In the injection molding industry, knowing injection just makes you entry‑level. A true expert masters back pressure!
In injection molding, when problems arise, many immediately think of adjusting injection pressure, injection speed, or mold temperature.
But I believe: “Those who truly understand injection molding don’t only focus on the forward play (injection); they also pay close attention to the rear guard (back pressure).”
Back pressure is the force applied by the hydraulic cylinder or servo motor to resist the screw’s backward movement as it retracts during plasticizing.
This parameter is often ignored, but when it’s off, it can drive you crazy.
Forget the fancy theories—let’s get straight to practical advice. Let’s talk about the biggest pitfalls in back pressure setting and how to adjust it properly.
1. Back Pressure Too Low
Some eager young operators, chasing “speed,” set the plasticizing speed as fast as possible and the back pressure very low, thinking this reduces plasticizing time and shortens the cycle.
To their surprise, the molded parts come out full of silver streaks or gas marks.
Cut open a large, thick part, and inside you’ll find many tiny bubbles.
Root cause analysis:
Plastic pellets drop from the hopper into the barrel, are drawn in by the rotating screw, sheared, and melted.
When back pressure is too low, the screw retracts too easily, and the air trapped between the pellets cannot be forced out.
The melt, full of trapped air, is conveyed to the front of the screw and then injected into the cavity.
Small bubbles that burst on the surface become silver streaks; those that stay inside become internal voids. (See “Don’t just blindly rely on drying + back pressure to fight silver streaks in injection molding—over‑adjusting only makes things worse.”)
Even worse, because of the low back pressure, the melt isn’t compacted.
The density of the melt in each shot varies—some shots contain more trapped air, some less—causing shot weight to fluctuate and dimensions to drift. No matter how you adjust injection or holding pressure afterwards, you can’t keep it under control.
Corrective actions:
Watch the plasticizing stage carefully. If the screw retracts in a jerky, unsteady manner, sounds hollow, or the cushion size is unstable (2–3 mm fluctuation), don’t hesitate—increase back pressure. Add about 0.5 MPa at a time until the entire plasticizing process becomes smooth and the cushion variation falls within 0.5 mm.
2. Back Pressure Too High
Some over‑correct from the previous mistake and then max out back pressure, assuming the more compact the melt, the better. As a result, the nozzle constantly drools, or the parts start showing black specks or yellowing.
Root cause analysis:
With excessively high back pressure, the screw struggles to retract, spinning almost in place, building up intense shear heat.
Excessive shear heat: the material gets over‑sheared and overheats, degrading and burning right inside the barrel. The molded parts show black streaks, yellowing, or even become brittle—especially with heat‑sensitive materials like PVC, POM, PC, PC/ABS.
Excessive melt compression: the melt pool becomes over‑compressed. When the mold opens, with no sprue break to block the nozzle, the built‑up pressure releases instantly, causing the melt to drool from the nozzle into the sprue bushing. It solidifies into a cold slug and causes cold slug marks or short shots in the next cycle. With nylon, it can completely block the nozzle and prevent injection. (See “The hazards of cold slugs and improvement measures – Part 2.”)
Corrective actions:
If you notice the front barrel temperature is higher than the setpoint, or plasticizing time is unusually long, or the screw just spins without retracting, the back pressure is too high. Reduce it immediately.
If you need a high back pressure to vent gases from the barrel but the nozzle starts drooling, increase the decompression (suck‑back) distance appropriately to relieve the pressure in the melt pool.
For special products that absolutely cannot tolerate drooling or cold slugs, a shut‑off nozzle can be used.
3. Back Pressure Setting for Glass‑Fiber Reinforced Materials
Glass‑fiber filled materials are more and more common—such as PA+GF, PP+GF, PBT+GF…
When running fiber‑reinforced materials, you must control back pressure carefully. Otherwise, part strength drops and your screw wears out fast.
Root cause analysis:
Glass‑fiber reinforced compounds contain fibers of a certain length that form a reinforcing network.
Some try to improve mixing by raising back pressure, unaware that under intense shearing, the glass fibers—originally of a certain length—get chopped shorter and shorter, even crushed into powder.
When long fibers become short fibers or powder, the parts fail tensile or drop tests, leading to customer returns.
Moreover, the crushed glass fibers, which are very hard, act like sandpaper particles under high back pressure, aggressively wearing the screw and the three‑piece set.
Corrective actions:
For fiber‑reinforced materials (especially those with a high glass content), back pressure should not be set too high. As long as the material is adequately mixed and free of bubbles, keep it as low as possible.
A back pressure of 0.5–1.5 MPa is generally sufficient. Also, don’t run the screw speed too fast—it’s better to have a slightly longer plasticizing time than to destroy the fiber length and reduce part strength.
