When we quote a mold or set up a trial run, the first thing on the table is the material. Take PE and PP—both are semi-crystalline and shrink like crazy. LDPE runs at 160–220°C with a mold temperature of 20–40°C, while HDPE needs 180–240°C. PP is similar: 200–280°C melt, 30–80°C mold. The catch is shrinkage: 1.5–3.5% for PE and 1.0–2.5% for PP. That means your cavity dimensions must account for post-molding contraction. For a 100mm long part, you could lose 2–3mm. We usually add 0.5–1° of draft per side and use a taper on core pins to avoid scuffing. Gate design? A direct gate or a fan gate works well for PP, but avoid pinpoint gates on thick walls—shear heat will degrade the material.

PVC and PC are a different animal. Rigid PVC processes at 160–190°C, but it's thermally sensitive—residence time over 5 minutes at 200°C releases HCl, which corrodes the screw and barrel. So we specify chrome-plated or nitrided screws and streamlined flow paths with no dead spots. Mold temperature for PVC is 30–60°C. PC, on the other hand, is amorphous and needs high heat: 280–320°C melt, 80–120°C mold. Its viscosity is high, so injection pressures often hit 100–150 MPa. For PC, we use large gates (e.g., 0.75–1.0 times wall thickness) and polished cores to reduce stress. Drying is non-negotiable: PC absorbs moisture, so 120°C for 4 hours is standard before molding.

From a mold sourcing perspective, these parameters directly affect steel choice and cooling layout. For PE/PP, we often use P20 or 718H with simple baffled cooling. For PVC, stainless steel or chrome-plated inserts prevent corrosion. For PC, we go with hardened H13 or S136 to handle high pressure and heat. If you're comparing quotes from different mold makers, ask for their cooling time calculations—a 2mm wall in PP might cool in 8 seconds, but PC takes 15–20 seconds. That difference adds up over a million cycles. For more mold sourcing data and supplier comparisons, visit MoldWorld at www.moldw.com.