Mastering Cooling Time：Achieving High-Efficiency, Low-Defect Injection Molding Processes

In injection molding, cooling time accounts for most of the cycle, typically 70% to 85%, and directly affects product quality, mold longevity, and production efficiency. Although often perceived as passive waiting, the cooling stage is, in fact, one of the most optimization-ready phases of the entire process. Therefore, the ability to accurately predict and control cooling time has become a key factor in enhancing the overall performance of injection molding operations.

 

 

 

Cooling time refers to the duration from the end of the packing phase until the molded part has gained sufficient rigidity to be safely ejected. Although the molten plastic begins to cool immediately after injection into the mold cavity, in practice, the cooling phase is typically defined as the period from the end of packing to the actual mold opening and part ejection. During this stage, the material must cool from its melt temperature to a level at or below its Heat Deflection Temperature (HDT) to ensure the part can be ejected without deformation.

 

Condition	Result	Possible Defects
Insufficient Cooling	Part ejected before full solidification	Warpage, deformation, sink marks, and stress cracking
Excessive Cooling	No additional cooling benefit	Wasted cycle time, reduced productivity, higher energy cost

Further reading： Injection Molding Cycle Time: The Key to Faster and Smarter Manufacturing

Further reading： Common‌ Injection Molding Defects: Causes, Types, and Solutions

 

 

 

 

 

• ：Cooling time (seconds)
• ：Maximum wall thickness of the part (mm)
• ：Thermal diffusivity of the material (mm²/s)
• ：Melt temperature (°C)
• ：Mold temperature (°C)
• ：Ejection temperature (°C), typically near HDT

* The wall thickness squared (h²) has the greatest impact on cooling time. Even slight increases in thickness can significantly extend the required cooling duration.

 

 

 

• Uniform Wall Thickness： Thicker sections take longer to cool. Designing parts with uniform thickness helps minimize uneven cooling and deformation.
• Ribs & Bosses： These localized thick sections act as heat sinks, prolonging the cooling process.

 

• Plastics with low thermal conductivity (e.g. ABS) require longer cooling； plastics with high thermal conductivity (e.g. PA6) cool more rapidly.
• Crystalline plastics (e.g. POM, PP) must cool below their crystallization point for ejection, extending cooling time.
• Amorphous plastics (e.g. PC, PS) only need to cool below HDT, requiring less time.

 

• Cooling Channel Layout： Cooling lines should closely follow the mold cavity contour, be symmetrically arranged, and ensure adequate flow rate and pressure differential.
• Thermal Insulation Components： Reduce heat transfer back into the mold structure, helping maintain consistent mold temperatures.

 

• Melt Temperature： Excessive melt temperatures prolong cooling；settings should follow material supplier recommendations.
• Mold Temperature： Higher mold temperatures increase cooling time, while overly low temperatures may cause weld lines or surface defects；balance is key.
• Separation of Packing and Cooling Phases： Properly defining the end of packing and the beginning of cooling ensures accurate time management and process control.

 

 

 

1. Conformal Cooling Channels via 3D Printing

Metal 3D printing enables the creation of complex, conformal cooling paths that are difficult to achieve with traditional machining. Ideal for small, thin-walled, or multi-cavity molds, conformal channels evenly distribute coolant, enhance heat exchange, prevent localized overheating, and shorten cooling times, improving both dimensional stability and surface quality.

 

2. Microcellular Foaming Technology

Incorporating microcellular bubbles lowers the plastic's density and heat capacity, accelerating cooling and reducing cycle time. Additionally, it mitigates sink marks and improves surface appearance while saving material, aligning with sustainable manufacturing trends.

 

Further reading： Microcellular Foaming – Revolutionizing Lightweight Injection Molding

 

3. Mold Anti-Condensation Devices

When mold temperatures are reduced, condensation can form due to thermal gradients, jeopardizing process stability and mold integrity. Low-dew-point dry air systems keep mold surfaces dry, preventing condensation and ensuring consistent part quality.

 

4. Dry Coolers

Closed-loop air-to-water systems significantly reduce water loss and scaling issues. These systems lower energy consumption, minimize environmental risks, and simplify cooling system maintenance.

 

1. Water Filtration and Softening Systems

Removing minerals such as calcium and magnesium prevents scale buildup and avoids blockages in cooling channels, preserving thermal transfer efficiency.

 

2. Air-Water Low-Pressure Cleaning Systems

Periodic cleaning with air-water mixtures removes contaminants without chemicals, an environmentally friendly and effective way to keep cooling channels clear.

 

 

Cooling time not only dominates the molding cycle but also has a profound impact on product quality, mold life, and production output. Though it occurs in the later stages of the process, cooling represents the critical bridge between design intentions and manufacturing execution. Only by systematically integrating cooling time into the broader process strategy can manufacturers enhance both product consistency and productivity, delivering stable, reliable, and competitive manufacturing outcomes.

 

 

• Group Name: Huarong Group
• Brand: Huarong, Yuhdak, Nanrong
• Service Offerings: Injection Molding Machine, Vertical Injection Molding Machine, Injection Molding Automation
• Tel: +886-6-7956777
• Address: No.21-6, Zhongzhou, Chin An Vil., Xigang Dist., Tainan City 72351, Taiwan
• Official Website: https://www.huarong.com.tw/