Injection Molding Back Pressure Parameter Reference Guide Classified by Material
Back pressure refers to the reaction pressure generated when the injection screw retracts during plasticization. Its core functions focus on three practical aspects: compacting melt to remove moisture and gas, improving mixing uniformity of modified materials and colorants, and stabilizing melt viscosity to maintain consistent filling performance. In actual parameter setting, three basic principles must be followed. Low viscosity and heat-sensitive materials shall adopt low back pressure to avoid thermal decomposition; high viscosity, transparent and fiber-reinforced materials need appropriately increased back pressure to ensure sufficient plasticization. Appearance and precision parts require stricter back pressure control, while ordinary structural parts can adopt moderate parameters to balance production efficiency. Meanwhile, it is necessary to match the load of injection molding equipment to prevent long-term overload wear of the screw and barrel. The unit conversion shall be unified on site, with 1 MPa approximately equal to 10 bar for rapid parameter adjustment.
Different types of plastics have obvious differences in melting performance and high temperature resistance, so back pressure cannot be applied universally. General plastics such as PP, PE and PS feature good fluidity, and the back pressure shall be kept low and stable to prevent shear overheating; appropriate adjustment is allowed during color mixing to optimize mixing effect. Engineering plastics including ABS, PA and PBT require medium back pressure, and sufficient raw material drying must be completed in advance to avoid bubble defects. High viscosity transparent materials such as PC and PMMA rely on high back pressure to eliminate tiny bubbles and improve surface finish, which is the key to qualified transparent parts. For thermally sensitive materials represented by POM and PVC, ultra-low back pressure is mandatory to suppress pyrolysis and prevent corrosive gas and yellowing. For glass fiber filling, mineral filling and flame retardant modified materials, medium and high back pressure is conducive to uniform dispersion of fillers and reducing floating fiber and surface blemishes.
| Material Category | Typical Material | Back Pressure(MPa) | Back Pressure(bar) | Key Notes |
|---|---|---|---|---|
| General Plastic | PP | 0.8–3 | 8–30 | High fluidity, adjustable for color mixing |
| General Plastic | PE | 0.5–2.5 | 5–25 | Low viscosity, prevent shear overheating |
| General Plastic | PS | 2–5 | 20–50 | Medium and low back pressure for general use |
| Engineering Plastic | ABS | 3–8 | 30–80 | Enhance surface gloss and exhaust |
| Engineering Plastic | PA/PA66 | 3–8 | 30–80 | Adequate drying required |
| Engineering Plastic | PBT/PET | 4–9 | 40–90 | Crystalline material, avoid degradation |
| High-viscosity & Transparent | PC | 8–15 | 80–150 | High back pressure for defoaming |
| High-viscosity & Transparent | PMMA | 8–14 | 80–140 | Eliminate silver streaks for transparent parts |
| Heat-sensitive Material | POM | 0.5–3 | 5–30 | Low back pressure to prevent decomposition |
| Heat-sensitive Material | PVC | 0.3–2 | 3–20 | Ultra-low back pressure for anti-corrosion |
| Modified & Reinforced | Glass Fiber Filled | 7–15 | 70–150 | Optimize fiber dispersion |
| Modified & Reinforced | Mineral Filled | 5–12 | 50–120 | Improve mixing uniformity |
| Modified & Reinforced | Flame Retardant | 4–10 | 40–100 | Prevent additive discoloration |
On the basis of standard reference values, back pressure needs fine adjustment according to raw material brands, wall thickness and production environment. When processing general plastics, operators should take low back pressure as the baseline and only increase parameters slightly for color mixing requirements. Nylon and crystalline engineering plastics should focus on drying and temperature coordination, avoiding excessive back pressure leading to material embrittlement. Transparent products must maintain stable high back pressure and matched low screw speed to reduce shear heat and ensure high light transmittance. In the production of easily decomposed materials, minimize back pressure and shorten the residence time of materials in the barrel. Modified reinforced materials should control the upper limit of back pressure, preventing excessive shear from breaking fiber structure and reducing mechanical properties of finished products.
