Shenzhen Alu Rapid Prototype Precision Co., Ltd.

In injection molding, the cushion refers to the small amount of molten plastic that remains in the barrel ahead of the screw after the injection and pack-and-hold (compensation) phases. It's typically measured as a linear distance (e.g., 0.1–0.2 inches or 3–5 mm) and acts as a buffer to transmit pressure into the mold cavities, ensuring complete filling and compensating for shrinkage without overpacking. A consistent cushion promotes shot-to-shot repeatability and part quality, but if it's too small (or absent), it can lead to short shots, underpacking, or screw damage. Increasing the cushion involves adjusting process parameters to leave more material in the barrel, but keep it small to avoid issues like pressure loss from melt compressibility or degraded material entering the part.

Methods to Increase Cushion

Here are proven ways to increase the cushion if it's too low, based on standard injection molding practices. Always monitor the cushion position via machine controls and make incremental adjustments while testing part quality.

1.Decrease Compensation (Pack-and-Hold) Pressure or Time

Lower the pressure or duration of the pack-and-hold phase to reduce how far the screw advances into the barrel. This limits material displacement, leaving more melt as cushion. For example, if your current setting pushes the screw too far forward, start by reducing pressure by 10–20% and observe the cushion reading. This ensures sufficient pressure transmission without bottoming out the screw.

2.Adjust the Transfer Position Forward

Move the transfer position (the switch from velocity-controlled injection to pressure-controlled pack) closer to the screw's forward limit (e.g., from 0.80 inches to 0.60 inches). This injects less volume during the high-speed phase, resulting in more remaining material for the cushion. Recalibrate shot size afterward to avoid underfilling the mold.

3.Reduce the Shot Size (Metering Stroke)

Slightly decrease the dosed shot volume by starting the screw farther back in the barrel. This increases the amount of material ahead of the screw post-injection, naturally building a larger cushion. Aim for a 5–10% reduction initially, then fine-tune based on cavity pressure data to maintain part dimensions.

4.Increase Back Pressure During Screw Recovery

Raise the back pressure on the screw during the recovery (plasticizing) phase to densify the melt more consistently. This helps deliver a uniform shot volume each cycle, preventing cushion drift and allowing for a more stable, larger cushion. Start with a 200–500 psi increase and monitor for overheating.

5.Use a Longer Screw or Verify Screw/Barrel Condition

If hardware-related, install a longer screw to inherently leave more space for cushion, or inspect for wear in the screw, barrel, or check ring (e.g., via a blockage test with an aluminum or wood plug in the nozzle). Worn components can cause inconsistent recovery, reducing cushion—replacing them restores capacity for a larger, repeatable cushion.