What is pressure reduction in injection molding?

Decompression in injection molding (also called suck-back) is a controlled backward movement of the screw after plasticizing (dosing/metering) and/or after the injection & holding phase.Its main purpose is to relieve residual pressure in the melt in front of the screw, preventing several common problems.

Main Types of Decompression

1.Decompression after dosing/plasticizing (most common)

→ Happens right after the screw finishes rotating and preparing the shot volume

→ The screw moves back a few millimeters (typically 2–15% of screw diameter)

→ This reduces pressure so the material doesn't drool/string from the nozzle when the mold opens or during mold movement

Here are some typical screw positions during the cycle (including decompression phase):

2.Decompression after injection/holding (less common, used in specific cases)

→ After packing/holding phase, a small suck-back is done before mold opening

→ Helps reduce nozzle drool and gate vestige in some hot-runner systems

Why is Decompression Important? (Key Benefits)

• Prevents drooling and stringing from the nozzle → cleaner molding area, less waste

• Stops material from leaking into the sprue/gate when the mold is open

• Helps maintain consistent shot weight by keeping the check ring (non-return valve) seated forward

• Reduces wear on the check ring / non-return valve

• In hot-runner molds → very important for controlling pressure and avoiding overpacking

Typical Settings & Rules of Thumb

• Distance: Usually 2–10 mm (most often 3–8 mm), or ≈ 5–15% of screw diameter

• Rule of thumb: Often set to slightly more than the travel distance of the check ring (≈1.0–1.5× check ring stroke)

• Too little → drooling, inconsistent shot size, pressure spikes

• Too much → sucks in air → splay, bubbles, voids, silver streaks, inconsistent filling

In short:

• Decompression is a small but very important process parameter.

• When set correctly → cleaner production, more stable process, longer life of check ring and more consistent parts.

• When ignored or set wrong → drooling, splay, weight variation, and unnecessary defects.Many experienced molders consider proper decompression one of the "hidden" keys to a truly robust and repeatable injection molding process.