Injection molding uses several pressure measurements and one clamp-force rating. Classify each value by location, units and control phase before calculating, transferring or troubleshooting a process.
Plastic injection molding uses several different pressures, and confusing them can lead to incorrect troubleshooting. Injection pressure drives the melt into the mold, holding or packing pressure compensates for shrinkage, back pressure conditions the next shot, cavity pressure records what the polymer experiences, and clamp force keeps the mold closed. Each value has a different location, purpose and risk limit.
Related CKMOLD resources: production injection molding, mold testing and validation, mold design.
What Pressures Are Controlled in Injection Molding?
Pressure values are not interchangeable until the measurement location and conversion are defined. Machine hydraulic pressure, calculated plastic pressure at the screw, nozzle pressure, predicted cavity pressure and sensor-measured cavity pressure describe different points. A displayed value also depends on machine architecture, screw diameter and control convention.
- Injection/fill pressure: the demand resulting from the commanded fill velocity, material, geometry and flow restrictions, limited by the machine.
- Hold/pack pressure: applied after transfer to compensate shrinkage while the gate remains effective.
- Back pressure: resistance during screw recovery that affects melting/mixing, recovery and thermal history.
- Cavity pressure: local pressure inside the cavity; its time/location trace can show fill, transfer, pack and gate-seal behavior.
- Clamp force: force holding the parting line closed, not a melt-pressure setting.
Primary sources checked: Autodesk — Clamp Force Prediction (pressure distribution, projected area and clamp-force modeling); Autodesk — DOE Quality Criteria (optimization objectives and machine-limit context). The cited scope still has to be matched to the exact grade, tool, product and destination market.
How Are Plastic Pressure and Clamp Demand Estimated?
Where a hydraulic machine reports cylinder pressure, conversion to theoretical plastic pressure requires the machine’s hydraulic/piston area, screw area and any documented losses or calibration. Obtain the machine manufacturer’s intensification/conversion data; do not use one ratio for every press.
A first clamp estimate is projected area multiplied by an appropriate cavity-pressure assumption, summed across cavities and runners, plus justified margin. Detailed analysis integrates a non-uniform pressure distribution over projected area. Slides, stack tools, preloads, orientation and overlapping projected surfaces require care. Confirm with tool behavior and quality evidence.
What Determines Fill-Pressure Demand?
Demand depends on the exact material viscosity at its processing condition, wall and flow length, gate/runner/hot-runner restrictions, fill time/profile, temperature history, venting and cavity count. Thicker walls generally reduce flow resistance when other factors are comparable; they can create different packing and cooling risks.
Specify a controlled velocity profile and a pressure limit that protects the machine/tool without hiding a short shot. Record peak pressure, fill time, transfer position/pressure and cushion alongside part evidence.
What Is V/P Transfer?
V/P transfer is the change from velocity-controlled filling to pressure-controlled packing. It may be triggered by screw position, cavity pressure, hydraulic/plastic pressure, time or a validated combination. Transfer too late can overpack or flash; too early can leave a short shot or underpacked region.
Establish transfer using a fill-only study, repeatable machine response and cavity/part evidence. Avoid a universal “percent full” rule—compressibility, machine response and geometry make it project-specific.
Why Do Two Machines Show Different Pressure Numbers?
They may report different physical quantities, use different screw diameters, intensification ratios, sensor locations, units, filtering or calibration. Before transferring a process, document screw/nozzle, shot size, velocity in a comparable volumetric basis, plastic-pressure conversion, transfer, hold profile and actual part/cavity response.
How Should Back Pressure Be Set?
Use enough back pressure to support repeatable plasticizing, mixing and shot preparation for the grade and machine, then check recovery time, melt temperature, residence and degradation. Higher is not automatically better: it can add shear/heat, recovery time and wear. Supplier guidance and controlled trials should set the window.
Injection and Peak Pressure
Injection pressure is the pressure needed to move the melt through the machine and mold flow system during filling. The displayed number may be hydraulic pressure, screw pressure or a controller limit, depending on the machine. Compare readings only when the machine’s measurement point and conversion are understood.
Holding or Packing Pressure
After the cavity is nearly full, holding pressure controls how much material enters while the gate remains open. It affects part weight, sink, shrinkage, dimensions and residual stress. Holding pressure that lasts beyond gate freeze may add little to the part while increasing stress or cycle time.
Back Pressure and Plasticizing
Back pressure acts during screw recovery and helps mix, melt and meter the next shot. It can improve consistency, but excessive back pressure may raise melt temperature, increase residence time, damage sensitive resin or reduce recovery speed. The setting should be linked to melt quality and shot-to-shot repeatability.
Cavity Pressure and Clamp Force
Cavity pressure is the pressure transferred to the polymer inside the cavity and is often more directly related to part formation than a machine display. Clamp force resists mold opening and flash. The required clamp force depends on projected area, cavity pressure, mold condition and a reasonable safety margin, not on a universal multiplier.
Use Pressure Traces for Diagnosis
A pressure profile should be reviewed with fill time, screw position, cushion, part weight, gate freeze and defect location. A rising peak may indicate colder material, a blocked gate, poor venting or wear. A falling pressure may reflect leakage, short filling, an open check ring or a changed material condition.
Plastic Injection Molding Pressures Classified and Controlled: Buyer Review Checklist
- Identify the measurement point and meaning of every pressure value on the machine.
- Separate filling, packing, back pressure, cavity pressure and clamp-force decisions.
- Relate pressure changes to fill time, cushion, weight, gate freeze and defect location.
- Check flash, stress, venting, gate restriction and machine reserve before raising limits.
- Store pressure traces with the approved material, mold revision and process window.
FAQ
What is the difference between injection pressure and clamp force?
Injection pressure moves plastic into the cavity; clamp force holds the mold closed against the separating force created by cavity pressure.
Why can two machines show different injection pressures?
Machines may measure hydraulic or screw pressure at different points, and their screw diameter, control system and conversion may differ.
Conclusion
Classify each pressure by location, units and control phase before comparing values. Establish fill, transfer, pack and recovery from the actual resin/tool/machine, and verify them with pressure traces, part weight, dimensions and defect evidence. Estimate clamp demand from projected area and pressure distribution, not a universal tonnage factor.