Tips To Machine Engineering Plastics

A lot more component designers these days are looking for ways to replace metal parts with ones made from engineering plastics, and so more and more engineering /machining companies are getting more and more requests to quote for jobs that call for machining plastics, the other upside is that these jobs can also be a bit more lucrative.

The machines and tooling/cutters etc needed are pretty much identical, but some of the machining techniques are not exactly the same as for machining metals. Not only that but machining one type of plastic can often be different to machining another.

Since most plastics aren’t overly excited on changes in temperature or humidity, keeping humidity down and temperatures constant under most of our roofs is pretty difficult if not impossible. Also most orders/jobs can be low-volume so experience can actually count for more when it comes to those critical speeds and feeds.

Plastics can be a bit inconsistent, which means you have to adjust those speeds/rates sometimes as you go.  Bet there’s plenty of scrapped jobs in landfill sites that can be put down to “experience”. Machining a new engineering plastic for the first time though may warrant a test piece or maybe sample just to get some feedback from it before quoting.

Quoting on engineering plastics jobs can sometimes be seen as a bit more interesting than metals jobs, not just because of the size range but because of the different and varied materials available. Metals are often sold by “random length” and the price varies almost on a daily basis, where plastics are sold by the sheet or bar, come in multiple different sheet/rod sizes, so you tend to know where you are and the price tends to be fairly stable

Cost per material can vary a bit though; Nylon for example would be at one price where as something like PEEK would be a whole different ball game. Precision ground rod or planed sheet can add a bit to the price because of those one off set-up costs, but these can easily be offset because your material arrives virtually “finished”

Most engineering plastics in comparison with metals though have a higher expansion/contraction rate and if your unit/building is not at an even temperature, some plastics just might not machine the same in winter as they do in summer. (Chance would be a fine thing)

Its also worth insisting on a quality brand of engineering plastic we reckon, as they’re always annealed or even double annealed if necessary at the factory (even these can still vary a bit from batch to batch) but it pays in the long run as you’re a lot less likely to have any quality issues (these can easily wipe out any initial savings) an experienced engineer can usually tell the difference straight away almost as soon as the machining starts

Data sheets are brilliant and can offer some real insight on machining although the best way to really discover how a plastic machines, is to actually machine it and experiment with different techniques. One tip on data sheets is to look for the “hardness”. The higher the reading or the harder the plastic as such, the easier it tends to be to machine, this is because the material will tend to cut or chip nicely. Really soft materials such as Polyurethane will often machine better if it’s frozen somehow whilst it’s being machined (if this is potentially doable) So therefore softer plastics do tend to prove a bit of a challenge to machine.

Another tip to be aware of is if the plastic contains glass, such as glass filled nylon or GF PTFE etc. This stuff causes a higher wear rate on cutters. Most basic plastics can actually be machined with high-speed steel cutters, but glass filled plastics need carbide or even ceramic if possible to counter-act the abrasiveness that this glass additive often causes. Remember though that cutting tools always need to be razor sharp for machining engineering plastics.

Machining Tufnol type plastics throws a bit of an anomaly into the mix in that not only does the lamination direction aspect need allowances made, but also the dust element which could do with some form of extraction to prevent that yellow dust getting everywhere.

Absorbing moisture from either coolant or actually just from the atmosphere around the place is a common factor. Some are a bit more prone than others, materials though such as PET or Acetal have really low absorption numbers, but the most popular engineering plastic of all, Nylon, could potentially need a few expansion allowances, particularly if its a precision component with fine tolerances.

Some engineering plastics though can actually absorb moisture and expand whilst being machined, (via the coolant) guess what, they’re just as likely to contract back as they dry out over a period of time, which can makes things a bit more tricky to calculate. If tight tolerances are critical on nylon it may be worth looking into the possibility of cooling with air, to avoid the expansion issue that coolant can create.

Feeds and speeds are bit of a dark art, in so much as the factors involved, such as: the actual machine itself, the cutters as well as type of plastic being machined; some go with high cutting speeds where others prefer to go for lower cutting speeds, there are no hard and fast rules here really other than everyone finds what works for them on their machines.

One tip is “you can't allow hardly any rubbing to take place”, (where have you heard that before) this is because it will cause the dreaded heat to build up and possibly cause the plastic to melt, this will quickly stick to the tool face causing a poor finish. One tip is to get the chips out of the way and try to avoid those long strands forming which wrap round the tooling and the chuck. Another tip is to “cut the plastic quickly” so that the heat that’s being generated in the actual tool itself doesn’t hang around to radiate its heat into the actual plastic. The objective is to find that happy medium, generally though, slower speed and a higher feed rate than perhaps steel is a good starting point.

At really high speeds the plastic may melt and stick to either the cutter or the material itself, slow speeds though can have the same effect because the cutter is sitting around the material too long passing its heat on. Either way, heat and engineering plastics should be avoided as they generally don’t tend to mix, not only from a machining point of view but also from a material performance point of view.