Two pressure settings control the entire fill and pack sequence. Set them wrong, and you’ll either short-shot the part or over-pack it into stress fractures. These settings interact in ways that confuse troubleshooting when operators treat them as independent variables. Understanding what each pressure does, when each applies, and how they work together transforms process optimization from trial-and-error into systematic engineering.
The Two-Stage Process
Injection molding fills cavities in two distinct stages, each with different objectives and control strategies.
Stage one (injection/fill) pushes molten plastic from the barrel through the nozzle, runner system, and gate into the mold cavity. The objective is to fill the cavity quickly, before material at the flow front freezes and causes short shots or flow defects. This stage operates under velocity control: the screw advances at a programmed speed, with pressure rising as needed to achieve that speed against the resistance of the flow path.
Stage two (pack/hold) compensates for material shrinkage as the plastic cools and solidifies. Plastic contracts 1 to 4 percent by volume as it solidifies. Without additional material to compensate, this shrinkage creates sink marks, voids, and dimensional variation. This stage operates under pressure control: the screw maintains a programmed pressure that forces additional material into the cavity to replace volume lost to shrinkage.
Both stages are necessary for quality parts. Velocity-controlled fill ensures complete cavity filling without defects. Pressure-controlled pack ensures dimensional stability and eliminates shrinkage defects. Combining these into a single pressure setting cannot optimize both objectives simultaneously.
The transition between stages, called switchover or transfer, is one of the most critical process parameters. Getting switchover right determines whether fill completes properly before pack begins.
Injection Pressure
Injection pressure during the fill stage provides the force needed to push material at the programmed velocity. The actual pressure observed depends on the resistance the material encounters.
Pressure-limited filling occurs when injection pressure reaches the machine’s set limit before achieving the target injection speed. The screw slows below programmed velocity to stay within the pressure limit. This situation indicates either that the pressure limit is set too low or that the flow path creates excessive resistance. Parts may fill incompletely or show flow hesitation marks.
Velocity-limited filling is the preferred condition. Injection pressure stays below the set limit, with the screw advancing at the programmed speed. Pressure rises naturally as the cavity fills and resistance increases. The pressure limit acts as a safety cap, protecting the mold and machine from excessive force.
Typical injection pressure ranges depend on material viscosity, wall thickness, and flow length. Thin-wall parts may require 15,000 to 20,000 psi at the nozzle. Standard wall parts typically run 8,000 to 15,000 psi. Thick-wall parts with short flow paths might need only 5,000 to 8,000 psi. These are barrel pressures; cavity pressures are lower due to pressure drop through the flow system.
Injection speed interacts with pressure requirements. Faster injection fills before material freezes but requires higher pressure to achieve the velocity. Slower injection reduces pressure requirements but risks premature freeze-off. The process window for acceptable parts often depends on finding speed and pressure combinations that work together.
When troubleshooting fill problems, distinguish between pressure and velocity effects. If parts short-shot but pressure reaches its limit, the problem is either insufficient pressure capability or excessive flow restriction. If parts short-shot with pressure below the limit, the problem is likely injection speed or material temperature.
Switchover Point
The transition from velocity-controlled fill to pressure-controlled pack determines process stability. Several methods exist for triggering this transition.
Position-based switchover transfers control when the screw reaches a specific position. Because screw position corresponds to material volume injected, this method provides consistent transfer at the same fill volume every shot. Position-based switchover is the preferred method for most applications because it accommodates normal material viscosity variation without affecting fill completeness.
The optimal switchover position is typically when the cavity is 95 to 98 percent filled by volume. At this point, the cavity is nearly full but a small cushion remains before the screw bottoms out. Setting switchover too early leaves the cavity underfilled when pack pressure begins, causing short shots or inconsistent dimensions. Setting switchover too late causes pressure spikes as velocity control tries to pack material against an already-full cavity.
Pressure-based switchover transfers control when cavity or nozzle pressure reaches a target value. This approach accommodates parts with variable shot weight or molds with pressure-sensitive features. However, pressure varies with material viscosity, so cold material (higher viscosity) transfers earlier in fill than hot material. This variation can affect part consistency.
Time-based switchover transfers after a fixed duration from injection start. This simplest method works only when all other variables remain constant. Any variation in material, temperature, or machine response causes fill volume variation at transfer. Time-based switchover is rarely appropriate for production processes.
Determining optimal switchover uses a short-shot study. Fill the cavity at progressively longer positions until just short of complete fill. That position is approximately 95 to 98 percent fill volume. Add pack pressure and confirm complete filling without excessive pressure spike.
Hold Pressure
Hold pressure (also called pack pressure or second-stage pressure) forces additional material into the cavity to compensate for shrinkage during solidification.
