Choosing your weapon

Last time I talked very briefly about what polymers are and why it’s relatively difficult to mold them into useful parts. The essential point to remember was that resins don’t behave like you’d expect when melted (“non-Newtonian flow”) and that the same heat that melts the resin, degrades it.

Extrusion and injection molding are similar until the last few inches of the resin’s molten journey through the machine, so it’s a good place to start. Last time I described extrusion as similar to squeezing toothpaste from a tube, an analogy that’s a little simplistic. Real-world machines have several configurations, but for most general purpose extruders and molding presses the basics are the same. A long horizontal tube surrounds a rotating screw that pulls the resin along its length. The tube (barrel) is heated, melting the resin, which at the end of its journey is extruded through a die into a useful profile like a pasta machine, or in the case of injection molding, is squirted into a cooled mold.

Molding requires an additional step to push the melt into the molds, making it a discrete, timed process rather than a continuous flow operation like extrusion. Most machines use a single screw, but some special extruders can have two. Injection presses can use a hypodermic needle-like “shooting pot” to fill the mold, but in most machines, the screw is arranged to move along its length, doing the mixing and the shooting.

How the screw is driven is an important consideration today. Hydraulics are traditional, but all-electric and hybrid designs are available. Electrics are relative newcomers and offer energy savings and simplicity, but are typically more expensive than hydraulic machines. Which is better? It depends on what you’re molding, and where, but the operating principles are the same as the resin sees it.

Controlling it all is a computer or PLC which uses various sensors to give the operator control over melt temperature down the barrel, the speed of travel through the machine, and critically, the time profile of the melt filling and packing of the mold. Older machines do these functions with mechanical timers and thermostats, but they’re rarely seen today. Why are there so many different sizes and models? Because, as we’ll see over the next few issues, the choice of equipment for the most cost effective molding flows backward from the material and design of the finished part required. The length of the swizzle stick can literally determine the tonnage (power) and size of a machine that costs as much as a house. It’s pennies that count at the shipping door, but it can cost millions on the production floor. Stay tuned.