Views: 0 Author: Site Editor Publish Time: 2026-07-16 Origin: Site
Pressure tuning is the core of injection molding process optimization. Improper injection pressure and holding pressure matching directly causes common defects: short shots, flash, sink marks, voids, warpage, and residual stress cracking. The most critical tuning principle is distinguishing thin-wall and thick-wall parts, as their filling logic, cooling characteristics and defect mechanisms are completely different.
This blog shares practical, machine-ready pressure & holding parameter matching rules for mainstream plastics (PP, ABS, PC, PA66, POM), covering standard thin-wall and thick-wall molding scenarios.
Thin-wall cavities feature long flow paths and rapid melt cooling. The melt tends to freeze halfway during filling, leading to incomplete filling. The tuning priority is high-pressure, high-speed fast filling. Meanwhile, short and low holding pressure is required to avoid flash, excessive residual stress and warpage.
Thick-wall parts have large cavity volume and slow cooling speed. The main defects are volume shrinkage, surface sink marks and internal vacuum voids. Different from thin-wall molding, it adopts medium-low injection pressure + multi-stage gradual reduced high holding pressure. Do not fully fill the cavity during injection; reserve proper melt volume for shrinkage compensation.
Injection phase: Speed control dominates, with high pressure as the upper limit to ensure full filling before melt solidification. Holding phase: Adopt two-stage short holding to stabilize cavity pressure and prevent backflow, avoiding long-time pressure retention that causes stress concentration.
1st stage holding: 50%–70% of peak injection pressure, 1–3s
2nd stage holding: 30%–45% of peak injection pressure, 1–2s
Injection phase: Fill 90% of the cavity with medium-low pressure and low speed to avoid trapped gas, scorch and internal stress. Holding phase: Multi-stage descending pressure for continuous shrinkage compensation, eliminating sink marks and internal voids.
1st stage holding: 75%–95% of injection pressure, 3–8s (primary shrinkage compensation)
2nd stage holding: 50%–65% of 1st stage holding pressure, 5–15s (stable pressure & void elimination)
3rd stage holding (optional for ultra-thick parts): 20%–40% low pressure, maintain until gate freezing
PP has excellent fluidity and large shrinkage rate. It is prone to flash under high holding pressure and sink marks on thick surfaces.
Thin-wall (≤1.5mm): Injection pressure 100%–120%; 1st holding 55%–65% (1–2s); 2nd holding 35%–45% (1–2s)
Thick-wall (≥3mm): Injection pressure 65%–75%; 1st holding 80%–90% (3–6s); 2nd holding 50%–60% (6–12s); final low-pressure maintenance
ABS has balanced fluidity, sensitive to residual stress. Excessive holding pressure causes cracking and paint peeling in post-processing.
Thin-wall: Injection pressure 95%–115%; 1st holding 60%–70% (2–3s); 2nd holding 40%–50% (1–2s)
Thick-wall: Injection pressure 70%–80%; 1st holding 80%–95% (4–7s); 2nd holding 55%–65% (7–15s)
High viscosity, poor fluidity and fast cooling. Requires the highest injection pressure among common materials; thick parts need slow pressure release to avoid stress cracking.
Thin-wall: Injection pressure 110%–130%; 1st holding 65%–75% (2–3s); 2nd holding 45%–55% (1–2s)
Thick-wall: Injection pressure 80%–90%; 1st holding 85%–95% (5–8s); 2nd holding 60%–70% (8–18s), extend cooling time properly
Fast cooling & curing speed, short holding window, large shrinkage rate. Moisture failure easily causes bubbles.
Thin-wall: Injection pressure 100%–120%; 1st holding 60%–70% (1.5–3s); 2nd holding 40%–50% (1–2s)
Thick-wall: Injection pressure 75%–85%; 1st holding 80%–90% (4–7s); 2nd holding 50%–65% (6–14s)
Maximum shrinkage rate among common plastics, easy to decompose under high pressure and high temperature, prone to flash.
Thin-wall: Injection pressure 90%–110%; low overall holding pressure (1st 50%–60%, 2nd 30%–40%) with short holding time
Thick-wall: Injection pressure 65%–75%; 1st holding 75%–85% (3–5s); 2nd holding 45%–55% (5–10s)
Short shot / incomplete filling: Increase injection pressure upper limit and injection speed (do not adjust holding pressure)
Flash / burrs: Reduce holding pressure and shorten holding time
Warpage: Decrease secondary holding pressure and total holding duration to reduce residual stress
Surface sink marks: Raise primary holding pressure and extend holding time
Internal voids: Reduce injection speed, adopt gradual pressure relief, extend cooling time
Gate whitening / stress marks: Reduce final-stage holding pressure to release gate residual pressure
Burn marks: Lower injection pressure & speed, add multi-stage injection to improve mold ventilation
Thin-wall molding: Prioritize speed and high injection pressure to fill the mold quickly; keep holding pressure low and time short to avoid deformation and flash.
Thick-wall molding: Adopt moderate injection pressure for smooth filling; rely on multi-stage descending long holding pressure to compensate shrinkage and eliminate internal defects.
Material characteristics determine the pressure upper limit and holding ratio: PC requires ultra-high injection pressure, PP/PA need sufficient shrinkage compensation, and POM/ABS avoid excessive holding stress.
