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Mould design: part 2 | DFM

In the second part in this three-part discussion on mould design, André Eichhorn provides some advice on avoiding problems due to poor venting and cooling of the cavity

Mould venting and cooling

As stated in the fi rst part of this discussion on effective and structured mould design, venting of the cavity is very often overlooked at the design stage and only given any real consideration once the tooling has been built and problems are experienced in production. However, implementing venting features on the tool structure after the tool has been built can be complicated and sometimes even impossible due to the space restrictions imposed by features and components such as cooling channels. Venting issues do not only produce bad quality parts

but can also put the mould tool itself at risk. The most familiar effect of poor venting on the part geometry is the burn mark (Figure 1), which actually shows up as a black spot on the plastic part. This black area is a thin carbon deposit created where the plastic is literally burned due to high temperature compressed gases trapped at the end of fi ll. While the burn marks are most evident on the plastic

component, the chemical action of these trapped gases at high temperatures can actually etch away the steel in these areas. This will eventually create an undercut, which will need to be corrected to avoid the part sticking during demoulding. The decision not to install a low cost venting pin or feature in a hard-to-fi ll area can end up costing several thousand Euros to rebuild a cavity. Venting may be required not only to eliminate gas

traps but also where weld lines occur or at very thin wall sections. Ensuring during the DFM process that venting features can be placed saves a lot of trouble as well as cost. Analysis tools such as fl ow study applica- tions will help to determine sensible areas on the component structure where venting would be required. April 2013 | INJECTION WORLD 47

Venting issues are most often created by the component geometry and Figure 2 shows an example where a rib design can be improved to ease the fi lling and avoid a gas trap on the fl ow path end simply by implementing a chamfer on the edge of the rib. Having a good understanding of the tooling technol-

ogy and venting requirements of moulding materials will help greatly during the design phase of the plastic component. Venting can be achieved in several ways, such as venting pins, ejectors, special designed core splits, or venting channels on the main parting lines which can be connected to the component geometry with small venting grooves. Three key factors need to be taken into consideration while setting up a proper venting system on a tool: dimensioning the vents, keeping them clear, and venting the gas.

Dimensioning the venting grooves All moulding materials will have different fl ow behav- iour so there will also be different requirements for the

Figure 1: Burn marks caused on the ribs of this moulding are due to trapped gas

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