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Basically, there are two areas within the molding process where back-flow typically occurs:

1) Transfer From Fill To Pack: If the mold is completely full during first stage, the mold will begin to pressurize as the machine tries to continue pack the mold with first stage fill. When the machine transfers to pack using a lower pressure, a little amount of the pressurized material within the mold will tend to back-flow. In some cases, this back flow will cause sinks or voids on the molded part… in other cases, you may see delamination occur. If you are using a pressure monitoring system, you may see a sharp drop in cavity pressure at the point where the machine transfers from fill to pack, especially near the gate.

2) At The End Of Pack: Insufficient packing time results in a gate which is not completely sealed. As a result, some of the pressure in the mold cavity will become relieved by a small back-flow of material across the gate into the runner system. This most often results in sinks or voids near the gate area. If you are using a pressure monitoring system, you should see a significant drop in pressure at the cavity pressure sensor near the gate when the packing pressure times out.

Keep in mind… if your process is properly established, you should notice a nice gradual rise and fall of pressure within the mold cavity and little to no back-flow within the mold cavity.

-Andy

A standard cutoff is helpful when initially establishing your fill… but you ultimately want to target a percentage of shot size. When you target a specific cushion value, then you tend to have the same amount of material, and pressure loss, in front of the screw during pack.

Many companies who mold similar products in similar sized machines sucessfully target a specific cushion value. Most molders have the greatest success in targeting a percentage, such as 5% or 10%, of the full shot size.

The transfer position should not have a significant influence on screw and barrel wear… but focusing on a cushion rather than a specific transfer will improve your ability to maintain a solid and reliable injection molding process.

-Andy