Holding pressure, also called packing pressure, is a key phase of injection molding. It comes right after the cavity fills. The screw keeps pushing melt into the mold at lower pressure. This extra plastic compensates for shrinkage as the part cools. Without holding pressure, parts develop sink marks and voids. The phase ends when the gate freezes shut. This article explains how holding pressure works and how to set it. The ideas below apply to most thermoplastics and standard molding machines. Engineers also call this phase the packing phase. The terms holding and packing are often used interchangeably. Both describe pressure applied after fill. This phase usually lasts a few seconds on small parts. Large thick parts may need much longer.
The Injection Cycle in Order
An injection cycle has clear stages. Plastic melts in the heated barrel. The screw rotates and builds a shot of melt. Injection fills the cavity at high speed. Then holding pressure packs the part. Cooling follows while the screw prepares the next shot. Finally the mold opens and ejectors push the part out. Holding sits between filling and cooling. It overlaps with the start of cooling. Understanding this order helps diagnose defects. Each stage leaves its own signature on the part.
What Holding Pressure Does
During filling, the screw moves fast to fill the cavity. Once the cavity is full, the machine switches to holding pressure. The screw now moves slowly forward. Melt continues to flow into the cavity at a low rate. This flow packs more plastic into the part. As the plastic cools, it shrinks. The packed melt fills the space that shrinkage creates. Density rises across the part. Higher pressure near the gate creates denser regions. Areas far from the gate stay slightly less dense. This density pattern affects part strength and warpage. Flow during holding is slow and pressure-driven. Engineers call it pressure flow. Viscosity rises fast as the mold wall cools the melt. That is why holding pressure must overcome growing resistance.
Holding Time and Gate Seal
Holding pressure stays on until the gate seals. The gate is the narrow entry to the cavity. Plastic there freezes before the rest of the part. Once it freezes, no more melt can enter. Extra holding time after that point wastes cycle time. It can also lock stress into the part. Too little holding time causes the opposite problem. The gate stays open while the part shrinks. Melt flows back toward the nozzle. Sink marks and voids appear on the surface. The right holding time ends just after gate seal. Technicians find it by weighing parts at different holding times. Weight stops rising once the gate is sealed.
Effects on Part Quality
Holding pressure shapes final part quality. Enough packing prevents sink marks on thick sections. It keeps dimensions stable from shot to shot. Part weight stays consistent, which helps quality control. Too much holding pressure causes its own defects. The mold may flash at the parting line. Parts can warp from uneven density. Ejection gets harder when parts pack too tightly. High pressure also raises clamp force needs. Finding the balance takes trials. Small pressure steps reveal the best window. Good process sheets record the winning settings. Weight variation is a useful alarm. A sudden drop may mean a blocked gate or low pressure. A rise may mean flashing. Operators who track weight catch problems early. Dimensional checks confirm what weight suggests.
Setting Holding Pressure in Practice
Most molders start with holding pressure at 50 to 70 percent of injection pressure. They adjust from there based on part appearance. Thick parts need higher packing pressure. Thin parts need less. Semi-crystalline plastics like PP and PA shrink more than amorphous ones like ABS. They need longer holding times. Multi-stage packing profiles help too. The first stage packs at higher pressure. Later stages drop pressure as the gate starts to freeze. Modern machines store these profiles by mold number. That makes repeat setups fast and consistent. Material suppliers publish shrinkage data for each grade. Mold designers use it to size the steel. The process engineer fine-tunes from there. No table replaces a real trial on the actual mold. Record melt temperature alongside pressure settings. Both affect viscosity and shrinkage together.
Clamping Force and Mold Safety
Holding pressure pushes outward on the mold. Cavity pressure transfers through the solidifying plastic to the mold walls. This force tries to open the mold slightly. The clamp unit must resist it. A small opening helps venting in some cases. But too much force causes flash and parting-line damage. The injection machine needs enough clamp tonnage for the job. Projected part area times cavity pressure sets the requirement. Choosing a press with margin protects both the mold and the parts. Undersized presses show telltale flash on every shot. The fix is a larger machine, not lower pressure. Lower pressure would only bring back sink marks.
Key Points to Remember
Holding pressure compensates for cooling shrinkage. It runs from fill end to gate seal. Time and pressure both need the right values. Part weight is the best guide during setup. Stable weight means a stable process. Record the final settings for the next run. Share these settings with the whole shift. Consistent setups keep quality steady around the clock. Good documentation turns one good run into every good run.
