Canadian Plastics

Squirt Guns and Shooting Pots

By Jim Anderton   



One of the everlasting joys of childhood, for my money at least, is the squirt gun. For many suitably armed children --including myself -- dispensing just enough pressurized water to annoy the chosen...

One of the everlasting joys of childhood, for my money at least, is the squirt gun. For many suitably armed children –including myself — dispensing just enough pressurized water to annoy the chosen target without inflicting any meaningful damage conjures up feelings of power, a rare experience for kids in their adult-dominated lives.

In my case, Dad was a favourite target, and to his chagrin it didn’t take me long to figure out the engineering behind this hydraulic technology. Pressure equals range, but volume is the real goal; combine both and you get pure joy.

In a lot of ways, the experience is the same for injection molders: once you’ve selected the right screw type and profile, length/diameter ratio, and optimal speeds and heat settings for your resin, the next step is to throw it into the mold, right? When everything goes as planned, a feeling of satisfaction ensues.

Unfortunately, getting that ‘shot’ into the mold isn’t as easy as filling your ice cube tray — or water gun for that matter. Most presses use a reciprocating screw design, meaning the same screw that rotates to melt, convey, plasticate and homogenize the resin stops spinning and strokes forward to push the shot into the mold, rather like a big hypodermic syringe.

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It wasn’t always this way. Prehistoric machines — those from 1940s and early 50s — used a ‘stuffer’ or injection cylinder with a piston to cram the resin into the mold.

I once scrapped an old machine — circa 1945 — I found in the back of the shop that was hand-operated. I think the shot size was about an ounce and the residual material looked like a styrene. The minuscule shot size was largely determined by the amount of force the operator could apply by pulling a large lever. Eight-and-a-half hours on this plunger machine at 85 cents an hour was an operator’s life back then, but the limitations of both shot size and resin capability were primary reasons why modern presses relegated these clunkers to the annals of plastics history.

Once the value of the screw for resin processing was recognized, early machines extruded the shot into the cylinder or ‘shooting pot’ where the familiar piston stuffed resin into the mold.

Today’s reciprocating screw machines supposedly cut out the middleman — the shooting pot — reducing machine size, weight and complexity. But that’s not necessarily the case.

Rotating and translating a precision piece like a screw takes serious engineering, both mechanically and on the control sequencing side.

From a throughput perspective, a screw that’s shooting isn’t processing resin either, so a portion of the possible output is lost for that part of the machine cycle. And what better way to optimize barrel throughput and shot size than to specify each to match machine, resin and cavity/runner volume?

In fact, for molding large parts, plunger-type machines never really went away. That’s because they offer a major advantage for machine builders: they can be designed so that more than one barrel can feed a large shooting cylinder, making very big custom machines practical.

The best example I’ve seen was during an experiment concocted by Chrysler and Husky, where half a car body was molded in a single mold Husky unit approximately the size of my house.

For most processes, however, reciprocating machines reign supreme, a trend that all-electric presses may reinforce.

Regardless, I still like plunger technology because of its simplicity. I wonder if we’ll see it return to small- and medium-sized machines as part of a modular press system that lets molders mix and match resin throughput rates, shot weights and platen sizes in some sort of quick-change arrangement.

And if you’re wondering, the best squirt gun I can recall was the ‘Green Hornet’. It was compact, had lots of injection pressure and volume in a lightweight, low cost package…the right machine for the job. I’m sure my dad, however, would disagree.

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