How to select plastic resin for molding (scientific method)
Updated: Jul 6, 2021
Last week, I walked through my 'easy button' method for selecting plastics. It's a *very* simplified method, but it should get you in the right material family.
Let's say we're designing a pair of safety glasses.
We want a premium offering. What plastic should we select for the lens? My knee-jerk response: let's use what other safety glasses manufacturers use: PC (Polycarbonate) But is there anything to back up this decision? Maybe science can help support our decision. Scientific Method: I'm going to walk through this selection guide from Curbell Plastics. First we can select by general characteristics:
Amorphous - softens over wide temp range; formability; transparency; bonds well; good fatigue resistance
Semicrystalline - sharp melting point; poor formability; opaque; difficult to bond; poor fatigue resistance
Imidized Materials - best for high-temp; best for durability/bearing/wear; good chemical resistance
We want the safety glass lenses to be transparent, so we select Amorphous Thermoplastics So, which plastic resins fall into that category? Well, there's common plastics like ABS, PETG, PVC, PC, PS/HIPS, Acrylic, and Kydex And a few (more obscure ones) Noryl, Polysulfone, Radel and Ultem We can sort these resins by checking them against our requirements: Transparency Since optical clarity is important, we'll lean towards Acrylic, Polycarbonate, PETG, PS/HIPS Most other resins in this class are tinted amber or milky white at their natural state. Cost If low cost is important, we'll lean towards Acrylic, PVC, Kydex and PETG Moderate cost resins are Polycarbonate, PS/HIPS, and ABS If we can tolerate higher cost, we'll pick Ultem, Radel, Polysulfone, and Noryl Temperature Resistance The temperature rating closely matches the cost scale, but PC moves up the charts slightly over Noryl. Impact Toughness, Tensile Strength and Flexural Modulus Most plastics land in the same range of Flexural Modulus, (300-400ksi) unless chopped fibers are added. A few surprising outliers: Ultem and Acrylic have exceptionally low IZOD Impact Toughness HIPS has a very low Tensile Strength (lower than ABS, PVC, and Kydex)! FDA-Compliant Material Required? Make sure to confirm if material/resin must be compliant to FDA regulations. Understanding this early will help us be successful. Using a guide like this one from Curbell Plastics, it's clear (pardon the pun) why so many companies select PC for their Safety Glasses. Using this guide, it helps us understand high-level material properties and cost tradeoffs. I hope this brief overview was helpful. For your next plastics selection, I recommend using a guide like this one. I'd love to help you design for injection molding. Call me if you have any questions on your next plastics project: 763-229-9516
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