Features of the exhaust slot design for injection molds!

Injection Molds

An injection mold is a tool for producing plastic products; it is also the tool that gives plastic products their complete structure and precise dimensions. Injection molding is a processing method used for mass production of certain complex shaped parts. Specifically, the heat-melted plastic is injected into the mold cavity at high pressure by an injection molding machine, and after cooling and curing, the molded product is obtained.

Mold Composition

Although the structure of the mold may vary a lot due to different plastic varieties and properties, shapes and structures of plastic products and types of injection machines, the basic structure is the same. The mold is mainly composed of pouring system, temperature regulating system, molding parts and structural parts. Among them, the pouring system and molding parts are the parts in direct contact with plastic and change with plastic and products, which are the most complicated and changeable parts in the mold and require the highest processing finish and precision.

The injection molding tool consists of two parts, the moving mold and the fixed mold, which are mounted on the moving template of the injection molding machine and the fixed mold on the fixed template of the injection molding machine. During the injection molding process, the moving mold and the fixed mold are closed to form the pouring system and the cavity, and when the mold is opened, the moving mold and the fixed mold are separated in order to remove the plastic products. In order to reduce the heavy workload of mold design and manufacturing, most of the injection molds use standard mold frames.

Exhaust port

It is a kind of slot-shaped air outlet in the mold to discharge the original and molten material into the gas. When the melt is injected into the cavity, the original air in the cavity and the gas brought in by the melt must be discharged at the end of the material flow to the outside of the mold through the vent, otherwise the product will be made with air holes, poor connection, mold filling, and even the accumulated air will be burned due to the high temperature generated by the compression. In general, the exhaust hole can be located either at the end of the melt flow in the cavity, or on the parting surface of the mold. The latter is opened on the side of the concave mold 0.03-0.2mm deep, 1.5-6mm wide shallow groove.

During injection, there will not be much melt oozing out of the venting hole because the melt will cool and solidify there blocking the channel. The venting hole should not be open to the operator to prevent the melt from accidentally ejecting and injuring people. In addition, you can also use the clearance between the ejector rod and the ejector hole, the clearance between the ejector block and the stripper plate and the core, etc. to vent the air.

1、The role of the exhaust tank

The role of the exhaust slot is twofold: one is to exclude the air in the mold cavity when injecting the molten material; the other is to exclude the various gases generated during the heating process of the material. The more thin-walled products, the farther away from the gate, the more important is the opening of the exhaust slot.

In addition, for small parts or precision parts, also pay attention to the opening of the exhaust slot, because it can avoid the product surface burns and insufficient injection volume, but also to eliminate various defects in the product, reduce mold pollution, etc..

So, what is the adequate venting of the mold cavity? Generally speaking, if the melt is injected at the highest injection rate and no scorched spots are left on the product, the venting in the mold cavity is considered adequate.

2、Exhaust method

There are many ways to exhaust the mold cavity, but each method must ensure that: the exhaust slot should be designed in such a way that the size of the exhaust slot can prevent the material from overflowing into the slot while exhausting; secondly, it should also prevent blockage. In addition, it is harmful to have too many exhaust slots. Because, if the clamping pressure acting on the parting surface of the mold cavity without venting slots is very large, it is easy to cause cold flow or cracking 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 be achieved by setting an exhaust slot at the end of the material flow of the casting system and leaving a gap along the perimeter of the ejector rod, because the depth, width and location of the exhaust slot will affect the beauty and accuracy of the product if it is not properly selected to produce a flying burr. Therefore, the size of the above-mentioned gap is limited to prevent flying edges around the ejector rod.

Special attention should be paid here to the fact that the exhaust of such parts as gears may be undesirable even for the smallest flying edges, and that the exhaust of such parts is best done in the following manner.

(1) Thorough removal of gas from the flow channel.

(2) The surface of the parting surface is shot peened with silicon carbide abrasive with a particle size of 200#. In addition, the exhaust slot is opened at the end of the material flow of the casting system, mainly the exhaust slot at the end of the manifold, the width of which should be equal to the width of the manifold, and the height varies depending on the material.

3、Design Methodology

Based on years of experience in injection mold design and product mold trials, this article briefly introduces several designs of exhaust slots. For the complex geometry of the product mold, the opening of the exhaust slot is best to be determined after several mold trials. The biggest disadvantage of the overall structure of the mold design is the poor exhaust.
 For the overall cavity core, there are several exhaust methods:

1) the use of the cavity slot or insert installation parts;

2) the use of the side of the insert seam;

3) locally made spiral shape;

4) in the longitudinal position of the slatted heart with a slot, open process holes;

5) when the exhaust is extremely difficult.
6) when the exhaust is extremely difficult, the use of inlay structure, etc.; If some of the dead ends of the mold is not easy to open the exhaust slot, the first should not affect the appearance and accuracy of the product, the mold should be properly changed to inlay processing, so that not only conducive to the processing of exhaust slot, sometimes can also improve the original processing difficulties and ease of maintenance.

4、Exhaust tank design for thermoset plastic molding

Venting is more important for thermoset materials than for thermoplastic materials. First of all, all the manifolds in front of the gate should be exhausted. The width of the venting slot should be equal to the width of the manifold and the height is 0.12 mm. all around the cavity should be venting, each venting slot should be 25 mm apart, the width is 6.5 mm and the height is 0.075-0.16 mm, depending on the flow of the material. The softer material should take a lower value. The ejector rod should be enlarged as much as possible, and in most occasions, 3-4 planes of 0.05mm high should be ground on the cylindrical surface of the ejector rod, and the direction of the grinding marks should be along the length of the ejector rod. The grinding should be carried out with a finer grit grinding wheel. The end face of the ejector should be chamfered by 0.12 mm so that the flange, if formed, will adhere to the part.

5、Conclusion

Appropriate opening of the exhaust tank can greatly reduce the injection pressure, injection time, holding time and clamping pressure, so that the molding of plastic parts from difficult to easy, thus improving production efficiency, reducing production costs and reducing the energy consumption of the machine.