This article breaks down the key injection molding parameters for common thermoplastics, offering practical reference values and troubleshooting insights for mold engineers.

In everyday injection molding production, the process window is dictated by the polymer’s rheology and thermal stability. For amorphous resins like ABS and PC, melt temperature is the dominant control variable. ABS typically runs at 200–240°C, with mold temperature held at 40–80°C to balance surface gloss and weld-line strength. PC, on the other hand, requires a higher melt range of 270–320°C and a mold temperature of 80–120°C to reduce residual stress and prevent cracking. For semi-crystalline materials such as PA66 and POM, mold temperature plays a more critical role in determining crystallinity and shrinkage. PA66 with 30% glass fiber is processed at 280–300°C melt, with mold at 80–120°C; POM needs a narrower melt window of 190–210°C to avoid thermal degradation and formaldehyde release. Keeping these base numbers handy is the first step to stable molding.

Injection speed and pressure must be matched to the part geometry and flow length. For thin-wall housings (1.0–1.5 mm wall thickness), a fast injection speed (60–120 mm/s) with high first-stage pressure (80–140 MPa) is necessary to prevent short shots and flow marks. Conversely, thick-wall optical parts require slow speed (20–40 mm/s) and lower pressure (60–90 MPa) to avoid sink marks and internal voids. Packing pressure is typically set at 50–80% of injection pressure, with hold time calculated from gate freeze-off—for ABS, 0.5–1.0 s per mm of wall thickness; for POM, 1.0–1.5 s per mm. Back pressure on the screw should be kept at 3–8 MPa for most resins, but for PA with high viscosity, 8–15 MPa is recommended to improve plasticizing homogeneity and venting.

Cooling time is often underestimated but heavily impacts cycle time and warpage. A general rule is that cooling time equals 1.5–2.5 times the square of the wall thickness (in mm) divided by the thermal diffusivity of the polymer. For a 2.5 mm ABS part, this gives roughly 15–20 s; for PC, 25–35 s due to lower thermal conductivity. Using conformal cooling channels can reduce this by 20–30% while improving temperature uniformity. Also, remember to adjust the barrel temperature profile—lower rear zone, higher front zone—by 10–20°C gradient to avoid material degradation. If you’re sourcing molds or need process-specific tooling advice, visit MoldWorld (www.moldw.com) for verified mold suppliers and technical resources.