Overcoming standard mould venting limitations: A different approach

Related Vendors

HASCO Hasenclever GmbH + Co KG

Planfuro Global

Nonstandard Venting Solutions

Typical vent locations may include, for example, between the parting lines, pins or ejector pins, or sliding elements such as sliders and cams. When parting lines and sliding elements don’t allow for enough venting, the mould cavity can be divided into inserts to increase the number of possible venting areas.

International Mold Steel

The potential of enhanced venting materials

Material viscosity limits the recommended size of vents that should be used in a mould. The lower the viscosity, the smaller the recommended vent. In fact, industry standards for vent size are determined by the material to be injected into the mould. Viscosity is also a function of melt temperature, and the viscosity of material flowing into a small vent changes according to injection pressure. Gases produced during moulding also impact venting. Materials such as polyamide (PA), polyphenylene sulfide (PPS), polyetherether ketone (PEEK), polycarbonate (PC) with glass fiber, polyethylene terephthalate (PET), polyoxymethylene (POM) and materials with flame-retardant properties produce gases during moulding. This gas production generates oily deposits that contaminate venting grooves, channels and the cavity surface, causing part defects. Venting gases produced by the melt can reduce mould cleaning frequency and mould cavity contamination.

Gallery

Although there are some guidelines for designing and machining vents to optimise venting capacity without removing excessive parting-line bearing surface, standard venting systems offer limited venting capacity and performance. There are a few nonstandard venting solutions, however, that can be added to cavity venting to improve mould performance: Porous sintered materials can be inserted into the cavity to act as a vent. This reduces injection pressure, and therefore scrap and reject rates. Keep in mind that the thicker the porous material, the smaller the venting capacity, so proper venting channels must be machined to collect the gases. Frequent cleaning of vents is also necessary, as gas deposits can build up and partially or completely block the removal of air and gas from the cavity.

Vacuum technology can remove the air from the cavity before the injection process. Although this addresses the problem upstream, it also causes limitations in application and performance. For example, to draw the air from the cavity and generate a vacuum (no air inside the cavity), the cavity must be completely sealed. This is costly and sometimes not possible in moulds with sliding elements. And although vacuum technology can solve air problems, it is not effective in dealing with gas produced during cavity fill. The intake valve closes before filling, so gases can’t find a way out of the cavity.

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