Canadian Plastics

Getting Small Parts Moving With Air

By Jim Anderton, technical editor   

It's hard enough to make money in the injection business, but what do you do when you just can't get parts away from the machine reliably? Most of the smaller equipment I worked on moved parts by grav...

It’s hard enough to make money in the injection business, but what do you do when you just can’t get parts away from the machine reliably? Most of the smaller equipment I worked on moved parts by gravity feed onto an exotic duct-tape and cardboard chute, which sent most of the parts onto a conveyor for downstream processing. On the cold-runner molds, step one of “downstream” included hand picking the occasional sprue from the good parts. Creating separate pathways for each can be difficult without a lot of trial and error, but are there other options? A typical multi-cavity balanced mold will generate a spider-like sprue whose aspect ratio can work to your advantage in separating waste from parts. The answer to the problem could be cheap and available on the shop floor: air.

Blow offs aren’t new, but consider what you can do with a little puff directed the right way. Timing can be by light/solenoid/pilot valve, or if you prefer the simpler things in life, a cam operated valve. And that’s the big secret: you often don’t need staggering pressure or volume to nudge parts into the right direction. Pilot air, for those of you with clean fingernails but no engineering degree, is air used to control larger volumes or pressures of air, similar to the way the low-voltage circuit in a home heating system thermostat controls big things in the furnace.

Pneumatic systems use pilot air circuits to control power actuators like cylinders and air motors, and since they don’t have to do anything more than open and close the much bigger air supply into the muscles of the system, tiny volumes are enough. As a result, their control circuits are relatively light duty. That low-pressure, low volume air can be manifolded at the parting line to both direct parts into chutes or conveyors and can be used for a little post-mold cooling as well. The trick is to keep it simple, and flexible. I use conventional poly airline and dry-break connectors, along with 3/4 inch copper plumbing line and fittings wherever possible. The copper works because pressures are low, a wide variety of fittings are readily available, often in-house, it’s easy to work, solderable, and best of all, really cheap.

Need a $20 air curtain? A series of tees capped with short tubes hammered flat at the end works, as does a vertical run perforated with holes. PVC and ABS pipe can be made to work, but copper is easily adapted to 1/4 NPT fittings to adapt it to common airline components. The flexible stuff is just rigid enough to hold its shape, and isn’t so heavy that it needs a lot of bracketry to keep it supported. One caveat, though: don’t use plumbing valves for regulation. Why? They’re just too coarse for convenient flow control, and since we’re talking about a relatively gentle puff, use a needle valve for best control. Naturally, you want clean dry air here, so a filter/regulator set upstream is a great idea. Chances are you’re mounting a system on the backside of the machine, away from operator/inspector, so a self-draining filter might be a good idea, too.


If your process is well-engineered in the first place, you probably won’t need to improvise a blow-off system, but for short runs with finicky resins in difficult molds, a “quick and dirty” system can be set up fast, and taken down as quickly when you need it. It isn’t pretty, but it works.


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