Process Spotlight: Compression Molding

Compression molding is a technique used for molding that involves a slightly different process from injection molding
Compression molding is a technique used for molding that involves a slightly different process from injection molding. Here a mold is used to shape thermoset resins which are not injected but rather placed directly into the mold before pressure is subsequently applied thereby forcing it to take the shape of the mold. There are many applications for compression molding which is mostly suited to making large and relatively intricate parts. It can be seen used commonly in soft plastic and rubber products such as bottle caps, mechanical parts, rubber car mats, wellington boots, buckets, plastic crockery, knobs and handles.

The Process

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The process of compression molding is relatively easy to understand. First a thermoset resin (rubber or soft plastics: silicone, pvc…), will be closed into a pre-heated mold. This mold will then be heated further to soften the material inside while pressure is applied in order to force it into the all corners and contours thus giving it the desired shape. It will then be allowed to cool and harden before being removed. The toughness/flexibility of the finished product will vary depending on the material measured by ‘durometer’.

Once removed, the hardened material will have a certain amount of ‘flash’ – or excess material – often where the two halves of the mold met and the material has spilled out. This will then need to be trimmed off in order to leave only the desired shape. If you have a pair of wellington boots then look closely around the bottom and you might notice some frayed plastic going all the way around. If so, then your willies were probably created using a compression mold and that skirting is where the flash used to be.

To visualise the process, imagine that you’ve taken a lump of Play-Doh then sandwiched it between two halves of a plastic egg. What you would be left with is the rough shape of an egg with some excess around the edge where the two halves of the egg met. In compression molding, that excess is called the flash and the shape would be more accurate due to the nature of the material and the amount of heat and pressure being applied. Getting the right amount of material for the process (the ‘shot’) is incredibly important to avoid waste and ensure a high-quality end product.

Advantages and Disadvantages

There are many benefits to using compression molding, the first being the low cost of the molds which makes it suitable for producing high shorter runs and for lower budgets. This is further aided by the minimal amount of waste material created by the process and by its simplicity which can help to keep labour costs down.

Unfortunately though there are also downsides though as well which prevent compression molding from being suitable in every situation. For one, compression molding is only possible with certain materials meaning that it can’t be used unless you want to create something with a soft plastic/rubber-like appearance and feel.

Compression molding is also considerably slower than injection molding and other techniques, and results in lower product consistency. If it is very important that your end product have high dimensional tolerance – then this may not be the right method for you. This can be improved by using more expensive sheet molding compounds versus bulk molding compounds, but it will still be less accurate than other common processes.

Compression molding can be used for fairly intricate parts, but particularly small and delicate elements may be troublesome and the mold can’t include undercuts. For very small parts the deflashing process may be difficult and require specific techniques such as ‘cryogenic’ deflashing.

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