Canadian Plastics

Mastering the black art of mold filling

By Jim Anderton, technical editor   



For several months I've been taking a quick and dirty hike through a typical injection molding press just to provide a sense of the basics of this important process....

For several months I’ve been taking a quick and dirty hike through a typical injection molding press just to provide a sense of the basics of this important process.

We’ve traveled from the hopper, down the length of the barrel, picking up heat, melting the resin and generally plasticating like Juicy Fruit in a kid’s mouth. Now it’s time to get down to the real business: mold filling.

You could spend a career studying this process. But at its most basic, it’s about squeezing in the molten polymer and dealing with shrinkage.

Unfortunately, commodity polymers in the melt don’t behave like conventional liquids, so packing a mold isn’t like filling an ice cube tray.

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There are two main problems to overcome: the way the melt resists flowing easily into the mold’s cavities and, of course, money.

In this case, money is represented by the need to keep the mold ice-cold so the parts solidify fast enough to keep the cycle time as short as possible.

Dealing with the second issue first, the cold internal mold passages freeze the molten polymer on contact. That’s a flow restriction, as is the rapidly cooling leading edge of the molten slug as it passes through the “runners” leading to the mold cavities. (More on this in a later column).

In a properly designed mold, the cavities fill smoothly, leaving a shell of solidified resin next to the mold walls and a molten core.

The temperature difference (or “gradient” as the engineers call it) defines both the overall cooling rate and the amount of shrinkage the part undergoes.

Shrinkage is obviously a very bad thing, and fighting back means maintaining pressure as the parts cool. Cavities are generally sized to accommodate the shrinkage, which could be half a per cent (linear) for an amorphous resin like polystyrene or as much as five per cent for a highly crystalline polymer like nylon, for example. And it gets better: many polymers change their degree of crystallinity with temperature, so the rate of cooling is crucial.

By now you get the picture. To make money, you need to fill, pack and freeze as fast as possible. Rarely, however, can you choose your materials or product geometry to satisfy the need for speed, so adding a bigger chiller or shaving a fraction of a second off the hold portion of the machine cycle isn’t a certain path to success.

How about speeding up the front end of the process by adding heat (either directly or with screw speed) to lower the resin’s viscosity? You can get more throughput by stepping on the gas pedal, but the extra heat has to be removed in the mold, so it’s not as easy as it seems.

Like many aspects of molding, experimenting with one parameter affects many others, which is one of the reasons why injection molding developed a reputation as a “black art” before mold analysis software and computer control. In many shops, it still is. There’s much more to say about molds and mold filling; it doesn’t need a PhD, but when you’re troubleshooting, it can sure seem that way.

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