Canadian Plastics

Inside the Barrel

By Jim Anderton,technical editor   



Last time I promised I'd take a quick look inside the barrel of a typical injection molding machine and describe what's going on inside....

Last time I promised I’d take a quick look inside the barrel of a typical injection molding machine and describe what’s going on inside.

That’s actually a nearly impossible task to do in a reasonable amount of time because the mysteries of melting and moving resin “down the pipe” and into the mold is so complex. In fact, 60 years into modern molding technology, research is still going on to discover the inner workings. Why? The primary reason is the nature of the raw materials, especially thermoplastic resins.

Highly crystalline solids like ice, for example, melt at a sharply defined temperature and go straight into the liquid state. It’s a fact which makes ice cube production possible without a quarter of a million dollars worth of machinery.

Thermoplastics, however, soften gradually and flow in complex, or “non-Newtonian,” ways. Molten resin flow is dependent upon temperature, pressure and the amount of shearing action of the screw flights, among others.

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Barrel temperature takes centre stage when controlling the process. But the pitch and profile of the screw also matter greatly, as does the speed at which it rotates. The diameter of the barrel (often expressed as the ratio of barrel length to diameter, or “L/D”), wear, additives and a dozen or so other parameters affect product quality and productivity.

In a perfect world, a press would mold a single, neat resin, running at maybe, 80% of the machine’s maximum output, and cycle slowly enough to accelerate, stop the screw gently and progressively. That perfect system would include a press with an optimal L/D and screw design for that single resin.

In the real world, however, compromise is the norm, resulting in less than optimal screws, barrels and operating parameters. Running machines flat-out for maximum productivity is the new normal, and if your process control software can tweak the process shot-by-shot, it’s a good way to make money. Let it get out of control and the results can be horrendous.

For example, a new mold design might shorten the cycle time of a machine by perhaps 10%. From the machine’s perspective, the mold is a big catcher’s mitt at the end of an otherwise comfortable extruder, so more parts mean more resin throughput per unit time.

More pounds per hour down the barrel can be accomplished by speeding up the screw, right? If only it was that simple! Faster screw speed means more shearing action of the resin and more internal heat generation. In many cases, the external heater bands supply only a third of the heat needed, so the ability of the PLC or computer to adjust melt temperature can get marginal in a hurry. Faster rotation also results in less polymer residence time in the barrel.

If the resin (perhaps some grades of ABS, for example) requires a vented barrel to release evolved gases, will all the gas escape? Is the profile of the screw efficient at mixing and conveying the resin at the new higher speed? These are just a few of the variables that can be introduced by simply increasing the throughput rate by changing one parameter. A book could be written on screws and barrels alone, but for the processor, the simplest message is that there’s really no such thing as a “universal” screw.

My advice: Tap your resin supplier’s knowledge about screw profiles early in the part development process. Before quotation is a good idea, since the customer may want to tweak the resin formulation a little, but you can’t rebuild you press with the click of a mouse. There’s a lot more to say about this issue, so stay tuned.

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