Plastic Machining: Using a Metal Machining Company to Produce Plastic Machined Parts: The Risks

Engineers and buyers often reach out to metal machining companies to produce plastic machined parts, thinking, “They use the same manufacturing processes, don’t they? Does it really matter?”

It does matter. If you’ve wondered whether you should have a metal machinist produce your plastic machined parts, The Short Answer Is – No, You Should Not

Here’s why:

Cutting fluids for each material are crucial: Equipment used to machine metal – even if only used for metal occasionally – can contaminate parts with oil-based cutting fluids. Many plastics are also very sensitive to petroleum based cutting fluids and will degrade if they come into contact with these fluids. Additionally, many plastics are hydroscopic and will absorb the cutting oils. If the parts are being manufactured for FDA-approved uses or medical applications, they will not meet standards.

Metal fragments contaminate plastic parts: It is 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 plastic machined parts.

Metal shops are experts in metal, not plastic: Metal machining companies rarely have any in-depth knowledge of plastic materials, or more specifically, which material to use in an application. Plastic machinining specialists know what plastics are best for any function, and can produce the plastic part you need without the “trial-and-error” processes a metal machinist would use. A good plastic machining firm will have the material knowledge and the right machining processes to consistently give you the highest quality plastic machined parts.

If you’ve got custom plastic machined components, EPP Corporation can help.


  • George K

    I couldn’t agree more. Over and above the comments above, a metal machinist really has no concept about a plastic material’s properties, how it reacts to cutting tools, what type of cutting tool to use and, originally coming from a metal background myself, I can attest to this “ignorance” personally. A metal machinist doesn’t even know that he is about to waste time and money making a part because he approaches plastics in his “metal” frame of mind. In no way am I suggesting that metal machinists are incompetent; far from it; metal machinists are experts in their own right – it’s just that they don’t realise that machining plastics is a process almost 180-degrees-removed from what they are used to and that, most of the rules they are accustomed to, do not apply to plastics at all. In my experience, a carpenter would make a better machinist of plastics than a metal machinist. That is … until they see the light of working with engineering plastics – then it becomes VERY difficult to go back to machining metals.

    • EPP Corp

      Thank you for your detailed and very insightful response. It is great to hear from someone with actual real world experience in the subject matter. Most individuals looking for plastic machined parts feel as though there is no difference in machining metal vs. plastic, and are disappointed with results from using a metal vendor for plastic parts. Experts in their fields are going to garner the best results. Metal machining vendors for metal parts, and plastic machining vendors for plastic parts.

  • Tom Jack

    Amen. The key here is GMP, quality, and HAACP. Metal contamination is just one of the many problems metal manufacturers can encounter when they try to machine plastics. Plus, they do not understand the material or chemical properties of plastics. Many molders have plastic fabricating capabilities. Look for a plastics expert with a controlled, measured process.
    Now that I have sung the praises of pure plastic manufacturing, does EPP have room on their staff for a true believer? Let me know.

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