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Machining Engineering Plastics

Plastics have become significantly more popular as a choice of engineering material, so how does the machining of plastics differ from that of metal?

Variation

Plastics can be inconsistent in the same manner as cast and forged materials, this means there may be a level of adjustment needed when machining.

Environmental Sensitivity

One of the key differences in the machining of plastics when compared to metals is their ‘sensitive’ nature in relation to the environment around them. Plastics have a higher coefficient of thermal expansion and are sensitive to changes in temperature and humidity. Keeping these at a consistent level represents a serious challenge for many machinists. Most engineering plastics have a higher expansion and contraction rate to the point where they would machine differently in a hot summer then they would in a cold winter. Changing the temperture may result in the cracking of the plastic. 

Some engineering plastics are also absorbent to moisture from coolant which may be used in the machining process or from the environment itself. Nylon, one of the more popular engineering plastics is quite absorbent and as such will expand and contract over time. These factors all means that where tight tolerances are required, engineering plastics may be unsuitable and certainly the tolerances that are achievable are considerably larger than those of metal components.

Cutting Tools

Generally speaking the harder the plastic the easier it will be to machine. Although plastics can be considered as soft in comparison to their metal counterparts, cutting tools will need to be razor sharp in order to get the best results and climb milling is often recommended in order to reduce heat build up. One thing to look out for is any plastic which contains glass which may be the case for Nylon or GF PTFE. These plastics are more abrasive to cut and as such carbide cutting tools will be needed to prevent rapid wear breakdown.

Cutting Speeds

There will always be an amount of experimentation needed to find the optimum cutting speed when working with a new engineering plastic and where possible we would recommend doing test pieces with any offcut/scrap material you can find to reduce costs. The key consideration is to avoid any heat building up and often high cutting speeds are recommended so as the heat that is generated in the actual tool doesn’t hang around and transfer to the plastic. Remember though that at really high cutting speeds that plastic may in fact melt and stick to the cutter.

Machining of Plastics

Machining of Plastics takes specialist knowledge and is certainly a different process to normal metal machining. Where you don’t have experience it may be wise to use a sub-contractor like Bronte Precision who have a lot of experience in working with Nylon, Polyethylene, Acetal and more. We are fully ISO accredited and based in Bradford, but serve the whole UK.