Mould design Mould venting: a different approach

A guest post by Vincent Hua*

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In mould construction economics, the number of vents for mould venting is often overlooked. However, to produce quality parts and avoid defects, mould venting is crucial. Failing to produce adequate mould venting can lead to poorly finished parts.

A standard mould venting procedure involves machining pathways between mechanical elements of moulds through which air can escape.
A standard mould venting procedure involves machining pathways between mechanical elements of moulds through which air can escape.
(Source: © robynmac -

During moulding, air needs a way to escape from the mould, or else the melt will likely result in air compression. The air that once gets trapped in the cavity will lead to the diesel effect — a process resulting from a combination of high temperature and melt pressure. In other words, poor venting leads to various defects that may hinder the overall quality of the finished plastic part.

For instance, heat generated and trapped in the cavity through air compression may burn the plastic, adding pores and looseness to the part. In addition, a combination of inadequate mould venting, carbonization, and diesel effect can cause excessive plastic on the injected region.

Things to consider while moulding

During moulding it is essential to ensure that the finished plastic part is of superior quality. Thus, there are certain factors that one needs to be wary of while engaging in the mould venting procedure.

  • Vents should be placed anywhere along the parting line. More specifically, they should be placed where the injection mould is located.
  • In case semi-crystalline polymers are used for injection moulding, it is essential to apply increased injection pressure or pack pressure to the material once the filing is complete. Increasing pack pressure allows polymer chains to rearrange while freezing.
  • Packing is an essential element of mould venting. The longer the filling time, the cooler the materials will become, leading to poor packing. Therefore, the center core flow part of the melted material should be identified beforehand to reduce the filling time.
  • Not correctly done cavity filling can increase the likelihood of air trapped between two plastic flow parts. To avoid this, software analyses that are good at predicting critical spots should be performed for the right placement of mould venting.
  • Material viscosity is a function of melt temperature. Depending on the material viscosity, the limits of vent size will vary.
  • Materials used in mould venting should be properly considered. Some materials lead to the production of various gases that causes oily deposits, which may contaminate cavity surfaces, grooves, and channels.

Standard mould venting solutions

A standard mould venting procedure involves machining pathways between mechanical elements of moulds through which air can escape. The typical venting areas could be between parting lines, sliders, camps, or ejector pins that sufficiently allow air to escape.

In places where parting lines and sliding elements cannot be permitted for proper venting, the number of potential venting areas can be increased by dividing the mould cavity into inserts. Rheology or filling analyses (more commonly known as the trial-and-error method) should be employed to determine the proper venting size. Experts provide various recommendations to improve mould performance, which are:

  • Porous sintered materials are good at reducing injection pressure. Thus, inserting such materials into the mould cavity to serve as a vent can reject rates.
  • Frequently cleaning vents can reduce the chances of eliminating built-up gas deposits from valves.
  • It's preferable to use vacuum technology to remove the air from the cavity before injecting plastics.
  • An overflow system can also be created to connect the cavity via the channel and allow the gas to exit from the mould.

A different approach to mould venting

Alternative venting valves are equipped to overcome the shortcomings of standard mould venting valves and consequently improve the overall mould performance. Unlike static venting valves, dynamic venting valves have their advantages.

Such dynamic venting valves can be connected via an external channel (such as the cold runner, if there is one) or even internally through the mould cavity. Such valves allow air and resin gases to quickly leave the mould cavity through a venting channel of a few square millimeters.

External mould venting valve

External mould venting valves have a body consisting of a guide for a slide. Spring has also been inserted to increase the mould cavity's venting capacity. The venting area between the slide and the body connects the wide venting channels. The mechanical design of such valves ensures that a conical closure is obtained when the injected plastic reaches the vent.

Doing so prevents the plastic from flowing into the valve. The performance of the external mould venting valve is also better than the standard valve because there is flexibility for the valve to be located in different positions. The main advantage of external mould venting is its ability to vent out gases during the moulding procedure. Due to the valve's lower air counter pressure, injecting time, part atheistic, and other mechanical characteristics are highly optimised.

Internal mould venting valve

Compared to an external mould valve, an internal mould venting valve is internally placed and has an outer sleeve, a central pin, and a sliding shaft that allows the gas to escape from the venting hole of the mould. The venting hole automatically closes once the pressure of the injected plastic reaches its threshold.

Various mechanical components of the valves are responsible for making such valves function optimally. When the filing is performed, a spring pushes the sliding shaft into an open position that helps gases flow through the central hole. As soon as the injected plastic meets the valve, the plastic applies pressure on the surface of the shaft, thereby compressing the spring and closing the valve channels.

When in a closed position, the central pin of the valve ensures that the venting hole is completely sealed off so that the plastic doesn't unnecessarily fill the valve. The overall size of the valve is determined by the part thickness and various aesthetic requirements.


From burn spots to weld lines and poor mechanical properties to compromised surface finish, inadequate venting results in various moulding failures. This is why mould venting should be adequately paid attention to, among other moulding techniques, including the hydraulic pressing machine.

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Newer approaches to mould venting thus provide quality mould venting solutions that drastically improve the overall performance and reduce the chances of diminished mechanical features of the finished part.

* Vincent Hua is the Marketing Manager at Tsinfa.