Canadian Plastics

When Time Isn’t on Your Side

"Waste of time", in a typical shop floor environment, has many meanings.

October 1, 2006   By Jim Anderton, technical editor



“Waste of time”, in a typical shop floor environment, has many meanings.

Coming out of the mouth of a millwright or set up tech, it can mean “I don’t want to follow the new safety procedure” or even “I wish that engineer would go back to his office and leave me alone”.

To an injection molding machine, however, wasted time means the portions of the machine cycle that don’t produce parts, often the time it takes to open and close the mold.

While mold movement is the big waste, it’s also possible to bleed dollars with slow mold filling, excessive hold time or prolonged cooling.

Everyone wants to improve throughput, but what do you tackle first? The best time to start thinking about this is during the job quoting process. Your available press has the tonnage and the platen size to handle the mold, and the dry cycle time is a known quantity, but there are a lot more variables that can affect overall cycle time.

Obviously, the material and shot size are factors, but how about gating? A reasonably balanced hot runner system feeding valve gates can flow resin fast, but what if the part geometry requires a yawning mold open cycle to eject the part?

Or how about a poorly gated part with an odd geometry or aspect ratio that needs oddball filling parameters?

I recall an example where a polypropylene (PP) part suffered terrible jetting due to an outrageous gate design (cold runner mold) but the customer wouldn’t spend the money to fix the problem in that medium-volume mold. The fix? Fill slowly, of course, and renegotiate the contract.

When I started in injection molding, “robotics” meant “sprue picker” and the hissing (they were pneumatic) monsters slowed the cycle time sufficiently that we considered exotica like vibratory separation of parts as a second op to avoid them.

Today’s units are low-inertia, keyed to the mold cycle, and when properly designed, can clear a mold much faster than straight eject-and-fall in many applications. The really big volume/low margin molders in the packaging segment make money with purpose-built presses in which the robotics are integral, and fast enough to allow in-mold labeling without a productivity penalty. Husky, for example, has some equipment for this segment that will blur your vision.

That’s fine for the heavyweights, but what can a smaller shop do with a mold that’s an unknown quantity? Naturally, it helps to talk with the molder that had the job previously, but you have to ask yourself why did they give up the job in the first place?

One red flag for me is when a good shop with excellent statistical process control (SPC) and quality assurance programs gets out. Another is when an important job parameter changes, examples being a switch in material grade, or a change in regrind or filler loading. I’ve never seen a cheaper material substitution result in a faster cycle time (yeah, I know it can be achieved) but I’ve seen plenty of problems that were “solved” with a bigger cushion and a longer hold and/or cooling time. I’ve also tried to win back some of the productivity loss by pushing process temperatures to the limit and tweaking cooling systems, something modern SPC systems probably would choke on.

Of course, if the previous shop was molding on junk and thinks SPC is a form of cancer, you might be able to step in and make a job work.

Any way you do it, consider taking the jobs that are on the table, because picking up scraps from the floor rarely tastes good.


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