Every 1 bar of unnecessary pressure can add 7–8% to a compressor’s power draw. In a plastic injection molding plant that runs presses around the clock, that small overshoot quietly becomes a six-figure energy bill. Screw compressor sizing for a plastic injection molding plant is the single biggest energy lever available, yet many buyers still size from motor nameplate alone. This guide uses a demand-first method so you stop paying for air the presses never use.
Why Nameplate Horsepower Leads to Oversizing
Injection molding air demand is not continuous. It spikes during mold open and close, ejection, robot end-of-arm actuation, air blast, and pneumatic valve cycling, then collapses while the plastic cools and the clamp holds. A press with large cylinders may pull 300–600 L/min for a few seconds each cycle and almost nothing in between. Buying a compressor to match total installed machine kW ignores that duty cycle and leaves the unit running unloaded or short-cycling.
- Map every air consumer: valve actuation, robots, ejection, blow-off, vacuum loaders, conveying and purge.
- Record peak simultaneous demand, not the sum of every installed tool.
- Measure pressure at the point of use; a 1 bar drop across dryers, filters and piping forces the whole system to run higher.
Pressure: The Hidden 7–8% Tax
Local Panrui field sizing data shows every extra 1 bar of pressure adds 7–8% to compressor energy consumption. Many molding plants run 8–9 bar plant pressure because one legacy machine needs it, even when modern machines only require 6.5–7 bar at the point of use. That 1.5–2 bar cushion is pure waste.
A fixed-speed unit holds a wide pressure band, often around 0.2 MPa, so it must start from a higher setpoint to avoid dropping the line. A variable speed screw compressor with pressure customization holds the band near 0.01 MPa, letting you lower the whole system setpoint and capture most of the savings.
Match the Duty Cycle with Variable Speed
Molding air demand follows cycle time, not a flat line. Fixed-speed machines that load and unload waste 10–26% of power during unload. A permanent-magnet VSD unit tracks demand instead and typically delivers 20–50% energy savings on fluctuating loads. Size for real average demand plus peak coincidence, then let the drive ride the curve. For the full selection guide, see the rotary screw air compressor pillar.
When Two-Stage Sizing Pays Off
On lines running many presses or 24-hour production, the compressor never gets a chance to recover. A two-stage screw design splits compression into two steps and cuts compression work by roughly 15% versus a single-stage unit at the same output. One Shenzhen plastic hardware manufacturer now runs Panrui PRSF two-stage permanent-magnet VSD screw units and reports lower electricity cost and stable pressure after replacement. Explore the two-stage series for continuous multi-press plants.
Six-Step Sizing Checklist
- Calculate total FAD in m³/min or cfm at the required discharge pressure.
- Apply a peak coincidence factor: which consumers run at the same time?
- Confirm dew point and oil-carryover limits for the molding and packaging process.
- Add piping, dryer and filter pressure drop to the point-of-use requirement.
- Review duty cycle and shift pattern to choose fixed-speed or VSD.
- Add 10–20% growth headroom above measured peak demand.
Frequently Asked Questions
What pressure does a plastic injection molding plant need? Most standard machines need 6.5–7 bar at the point of use. Set discharge high enough to cover dryer, filter and piping losses—commonly 7.5–8.5 bar—not an arbitrary 10 bar.
Should I install one large compressor or several smaller units? Several VSD units usually size better than one oversized fixed-speed machine because they cover turndown, peak demand and maintenance windows with less idle energy.
Does oversizing really cost that much? Yes. Because every extra bar costs roughly 7–8% more power, a system running 1.5 bar above need can waste more than 10% of its compressed air energy before counting leaks or unload losses.
Conclusion
Screw compressor sizing for a plastic injection molding plant is a demand, pressure and duty-cycle exercise, not a kW-per-machine guess. Lower the pressure band, match the load curve with a variable speed drive, and keep two-stage units on the table for continuous multi-press lines. Contact Panrui for a load profile review and a sizing calculation built on your actual cycle data.
