How To Choose The Injection gate Of The Injection Mold ?

- Jul 01, 2019-

How to choose the injection gate of the injection mold ?


1. 5 requirements for gate location

1) Appearance requirements (gate marks, weld lines)

2) Product function requirements

3) Mold processing requirements

4) Warpage deformation of the product

5) The gate is not easy to remove

6) The molding process is easy to control


2. Impact on production and function

1) The flow length determines the injection pressure, the clamping force, and the shortened full flow length of the product can reduce the injection pressure and clamping force.

2) The position of the gate will affect the holding pressure, the pressure of the holding pressure, and whether the holding pressure is balanced. Keep the gate away from the position of the product (such as the bearing) to avoid residual stress. The gate position must consider the exhaust to avoid Accumulation occurs, do not place the gate in a weaker place or in the embedding to avoid misalignment.


3. Tips for choosing a gate location

(1) gate

The gate is a short groove with a small cross-sectional area for connecting the flow path to the cavity. The cross-sectional area is so small that the purpose is to achieve the following effects:

1) soon after the mold hole is injected, the gate is cold junction

2) Simple water removal

3) Only after the water outlet is completed, only a few traces are left

4) Make the filling of multiple cavity points easier to control

5) Reduce the excessive amount of filler


(2) Gate location and size

1) Place the gate at the thickest part of the product and pour it from the thickest part to provide better filling and pressure retention. If the pressure is insufficient, the thinner area will solidify faster than the thicker area, avoiding the gate being placed at a sudden change in thickness to avoid hysteresis or short shots.

2) If possible, pouring from the center of the product, placing the gate in the center of the product can provide the same length of flow, the length of the flow will affect the required injection pressure, and the central pouring will make the pressure in all directions uniform. Uneven volume shrinkage can be avoided.

3) When the plastic flows into the flow path, the plastic first heats down (cools) and solidifies near the die surface. When the plastic flows forward, it flows only through the solidified plastic layer. Moreover, since the plastic is a low heat transfer material, the solid plastic forms a green layer and the retaining layer can still flow. Therefore, in the ideal case, the gate should be placed in the cross-flow layer position to make the best plastic flow effect. This situation is most common in circular and hexagonal cross-flow paths. However, trapezoidal cross-flow paths do not achieve this effect because the gate cannot be placed in the middle of the flow path.

   

When determining the location of the gate, the following principles should be followed:

1) The rubber injected into each part of the cavity should be as even as possible;

2) The rubber compound injected into the mold shall maintain a uniform and stable flow front at all stages of the injection process;

3) Consideration should be given to possible occurrences of weld marks, bubbles, pockets, imaginary position, insufficient shot glue and glue spray;

4) The water removal operation should be as easy as possible, preferably automatic operation;

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


There are no hard and fast rules for designing gates. Most of them are based on experience, but there are two basic elements that need to be compromised:

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 when the plastic passes.

2) The gate must be narrow so that it can be easily cold-knotted and prevent excessive plastic from flowing backwards. Therefore, the gate should be in the center of the flow channel, and its cross-section should be as circular as possible. However, the gate switch is usually determined by the switch of the module.

(3) Gate size

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

1) Rubber flow characteristics

2) Thickness of the module

3) The amount of rubber injected into the cavity

4) Melting temperature

5) mold temperature


4. Balance of the gate

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

There are two ways to balance the gate: changing the length of the gate channel and changing the cross-sectional area of the gate. In another case, when the cavity has a different projected area, the gate also needs to be balanced. At this time, to determine the size of the gate, it is necessary to first determine the size of one of the gates, find the ratio of its gate cavity volume, and apply this ratio to its gate and each corresponding cavity. In comparison, the size of each gate can be determined successively. After the actual test, the balance operation of the gate can be completed.


5. Direct gate or large nozzle

The sprue supplies plastic directly to the finished product, and the sprue adheres to the finished product. In the two-plate mold, the big nozzle is usually one out, but in the design of the three-plate mold or the hot runner mold, there can be more than one beer.

Disadvantages: The formation of a stencil on the surface of the finished product will affect the appearance of the finished product, while the size of the stencil is in the fine-diameter hole of the nozzle.


6. The draft angle of the nozzle, the length of the nozzle

Therefore, the large water-jet printing can be reduced, as long as the size of the above-mentioned nozzle is reduced. However, the diameter of the nozzle is affected by the diameter of the nozzle, and the nozzle is easy to be out of the mold, and the draft angle cannot be less than 3 degrees, so only The length of the nozzle can be shortened.


Gate selection:

The gate is the connecting part of the flow channel and the cavity, and is also the last part of the injection molding system. Its basic function is:

1) The molten plastic from the flow path enters the filled cavity at the fastest speed.

2) After the cavity is filled, the gate can be quickly cooled and closed to prevent the plastic cavity from flowing back without cooling.


7. Summary

  The design of the gate is related to the size of the plastic part, the shape of the mold, the injection process conditions and the performance of the plastic part. However, in terms of basic functions, the gate section is small and the length is short, because only in this way can the requirements of increasing the flow rate, rapid cooling and closing, and the separation of the plastic parts and the minimum gate residue are satisfied.


The main points of the gate design can be summarized as follows:

1) The gate is opened in a thick section of the plastic part, so that the molten material flows into the thin section from the thick material section to ensure complete filling;

2) The choice of gate location should minimize the plastic filling process to reduce pressure loss;

3) The choice of gate location should be beneficial to eliminate air in the cavity;

4) The gate should not be allowed to flush the melt directly into the cavity, otherwise it will generate a swirling flow, leaving traces of the rotation on the plastic part, especially the narrow gate is more prone to such defects;

5) The choice of gate location should prevent the formation of seam line on the plastic surface, especially in the ring or cylindrical plastic parts, the cold well should be added at the melt casting of the surface of the gate. ;

6) The gate position of the injection mold with the elongated core should be far from the forming core, so that the forming core is not deformed by the flow;

7) When forming large or flat plastic parts, a double gate can be used to prevent warpage, deformation and lack of material;

8) The gate should be opened as far as possible without affecting the appearance of the plastic part, such as the bottom of the edge;

9) The size of the gate depends on the size, shape and plastic properties of the plastic part;

10) When designing multiple cavity injection molds, consider the balance of the flow channels to consider the balance of the gates, and try to make the molten materials uniformly charge at the same time.


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