Canadian Plastics

When is closed-loop control not closed-loop?

Upgrading an old machine with closed-loop controllers may not accomplish much if your operators continue to run the machine in an open-loop mode.The basic concept behind closed-loop control is the abi...

December 1, 2000   Canadian Plastics



Upgrading an old machine with closed-loop controllers may not accomplish much if your operators continue to run the machine in an open-loop mode.

The basic concept behind closed-loop control is the ability to make continuous, automatic adjustments to machine parameters, such as ram speed or platen travel, in response to small variations, for example pressure, during the injection cycle. With the right controller and set-up, the net effect is greater repeatability of machine performance from shot to shot.

The problem occurs when molders set up their machines to completely fill the part during injection. This leaves little margin for error, and any small change in viscosity or resistance near the programmed cut-off point could cause the controller to read the mold as filled, thus causing the ram to overshoot or undershoot.

A new theory developed by RJG Technologies, Traverse City, MI, called decoupled or scientific molding, says this scenario can be avoided by programming the machine to fill 90 to 95 percent of the part during injection, then fully packing out during pack and hold. The final filling of the mold is decoupled from the high-velocity injection, allowing the mold to be filled under much lower, controlled pressure.

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