Why the venting is so important for a plastic injection mold?

The venting slot serves two purposes: 1. Expel air from the plastic injection mold cavity during the injection process of the molten plastic material; 2. Get rid of the various gases produced during injection molding processing. The setup of venting slots is of great importance, especially for thin-walled products or the locations far away from the gate. In addition, close attention should also be paid to venting slots for the production of small-size or precision parts, because they are able to protect the products from surface burns, insufficient filling, as well as other defects.

So, what is sufficient air venting for plastic injection molding? Generally speaking, if no burn marks are left on the product at the highest molten plastic injection speed, then the venting effect of the mold cavity will be considered sufficient.

 

Venting Methods:

There are lots of ways to vent the mold cavity, but each of them has to guarantee that: while performing venting functions, the size of the vent slot needs to be able to prevent material from entering the slot, as well as clogging at the same time. However, if there are too many slots, it will do more harm than good, because if the clamping force against the mold cavity parting surface area without vent slots is too high, the cavity material will be prone to cracks, which is very dangerous. In addition to being designed on the parting surface of the mold cavity, the vent slot can also be machined in the end section of the runner system. The clearance around the ejector can also act as a way to let the trapped air out. If the height, width and position of the vent are not appropriately defined, flash will be caused, thus affecting product aesthetics and precision. As a result, the clearance design should be able to prevent flash from occurring around the ejector pin. In particular, it is worth noting that molded parts like gears expect no flash at all. Therefore, the following venting methods should be employed:

  1. Completely eliminate the air in the runner;
  2. Apply peening treatment to the parting surface with the 200# silicon carbide abrasive, and open vent slots in the end section of the runner system, mainly referring to machining slots in the end section of the sub-channel, of which the width should be equal to that of the sub-channel while the height may vary from material from material.

Design Approach:

Based on the years of injection mold design and mold trial experience, this article is aimed to generally explain the design principles of several slot types. For parts with complicated geometric shapes, the vent slot positions should be identified after several mold trials. If a mold design adopts the integrated structure, poor ventilation will be its biggest disadvantage. So, for molds with integrated cavity and core, the following venting methods can be adopted:

(1) Make use of the slot or insert location in the mold cavity;

(2) Make use of the lateral insert crevice;

(3) Machine the local part into the spiral shape;

(4) When it is extremely difficult to expel the air out of the mold, an insert should be adopted. If it is not easy to machine a vent slot in some locations of a mold, such as in the corners, the insert molding process may be appropriately applied on condition that product appearance and precision are not affected. This method not only helps with venting, but is also able to lower the difficulty level for machining, and convenient for maintenance, too.

Vent Slot Design Dimensions:

The width of the vent slot ranges from 1.5 to 6mm, while the depth design should be able guarantee that the plastic material will not get into the slot to cause flash. Its value is dependent on the viscosity of the molten plastic, but usually its applicable range is from 0.013 to 0.05mm.

 

Conclusion:

Appropriately designed venting slots are able to drastically reduce injection pressure, injection time, pressure holding time and clamping force, thus making the plastic injection molding process much easier by improving production efficiency, lowering production costs and saving the energy consumed by machine.

 

 

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