Saturday 31 March 2018

Polymeric Material Selection: A Critical Factor In Making Successful Plastics Parts

Holding a high quality plastic part in your hands is a result of several product development steps. Having your product development and production process strategy properly aligned is half the success. The other half comes by considering five factors which influence your outcome in having best in class injection moulded parts (Figure 1) [1]:

  1. Part design: there are design rules for plastics part, especially for injection moulded parts which need to be followed. Polymers have anisotropic behavior compared to isotropic metal parts.
  2. Material selection: once application requirements are established and the base design of the part is done, selecting the material can start.
  3. Mould design and construction: designed in a way that the mould can withstand the moulding process and the polymer.
  4. Moulding machine selection: when the mould design is completed, the injection moulding machine selection should be done or it can be done in parallel to the mould design, depending on the data available.
  5. Moulding process: optimization of the process is the last step and often done incorrectly or not at all.

Figure 1: “From Art to Part”: Material Selection as one critical factor in successful plastic part production [1].
In this post, I keep the focus on the “material selection” factor, since it is a vital one. After the basic part design is done, it is time to review the part performance requirements. In general, a separation of material selection based on performance, processing, and costs can be done. Following questions you need to answer for your part:

- Which areas of performance do I need to consider for this application?
- Are mechanical performance criteria (strength, stiffness, toughness) dominating?
- Are electrical performance criteria (insulating polymers vs. conductive polymers) dominating?
- Are environmental effects (temperature, chemicals, radiation, time) dominating?

Furthermore, tolerance criteria on the part itself need to be taken into account. In case you have a tight tolerance part, low shrinkage materials are the preferred choice. Having thick sections in your part, filled polymers can help obtaining a good filled part.

After gathering all the data which is needed to answer the questions from above, you can start your material selection procedure and make your material shortlist for decision making. Usually, a typical material selection procedure covers three steps [3]:

  1. Application screening
  2. Generic family and specific grade identification
  3. Process selection and cost analysis
These three steps reconcile with the five critical factors for the making successful plastics parts.
Here are 9 more tips what can be considered in the phase of material selection [2]:
  1. Stress/strain curve: for plastics the stress/strain behavior is usually not linear up to yield. There are cases where the yield may be very slight or does not exist at all.
  2. Modulus of elasticity in tension vs. compression: the E-modulus in tension is not necessarily the same as that in compression.
  3. Young’s modulus (E-modulus): the plastic modulus of elasticity is low compared to that of metals.
  4. Plastics show anisotropic behavior: injection moulded parts made out of fiber reinforced plastics demonstrate anisotropic behavior.
  5. Mechanical behavior: in plastics parts mechanical behavior is influenced by the rate of straining of the material. It is a function of temperature and time as well.
  6. Creeping: in comparison to metals, plastic parts creep under load with time.
  7. Reduction in strength: plastic parts show a decrease in the strength with time. This is the case with static loads too.
  8. Environmental conditions: material properties of polymer-based products may change in certain environmental conditions.
  9. Additive package: most plastics have an additive package consisting out of heat stabilizers, fillers and glass reinforcements and this must be considered when specifying the material.
Once the material is chosen, the mould design (factor 3), injection moulding machine selection (factor 4) and processing (factor 5) can kick off.
Since there is not always a full engineering of the material properties needed, time saving material selection tips can help. Here are some rules of thumb for making an educated guess on plastics material selection [4]:
  • Trying out acrylonitrile butadiene styrene (ABS): it works for many applications and is in a reasonable price range. It is strong and relatively though, combined with a low melting point and good processing properties.
  • For a cheap solution and when surface aesthetics are not critical, polypropylene (PP) will do the job.
  • For having increased temperature resistance as well as higher impact resistance, polycarbonate (PC) is the next best candidate going from ABS.
  • For having a good overall aesthetics and transparency, polymethylmethacrylate (PMMA) is your material of choice. The downside is that it can be too brittle for certain applications. Considering a transparent PC, it will be tougher than PMMA, however the surface aesthetics might not fulfill your set of needs.
  • For higher engineering demands, aliphatic nylons are the best way to go. Particularly, the polyamide 6.6-GF30 is well established in lots of engineering applications, especially in Automotive. When higher temperatures are needed (120-140°C), aromatic polyamides (e.g. polyphthalamide (PPA)) will do the job.
Apart of the aforementioned guides, I developed a systematic way of selecting polymers which uses a funnel method. Here you can read an introduction and my book on this topic is available here . 
Success with your next material selection!
Thank you for reading!

Herwig Juster

Interested to talk with me about your polymer material selection, sustainability, and part design needs - here you can contact me 

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[1] Distinctive Plastics Inc.:  The 5 critical factors to produce a succesfull injection moulded product, 2011
[2] B. S. Benjamin, "Structural Design with Plastics," Van Nostrand-Reinhold, 1961.
[3] Paul F. Kusy: Plastics Material Selection Guide, 1976

[4] Proto Labs: Materials Matter – The Material Selection Process

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