Proper Setting of Back Pressure in Injection Molding
2026-01-17 13:41:21
Injection Molding
Back pressure is the resistance exerted on the screw during plasticizing. It is a key parameter that affects melt quality, surface appearance, and dimensional stability. Proper back pressure improves melt homogenization, removes gases, and stabilizes shot volume. Excessive back pressure can cause degradation and wear, while insufficient back pressure leads to poor plasticization, silver streaks, and short shots.
1. Key Functions of Back Pressure
Back pressure improves melt uniformity by increasing shear, which enhances melting and dispersion of additives. It also helps remove moisture, air, and volatile gases from the barrel, reducing bubbles and burn marks. Proper back pressure stabilizes shot volume, improving dimensional repeatability. Moderate back pressure can also slightly increase melt temperature and reduce viscosity, aiding flow into thin or complex cavities.
2. Factors Affecting Back Pressure Settings
2.1 Material Characteristics
Low‑viscosity and heat‑sensitive materials such as PVC, POM, and PA require low back pressure (0.5–3 MPa) to prevent degradation. Medium‑viscosity materials such as PP, PE, and ABS work well with 3–8 MPa. High‑viscosity materials such as PC, PMMA, and glass‑fiber reinforced grades may need 8–15 MPa to improve mixing, but must stay within thermal limits.
2.2 Part Quality Requirements
High‑surface‑quality parts, such as transparent or high‑gloss components, may need slightly higher back pressure (2–3 MPa above standard) to ensure good degassing. Tight‑tolerance parts require stable medium‑low back pressure to avoid shot‑to‑shot variation. General structural parts should use the lowest effective back pressure to balance quality and efficiency.
2.3 Machine and Mold Conditions
Standard screws require lower back pressure, while mixing or barrier screws can handle 10–15 MPa. Complex or thin‑walled molds benefit from slightly higher back pressure to improve flow, while simple or thick‑walled molds can use lower back pressure to reduce cycle time.
3. Setting Principles and Practical Methods
3.1 Basic Principles
Back pressure should be adjusted gradually from low to high, starting at the material’s recommended minimum. Increase in 0.5–1 MPa steps while checking part quality. The upper limit is determined by the absence of yellowing or charring. Back pressure must be matched to screw speed: higher speeds require lower back pressure to avoid excessive shear.
3.2 Practical Guidelines
Heat‑sensitive materials (PVC, POM, PA) should use 0.5–3 MPa, with gas reduction achieved mainly through drying. Standard PP, PE, and ABS parts typically use 3–8 MPa; adding colorants may require a 1–2 MPa increase. Transparent PC and PMMA parts often need 8–12 MPa, provided the material is well dried. Glass‑fiber reinforced materials may use 10–15 MPa to improve fiber dispersion. Thin‑walled or complex parts can use 2–3 MPa above standard to enhance filling.
3.3 Troubleshooting
Silver streaks or bubbles may be fixed by increasing back pressure 1–2 MPa after confirming proper drying. Degradation or yellowing requires reducing back pressure 2–3 MPa and lowering screw speed or temperature. Poor plasticization or color differences can be improved by increasing back pressure slightly, provided no degradation occurs. Unstable shot volume may result from fluctuating back pressure or a worn check ring.
4. Important Considerations
Monitor plasticizing time and motor load when adjusting back pressure; high back pressure increases cycle time and energy use. Confirm whether the machine displays MPa or bar. Before shutdown, set back pressure to 0 MPa and purge the barrel. For heat‑sensitive materials, use a metering color feeder to minimize decomposition.
Summary
Proper back pressure requires balancing material characteristics, part requirements, and machine conditions. Low back pressure suits heat‑sensitive materials, while higher back pressure improves mixing for high‑viscosity or reinforced materials. High‑quality or complex parts may need slightly higher back pressure, while standard parts use lower values for efficiency. Settings should start at the recommended minimum and be optimized through trial molding.
