Design method of exhaust slot of injection mold

During injection molding, there will not be a lot of molten material seepage from the exhaust hole, because the molten material will cool and solidify there and block the channel. The opening position of the exhaust port shall not face the operator to prevent accidental ejection of molten material from hurting people. In addition, the matching clearance between the ejector rod and the ejector hole, and the matching clearance between the ejector block, the stripping template and the core can also be used for exhaust.

1. Function of exhaust slot

There are two main functions of the exhaust slot:

a. Expel the air in the mold cavity when injecting molten material;

b. Remove all kinds of gases generated during the heating process of materials.

The more thin-walled products, the more far away from the gate, the opening of exhaust slot is particularly important.

In addition, for small parts or precision parts, we should also pay attention to the opening of exhaust slot, because it can not only avoid product surface burns and insufficient injection volume, but also eliminate various defects of products and reduce mold pollution.  

So, how can the exhaust of the mold cavity be sufficient? Generally speaking, if the melt is injected at the highest injection rate, but no focal spots are left on the product, it can be considered that the exhaust in the mold cavity is sufficient.

2. Exhaust mode

There are many methods for cavity exhaust, but each method must ensure that: while the exhaust slot is venting, its size design should prevent materials from overflowing into the slot; Secondly, it is necessary to prevent blockage.

Therefore, the height of the exhaust slot with a length of more than 6-12mm shall be about 0.25-0.4mm measured from the surface of the die cavity to the outer edge of the die cavity. In addition, too many exhaust slots are harmful. Because if the mold locking pressure acting on the part of the mold cavity with no exhaust groove on the parting surface is very large, it is easy to cause cold flow or crack of the mold cavity material, which is very dangerous.

In addition to venting the mold cavity on the parting surface, the purpose of venting can also be achieved by setting an exhaust slot at the end of the material flow of the gating system and leaving a gap around the ejector rod, because the selection of the opening depth, width and position of the exhaust slot will affect the beauty and accuracy of the products if it is inappropriate. Therefore, the size of the above gap is limited to preventing flash around the ejector rod.

It should be noted here that even the smallest flash may not be desirable when exhausting parts such as gears. It is best to exhaust such parts in the following ways:

a. Completely remove the gas in the flow channel;

b. The matching surface of parting surface is shot peened with silicon carbide abrasive with particle size of 200#.

c. An exhaust slot is set at the end of the material flow of the gating system, which mainly refers to the exhaust slot at the end of the runner. Its width shall be equal to the width of the splitter, and the height varies depending on the material.

3. Design method

For product molds with complex geometry, it is best to determine the opening of exhaust slot after several mold trials. The biggest disadvantage of the overall structure in the mold structure design is poor exhaust. For the integral cavity and core, there are the following exhaust methods:

a. Use the groove of the cavity or the installation position of the insert;

b. Use side insert joints;

c. Partially made into spiral shape;

d. Install the strip center with groove and open the process hole in the longitudinal position.

When the exhaust is extremely difficult, the mosaic structure shall be adopted; If it is difficult to open the exhaust slot in the dead corner of some molds, first of all, the molds should be changed to inlay processing without affecting the appearance and accuracy of the products. In this way, it is not only conducive to the processing of the exhaust slot, but also can improve the original processing difficulty and facilitate maintenance.

4. Conclusion

Properly setting an exhaust slot can greatly reduce the injection pressure, injection time, pressure holding time and mold locking pressure, and make the molding of plastic parts from difficult to easy, so as to improve the production efficiency, reduce the production cost and reduce the energy consumption of the machine.