, just use a longer nozzle.
Gate selection: Gate is connecting part of runner and cavity, and also last part of injection mold feeding system. Its basic functions are:
1) Make molten plastic from runner enter filling cavity at the fastest speed.
2) After cavity is full, gate can be quickly cooled and sealed to prevent cavity from returning to uncooled plastic.

Eighth, summary

Design of gate is related to size of plastic part, shape of mold structure, injection process conditions and performance of plastic part. However, according to basic functions of above two sentences, gate section is small and length is short, because only in this way can it meet requirements of increasing flow rate, rapid cooling and sealing, facilitating separation of plastic parts and minimum gate residue.
Main points of gate design can be summarized as follows:
1. Gate is opened in thicker section of plastic part, so that melt flows from thick section into thin section to ensure complete mold filling;
2. Selection of gate position should make plastic filling process the shortest to reduce pressure loss;
3. Selection of gate position should help to remove air in cavity;
4. It is not advisable for gate to make melt directly rush into cavity, otherwise it will produce a swirling flow, leaving swirling marks on plastic part, especially narrow gate is more prone to such defects;
5. Choice of gate position should prevent production of splicing lines on plastic surface, especially in circular or cylindrical plastic parts, cold material well should be opened at melt pouring on the surface of gate. ;
6. Gate position of injection mold with a slender core should be far away from molding core, so that molding core is not deformed by flow of material;
7. When forming large or flat plastic parts, in order to prevent warping, deformation and lack of material, multiple gates can be used;
8. Gate should be opened at a position that does not affect appearance of plastic part, such as bottom of edge;
9. Size of gate depends on size and shape of plastic part, performance of plastic;
10. When designing multiple-cavity injection molds, consider balance of gate in combination with balance of runner, and try to fill molten material uniformly at the same time.

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