Back pressure in injection molding is the resistance the machine applies against the screw while it turns and moves backward during screw recovery. The melt collecting in front of the screw has to build up that pressure before it can push the screw back, so it is compacted and mixed more thoroughly. Higher back pressure gives a denser, more even melt and more consistent shots, but it also lengthens recovery, adds shear heat and increases wear on the screw, the check ring and the barrel.
Back pressure acts on the screw, item 4 in the drawing. For how screw recovery fits into the cycle, see the barrel and screw guide.
How the machine creates back pressure
During recovery, the melt pushes the screw backward, and the screw pushes the piston of the injection cylinder back with it.
- Hydraulic machines. The oil pushed out of the injection cylinder has to pass a pressure control valve. Depending on the machine, this is a proportional valve set from the controller or an adjustable back pressure valve set by hand. The valve holds the oil at the set pressure, and that pressure resists the screw's movement.
- Electric machines. The injection servo motor holds a controlled force against the screw as it moves back, and the setting is often entered directly as melt pressure.
Hydraulic and specific back pressure
On hydraulic machines the setting is usually an oil pressure. The pressure in the melt, called the specific back pressure, is higher: it equals the hydraulic pressure multiplied by the intensification ratio, the area of the injection piston divided by the cross-section of the screw. The ratio is typically about 10:1, but it differs between machines and between screw diameters on the same injection unit, so take it from the machine data. Always state which kind of pressure you mean when you compare settings between machines.
What back pressure does
| Too low | Too high | |
|---|---|---|
| Melt | Air and gas stay in the melt, giving splay and bubbles; unmelted pellets; poor color mixing | Extra shear heat raises the melt temperature; heat-sensitive materials degrade, giving burn marks and black specks |
| Shot | Shot weight varies from shot to shot | Consistent, but the nozzle may drool |
| Cycle | Short recovery | Long recovery, which can become longer than the cooling time and extend the cycle |
| Materials with fibers | Fibers less damaged | Glass fibers are broken into shorter lengths |
| Wear | Low | More wear on the screw, check ring and barrel |
Back pressure also works with screw speed: both add shear heat, so a change in one often calls for a change in the other.
How to set back pressure
- Start from the material supplier's recommendation, or from a low value as the machine manual suggests.
- Set the screw speed so that recovery finishes before the end of the cooling time, with some time to spare.
- Raise the back pressure in small steps, watching shot weight, cushion, color mixing and splay. Stop when the parts are consistent.
- After each change, check the recovery time and the melt temperature, for example by measuring an air shot with a melt probe.
- Set decompression (suck-back) after recovery to relieve the pressure in front of the screw and stop drool. Too much decompression draws air into the nozzle.
- If the controller allows a back pressure profile over the stroke, use it only when a single value does not work.
Back pressure problems and their causes
- Recovery time gets longer at the same settings. Feeding problems at the hopper and feed throat, a cold heating zone, or a worn screw and barrel.
- The back pressure reading fluctuates or will not hold the set value. On hydraulic machines, a sticking or worn pressure control valve, contaminated oil, or a faulty pressure transducer; on electric machines, a drive or sensor fault.
- The screw turns but does not move back. No material is reaching the screw: an empty hopper, a closed slide gate or bridged pellets.
- Drool and stringing. Back pressure too high, decompression too low, or the nozzle too hot. See nozzle drooling, stringing and blockages.
- Splay or bubbles that back pressure does not cure. Often moisture in the material, which needs drying rather than more back pressure.
Before changing settings to make up for a machine that has become inconsistent, check the check ring: a leaking non-return valve causes unstable cushions that no back pressure setting will fix.
Parts that control back pressure
On hydraulic machines, back pressure depends on the pressure control valve, the injection cylinder seals and the oil condition. See the proportional pressure and flow valves we supply. Request a quote with the machine model and the valve or cylinder details.
Updated by the IMSPARES technical team.
