Custom Rubber Mouldings in Aerospace Applications

A mould might be described simplistically as at least two pieces of material (typically steel), which when fitted together form a cavity, resembling the shape of the product. This would be a very basic mould. Moulding is by far the most important curing process, where uncross-linked rubber is placed into a heated mould, which gives it the final product shape, and then vulcanizes the material.

The mould

It can vary in size from a clenched fist to that of an automobile, and can have a single cavity to make one product at a time, or enough cavities to make a hundred or more. Most custom rubber mouldings are based on introducing a solid compound into a mould, although urethanes and silicones can be introduced as solids or liquids. It takes a fairly high mechanical pressure, to close the mould, and thus form the product shape; this pressure is provided by a press. Thus the mould must be strong enough to avoid being crushed. Tool steel hardened to a Rockwell C hardness of about 60 might be needed.

Mould design

A basic compression mould design must have the two halves of the mould register (fit accurately together). Often, pins built into the top section fit snugly into holes drilled into the bottom half. Any looseness between the pin and the hole may cause the top half of the product to be out of alignment with the bottom half. If the fit is too tight, attempts to manually open the mould may prove difficult.

Since a number of compound materials expand with heat (the raw gum elastomer is of primary concern) by at least an order of magnitude more than steel, they will also shrink correspondingly as they cool when taken from the steel mould. Thus the mould dimensions are typically designed to be around 1.5% (based on linear dimensions) greater than those required in the rubber product, to compensate for the difference in expansion between the rubber and steel. This percentage vulcanizate shrinkage might be greater for FKM and silicone compounds and less for compounds with high amounts of filler.

Overflow (flash or spew) grooves are machined around the mould cavity. In theory, this is to contain rubber in excess of the cavity volume. In practice for compression moulds, it is not unusual to see during mould closure, material filling the cavity, then spilling out of the overflow grooves, and even across an area outside the grooves known as the land, and then out of the mould. This excess material is known as flash.

Introducing compound to the mould

There are different ways of introducing compound into the mould, some of which involve modifications to the basic design. They each confer certain advantages not found in the others.

a) In the most basic design, pieces of rubber compound are placed in the bottom cavity and compressed using the top half of the mould.

b) The first modification is transfer moulding, which can be visualized as drilling holes through the outside of the top mould half of a compression mould through to the cavity. Thus the mould can stay closed while rubber compound is introduced through these holes into the cavity by using the force exerted by the press platen.

c) If a separate device is used, not related to the press platen, which injects the compound through the holes, this would be rubber injection moulding.

Post time: 02-23-2017