Injection Molding Cushion Variation: How to Diagnose an Unstable Shot

The shot-size setting stays the same, but some cycles leave almost no cushion and produce lighter parts. Before increasing the shot size, compare the screw position and part weight from the same cycles. That pairing helps distinguish a screw that reaches its forward limit from a feeding problem, inconsistent check-valve sealing, or a change in how the mold fills.

What the cushion reading tells you

In a reciprocating-screw injection molding machine, cushion is the melt remaining ahead of the screw near the end of packing and holding. The controller commonly represents it as a screw position or a calculated volume. If the screw reaches its mechanical forward limit while the cavity still needs packing, it loses the travel required to sustain pressure on the melt. Check whether the machine displays the minimum position reached during hold or the position at the end of hold, and verify how it defines zero. The transfer position is a separate reading taken as the machine changes from filling to pressure control. RJG explanation of cushion and screw position

Record consecutive cycles before making a change

Start with the approved settings and collect a sequence of consecutive shots once the press and mold are at steady conditions. Record the cycle number, actual screw position after recovery, actual transfer position, fill time, peak injection pressure and cushion. Weigh each part from the same cycles and record cavity IDs. With a cold runner, note whether the recorded weight includes the runner. Keep startup or restart samples separate so they can be compared with steady production.

A worn or contaminated check ring can allow melt to flow backward across the screw tip. A fluctuating cushion is one clue. Compare it with part weight, pressure and the stage of the cycle where the traces first diverge before assigning a cause. Avient processing guidance on cushion and check-ring sealing

Worked example with a four-cavity mold

Consider a four-cavity housing mold. At the same indicated recovery and transfer positions, most shots finish with a 4 mm cushion. On occasional cycles the displayed cushion falls to 1 mm and parts from all four cavities weigh less. The team checks the pressure and position traces and finds that the screw moves farther forward during hold on the low-weight shots. The shared pattern makes a press-side issue worth investigating. Maintenance checks valve sealing before the team changes mold steel or increases the shot size.

If the cushion stays near 4 mm and only cavity 3 loses weight, the first comparison changes. The team inspects that cavity's gate and flow path and weighs all four parts individually. The other cavities can mask a local change in the combined weight. The numbers in this example illustrate the comparison method. Set production limits from the validated process for your own mold, resin and press.

Confirm the correction on parts

After correcting the suspected cause, run another consecutive series under comparable conditions. Check that the screw keeps usable travel through hold and that weight and critical dimensions repeat by cavity. Where production includes restarts, repeat the check after a representative stop. Set cushion alarms against the verified process baseline and define which parts to segregate when an alarm occurs.

Can you use the same cushion setting on a different press?

Compare the controller definitions and screw diameters first. The same position in millimeters corresponds to a different nominal volume on a different screw diameter. When transferring a mold, use volume as one setup reference and qualify the process again with part weight and dimensions by cavity.

For an injection mold review, share the resin grade, press and screw size, cavity layout, and cycle records that pair cushion readings with cavity weights. Include the drawing features or appearance limits affected by the variation. Request an injection molding review from Aluprototype.