04.16.2013

Clear Machined Plastic Choices – Part One

Many applications benefit from using clear machined plastic parts to help view the process or improve aesthetics. In the amorphous plastics family, the design engineer can select from 6 commonly available materials: acrylic, polycarbonate, Polysulfone, Ultem, PVC and COC/COP. While clear in varying degrees, the materials are all sensitive to stress cracking. Stress risers with high mechanical forces should be avoided. In particular, poor machining can lead to stress cracks after several months. All machined plastic parts require post polishing to return them to the original clarity. Without polishing, the machined plastic part is more translucent than fully clear.

Acrylic: One of the least expensive materials for machined plastic components, Acrylic is also the most clear. Transmitting nearly 92% of light, acrylic has no tint and looks similar to window glass. Fairly hard and scratch resistant, it can suffer from brittle failure. Acrylic has limited chemical resistance. It tolerates Isopropyl alcohol moderately well but will be degraded by stronger solvents.

  • Coefficient of Thermal Stability – .00004 in/in/F
  • High dielectric constant
  • Haze – 1%
  • UV stable
  • Continues service temp 175F, burst to 200F
  • Tensile strength 10,500 psi
  • Modulus of elasticity – 450,000 psi (tensile)
  • Flammability – 94HB
  • Available in many different colors

 

Polycarbonate: Also known as Lexan, polycarbonate is very impact resistant for machined plastic parts. This is not to be confused with stress cracking resistance, like other amorphous plastics it does not tolerate steady state stress well.  Available in machine grade (gray or purple tint) or window grade (tint less). Polycarbonate has better chemical resistance than acrylic but lower scratch resistance. Being slightly softer provides toughness and does not suffer from brittle failure. It is easy to polish after machining and makes an attractive part. Biocompatibility (USP class VI) allows Polycarbonate machined plastics parts to find uses in medical device applications.

  • Tensile strength at yield – 8900 psi
  • Tensile Modulus – 340,000 psi
  • Flexural Strength – 13,500
  • Flexural Modulus – 340,000
  • Heat deflection temperature – 270F at 264 psi
  • Flammability – V0 & 94HB
  • ½ the weight of Glass
  • Continuous Service temperature to 250F

Polysulfone: Often used in hot liquid applications, Polysulfone machined plastic components have higher continuous  service temperature than both Polycarbonate and acrylic. Sporting a light amber tint, Polysulfone machined plastic parts can be very clear though more expensive with the higher performance. Sensitive to stress cracking, helicoils thread inserts are strongly. Autoclave-friendly, it is available in many colors in USP class 6 for medical applications. Easily polished in a machined plastic part form.

Typical properties are:

  • Tensile strength at yield – 10,200
  • Modulus of Elasticity – 360,000 psi(tensile), 390,000 psi (flexural)
  • Continuous service temperature in air – 300F, bursts to 340F
  • Low creep under continuous load at elevated temperatures
  • Dielectric strength – 425 v/mil
  • Chemical resistance to mineral acids, alkali, and salt solutions, detergents and hydrocarbon oils
  • Resistant to hydrolysis
  • Can be repeatedly steam and dry heat sterilized
  • Excellent dimensional stability

End of Part One