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Guide to Machining Plastics

Explore Our Plastic Machining Guidelines

Select a machining process below for specific guidelines and recommendations specific to plastic material types.

General Guidelines

Learn about machining tolerances, general best practices for clamping and tooling, and recommended coolants for plastics.

Turning

Reference our turning guidelines for a wide range of plastics with complete parameters by material for speeds, feeds, and tool geometries.

Milling

Reference our milling guidelines for a wide range of plastics with complete parameters by material for speeds, feeds, and cutting tool types.

Drilling

Reference our drilling guidelines for a wide range of plastics with complete parameters by material for bit type, best practices, and special considerations.

Sawing

Reference our cutting / sawing guidelines for a wide range of plastics with complete parameters by material for saw type, material thickness, and other parameters.

Other Resources

Check out our complete resources for annealing, ASTM properties, chemical resistance of plastics, plastic identification, and more.

General Fabrication Guidelines

The following guidelines are presented for those machinists not familiar with the machining characteristics of plastics. They are intended as guidelines only, and may not represent the most optimum conditions for all parts. The troubleshooting quick reference in this section should be used to correct undesirable surface finishes or material responses during machining operations.

Machining Tolerances:

Most quality stock shape materials are stress relieved to ensure the highest degree of machinability and dimensional stability. However, the relative softness of plastics (compared to metals) generally results in greater difficulty maintaining tight tolerances during and after machining.

The machining tolerances that are required for thermoplastic parts are in general considerably larger than those normally applied to metal parts. This is because of the higher coefficient of thermal expansion, lower stiffness and higher elasticity, eventual swelling due to moisture absorption (mainly with nylons) and possible deformations caused by internal stress-relieving during and after machining. The latter phenomenon mainly occurs on parts where significant material removal occurs and/or machining causes asymmetric components (more material removed from one side versus the other.  In such cases, a “balanced” machining on both sides of the stock shape’s centerline may help to reduce warpage or bow.  A thermal treatment (stress-relieving) after pre-machining and prior to final machining of the part may also prove advantageous.

 A good rule of thumb for tolerances capable of being held with advanced polymer parts is 0.1 to 0.2%, or +/- .001” per inch of dimension.  Tighter tolerances are possible with very stable, reinforced materials.  However, wider tolerances may be required for softer, less stable polymers.

When machining plastic stock shapes, remember...

  • Thermal expansion is up to 10 times greater with plastics than metals
  • Plastics lose heat more slowly than metals, so avoid localized overheating
  • Softening (and melting) temperatures of plastics are much lower than metals
  • Plastics are much more elastic than metals

 

Because of these differences, you may wish to experiment with fixtures, tool materials, angles, speeds and feed rates to obtain optimum results.

Getting Started

Machining Forces / Clamping:

Machining forces are significantly lower for plastics versus metals, and plastics will deform more easily if clamped too tightly.  Therefore, clamping forces should be monitored carefully and one should consider using less force compared to those used for traditional metal machining.  Of course, take caution to avoid any unsafe condition where the plastic part could come out of the clamping tools.

As engineering plastics are not as rigid as metals, it is essential to support the work adequately during machining in order to prevent deflection or deformation, e.g. thin walled tubes often require the use of an internal plug at the chucked end in order to allow accurate machining of bushings with respect to roundness and tolerances.

Tooling:

Tools suitable for plastics must be used.  Dull cutting edges can lead to increased heat generation resulting in thermal expansion, poor surface finish, and inability to hold desired tolerances.  Tools should be adequately spaced to ensure that only the cutting edge comes in contact with the plastic.  High speed steel tools work well.  However, tungsten carbide (with ground cutting edges) or polycrystalline diamond (PCD) tooling is preferred for long production runs.  This is essential when machining glass and/or carbon fiber reinforced materials to ensure long tool life and good surface finish.

  • Positive tool geometries with ground peripheries are recommended.
  • Carbide tooling with polished top surfaces is suggested for optimum tool life and surface finish.
  • Diamond coated or polycrystalline tooling provides optimum surface finish when machining harder materials like Celazole® PBI.
  • Use adequate chip clearance to prevent clogging.
  • Adequately support the material to prevent vibration and restrict deflection away from the cutting tool.

Coolant Recommendations

Coolants are generally not required for most plastic machining operations (not including drilling and parting off). However, for optimum surface finishes and close tolerances, non-aromatic, water soluble coolants are suggested. Spray mists and pressurized air are also very effective means of cooling the cutting interface. General purpose petroleum based cutting fluids although suitable for metals and some plastics, may contribute to stress cracking of amorphous plastics such as Acrylic, Polycarbonate, Polysulfone, Ultem® PEI, and Radel® R PPSU.

Coolants are strongly suggested during drilling operations, especially with notch sensitive materials such as Ertalyte® PET-P, Torlon® PAI, Vespel® PI, Celazole® PBI and glass or carbon reinforced products.

In addition to minimizing localized part heat- up, coolants prolong tool life. Example (flood) coolants suitable for most plastics include:

Polycut (Tullco, Savannah, GA)

A generally suitable mist coolant is Astro-Mist 2001A (Monroe Fluid Technology, Hilton, NY).

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