Injection cooling time
The injection molding cooling time accounts for the largest portion of the molding cycle (typically 50%-70%), directly impacting production efficiency and part quality. Its core objective is to cool the part below its thermal deformation temperature to ensure dimensional stability after demolding. Cooling time should be calculated based on the part’s maximum wall thickness. For parts with uniform wall thickness, the formula t = (s² × α) / π² can be used (s is the maximum wall thickness, α is the thermal diffusivity). For a 3mm-thick PP part with a thermal diffusivity of 0.12 cm²/s, the cooling time is calculated to be approximately 15 seconds. In actual production, 18 seconds is used, taking into account mold cooling efficiency, to ensure a stable shrinkage of 1.5% ± 0.1% after demolding. Insufficient cooling time can lead to part warping (for example, reducing the cooling time of an ABS housing from 20 seconds to 15 seconds increased the warpage from 0.3mm to 0.8mm). Excessive cooling time can reduce production efficiency. For example, for a PC lens, a 5-second increase in cooling time reduced daily production capacity by 800 units.
The material properties of plastic parts are key factors in determining cooling time. Crystalline and amorphous plastics have significantly different cooling requirements due to their differing thermal conductivity. Crystalline plastics (such as PA66) require longer cooling times to fully crystallize. For example, a 4mm-thick PA66 gear requires 30 seconds to achieve a crystallinity of at least 75%. If this cooling time is shortened to 20 seconds, the crystallinity drops to 60%, resulting in a 20% decrease in dimensional stability. Amorphous plastics (such as PC) rely primarily on thermal conductivity to cool down, eliminating the need for crystallization. With the same wall thickness, cooling times are 30% shorter than those of PA66. A 4mm-thick PC lampshade requires only 21 seconds to cool. The thermal conductivity of the material also influences cooling efficiency. POM (0.3W /(m・K) ) cools 50% faster than ABS ( 0.2W/(m・K) ). With the same wall thickness, cooling time can be nearly halved. For example, a 2mm- thick POM part can cool in 8 seconds, while ABS requires 12 seconds.
The design of the mold cooling system directly impacts cooling time. Proper water channel layout and sizing can significantly shorten the cooling cycle. The distance between the water channel and the cavity surface should be controlled at 1.5-2 times the channel diameter (e.g., an 8mm diameter water channel should be 12-16mm from the cavity). By optimizing the water channel placement, a washing machine panel mold reduced cooling time from 45 seconds to 32 seconds. Water channel diameter should be selected based on part size. Small parts (<100g) use 6-8mm channels, while large parts (>500g) require 10-12mm channels. Increasing the water channel diameter from 8mm to 12mm in a car bumper mold reduced cooling time by 25%. Using conformal water channels (3D printing) improves cooling efficiency by 40% compared to traditional straight channels. For a complex curved part, cooling time was reduced from 35 seconds to 21 seconds, and temperature differences between different areas were reduced from ±5°C to ±2°C, effectively reducing warping caused by uneven cooling.
The setting of process parameters needs to be optimized in conjunction with the cooling time. Mold temperature, melt temperature, and cooling time form a triangular relationship that influences each other. Increasing mold temperature will extend the cooling time. For example, when the mold temperature of a PP plastic part increases from 50°C to 70°C, the cooling time needs to increase from 15 seconds to 20 seconds, but it can reduce internal stress (from 25MPa to 15MPa). Excessively high melt temperature will significantly increase the cooling load. When the melt temperature of an ABS material increases from 220°C to 240°C, the cooling time needs to increase by 8 seconds to ensure the demolding temperature. Therefore, the melt temperature should be reduced as much as possible while meeting the fluidity requirements. The temperature and flow rate of the cooling medium are also crucial. Usually, the cooling water temperature is controlled at 15-25°C and the flow rate is 1.5-3m/s. In one case, by reducing the water temperature by 5°C and increasing the flow rate by 0.5m/s, the cooling time was shortened by 10%, and the increase in energy consumption was controlled within 8%.
Optimizing cooling time requires balancing production efficiency and part quality. Finding the optimal balance is achieved through CAE simulation and field testing. The initial cooling time for a mobile phone casing mold was set at 25 seconds. Moldflow simulation analysis revealed that 20 seconds would meet demolding requirements. Mold trials verified that the dimensional deviation of the parts cooled for 20 seconds (±0.05mm) was minimal compared to 25 seconds (±0.04mm), yet daily production capacity increased by 20%. For thick-walled parts (>10mm), stepped cooling can be employed: first rapidly cooling to above Tg (e.g., 150°C for PC), followed by slower cooling to reduce stress. This approach reduced the total cooling time for a 15mm-thick PC part from 60 seconds to 45 seconds, while also reducing internal stress by 30%. Cooling time can be monitored using an infrared thermometer to ensure that the demoulding temperature of plastic parts is 10-20°C below Tg (e.g. PC demoulding temperature < 140°C). Through real-time monitoring, a production line has controlled the cooling time fluctuation to ±1 second, and the dimensional stability CPK value has been improved from 1.2 to 1.6.
