Casting and Machining Plastics


Reading Assignment


Casting Plastics

  •  Simplest of the shape-forming processes
  • No fillers and no pressure is required
  • Thermoplastics are the main type of polymer that can be casted
  • Some thermosets can also be cast


Materials typically cast (initially in liquid form)

  • Nylon (Polyamide)
  • Polyurethane
  • polyesters
  • Acrylics
  • Epoxies
  • PVC Plastisols

Requires a mold or die.

Molds can be simple to complex.

  • Lead on steel mandrel
  • Latex or silicone
  • Aluminum
  • Steel

Often a catalyst is used to air in the curing process.
Size and weight range from less than one ounce to several tons

Dip Casting

Uses liquid plastisol or a fluidized powder.

Machining Plastics

Machining Plastics

  • Plastics can undergo many of the same processes of metals
  • Milling, sawing, drilling, and threading
  • General characteristics of plastics that affect machining
  • Poor thermal conductors
  • Soft and may clog tooling
  • Softening may reduce the precision of the final dimensions of thermoplastics
  • Thermosets can have more precise dimensions because of its rigidity


Tooling Considerations for Machining Plastics

  • High temperatures may develop at the cutting point and cause the tools to be hot
  • Carbide tools may be preferred over high-speed tool steels if high-speed cutting is performed
  • Coolants can be used to keep temperatures down
  • Water, soluble oil and water, weak solutions of sodium silicate
  • Lasers may be used for cutting operations

Machining processes are sometimes used as secondary operations on molded and
formed parts, but are most commonly used in the fabrication of flat plastic
sheet, or rod and bar stock into finished products.
There are several key differences between machining plastics and metals:

  • Plastics exhibit greater thermal expansion, up to 10 times more than that of
  • metals
  • Plastics dissipate heat at a slower rate than metals
  • Plastics have more elasticity than metals
  • Plastics have lower melting temperatures than metals

These differences require changes in both cutting tool speeds and feed rates.
Cutting tool materials also affect the machining of plastics. High speed steel
cutting tools can be used in short run production, but tungsten carbide or
diamond tipped tooling are required for higher production rates.
The range of plastic materials respond to machining differently and are
typically divided into three main categories:

  • Soft plastics, such as polyethylene and polypropylene, which will produce long curly chips when machined
  • Hard plastics, including Acrylonitrile Butadiene Styrene, or ‘ABS’ and polycarbonate, which result in short crystal like shards when machined
  • Reinforced plastics, which contain abrasive fibers requiring very sharp tooling of specific geometries for machining

Because of heat buildup both in part and in the tool bit, coolants are often
used to control temperatures. Coolant mediums can include:

  • Clean compressed air
  • Mist sprays
  • Water soluble oils
  • Light cutting oils


Careful selection of a coolant is needed to avoid any adverse reactions with the
plastic being machined.

Harvry tool plastic cutting end mills (link)

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