Last updated: 2026-09-05 · 5 min read
Mold Temperature in Injection Molding — How Does It Affect Surface Finish, Cycle Time, Shrinkage and Warpage, and What Should You Set It At for Each Resin?
Mold temperature controls how fast the melt cools — a cold mold (30–50°C) freezes the surface instantly, capturing fine texture but risking weld lines and flow marks. A hot mold (80–130°C) keeps the melt fluid longer, improving surface gloss and fiber flow but adding 10–20% to cycle time. Set mold temperature per the resin data sheet — not by guesswork.
- Mold temperature directly affects surface quality. At low mold temp (30°C), the melt freezes on contact — surface texture is replicated precisely (good for matte or textured surfaces), but the frozen skin traps flow marks, weld lines and jetting. At high mold temp (100°C), the melt stays fluid longer — flow marks heal, weld lines gain strength (up to 30% higher), and the surface develops a glossy finish. For automotive interior parts (grain texture): mold temp must be 60–80°C to replicate the grain pattern uniformly.
- Mold temperature affects shrinkage and warpage differently for amorphous vs. semi-crystalline resins. For amorphous (ABS, PC, PMMA): higher mold temp means slower cooling, more uniform shrinkage, less warpage — but longer cycle. For semi-crystalline (PP, nylon, POM): higher mold temp increases crystallinity — the part shrinks more (higher crystallization) and becomes stiffer but more brittle. For glass-filled nylon: mold temp 80°C vs. 40°C — warpage drops 25% (more uniform cooling), but shrinkage increases 0.1%.
- Cycle time is driven by cooling time — which depends on wall thickness squared. For a 2mm wall: cooling time is 8 seconds at 40°C mold temp, 12 seconds at 80°C mold temp — a 50% increase. For a 4mm wall: 25 seconds at 40°C, 38 seconds at 80°C — also 50% more. The trade-off: higher mold temp improves quality but costs 10–20% more per part in cycle time. For high-volume production (100k+ shots), the cycle time cost dominates. For cosmetic or structural parts, quality comes first.
What Mold Temperature Should You Use for Each Common Resin?
| Resin | Mold Temp Range | Recommended | Effect of Too Low | Effect of Too High |
|---|---|---|---|---|
| ABS | 40–80°C | 60°C | Flow marks, poor gloss | Longer cycle, sink marks |
| PC | 80–130°C | 100°C | Stress cracking, weld lines | Sticks to mold, long cycle |
| PP | 30–70°C | 50°C | High shrinkage, warpage | Excessive crystallization |
| Nylon (PA66) | 60–120°C | 80°C | Brittle, poor surface | Long cycle, post-shrinkage |
| POM | 60–100°C | 80°C | Sink marks, voids | Warpage, longer cycle |
Mold temperature is not a single number — it is a range, and uniformity matters more than the absolute value. A ±5°C variation across the cavity causes differential cooling, which creates warpage. Use a mold temperature controller (TCU) with separate circuits for core and cavity — set each circuit to the same target and verify with a thermocouple at the mold surface before production starts.
TCU (Temperature Control Unit) — a device that circulates heated or cooled fluid through the mold’s cooling circuits to maintain a stable mold temperature. TCUs control temperature to ±1°C and support separate circuits for core and cavity. Without a TCU, mold temperature drifts as production heat builds up — causing dimensional variation and warpage. The fiber orientation warpage guide shows how mold temperature affects fiber alignment in glass-filled resins.
What Three Rules Guide Mold Temperature Selection?
- Start with the resin manufacturer’s recommended mold temperature — this is the single most reliable reference. The resin data sheet specifies the mold temperature range based on the polymer’s thermal properties. For PC: 80–130°C (high because PC is amorphous and benefits from slow cooling to reduce residual stress). For PP: 30–70°C (low because PP is semi-crystalline and fast cooling reduces crystallinity and shrinkage). Deviating from the recommended range causes defects that no amount of process tuning can fix. The jetting guide shows how mold temperature also affects flow mark healing vs. jetting prevention.
- Use higher mold temperature for cosmetic surfaces — lower for hidden surfaces. If one side of the part is visible (exterior panel) and the other is hidden (interior rib), set the visible side 20°C higher to improve surface gloss and flow mark healing. The hidden side can be cooler — faster cooling on that side reduces cycle time without affecting appearance. For textured surfaces: mold temp must be at the high end of the resin range to replicate the texture uniformly. A 10°C drop below the recommended minimum causes grain pattern defects.
