Older, pressure-controlled molding machines have no control over the material shear rate. Since the uses a set pressure, any attempt to short shot by time or position will result in 1st stage variation. Every time the material viscosity shifts upward, the part will be significantly short. Adversely, any drop in viscosity will result in a complete fill or flash during 1st stage. The 2nd stage pressure cannot be properly configured to provide the same final product when the mold shorts as well as when the mold fills completely. This condition requires constant adjustment whenever variation occurs.

If you configure the process to completely fill the mold during 1st stage with older, open-loop molding machines, then then you can can have a relatively consistent final part when 2nd stage pressure is applied.

Keep in mind the simplicity of open-loop molding machines can be very appealing, but they have little control over the actual process.

-Andy