Back to Basics

It’s amazing how often I’m approached by experienced people in our industry who are looking for a basic understanding of how resin parts are made. Usually, they’re a little defensive, embarrassed, or both. If this describes you, don’t be, because you’re in good company.

That’s why this year, I’m starting series of “back to basics” columns that will (hopefully) give industry insiders who haven’t burned their fingers on hot resin a sense of what’s happening on the shop floor, without having to ask embarrassing questions. Believe me, I’ve been there too.

You don’t need a PhD in polymer science to get a great understanding of materials and processes. Before we look at making things from plastics, let’s define what the material is in the first place. “Plastic” is really a term for the state of a material, not the material itself. When a steel beam bends, engineers say it has “deformed plastically”. The proper name for the stuff we use is “polymer”, and in the pellets that we see on the shop floor, “resin”. There are two basic types: thermoplastic resins, and thermosetting resins. Thermoplastics melt when heated, a property that makes this type the majority of the polymers in use today. Polypropylene and polyethylene are examples. Thermosetting or “thermoset” resins go the other way; they harden when heated. The familiar brown phenolic light switch bodies and switch plates in older homes are examples. There are others (silicones come to mind immediately), but the vast majority in production are these two generic classes.

They’re chemically very different, but both are derived from petroleum, which is the main reason why resin raw material pricing is so dependent on the price of crude oil. By the way, you can make polymers out of things as diverse as soybeans, milk by-products and the gas that comes off rotting garbage, but so far oil is the cheapest basic ingredient. Of course there’s lots more to say about resin properties, but for now, it’s as simple as “thermoplastics melt when heated, thermosets set when heated”. You’re ready to move on.

The next step is to do something with your resin. I’ll restrict this discussion to the two most common processes for now, extrusion and injection molding, since they form the bulk of plastics (there’s that word again) processes in use today. Both are based on the same technology, and they’re both easy to understand. Extrusion goes on in every bathroom in the country. Squeeze your toothpaste tube, and notice how the toothpaste comes out in the shape of the tube’s round hole. That’s extrusion at it’s simplest, and it doesn’t take much imagination to figure that if you an make the hole into the shape of, say, the cross section of a piece of window frame, you can make loads of useful products as fast as you can squeeze out the hot resin.

Squirt the hot “melt” into a cold mold, and you have injection molding. Unfortunately, it’s not quite as simple as making ice cubes. The problem is the thermoplastic resin. It doesn’t flow like water when heated, and has a couple of other difficult properties. One is that the same heat that melts the resin can degrade it, creating anything from charred residue to poison gases. The other is that it flows in strange (“non-Newtonian” to scientists) ways when it’s inside the long barrel where it’s melted. That means that extruders and the similar barrels in injection molding machines have to perform a complex dance to keep the melt flowing into the extruder’s nozzle (called a “die”) or the injection molding machine’s mold without damage or degradation. More on that challenge next month.