The Mold Cavity Venting Methods

The thinner the wall of the product, and the farther it is from the gate, the more critical it becomes to establish venting channels.

Additionally, for small-sized or precision components, the creation of venting channels should be given high importance.

This is because venting channels not only prevent surface scorching and insufficient injection but also help eliminate various defects in the product and reduce mold contamination.

The role of venting channels primarily serves two purposes:

  • First is toexpel air from within the mold cavity during the injection of molten plastic.
  • Second is toeliminate various gases generated by the plastic during the heating process.

So, what constitutes adequate venting for the mold cavity? Generally, it can be considered sufficient if, when injecting molten material at the highest injection speed rate, there are no burn marks left on the product.

Mold Cavity Venting Methods (1)

1. Venting Methods

There are various methods for venting the mold cavity, but each method must ensure the following points:

(1) Venting channels, while allowing for the release of air, should be designed to prevent material overflow into the channels.

Additionally, they should prevent blockages.

Therefore, when measuring from the inner surface of the mold cavity towards the outer edge, the venting channel section, which should be 6-12mm in length or more, should have an increased height of 0.25-0.4mm.

(2) The number of venting channels should be adequate. Excessive venting channels can be detrimental. This is because if there is significant clampingpressure acting on the portions of the mold cavity’s parting surface without venting channels, it can easily lead to material freezing or cracking in the mold cavity, which is highly dangerous.

(3) In addition to venting the mold cavity on the parting surface, venting can also be achieved by placing venting channels at the end of the material flow in the gating system and leaving gaps around the ejection pins.

Improper choices in the depth, width, and placement of venting channels can result in flash or burrs that affect the aesthetics and precision of the product. Therefore, the size of these gaps should be limited to preventing flash around the ejection pins.

A critical point to note here is that for components like gears, even the tiniest bit of flash during venting is undesirable. It’s best to vent gear components using the following methods:

Thoroughly purge the gas.

Perform shot blasting treatment on the mating surface of the parting line using 200-grit silicon carbide abrasive.

Furthermore, opening venting channels at the end of the gating system mainly refers to venting channels at the end of the runner. The width of these channels should match the width of the runner, while the height may vary depending on the material.

2. Design Approaches

For molds of products with complex geometric shapes, it is advisable to determine the placement of venting channels after several trial mold runs. One major drawback of the overall structural design of molds is inadequate venting.

For integral mold cavities and cores, several venting methods can be employed:

(1) Utilizing slots or fixture installation points within the cavity.

(2) Employing seams in the side fixtures.

(3) Creating localized spiral shapes.

(4) Installing slotted bar cores or process holes at longitudinal positions.

When venting becomes extremely challenging, consider using insert assembly structures.

If some areas of the mold have inaccessible corners for venting channels, the first step should be to modify the mold for insert assembly processing, as long as it does not compromise the product’s appearance or precision. This approach not only facilitates the creation of venting channels but can also improve the existing manufacturing difficulties and make maintenance easier when necessary.

3. Design Dimensions for Venting Channels

Venting is even more critical for thermosetting materials compared to thermoplastic materials.

Firstly, venting should be provided in all runner channels before the gate. The width of the venting channel should match the width of the runner, and its height should be 0.12mm.

Venting should be incorporated all around the mold cavity, with venting channels spaced at intervals of 25mm.

They should have a width of 6.5mm and a height ranging from 0.075mm to 0.16mm, depending on the flowability of the material. Softer materials should use the lower end of this range.

Ejection pins should be enlarged as much as possible. And in most cases, 3 – 4 flat surfaces, each measuring 0.05mm in height, should be ground on the cylindrical surface of the ejection pin.

The grinding direction should be along the length of the ejection pin. Fine-grit grinding wheels should be used for this purpose. The end face of the ejection pin should be chamfered by 0.12mm so that any flash formed can adhere to the component.

Mold Cavity Venting Methods (2)

4. Conclusion

Properly incorporating venting channels can significantly reduce injection pressure, injection time, holding pressure time, and mold clamping pressure.

This transforms plastic molding from a challenging process into an easier one, ultimately enhancing production efficiency, reducing manufacturing costs, and lowering machine energy consumption.

In fact, venting can be achieved through various methods other than venting channels. There are several other methods for venting as well, including the following:

(1) Venting through Venting Channels

For molds used in the production of large and medium-sized plastic components, a significant amount of gas needs to be expelled.

Therefore, it is common practice to create venting channels, typically located on the concave side of the parting surface.

The positioning of venting channels is best at the end of the molten material flow. And their dimensions should ensure that gases can be smoothly released without material overflow.

Venting channels are usually around 3-5mm in width, less than 0.05mm in depth, and have a length typically ranging from 0.7-1.0mm.

(2) Venting through Parting Surface

For smaller molds, venting can be achieved by utilizing the gaps in the parting surface, with the parting surface positioned at the end of the molten plastic flow.

(3) Venting through Inserted and Assembled Components

For combination-type molds or cavities, venting can be accomplished through the gaps formed by their assembly.

(4) Venting through Ejection Pin Clearances

Venting can be achieved by utilizing the clearance between ejection pins and the mold base or core, or by intentionally increasing the gap between ejection pins and the mold base.

(5) Venting with Sintered Powdered Metal Blocks

Sintered powdered metal blocks are made from spherical particle alloys and have lower strength but a porous texture that allows gas to pass through.

Placing one of these alloy blocks in areas requiring venting can satisfy venting requirements. However, the diameter of the vent hole at its base should not be too large to prevent deformation due to mold cavity pressure.

(6) Venting through Vent Wells

On the exterior of the convergence point of molten plastic, create a cavity where gas can escape, providing effective venting.

(7) Forced Venting

In areas where gas is trapped, install venting rods. This method offers excellent venting results but may leave marks on the plastic component. Therefore, venting rods should be placed in concealed areas of the plastic component.

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