Plastics Selection Support

Thermoset or Thermoplastic?

Despite the many thousands of engineered materials available, plastics fall into two main types: thermoset or thermoplastic. The terms sound similar and both are appropriate for many applications but there is a notable difference in how they behave during processing:

Thermosets (e.g, liquid silicone rubber (LSR), epoxy, phenolic) join polymers on one application of heat and structure chemical bonds that harden permanently on cooling. The chemical reaction cannot be reversed, so thermoset parts cannot be re-shaped or re-melted. Thermosets have strength, chemical resistance, and temperature resistance that make them appropriate for medical implantables, electronic interfaces/touchpads, and wiring harnesses/housings.

Thermoplastics (e.g., thermoplastic elastomer (TPE), high-density polyethylene (HDPE), acrylonitrile butadiene styrene (ABS)) become plastic on heating and harden on cooling, without changing plastic chemistry. These resins can be re-melted, reused, and recycled.

Whether designated a thermoplastic or thermoset, plastics are made up of polymers — long chains of repeated molecular units. The ways in which these chains align determine the plastic’s macroscopic properties. In turn, the macroscopic properties dictate the category in which the resin falls. Specific to thermoplastics, these resin categories are amorphous or semi-crystalline.

Amorphous

When polymer chain orientations are random, the plastic lacks a clear structure and is deemed amorphous. 

General Advantages

• Softens over a broad range of temperatures
• Easy to thermoform
• Excellent bonding with adhesives and solvents

General Disadvantages

• Vulnerable to stress cracking
• Poor chemical resistance
• Appropriate for structural applications only

Semi-crystalline

When polymer chains arrange themselves in an orderly, densely packed fashion, the plastic is deemed semi-crystalline.

General Advantages

• Resistant to stress cracking
• Good fatigue resistance
• Appropriate for bearing, wear, and structural applications

General Disadvantages

• Sharp melting point
• Difficult to thermoform
• Poor bonding with adhesives and solvents

Additives and Alloys

Plastics’ characteristics can be changed by mixing or combining different types of polymers and by adding non-plastic materials. 

Additives are used to improve specific properties such as strength, rigidity, UV-resistance, and flame resistance — a versatility that’s proving particularly beneficial as more industries incorporate metal-to-plastic conversion into their production processes.

The most common categories of additives are particulate fillers and reinforcing fillers.

Particulate fillers can reduce costs and improve properties such as modulus, conductivity, heat and ultraviolet light resistance. Popular particulate fillers include:

• Mineral
• Silica
• Ceramic
• Carbon powder/fiber
• Glass microspheres/fibers
• Powdered metal

Reinforcing fillers improve mechanical properties. Popular reinforcing fillers include:

• Long glass fiber additives to improve stiffness and strength, increase temperature performance (up to 150℉), and create a moderate surface appearance
  
  
• Short glass fiber additives improve stiffness, increase temperature performance, and offer better aesthetics than long glass fibers. Since the glass content is 30% or lower, short glass fibers allow parts to look comparable to unreinforced plastic parts
  
  
• Carbon, stainless steel, and Kevlar fillers improve conductive and/or shielding properties