Understanding Two Stage Molding - Pack & Fill

Molding has 2 gear settings

• Fill stage

• Packing/Holding stage

Part is filled to approximately 95% with ram speed.

Packing pressure is applied for the last 5%.

This finishes the fill and compensates for the cooling shrinkage.

NOTE
Packing pressure is 75% - 85% of the max fill pressure.

The changeover (gear change) between these stages is called. . . .

VELOCITY PRESSURE transfer. ====> VPT

How do you decide when to change gears?
Great question.

The molding operator molds a series of short shots.
Changing the VPT settings until the mold is 95% full.

How do you know it’s 95% full?

Well, you know the weight of a full fill.
Weigh your short shorts until it’s 95% of the weight target.
Weight is key.

Now, the challenge is to repeat this weight for every VPT.

How do you do this?

There are a few options to signal and activate the gear change.
The change from Velocity (fill) to Pressure (pack).

Position
Screw position. How far forward does the screw plunge to activate the VPT?

Cavity pressure
When the resistance from the cavity pressure reaches X ===> VPT

Hydraulic pressure
When the resistance from the machine hydraulic reaches X ===> VPT

Nozzle pressure
When the resistance from the nozzle pressure reaches X ===> VPT

Most often used . . . . .

• Position

• Hydraulic pressure

There you have it.

A simple overview of mold filling.

Two stages.

Fill and Pack.

This process is often referred to as “Scientific Molding”

Now you know