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Plastic Parts

Machining Plastics? Call In A Specialist

The following article was written by Alex Curtiss, President of EPP Corp. and published in Purchasing Management Magazine. Alex has been in the plastics industry for more than 25 years. He has served on the board of directors of the National Association of Plastic Distributors. He has extensive experience with the application of plastic machining to a wide variety of industries. (Also see Molding Vs. Machining which accompanied the article).

Imagine this scenario: A relatively low-volume order for machined plastic parts comes across your desk. Quality and product liability are major issues - the parts have to meet critical tolerances, and maybe even FDA standards. The best possible product has to be obtained at the lowest possible cost.

Who are you going to call?

It might be easiest to contact a sheet, rod and tube distributor, but at some risk. The distributor would probably know the materials, but may be completely unversed in machining.

Another possibility would be a metal machining house. The source would likely be expert in machining, but would they know the materials well enough to use the right one? Would they use the appropriate machining methods?

Fortunately, in such a situation you are not limited to these two choices. A third choice is a firm that specializes in machined plastic parts. A plastic machinist will not only be conversant with materials and methods, but will also have equipment that is used solely on plastic.

A Big Concern

Unlike the metal machining house, the plastic machining vendor can consistently offer uncontaminated product that meets the stiffest standards. Contamination may sound like a small problem, but it can mean big trouble in machining plastic.

Equipment used to machine metal - even if only used for metal occasionally - can contaminate parts with oil-based cutting fluids. Many plastics are hydroscopic, so they pick up liquids they come in contact with. If the parts are being manufactured for FDA-approved uses, this is no small problem.

It is often difficult to adequately clean a machine that has been working on metals, especially if it has been working on stainless steel. This can lead to another contamination problem. If the plastic material is soft, residual metal fragments can become embedded in the parts.

Even if parts are not going to be used in an FDA-approved application, contamination is still a concern. Oil-based cutting fluids chemically attack many plastics. This, in turn, can lead to fatigue, crazing, and ultimately, parts failure. One manufacturer of food processing equipment found this out the hard way. A pressure plate, machined out of polycarbonate, exploded in its very first use. It was later determined that an oil-based cutting fluid, used by a metal shop when machining the pressure plates, had reacted with the polycarbonate material and caused the failure.

Fortunately, problems like this are easily avoided. A good plastic machining firm will have the material knowledge and the right machining processes to consistently give you quality parts.

Choosing: A Checklist

Not all plastic machining vendors are equipped to offer all parts; others may not be able to give you the best prices. The following guidelines can help you locate a fully-qualified, cost effective specialist:

• Material Knowledge

Quiz potential vendors on their knowledge of various materials and applications. Ask them to share the information they have from the manufacturers. Do they have any property charts or handbooks to give you? Specialists in plastic machining should have a wealth of up-to-date information.

Potential machining firms that are members of a national trade association, such as the National Association of Plastic Distributors, may have better access to reference materials and other resources.
Material knowledge is a critical area. Good plastic machining firms must be willing to help educate their customers and help them choose the best material for their application.

• Purchasing Practices

Just as a metal machine shop is unlikely to have the material knowledge you need, neither are they going to be able to purchase plastics in large enough quantities to provide you with the best price.
Find out if the vendor candidates can purchase materials directly from the manufacturer. If they rely on distributors for materials, make sure the vendor is up on the technical side of the business.

Your goal should be to make sure that your plastic machining firm's buying practices are as good as yours.

• Equipment

Ask about the type of equipment the potential vendor used. The well-equipped plastic machining vendor will look pretty much like a metal machining shop, but the bona fide specialist will have equipment that has been adjusted or re-built expressly for machining plastic material.
The right equipment will machine plastic using the correct speeds, feeds and tooling, and won't pose the threat of contamination faced when metal-machining equipment is utilized. The right equipment will also insure higher quality arts, with better finishes and less chance of chips, burrs and other imperfection.

• Quality and Price

Quality, of course, is directly related to price. Make sure the vendors you interview can provide the highest quality part at the lowest price. Can they machine and screw machine all plastic materials using conventional, CNC and tool and die procedure?

Make sure the supplier is familiar with each process you need, as well as with the specific plastic material you will be using. Experience, more than anything else, will help make sure a supplier offers you the most cost-effective part with the best quality.

• Looking For The Best

Selection of a truly qualified vendor is extremely important in today's business environment. The time it takes to ask the right questions will pay big dividends - helping you obtain high quality machined plastic parts. And even though the rules for service and quality seem to be continually evolving, getting the best part at the lowest possible price is still the golden rule of purchasing.

 
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