November 2010 Design Tip

Living in the Material World

Prototypes are for testing whether your design and the material you’ve chosen for your plastic part combine to provide the functionality you need. A prototype lets you see how the part looks, whether it can handle an impact, how dimensionally stable it is, and how it wears. If there are problems, it gives you the opportunity to redesign or choose another material before committing to the cost of full-scale manufacturing or, worse yet, taking a less-than-optimal product to market.

Resin choice can be an important part of the process and the resulting product.

Resin choice can be an important part of the process and the resulting product.

In some cases, the part’s function is simple and the choice of material is not that critical; ordinary, affordable, easy-to-mould ABS or polypropylene may be perfectly adequate. But the demands on many plastic parts today are far greater, and the thousands of other engineered resins are out there for a reason. In some cases, your requirements are so specific that resins will have to be custom blended to meet the need, but even when requirements are less rigid, material choice can be critical.

You can design your part and then go shopping in the vast “resin supermarket” for a suitable material. There are, however, good reasons to choose your resin, or at least narrow the choices, as early in the process as possible, perhaps even before you begin designing. Designing with resin in mind can speed the development process, simplify choices along the way, and reduce or eliminate the need for rework in the later stages of the process.

Clearly, functionality will be a major factor in resin choice, but other factors including mouldability should be considered as well. Knowing in advance the resin you will use can make a significant difference in the way you design a part. If, for example, the resin you choose is expensive, you can focus on eliminating unnecessary material in your design. If you need the strength of a resin that does not flow well in a mould—glass-filled nylon, for example—you will have to be particularly aware of narrow areas that can lead to voids or knit lines. Or if your part will serve as a bearing and requires the lubricity of an acetal resin like Delrin®, you can begin to design knowing that this resin is very sensitive to excess wall thickness.

Of course, design choices are rarely as simple as a single functional requirement or a single resin characteristic. Early decisions regarding resin may be based on a long list of requirements including strength, durability, hardness, flexibility, lubricity, electrical resistivity, chemical resistance, UV resistance, heat tolerance, flammability, colour, transparency, environmental challenges, and cost. Mouldability factors can include ease of flow, tendency to flash, ease of ejection, and likelihood of warp or sink. Obviously, not all of these characteristics will apply to any one part. But it does make sense to carefully review the requirements, determine which ones apply, and then rank them in terms of importance. Such a checklist can then be used to narrow the list of usable resins, possibly even identifying the one best candidate before even beginning to design the part.

Once the resin has been identified, the design and moulding process can then be adapted to that material’s requirements. For example:

  • Polystyrene is hard, clear, and inexpensive, but brittle, limiting its application or requiring that steps be taken to toughen the resin.
  • ABS is very affordable and impact resistant but is susceptible to sink, requiring that thick areas be avoided.
  • LCP is strong and fills thin features well but forms weak knit lines, affecting both part geometry and gate placement.
  • Nylon is affordable and strong, but absorbs water leading to dimensional and property change. This limits the applications in which it can be used.
Our online Protomold Resin Guide lists a sampling of resins, along with their mechanical properties, mouldability characteristics, and some brand names.

Our online Protomold Resin Guide lists a sampling of resins, along with their mechanical properties, mouldability characteristics, and some brand names.

Decisions don’t necessarily end with choice of resin. For specialized requirements, various additives can be used to extend the capabilities of the base material.

  • Short glass fibres can strengthen a resin and help prevent high-temperature creep. They can, however, make a resin more brittle and increase the tendency to warp as a part cools.
  • Long glass fibres provide greater strength and creep resistance, but can impede resin flow, particularly through thin areas.
  • Aramid (Kevlar®) fibres add strength, though not as much as glass, and are less abrasive than glass.
  • Carbon fibre can strengthen and stiffen a resin and aid in static dissipation, but is costly and can lead to warp.
  • Stainless steel fibres are used in electrical housings to reduce electromagnetic and radio frequency interference.
  • Mineral fillers—talc or clay—can increase hardness and reduce both cost and warp.
  • Glass beads and mica flakes add stiffness and reduce warp and shrinkage, but can be challenging to inject.
  • PTFE (Teflon®) and molybdenum disulfide, dry lubricants that function like graphite, can make plastic parts self-lubricating.
  • UV inhibitors help prevent material breakdown in outdoor applications.

Keep in mind that, while our Customer Service Engineers are always willing to address your questions at +44 (0) 1952 607447, we are not resin specialists. We can answer basic questions, but for detailed information on standard resins or custom blends, we may refer you to specialized resources. The final decision regarding resin choice for a specific application will be yours.

For information on Protomold-stocked resins and links to supplier sites, visit our materials page. For more detailed information on resins and their characteristics, see the PolyOne or RTP Plastics sites.

View our Protomold Resin Guide.

Read past Protomold Design Tips.