How to vent the plastic mold

In the injection molding, short-filling, trapped air burns, high internal stress, flow marks and weld lines and so on could be often seen. To solve those problems, the first consideration is the process setting, and the gate design also plays a very important role. Once both of these problems are solved, the possible remaining trouble could be mould trapped air, and the main solution is to engrave slots around the cavity.

design and function

Function of venting slot: First, when inject the melt Plastic, it could expel air in the cavity via the slot out of the plastic mold; the second is to expel air produced during heating plastic material.

The thinner the products wall is, the further away from the gate area the venting slot should be. The venting slot is particularly important for the thin wall plastic part molding. In addition to avoid the short-filling and the surface burns, it could also eliminate many defects and contamination of plastic mold.
So, what is sufficient air venting for plastic mold? In general, the melt injection with maximum speed leaves no burn spots on surface; it can be considered a sufficient venting.

Ways of venting

There are many venting ways, but each way shall ensure that the slot size is designed to prevent the material enter into the cavity, also to prevent clogging. The depth(X) of the slot:

Further, too many slots are rather harmful. Because, the clamping force against the section without machining for venting along the cavity is very is prone to cold material or crack, which is very dangerous.

Besides the venting slots along the parting surface of the mold cavity, it could also be set on the end of the main runner. The clearance along the top of the ejector pins could acts as another way to vent the trapped air. The wrong depth, width and position of venting slot could lead to the flash to affect the aesthetic surface. Therefore, the clearance design should prevent the flash around the top ends of ejector pins.

it should be particularly noted that: The gear injection molding should prevent any flash. The best ways are the following:

(1) To expel of the air in the runner totally;
(2) to treat the P/L surface by shot peening with particle size of 200 # of silicon carbide abrasive. In addition, the venting slots could be cut in at the end of injection system; it mainly refers to the end of runner branch. It should be equal to the width of the runner branch and its height depends on plastic materials.



For parts with complex geometry, the venting slot position could be decided after trials of plastic mold. If the core direct cut on B-plate, its biggest drawback is the poor air venting. For the core direct cut on B-plate or cavity direct cut on A-plate, there are some venting ways like:

  • The slots or insert on the cavity
  • Crevice between the insert and cavity
  • Local spiral shape
  • When it is extremely difficult to expel the air out of the plastic mold, insert should be adopted; keep in mind that the insert shouldn’t affect the precise and the surface looking. The insert could help to vent, improve the polishing effect and make the maintenance easy.



August 17, 2020 plastic mold

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