Tips for choosing the location of the gate for injection molds

The design of the gate is related to the size of the molded part. The shape of the mold structure, the injection process conditions and the performance of the molded part are all related. But in terms of the basic role, the gate cross-section should be small and the length should be short, because only in this way can it meet the requirements of increasing the flow rate, fast cooling and closing, easy separation of the plastic part, and minimum residual marks of the gate.


 

I. Five requirements to be met by the gate location

1. Appearance requirements (gate marks, fusion line)
2. Product function requirements
3. Mold processing requirements
4. Warping and deformation of the product
5. The gates can be easily removed
 

II. Impact on production and function

1. The flow length determines the injection pressure, clamping force, and whether the product is filled or not. The shortened flow length can reduce the injection pressure and clamping force.

2. The location of the gate will affect the holding pressure, the size of the holding pressure, whether the holding pressure is balanced or not, keep the gate away from the product stress location (such as at the bearing) to avoid residual stress, the location of the gate must consider the exhaust to avoid the occurrence of wind accumulation, do not place the gate at the weaker or embedded part of the product to avoid deflection.
 

III. Tips for choosing gate position

1. Gate
The gate is a short slot with a small cross-sectional area to connect the runner to the mold cavity. The cross-sectional area should be small to obtain the following effects.

(1) shortly after the mold cavity is injected, the gate is cold knot
(2) Easy to remove the water outlet
3）Once the water outlet is removed, only a small trace is left
4）Make it easier to control the filling of multiple mold cavities
5）Reduce the phenomenon of excessive filler

2. Gate position and size

1）Place the gate at the thickest part of the product, from the thickest part into the gate can provide better filling and pressure-holding effect. Avoid placing the gate at a sudden change in thickness to avoid hysteresis or short shots.

2) If possible, pour from the center of the product. Placing the gate in the center of the product provides an equal flow length, the size of which affects the required injection pressure.

(3) When the plastic flows into the runner, the plastic is the first to cool down and solidify near the mold surface. When the plastic flows forward again, only the solidified plastic layer flows through. Since plastic is a low heat transfer material, the solid plastic forms a green layer and the holding layer can still flow. Therefore, ideally, the gate should be located in the cross-runner layer for the best plastic flow effect. This is most often the case with round and hexagonal cross runners. However, trapezoidal cross runners cannot achieve this effect because the gate cannot be located in the middle of the runner.

When deciding on the location of the gate, the following principles should be adhered to:

1. the glues injected into each part of the mold cavity should be as even as possible.

2. the rubber injected into the mold, in all stages of the injection process, should maintain a uniform and stable flow front. 3.

3. the possibility of weld marks, air bubbles, pits, dents, false positions, under-injection, and spraying should be considered.

4. should be as easy as possible to make the operation of the water gate, preferably automatic.

5. The location of the gate should be compatible with all aspects.

There is no hard and fast rule to design the gate, most of them are based on experience, but there are two basic elements to be considered in compromise:

(1) The larger the cross-sectional area of the gate, the better, and the shorter the length of the channel, the better, to reduce the pressure loss of the plastic through.

(2) The gate must be thin and narrow so that it can be easily cooled and prevent the backflow of excess plastic. Therefore, the gate should be in the center of the runner, and its cross-section should be as round as possible. However, the opening and closing of the gate are usually determined by the opening and closing of the molded part. 3.

The size of the gate can be determined by the cross-sectional area and the length of the gate, the following factors can determine the optimum size of the gate:

1) Flow characteristics of the compound
2) The thickness of the molded part
3) The amount of compound to be injected into the cavity
4) Melting temperature
5) Mold temperature
 

IV. Balancing of gates

If a balanced runner system cannot be obtained, the following gate balancing method can be used to achieve the goal of uniform injection. This method is suitable for molds with a large number of cavities.

There are two methods of gate balancing: changing the length of the gate channel and changing the cross-sectional area of the gate. In another case, where the cavities have different projection areas, the gate also needs to be balanced.

In this case, to decide the size of the gate, one of the gates is sized and the ratio of its volume to the volume of the corresponding cavity is calculated and applied to the comparison between the gate and the corresponding cavity so that the size of each gate can be calculated successively. After the actual trial injection, the balance of the gates can be completed.
 

V. Direct gate or large watergate

The sprue supplies plastic directly to the finished product, and the sprue adheres to the finished product. In a two-plate mold, the gate is usually one at a time, but in a three-plate mold or hot runner mold, it is possible to have multiple gates.

Disadvantage: Forming a spout mark on the surface of the finished product will affect the appearance of the finished product, and the size of the spout mark depends on the fine diameter hole of the nozzle.
 

VI. The release angle of the nozzle, the length of the nozzle

Therefore, the large water mouth mark can be reduced, as long as the size of the above-mentioned nozzle change small. However, the diameter of the nozzle is affected by the diameter of the nozzle, and the water mouth to be easy out of the mold relationship, the release angle can not be less than 3 degrees, so only the length of the nozzle can be reduced, with a longer nozzle can be.