- Balance core and cavity temperatures to prevent warpage — do not let one side cool significantly faster. If the core is 40°C and the cavity is 80°C, the core side shrinks faster — the part warps toward the core. Keep core and cavity within ±10°C of each other. Use a TCU with independent circuits for each, and measure the actual surface temperature with a thermocouple before production — the machine readout shows coolant temperature, not mold surface temperature.
Crystallinity (in molding) — the degree to which semi-crystalline resins (PP, nylon, POM) form ordered crystal structures during cooling. Higher mold temperature = slower cooling = higher crystallinity = more shrinkage and stiffer but more brittle parts. For PP: mold temp 30°C gives 40% crystallinity; 70°C gives 60% — shrinkage increases 0.3%. The packing pressure guide covers how crystallinity affects post-mold shrinkage.
Mold temp: cold (30–50°C) = fast cycle, traps defects. Hot (80–130°C) = better surface, heals weld lines, +10–20% cycle. Set per resin data sheet. Uniformity ±5°C. Core and cavity within ±10°C. Cosmetic side hotter, hidden side cooler.
- Mold temp set per resin data sheet — ABS 60°C, PC 100°C, PP 50°C, nylon 80°C.
- Uniformity ±5°C across cavity — variation causes differential cooling and warpage.
- Core and cavity within ±10°C — measure with thermocouple, not TCU readout.
- Cosmetic side 20°C hotter — improves gloss, heals flow marks on visible surface.
- GF resins: warm mold (80–100°C) — reduces fiber alignment and warpage 20–30%.
Frequently Asked Questions
Why does my PC part have stress cracks even though the mold temperature is set to 100°C?
Check the actual mold surface temperature — the TCU readout shows coolant temperature, not mold steel temperature. If the cooling circuits are scaled or blocked, the actual surface temperature may be 60°C even though the TCU reads 100°C. Clean the cooling circuits and measure with a surface thermocouple. Also check: is the mold temperature uniform across the cavity? A ±15°C variation creates differential cooling and residual stress — the part cracks at the high-stress zone after ejection.
How does mold temperature affect glass-filled parts differently from unfilled parts?
For glass-filled resins, mold temperature controls fiber orientation. A cold mold (40°C) freezes fibers in high alignment near the wall — anisotropy is high, warpage is high. A warm mold (80–100°C) keeps melt fluid longer, allowing fibers to relax — anisotropy drops, warpage decreases 20–30%. For unfilled resins, mold temperature mainly affects surface quality and cycle time — there is no fiber orientation effect. The warpage mechanism is different: unfilled resins warp from differential cooling, not fiber alignment.
Can I reduce cycle time by lowering mold temperature below the recommended range?
Yes — lowering mold temperature by 20°C reduces cooling time by 15–25%. But the trade-off is quality: flow marks, weld lines, poor surface finish, and higher shrinkage variation. For a cosmetic part with a Class A surface requirement, the scrap rate from lowering mold temperature will exceed the cycle time savings. For a hidden structural bracket where appearance does not matter, lowering mold temperature is a valid cost reduction — as long as the parts still meet dimensional tolerance.
Key Takeaways
- Mold temp: cold = fast cycle, traps defects. Hot = better surface, longer cycle.
- Set per resin data sheet. ABS 60°C, PC 100°C, PP 50°C, nylon 80°C.
- Uniformity ±5°C across cavity. Core and cavity within ±10°C.
- Cosmetic side hotter, hidden side cooler. Measure with thermocouple.
Send the part drawing, resin grade and current mold temperature settings — the review checks whether mold temperature is set correctly for the resin and part geometry, and specifies the adjustments needed to eliminate surface defects and control warpage.
Sources: Plastics Technology — Mold Temperature Effects on Part Quality · Plastics Technology Online — Mold Temperature Control for Engineering Resins · Society of Plastics Engineers (SPE) — Mold Temperature and Cycle Time Technical Papers · BASF — Processing Guidelines for Engineering Resins · Wittmann — Mold Temperature Controllers: Technology and Applications
— Arthur, Tooling Engineer at aCatMould