Canadian Plastics

What’s the right tool for the job?

Readers who have been following my simplified, well, extremely simplified trip from resin pellets to parts may have noticed that up until now, I haven't formally described the differences between extr...

May 1, 2006   By Jim Anderton



Readers who have been following my simplified, well, extremely simplified trip from resin pellets to parts may have noticed that up until now, I haven’t formally described the differences between extrusion and injection processes.

That’s intentional, because at its simplest, the process that melts, mixes, meters and plasticizes the resin is the same. It’s what happens to that mystical semi-solid, semi-liquid polymer after the barrel that defines the process.

I like to think of extrusion as two-dimensional molding, but it has another simplifying beauty compared to injection molding, mainly that cooling, sizing and cutting processes are not generally integral to the machine. That’s not to say that there aren’t plenty of issues to confront in extrusion, but conceptually it’s still like toothpaste coming out of the tube — I’ll dive into that subject in another column.

With injection molding, the nozzle is the end of the road for the resin as far as the machine is concerned. At this point it’s on to the mold.

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Why isn’t the mold defined as part of the machine? It’s often owned by the molder’s customer, it’s built by a moldmaker who likely doesn’t know what specific press it will run on and can be relatively easily — although my knuckles disagree — moved from machine to machine.

The customer ownership and often unknown history of a mold are two of the major factors defining the relationship between a job shop and its customer. If you’re a captive operation, of course, you have no one to blame but yourself!

In either case, you need to get the mold into a press that’s right for the job. “Right” means profitable and that loaded word has both a “sales” and “overhead” component. The “sales” component is the machine’s ability to reliably produce parts with the shortest possible cycle time. The “overhead” is the cost of labour, power, wear, depreciation and downtime for maintenance, repairs and mold changeover.

Before these issues can be optimized, the machine has to meet the basic needs of the job:

* Does the platen size accommodate the mold? It seems obvious, but I’ve seen equipment without retractable tie bars that would take explosives to get the mold in and out.

* How is the mold clamped? It may be more profitable to run a smaller mold in an oversized, overrated press with quick-change hydraulic or magnetic clamping, for example, if frequent mold changes are necessary, despite penalties in cycle time and underused tonnage and shot size.

* Another factor is the mold’s actual shape, as opposed to its base dimensions.

* Are there core pulls or multi-stage ejection mechanisms to consider? Add-on hydraulics or pneumatics, for example, can foul tie-bars and guarding.

* Cooling can also be a factor, through external manifolding or the often seen “spaghetti” of flex lines.

* Even the mold stroke can rear up and bite.

I once encountered a relatively simple cold-runner mold that ejected the part gated for ease of second-op handling, like a plastic model kit. When the part ejected, about one in four would tumble and hang up in the mold because of marginal open clearance. The solution? I rigged a primitive cam-actuated air blast that blew upward to flip the part vertically as it stripped. It was noisy, dirty, consumed lots of air and had to be adjusted with each change-out, but it was the “hottest job in the shop”, so time was of the essence. Sound familiar?

The issues are too many to describe, but the moral of this story is that the little problems can turn a job into a money pit just as easily as the big ones, so take nothing for granted. If the job looks easy, but it’s been on more machines than WD-40, look closely, very closely!


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