The compensation mechanism works because material near the gate remains molten while areas further from the gate solidify. Pressure applied at the screw face transmits through this molten region, pushing material into the solidifying sections to replace volume lost to shrinkage. As the gate freezes, pressure transmission stops and the hold phase ends functionally even if the timer continues.
Typical hold pressure levels range from 40 to 80 percent of injection pressure, depending on material shrinkage characteristics and part geometry. High-shrinkage materials like polyethylene and polypropylene often require higher hold pressures (60 to 80 percent) than low-shrinkage materials like polystyrene or ABS (40 to 60 percent).
Hold time must extend until the gate freezes. Gate seal studies determine the minimum hold time: weigh parts with progressively longer hold times until part weight stabilizes. The time at which weight stops increasing indicates gate seal. Adding a small safety margin (10 to 15 percent) ensures consistent gate seal across normal process variation.
Multi-stage hold profiles provide different pressures at different times during the pack phase. High initial pressure packs thick sections near the gate while they’re accessible. Lower pressure later reduces stress near the gate as it freezes. Three to five pressure stages can optimize packing for complex parts, though single-stage hold works well for many applications.
Pack pressure too high causes over-packing near the gate. Symptoms include stuck parts (over-packing grips cores), stress whitening, flash at the parting line, and excessive residual stress that causes warpage after ejection or environmental stress cracking in service.
Pack pressure too low leaves shrinkage uncompensated. Symptoms include sink marks opposite thick sections, voids inside thick walls, dimensional variation as shrinkage varies with process conditions, and parts that measure correctly hot but shrink out of tolerance after cooling.
Process Window Optimization
Injection and hold pressure interact to define the process window where acceptable parts can be produced. Optimizing these parameters systematically produces robust, stable processes.
Decoupling fill and pack is the foundational principle of scientific molding. Set injection speed to fill the cavity just short of complete (95 to 98 percent) without pack pressure. Confirm acceptable fill using short shots. Then add pack pressure to complete filling and compensate for shrinkage. This separation allows independent optimization of each stage.
Fill optimization adjusts injection speed and pressure to achieve complete filling without defects. Faster fill reduces pressure requirements near the flow front but increases shear and may cause jetting. Slower fill allows thicker frozen layers but reduces shear stress. The optimal speed fills completely with minimum pressure and acceptable surface quality.
Pack optimization adjusts hold pressure and time to achieve target dimensions and eliminate shrinkage defects. A gate seal study determines minimum hold time. Pressure is then adjusted to achieve dimensional targets. Higher pressure reduces sink marks but increases stress and flash risk. Lower pressure allows sinks but reduces stress. The optimal pressure meets dimensional requirements without over-packing.
Process window mapping varies fill and pack parameters systematically while recording quality results. The acceptable region where good parts are produced defines the process window. Narrow windows indicate sensitive processes that require tight control. Wide windows indicate robust processes that tolerate variation.
Robustness testing verifies that the process produces acceptable parts despite normal variation. Simulate material lot changes, temperature variation, and machine differences within expected ranges. A process centered in its window with margin to all limits will run reliably in production.
Signs of Incorrect Settings
Recognizing symptoms of pressure-related problems accelerates troubleshooting.
| Symptom | Likely Cause | Adjustment |
|---|---|---|
| Short shots | Low injection pressure or speed | Increase pressure limit or injection speed |
| Flash | Excessive pack pressure | Reduce hold pressure |
| Sink marks | Insufficient pack pressure or time | Increase hold pressure or time |
| Voids (internal) | Pack pressure cannot reach thick sections | Increase pressure, modify gate, or redesign |
| Stress whitening | Over-packing | Reduce pack pressure |
| Part sticking | Over-packing grips core | Reduce pack pressure |
| Dimensional variation | Inconsistent packing | Verify gate seal, stabilize process |
| Warpage | Non-uniform packing or stress | Balance pack pressure, check gate location |
Troubleshooting pressure problems requires understanding whether the issue occurs during fill or pack. Fill problems (short shots, jetting, flow lines) respond to injection speed and pressure changes. Pack problems (sinks, voids, over-packing) respond to hold pressure and time changes. Confusing the two leads to ineffective adjustments.
Injection and hold pressure work together to fill the cavity and freeze a dimensionally stable part. Understanding their separate roles is the foundation of process optimization. Decoupling these stages, optimizing each independently, and verifying robustness produces processes that run consistently and yield quality parts.
Sources
- RJG Inc. “Scientific Molding Reference Guide.” https://rjginc.com/
- Beaumont, John. “Runner and Gating Design Handbook.” Hanser, 2004.
- Plastics Technology. “Process Optimization Strategies.” https://www.ptonline.com/
- Kazmer, David O. “Injection Mold Design Engineering.” Hanser, 2007.