DESIGNING

PLASTICCOMPONENTS
FOR POWER TRANSMISSION ENGINEERS
Justin Frohock
R&D Engineer

US.SUMITOMODRIVE.COM

Designing Plastic Components, for Power Transmission Engineers
Justin Frohock, Sumitomo Machinery Corporation of America

Steel, iron, and aluminum are the dominant materials in the mechanical power transmission industry forgood reason: high power density requires the high strength and stiffness of metallic materials. Plastics,
however, offer valuable features that should be utilized for good gearbox design. Their low density, lowcost, and corrosion resistance properties are ideal for low-stress applications such as fans and covers;
transparent plastics are ideal for oil expansion tanks to aid operators to visualize oil level andappearance from varying distances; high-strength plastics can be used as couplings between metallic
components to completely eliminate fretting wear. Despite their value, plastics are still usedinfrequently enough in the gear industry that engineers often mistakenly design plastic components as if
they were metallic.
Designing plastic components has never been easier. Guides to the basic concepts of plastics design,
such as wall thickness, draft, ribbing, etc. are readily available, and fully automated design analysis of acomponent can be generated at a number of rapid manufacturing websites. While these resources
allow anyone to easily design a manufacturable plastic component, they typically don’t provide guidance
about how a plastic component might interact within an assembly or environment.
Plastic materials have unique interactions that should be considered prior to design finalization. As
noted herein, there are some common pitfalls that should be avoided and mitigated during the first
iteration of a plastic prototype design to maximize success.

Figure 1: Left: Sumitomo Cyclo Cooling Fan. Center: Sumitomo Safety Cover. Right: Sumitomo Oil Fill Cup.

Thermal Expansion
Thermal expansion is a common consideration with metallic components when designing press fits,setting bearings, etc. But consider that plastic materials typically have a thermal expansion rate 5 to 10

times higher than steel: this can cause a number of issues. When plastic components operate on a shaft
or within a bore, resultant fits should be carefully calculated. Furthermore, plastic thermal expansion
rates are often high enough that dimensional change due to temperature should be included intolerance analyses or “stack-ups” to ensure the intended final fit. For example, depending on thespecific grade of plastic, a 50mm long component made of Nylon 66 will expand up to 0.25mm when
undergoing a 50°C temperature rise. Thermal expansion could be contributing to the tolerance stack-up
just as much as the tolerance itself! When analyzing thermal expansion, it’s vital to ensure data
accuracy: use the actual data sheets for the chosen material or standardized sources like CAMPUS(https://www.campusplastics.com/), as material properties of plastics can vary wildly even within thesame polymer family. [1]Water Absorption
Plastic components are injection molded, 3D-printed, or otherwise manufactured with the raw material
completely dry. After the component is released from the mold, it begins absorbing moisture from theambient air, until it achieves equilibrium with the environment. This is the state at which plastic
components are physically measured, and mechanical properties such as strength and hardness arereported. However, if a component’s operating environment has higher moisture content, e.g. it’ssubmerged in water or in an environment with heavy splashing, the plastic will absorb more water. Thisadditional water content impacts the component’s mechanical properties and causes it to physicallyswell. Most plastics only change slightly due to this phenomenon, but many families of Nylon (PA) are
affected significantly, in some cases growing as much as 1%. Referencing the previous Nylon example,
the 50mm long Nylon 66 component could grow 0.5mm from moisture content alone. This can cause
similar issues to thermal expansion, and needs to be accounted for by choosing the right material inapplications that may experience high moisture content. [2]Stress Relaxation
Plastic components undergo a phenomenon known as stress relaxation (not to be confused with creep).
When a plastic component is subjected to constant strain, the internal stress in the plastic will reduceover time. This has a number of effects, but the most common application of stress relaxation is in thebolted joint. Bolts generally stay tight because opposing internal stresses within the bolt and the boltedmembers cause resultant forces on the threads, which then stay in place due to frictional forces.
However, since plastics undergo stress relaxation, a traditional bolted joint with plastic members willlose its axial stress over time, and thus lose friction in the threads, allowing the bolt to loosen. Fastening
a plastic component generally requires use of compression limiters or thread-forming screws.
Compression limiters are metal tubes inserted into the component to act as the bolted member for
standard fasteners (see figure 2). Thread-forming screws for plastic are specially designed to retain theirclamping force in spite of stress relaxation. [3]Figure 2: Example of compression limiter use.

Knit & Meld Lines
Plastic components are injection molded by forcing molten plastic to flow through a mold under highpressure. Since the plastic is in a liquid state, the long polymer molecules intermix as they flow, andremain in an entangled state when they solidify. This creates much of the component’s material
strength, since molecules share loads over many intertwined neighbors. In a plastic component with ahollow feature such as a hole, molten plastic must flow around the hole, splitting into two fronts whichmeet on the opposite side. When the two fronts combine into a single flow front, the meeting point iscalled a “meld line”; when the flow fronts coincide at the end of the mold, the meeting point is called a“knit line” (see figure 3). At knit and meld lines there is significantly less intermixing of molecules, andthus the strength at the knit line may be much lower than the rest of the material. This is especiallypronounced using brittle materials or materials including glass fiber reinforcement, where the strength
at a knit or meld line can be less than half that of the general material. Thus, when designing hollow
components one should be especially aware of how the mold will fill, to ensure that knit lines don’t formin critical areas. [4]Figure 3: Meld and Knit Line Examples

Conclusion
Plastics components are used in the power transmission industry, but they certainly aren’t incorporated
as often as they could be. Plastics offer low cost, low weight, and feature benefits that can’t be attainedwith metals. When designing a supporting feature, consider utilizing a plastic design, keeping in mind
the unique design parameters that plastics demand. Use an online plastic design guideline or tool suchas those available from Xometry (https://www.xometry.com/) or ProtoLabs(https://www.protolabs.com/) to ensure that a component is manufacturable, and keep the above
concepts in mind when ensuring the component is compatible with its assembly and environment.

[1] McKeen, L. (2014). The Effect of Temperature and Other Factors on Plastics and Elastomers.
[2] Monson, L., Braunwarth, M., & Extrand, C. (2008). Moisture absorption by various polyamides andtheir associated dimensional changes. Journal of Applied Polymer Science, 107(1), 355-363.
[3] Ward, I., & Sweeney, J. (2012). Mechanical Properties of Solid Polymers (3. Aufl. ed.). New York:Wiley.
[4] Criens, R. (1986). Impact strength of knit-lines in injection moulded parts. Materials Chemistry andPhysics, 14(1), 69-83.