Designing plastic parts is deceptively complicated. There are many factors to consider along with the obvious part function, performance, and cosmetic requirements. The checklist below outlines most of the important factors affecting performance and cost for any given application. Not all of these items will be applicable to every part you design, but going through this list will undoubtedly give you a better understanding of your part and what it needs to do. This understanding will undoubtedly help you make changes to optimize the part design.
It’s a long list but don’t let that dissuade you. Every item on the list will affect part cost and performance.
Injection Molded Plastic Part Design Checklist (in no particular order):
- What is the function of the part?
- What is the expected lifetime of the part?
- What agency approvals are required? (UL, FDA, USDA, NSF, USP, SAE, MIL spec)
- Will the part be implanted in humans?
If so, biocompatibility is your first concern.
- What electrical characteristics are required and at what temperatures?
Some material properties of concern: Electrical Resistivity, Surface Resistance, Dielectric Constant, Dielectric Strength, Dissipation Factor, Arc Resistance, Comparative Tracking Index.
- Will the part be used in an optical system?
Some material properties of concern: Refractive Index, Gloss, Haze, Transmission (in desired spectrum, ie IR, visible, etc.)
- What temperature will the part see? And, for how long?
Some material properties of concern: Coefficient of Linear Expansion, Specific Heat Capacity, Thermal Conductivity, Maximum Service Temperature, Deflection Temperatures, Vicat Softening Point, Glass Transition Temperature, Flammability, Glow Wire Test.
- What chemicals will the part be exposed to?
Most material manufacturers test their materials with common chemicals. Contact individual suppliers for the results of their chemical compatibility testing.
- Is moisture resistance necessary?
Some material properties of concern: Water Absorption, Water Absorption at Equilibrium, Water Absorption at Saturation, Maximum Moisture Content.
- How will the part be assembled? Can parts be combined into one plastic part? Will one plastic part need to be divided into two or more?
- Is the assembly going to be permanent or one time only?
- Will adhesives be used?
Some resins require special adhesives.
- Will fasteners be used? Will threads be molded in?
- Does the part have a snap fit? Glass filled materials will require more force to close the snap fit, but will deflect less before breaking.
Some material properties of concern: Flexural Modulus, Flexural Yield Strength.
- Will the part be subjected to impact? If so, add rounds to the corners to minimize stress concentration.
Some material properties of concern: Izod Impact, Charpy Impact (Unnotched), Charpy Impact (Notched).
- Is surface appearance important?
If so, beware of weld lines, parting line, ejector location, wall thicknesses, surface texture, draft, and gate vestige.
- What color is required for the part? Is a specific match required or will the part be color coded? Some glass or mineral filled materials do not color as well as unfilled materials.
- Will the part be painted?
Some paints require a primer which may attack the molecular structure of the material. Some paints require a thermal cure so you will need to verify the material will withstand the oven cure temperture.
- Is weathering or UV exposure a factor?
Some material manufacturers test their materials for UV exposure. Contact individual suppliers for the results of their UV testing. If no testing has been done, plan on doing the UV testing yourself. UV exposure is often overlooked and be very detrimental to the physical properties of the part.
- What are the required tolerances? Can they be relaxed to make molding more
- What is the expected weight of the part? Will it be too light (or too heavy)?
- Is wear resistance required?
Some material properties of concern: Rockwell M Hardness, Rockwell R Hardness, Coefficient of Friction (Static), Coefficient of Friction (Dynamic). Surface finish is also a factor so adjust draft to allow for the desired finish within the tool and plan for no ejection on wear surfaces..
- Does the part need to be sterilized? With what methods (chemical, steam, radiation)?
This requirement is similar to chemical compatibility. Some materials are tested and results published by material manufacturers, others will need to be tested for your specific application.
- What is the worst possible situation the part will be in? (For example, will the part be outside for an extended period of time and intermittently put in water, or maybe see a constant high load while submerged in gasoline.) Parts should be tested in the worst case environment.
- Will the part be insert-molded or have a metal piece press-fit in the plastic part? There are tooling, process, and residual stress implications of insert molded features and press-fits.
- Is there a living hinge designed in the part? Be careful with living hinges designed for crystalline materials such as acetal.
- What loading and resulting stress will the part see? And, at what temperature and environment? Will the loading be continuous or intermittent?
Some material properties of concern: Ultimate Tensile Strength, Tensile Yield Strength, Flexural Modulus, Flexural Yield, Elongation at Yield, Elongation at Break, Tensile Creep Modulus, Deflection Temperture..
- What deflections are acceptable?
- Is the part moldable? Are there undercuts? Are there sections that are too thick or thin?
- Will the part be machined?
Some materials are more amenable to machining than others.
This list is intended to be a starting point for plastic part design and is not a comprehensive design guide. Your part in your specific application may have requirements not listed here. If that’s the case, please leave a comment. We would love to hear about it.
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