The Essence of the “Cooling Process” in Injection Molding Dies

Introduction

During winter, we have all experienced touching a metal handrail and feeling a more intense coldness than when touching a wooden one, despite the same external temperature. This everyday phenomenon serves as an excellent example, clearly illustrating the fundamental principles of mold cooling design in injection molding.

In injection molding, “cooling” significantly influences quality factors such as the molding cycle, dimensional accuracy, warpage, sink marks, and internal stresses. This article will explore what occurs in mold cooling, starting from why humans perceive metal as cold, organising the explanation both intuitively and from an engineering perspective.

1. Humans perceive coldness not through “temperature” but through 'heat transfer'

Even when metal and wood are at the same temperature, metal feels colder due to differences in thermal conductivity. While a person's hand is approximately 32–36°C, upon touching metal, the heat from the hand rapidly transfers to the metal. Conversely, wood is a poor conductor of heat, so the rate at which heat is drawn from the hand is slower, making it feel less cold.

The crucial point here is that the sensations of “cold” and “hot” experienced by humans are determined not by the object's temperature itself, but by how rapidly heat transfers.

2. This sensation is identical to the cooling phenomenon in injection molding

In injection molding, cooling of the molten resin commences the instant it fills the mold cavity. Molds are typically constructed from steel, a material with vastly superior thermal conductivity compared to resin.

This can be likened to:

Human hand = Molten resin

Metal handrail = Mold

The resin rapidly loses heat upon contact with the mold and begins to solidify. This ‘rate at which heat dissipates’ is one of the primary factors determining molding quality.

3. The role of mold cooling is not to “lower the temperature”

When discussing mold cooling, it is often mistakenly thought that the objective is “to lower the resin temperature”. However, the essence lies in how efficiently and uniformly the resin's heat can be removed.

Even when mold temperatures are identical,

the amount and rate of heat extracted from the resin varies significantly depending on:

・the position of the cooling circuit,

・the flow velocity of the cooling water,

・the mold material,

and wall thickness variations.

This follows the same principle as how thick metal plates and thin metal plates feel differently in terms of coldness, even when at the same temperature.

4. Problems arising from uneven cooling

Uneven cooling leads to the following defects:

• Warpage: Differential shrinkage due to differences in cooling rates

  Sink marks: Delayed cooling in thick-walled sections

  Internal stresses: Differences between rapidly cooled and slowly cooled areas

  Dimensional variation: Reduced stability between production batches

  
  

  This concept is analogous to how rapid cooling at a single point in metal leaves residual strain. In essence, cooling is not about “cooling intensely”, but about “dispersing heat evenly”.

  
  

  5. Cooling circuit design is the task of designing the “flow of heat”

  
  

  An excellent cooling circuit is not simply about placing numerous water channels. What is crucial is a design that consciously considers the flow of heat: resin → mold → cooling water.

• Bring cooling closer to thick-walled sections where heat concentrates

  Avoid creating areas where cooling is excessively effective

  Ensure flow velocity to prevent temperature variations

  
  

  All these points can be intuitively explained by understanding why metal feels cold.

  
  

  6. The Relationship Between Cycle Time Reduction and Cooling

  
  

  Within the injection molding cycle time, cooling time accounts for a very large proportion, sometimes as much as 60-70% of the total. Improving cooling efficiency directly shortens molding time and boosts productivity.

  
  

  However, forcibly intensifying cooling can lead to issues such as:

  The surface solidifying before the interior is fully cooled

  
  

  Increased internal stress

  
  

  Here too, the crucial factor is the “quality of the cooling method”.

  
  

  7. The Significance of Translating Familiar Sensations into Technology

  
  

  The fact that metal feels colder than wood is a phenomenon we can readily experience.

  However, underlying this lies the physical laws of heat transfer, which are also applied in injection molding.

  Possessing this intuitive understanding enables us to explain phenomena such as:

  
  

  Why cooling defects occur

  Why modifying the cooling circuit improves warping

  not just through mathematical formulas, but with a genuine sense of conviction.

  
  

  In Conclusion

  
  

  Cooling in injection molding is not merely an ancillary process; it is a core technology that determines molding quality and productivity. By starting from the reason metals feel cold, the essence of cooling design can be understood more comprehensively.

  
  

  Linking everyday sensations with engineering leads to a technical understanding that proves valuable on the shop floor.