Stack Molding: Don't Blow Your Stack!

By Keith Klamer


Plastic injection molding may be the most utilized industrial process in the world. And no matter what the end product - whether a drug vial or a car door handle - the plastic injection molding process is more or less the same. Plastic, in the form of granules or pellets, is fed into a hopper, heated, than "injected" or rammed into a mold containing a cavity which is the exact shape of the part to be made.

Similarly to other plastic injection molding companies the world over, Illinois plastic injection molding companies discovered many years ago that if you assemble several molds in a line on the same press, you can turbo-charge your efficiency by adding more cavities. This series of connected face molds is known collectively as a "stack mold." Each mold in the stack has the same shape and mass as the one next to it, so they all fit within the same tie bar assembly.

While there are caveats, output typically goes up in relation to the amount of molds in the stack. Just to give an example, a plastics company can roughly double their output per run by increasing the number of cavities on the same press by 100%.

There are numerous different types of stack molds, but some common elements include:

* A high level of production runs on fewer presses

* Many cores and cavites

* Indexable cores, to injection-mold multiple materials

* Possessing the ability to perform multiple operations in the same molding cycle

A short primer: a stack mold has a stationary "A" half, and at least two "B" or moving halves. They are similar to rotary molds that have been around for decades but usually are vertical rather than horizontal. This results in moving cores that rotate about a vertical axis, as opposed to a horizontal one. Moreover, the stack mold is very small as compared to other mold types.

Stack molds can be very pricey. This is why a high volume is necessary to compensate for their expense. Not to worry; in pharmaceutical or cosmetics molding, the break-even point is quickly reached. Auto and medical products are also frequently made with the help of stack molds.

Just think about the amazing amount of engineering and mold-making required, the money is understandable. Designing and making a stack mold is not for every company, or molders who just want to make quick buck. Plastic injection molding is not a breeze at any time but stack molding is a whole new dimension of competence in our industry.




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