Fundamentals of cooling time
Cooling is the most time-consuming part of the injection moulding cycle. It is the phase in which the injected polymer must cool in the mould from processing temperature to a temperature at which the part can be ejected dimensionally stable. While injection and holding phases are comparatively short, the cooling phase often dominates more than 60 % of the total cycle for thick-walled parts.
The physics follow heat conduction: the molten polymer gives up heat to the cooled mould. The speed depends essentially on three factors: the thermal conductivity of the plastic, the part geometry (especially wall thickness) and the temperature difference between melt and mould. Because plastics are very poor heat conductors compared with metals, cooling is a critical productivity bottleneck. Every second saved per cycle multiplies across thousands or millions of parts – yet cooling must not be cut so far that warpage, sink marks or insufficient strength appear. How grades differ by method is covered in Plastics grades by processing method.
Thermal conductivity of plastics: material differences
Plastics have an extremely low thermal conductivity compared with metals. Steel reaches about 50 W/(m·K) and aluminium about 200 W/(m·K), while unfilled thermoplastics sit between roughly 0.12 and 0.44 W/(m·K). This property largely determines how fast heat can travel from the part core to the cooled mould surface.
- Low (~0.12–0.20 W/(m·K)): PP ~0.12, PS ~0.13, PLA ~0.16, PVC ~0.17, PMMA ~0.18 – these cool comparatively slowly.
- Medium (~0.19–0.26 W/(m·K)): PA12 ~0.19; PC, PET, PETG ~0.20; PBT ~0.21; PA6 ~0.23; PA66, PEEK, PTFE ~0.25; PSU and TPU ~0.26.
- Higher (~0.31–0.44 W/(m·K)): POM ~0.31, PE-LD ~0.33, PE-HD ~0.44 – the highest among common unfilled thermoplastics.
In practice a PE-HD part cools faster than a comparable PP part at equal geometry. But thermal conductivity is only one factor; the decisive quantity is thermal diffusivity a = λ/(ρ·c_p), which also accounts for density ρ and specific heat capacity c_p.
The quadratic relationship: wall thickness as the dominant factor
The most important factor for cooling time is the part’s wall thickness. From the heat-conduction equation an approximately quadratic relationship follows: cooling time scales with the square of wall thickness (t ∝ s²). Doubling the wall raises cooling time to about four times; halving it cuts it to about a quarter.
This is because heat must diffuse from the core to the surface: doubling the wall doubles both the distance and the amount of heat to be removed. In design practice this gives a clear recommendation: aim for thin parts with constant wall thickness. Wall-thickness steps make the thickest spot govern the whole cooling time, while thinner areas have already frozen – causing uneven shrinkage and warpage. Replacing solid sections with ribs shortens the cycle considerably. The lower limit is set by flow-length and filling constraints; for many applications a wall thickness of 1.5–3.5 mm is favourable, depending on material, part size and function.
Thermal diffusivity and process conditions
Beyond thermal conductivity, thermal diffusivity a = λ/(ρ·c_p) is central: it describes how fast temperature changes propagate through the material; higher diffusivity means faster cooling.
Process conditions matter too. A larger mould temperature difference (lower mould temperature at a given melt temperature) speeds heat transfer – but there are limits: too cold a mould causes surface defects, poor filling, raised internal pressure or gloss problems. Higher melt temperature lengthens cooling (more heat to remove) but often improves filling and surface. Holding time, holding pressure and injection speed primarily affect part quality but can indirectly influence the effective cooling time; an overly long hold can lengthen cooling without further quality gain.
Tool design: cooling channels and tempering
Mould design is as decisive for cooling efficiency as material and geometry. Conventional drilled cooling channels should run close to the cavity surface, have an adequate diameter and cover the whole part evenly so that no area cools significantly slower than others. Hotspots – thicker walls or unfavourable channel positions – govern the overall cycle and should be minimised by targeted tempering. Complex geometries with cores, sliders or deep ribs need careful thermal analysis.
Modern toolmaking increasingly uses conformal cooling via additively manufactured inserts, enabling channel paths impossible with conventional drilling – cooling hard-to-reach areas efficiently and cutting cycle time by about 10–30 % on complex 3D geometries. Tempering units (flow rate, temperature stability, cooling power) must match the process heat load; insufficient cooling power causes creeping mould heat-up and longer cycles.
Material choice from a cooling perspective
Material choice affects not only mechanical, chemical and optical properties but also process economics. At high volumes thermal conductivity can be a relevant selection criterion if several materials meet the functional requirements. PE-HD (~0.44 W/(m·K)) offers the highest conductivity among common unfilled thermoplastics and thus potentially shorter cooling than PP (~0.12) – but the two differ greatly in stiffness, temperature and chemical resistance, so the choice must be functionally driven.
Filled and reinforced plastics (glass fibre, minerals, thermally conductive additives) can raise conductivity markedly – glass-fibre polyamides reach two to three times the unfilled value. Specific heat capacity also matters: materials with low heat capacity release less heat on cooling, shortening cooling time. Thermal diffusivity, combining conductivity, density and heat capacity, is a better basis than conductivity alone. Density also correlates with cooling time via mass: at equal geometry a denser material holds more heat to remove.
Metrology and process monitoring
Systematic optimisation needs precise metrology. In-mould temperature sensors monitor mould temperature at critical points and reveal thermal imbalances; infrared thermography measures part temperature contact-free right after ejection, helping to identify hotspots and optimise channel routing. Ideally the ejected part shows an even temperature distribution just above the minimum needed for dimensional stability.
The optimal cooling time is usually found experimentally by stepwise shortening with simultaneous quality checks: cut too far and deformation, sink marks or dimensional deviations appear. The minimum allowable cooling time is the one at which all quality criteria are just met. Process-monitoring systems with statistical evaluation enable continuous control, and injection-moulding simulation predicts cooling times and virtually optimises channels before the tool is built – saving time and cost.
Economic assessment and best practices
Reducing cooling time directly improves the economics of moulding. At typical cycle times of 20–60 s, every second saved is up to a 5 % productivity gain; across millions of parts even small optimisations add up via lower machine-hour cost, energy and higher output. Best practices:
- Design: minimise and even out wall thickness; replace solid sections with ribs; avoid wall-thickness steps.
- Tooling: position cooling channels near the cavity; adequate diameters; even tempering; conformal cooling for complex geometries.
- Process: mould temperature as low as acceptable surface quality allows; melt temperature to the necessary minimum; optimise holding time.
- Material: prefer higher thermal conductivity among functionally equivalent options; consider filled variants.
- Monitoring & simulation: validate cooling time experimentally; apply SPC; simulate cooling in the development phase.
A holistic view weighs cycle time against quality, tooling cost and process stability – the minimum cooling time is not optimal if scrap rises. Investments in conformal cooling often pay back within months at high volumes.
Summary
Cooling time is the dominant productivity factor in injection moulding and can exceed 60 % of the cycle for thick-walled parts. The quadratic link to wall thickness makes design the biggest lever. Unfilled-plastic conductivity ranges from about 0.12 W/(m·K) (PP) to 0.44 W/(m·K) (PE-HD), with thermal diffusivity setting the real cooling rate. Tool design (well-placed conventional or conformal channels) and process control (mould/melt temperature, holding time) must be matched to material and geometry. Future gains will come from simulation, additive tool inserts and intelligent process control. Related calculators are in the tools & calculators section.