Plastic Part Design

Plastic material selection starts by considering the functional requirements of the part including mechanical loads and the operating environment (temperature range, presence of water or chemicals, outdoor exposure, etc.). It is important for designers to recognize that the reported mechanical property values for a plastic material reflect the results of short term laboratory tests and are not indicative of the loads that can be sustained in actual use.

Most “real world” applications involve extended service life, and therefore long term behaviors such as creep, stress relaxation, and thermal degradation need to be considered when selecting a plastic material for an application. Additional considerations include requirements for transparency, compatibility with adhesives and/or surface finishes, and any friction and wear requirements.

Finally, flammability characteristics, electrical properties, and agency compliance requirements (FDA, USP Class VI, ISO 10993, etc.) need to be considered when selecting a plastic material for an application. Our experienced sales and technical teams are available to assist you with material selection challenges.

If you would like to do research yourself, before contacting Curbell, view our Plastic Selection Chart for Reliable Machinery and Parts to compare key characteristics of UHMW, acetal, nylon, PET, HDPE, PEEK, polycarbonate, and acrylic.

Plastics can be shaped via a number of processes including extrusion, injection molding, machining, and thermoforming.

Extrusion

• Plastic profiles can either be extruded on a thermoplastic extruder or machined via multiple passes through a precision shaper.

Injection Molding

• High volume plastic parts with thin walls, consistent wall thicknesses, and appropriate draft are typically produced via injection molding. It is important to note that injection molding requires expensive tooling that is often specific to a single polymer. This limits design flexibility in the event that material or part geometry changes are desired.

CNC Machining – Router, Mill, Lathe

• CNC machining (via router, mill or lathe) is often the best choice for plastic parts with thick or uneven walls or in cases where design flexibility is desired since machining generally does not require expensive tooling.
• CNC routers are outstanding machines for rapidly producing two dimensional parts from flat plastic sheet. Routers are able to manufacture parts more quickly and cost effectively than CNC mills, however routers generally require more open dimensional tolerances than CNC mills and they are for lighter duty work than mills.
• CNC lathes are used to create parts that are round in cross section.

Thermoforming

• Thermoforming is a process where plastic sheet is heated and softened and then stretched over a mold. Air is then evacuated from the system and atmospheric pressure forms the finished part. Thermoforming is a good choice for hollow parts such as kiosk fronts, machine guards, and medical equipment housings that have even wall thickness and sufficient draft for release from the mold. Thermoforming tooling tends to be relatively inexpensive since it does not have to withstand the temperatures and pressures associated with thermoplastic injection molding.

It is important that designers be sensitive to certain part geometry and dimensional tolerance considerations when developing plastic parts.

• Thermal Expansion: Plastics tend to have higher rates of thermal expansion than metals.
• Moisture Absorption: Plastics can change shape due to the absorption of water or other chemicals.
• Residual Stress and/or Creep: Plastics may grow, shrink, or warp due to residual stress or creep.

These properties require designers to specify wider dimensional tolerances for plastic parts compared with tolerance that are typical for metal parts.

Direct Contact with Mating Metal Parts and Thermal Expansion

If plastic components are to be used in direct contact with mating metal parts, it is important that assembly methods allow for thermal expansion mismatch between the metal and plastic components. This is often accomplished through the use of oversized or slotted mounting holes or by using flexible adhesives that allow the plastic parts to expand without joint failure.

Residual Stress or Creep

Stress concentrations are a particular concern for plastic parts, especially for amorphous thermoplastics such as acrylic and polycarbonate. These materials are more susceptible to environmental stress cracking than semi-crystalline thermoplastics or thermosetting plastics.  It is important to avoid features such as sharp 90 degree internal corners, which concentrate stresses in ways that can lead to part failure. Internal corners on plastic parts should be designed with generous radii whenever possible.

Fasteners with flat tapered heads and/or sharply pointed threads also create stress concentrations that can lead to environmental stress cracking and/or creep rupture. This is particularly problematic when fasteners are placed too close to the edges of plastic parts. Preferred for plastics parts are fasteners with:

• rounded threads
• flat (not tapered) contact surfaces
• fastener designs that include washers

Additionally, fasteners should be kept some distance from the edges of parts to minimize the chance of cracking during assembly or subsequent failure due to creep rupture.

Threaded metal inserts can be problematic if they are press fit into plastic parts or if they have sharp external threads, either of which can cause stress concentrations, environmental stress cracking, and/or creep rupture. Designers are encouraged to specify threaded inserts that have been specially designed for plastics. These inserts are often thermally or ultrasonically installed and they have geometries that limit stress concentrations. Additionally, it is important that metal inserts be free from lubricants or other chemicals that might be stress crack agents that could potentially degrade adjacent plastic surfaces.

Many plastics can be assembled with adhesives or solvent cements. However, some semi-crystalline plastics (PTFE, acetal) are engineered to have good chemical resistance, which limits their ability to bond. These materials may require surface treatment (corona, chemical etching, flame) prior to bonding.

Plastics can be joined using mechanical fasteners. Avoid mechanical fasteners that concentrate stresses in ways that can lead to part failure—i.e., environmental stress cracking, creep rupture.