Back Pressure: Consistency Is Bought in Heat

Back pressure is resistance during recovery, not during injection

Back pressure is applied while the screw is rotating and retracting, not while it is injecting. It is a hydraulic resistance on the screw that the melt must overcome to push the screw back, and its job is to compact the melt, squeeze out air and mix the material as it passes through the check ring.

The confusion is common because the number appears on the same screen as the injection pressure. Injection pressure drives the screw forward and fills the cavity. Back pressure resists the screw moving back and prepares the next shot. They act on the same screw at different times and they do opposite things.

With no back pressure the screw retracts as fast as the melt arrives, the check ring does not seat properly, and the shot volume is whatever happened to be in front of the screw. That is why zero back pressure produces the worst shot-to-shot variation on any machine.

Consistency has a floor and it is reached early

Shot weight variation against back pressure: 0.90 percent at zero, 0.50 at 5 bar, 0.30 at 10, 0.25 at 20 and 0.24 at 30. The first 10 bar removes two thirds of the variation. The next 20 bar removes 0.06 percentage points.

The mechanism behind the flattening is that the melt is already compact. Once the check ring has seated and the air has been squeezed out, more resistance does not make the melt more compact, it just shears it harder against the barrel wall. The shear generates heat, and the heat is what the rest of the curve costs.

That makes 10 to 15 bar the useful range for most materials. Above it the machine is working harder, the melt is hotter, and the shot weight is no more repeatable than it was at 10 bar.

The heat is not a side effect, it is the bill

Melt temperature rise against back pressure: 0 degrees at zero, 3 at 5 bar, 7 at 10, 16 at 20 and 26 at 30. The rise is not linear, because shear heating depends on the shear rate and on the viscosity at that shear rate, and both change as the pressure rises.

A 26 degree rise is enough to change the material. It shortens the cooling time needed, which sounds good, and it moves the melt further along its degradation curve, which does not. For a heat-sensitive material like PVC or a flame-retardant compound, 26 degrees is the difference between a stable process and one that produces black specks after an hour.

It also makes the process harder to control, because the shear heating depends on the screw speed and on the melt viscosity, which depends on the temperature. A high back pressure combined with a high screw speed is a feedback loop: more heat, lower viscosity, more flow, more shear, more heat.

Screw recovery is the other half of the setting

Back pressure is one of three settings that control recovery. The other two are screw speed and the decompression, or suck-back, after recovery finishes. All three affect the shot, and they are usually set by whoever was on the machine first.

Table 1 — The four settings that decide whether the next shot is the same as the last

The cushion is the one that gets ignored and matters most. A cushion of 3 to 6 mm means the machine had material left to pack with. A cushion near zero means the shot used everything available and the part was never packed, which shows up as sink and as short shots at the end of a run rather than at the start.

Setting it from the material, not from the last job

Back pressure is usually quoted as a hydraulic pressure, which is machine-specific. Two machines with different screw diameters will produce different melt pressures at the same hydraulic setting, so a value copied from another machine is meaningless. Set it as a percentage of the machine's maximum, then verify by the effect rather than by the number.

Table 2 — Starting points by material, to be verified on the machine

Then verify with a shot weight study: twenty shots at each setting, weighed, and the standard deviation calculated. The curve will flatten at a different pressure on every machine, and the flattening point is the setting, not the number in the table.

What to specify

A recovery specification has to describe the outcome as well as the settings.

• Back pressure as a percentage of machine maximum, not as an absolute hydraulic value.
• Screw speed, in rpm, with the recovery time it produces.
• Cushion, in mm, with the minimum acceptable value.
• Decompression, in mm, and whether the nozzle is shut off mechanically.
• Shot weight variation, as a standard deviation over twenty shots, which is the number the settings exist to control.

The shot weight number is the one to hold the process to. Settings drift between machines and between shifts, but a standard deviation under 0.3 percent means the process is doing what it was set up to do, whatever the numbers on the screen say.