Plastics and Polymers
About three years ago we had a really bad hailstorm that destroyed many of the roofs in my neighborhood. In addition to tearing up the shingles on many of the houses (which were covered by insurance), the storm also put holes in the plastic sheeting that covered part of my back deck (which was not covered).
I decided to make sure that no hailstorm could ever destroy my pergola roofing again; I went out and bought sheets of 3/16″ x 4′ x 8′ Lexan, a polycarbonate material. I had often used Lexan for machine guarding and found it to be nearly unbreakable.
After installing the sheets I started hearing loud banging sounds coming from my deck area as the days grew warmer. Because I had installed the sheets end to end, as the material heated the sheets would bow up, eventually pulling some of the screws out of the wood. The sounds were made as the edges popped up where the sheets joined.
It turns out that an 8′ long piece of Lexan can expand up to an inch or more when heated. I never noticed this when guarding machinery because the temperature usually doesn’t vary much in a typical machine environment, also the sheets are usually smaller and mounted differently. I will be overlapping the sheets and ensuring that the screw holes have plenty of clearance when I reinstall my roofing.
As part of chapter 5 in my book Industrial Automation: Hands On I included a list of sheet plastics and their properties. This chapter covers process systems and industrial machinery categories, but since many of these plastic materials are used in machine fabrication I included the properties as well as the manufacturing methods. The following is an excerpt of several of the materials from the book:
Acrylonitrile-butadiene-styrene (ABS): This thermoplastic material has good impact strength, formability, stiffness, and toughness. Good chemical and stress-cracking resistance. A good general-purpose, low-cost material. Easily thermoformed, strength is affected by temperature. Black is UV resistant, while white and natural colors are not. Applications include aircraft interior trim, tote bins and trays, cassette holders, automotive parts, and luggage. The maximum working temperature is 185°F, forming temperature 325 to 350°F.
ACETAL (Delrin): Excellent load-bearing qualities in tension and compression. Does not absorb a large amount of moisture. High-yield strength at elevated temperatures. Machinable, easily fabricated. Low-friction, high-wear resistance. Attacked by strong acids and oxidizing agents, resistant to a wide range of solvents. Not UV stabilized. Excellent material for bearings, gears, cams, and small parts. Meets FDA standards, USDA approved. Service temperature range 20 to 185°F, intermittent 200°F.
Acrylic: Completely transparent, flexible, resistant to breakage. Lightweight (half the weight of glass), virtually unaffected by exposure to nature, salt spray, corrosive atmospheres. Easy to fabricate, can be sawed with fine-tooth blades, drilled with plastic drills, sanded, and polished. Can be cemented with acrylic cement. Meets FDA standards, UV stabilized, UL 95 flammability rating. Used for inspection windows, sight gauges, windshields, meter faces, protective covers, safety shields, tanks, trays, and displays. Service temperature range –40 to 180°F, forming temperature 350°F.
Polycarbonate (Lexan): High-impact material, virtually unbreakable. UV stabilized, can be sawed with fine-tooth blades, drilled with plastic drills, sanded, and polished. Applications include greenhouses, window glazing, safety guards, chair mats, equipment enclosures, signs, and doors. Continuous service range –40 to 240°F.
Additional materials I listed in the book are CPVC (Chlorinated Polyvinyl Chloride), Nylon, Polyethylene LDPE (Low Density), Polyethylene HDPE (High Density), Polypropylene, Polyurethane, Polyvinyl Chloride (PVC), PVC Expanded Sheet (Sintra), Styrene, Teflon, and Ultra High Molecular Weight Polyethylene (UHMW). I also cover many of the properties of different metals in my book. When implementing these materials in machine building, it is important to keep all of the material properties in mind!