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Injection Mold Cooling Design

The design of the injection mold cooling system is very important. The cooling time takes up 70% to 80% of injection molding cycle, a well-designed cooling system can shorten the molding time and improve the productivity magnificently. Poor design of cooling system will extend molding time, increase production cost, and the injection mold temperature has great influence to the mold shrinkage, dimensional stability, deformation, internal stress and surface quality.
Injection mold cooling system

So what are the factors that matter to the cooling effective?
Plastic wall-thickness
Part with thicker wall would need longer cooling time. Generally, the cooling time is approximately proportional to the square and the thickness of plastic parts. If possible, propose to the part designer to minimum the wall thickness.

Mold steel
The higher the thermal conductivity of the injection mold steel, the better for heat transferring, the shorter cooling time needs. In practice, injection mold shop usually copper to replace steel on where the cooling line is not possible to do.

Cooling line layout
The closer the mold cavity goes to the cooling pipes, the greater the diameter of cooling pipes, the better the cooling effect, the shorter the cooling time is going to be. The designer should look out for all the possible to get maximum cooling effect.

Coolant
The nature of the coolant could be different, usually used coolant are oil and water. Viscosity and thermal conductivity of the coolant also affect the heat conduction effect of the plastic injection mold. The lower cooling fluid viscosity, the higher the thermal conductivity, the lower the temperature is, the better cooling effect.

Cooling system design rules:
  • Ensure cooling efficiency, achieve shortest the cooling line meanwhile get quality parts.
  • Ensure uniform cooling to avoid deformation.
  • Ease of manufacturing.
Some examples of injection mold cooling design
If possible, the number of cooling channels should be as many as possible, diameter of the cooling channel should be design as large as possible, cooling speed of A is faster than B as figure below. Diameter of cooling channel usually are 6-12mm.
Injection mold cooling channel

Cooling channels layout must be reasonable. When the wall thickness of part is uniform, the distance of each channel to the surface of parts should be even, which means the layout of channels should follow the actual geometry of the part, see figure A. When the thickness of the part is un-uniform, then thicker wall area need more cooling, see figure B, the injection mold cooling channel can be closer to the part to enhance the cooling effect.
Uniform and un-uniform injection mold cooling channel

Usually temperature of the sprue gate area are highest, so the cooling start from there would achieve the best cooling effective, see figure below.
Injection mold sprue gate

Re-think cooling, that is heat removal, as "Thermal Management" and not simply "Cooling". The engineering of the Thermal System is not trivial but is often over looked. This phase of the process cycle is the longest accounting for 70-80% or more of the time. This means there is a large financial incentive to reduce this and increase profit. However, the traditional way of treating cooling as less critical discounts that dimensional stability and quality is driven during this phase. Removing parts early leads to many issues so the balance is finding the best time and the best method to optimize cooling.

Look at cooling as "The goal of cooling is to remove heat from the part in an advantageous way to result in a low stressed part with the best dimensional and physical properties". Often this is treated as an even rate while in truth the cooling may need to be differential to pull the heat from difficult areas of the part as to allow for a steady state result. This requires a more thorough engineering approach including thermocouples to measure the thermal performance of the injection mold, using water circuits and manifolds to control temperatures and, perhaps, tool materials with tailored thermal properties. This list is not inclusive of a fully engineered thermal system but is a thought starter. Using this approach along with good engineering practice will go a long way of moving from designing injection molds (rule of thumb) and engineering tools (using solid practices) to getting efficient tools, improving processing margins and getting better profits.
How far should be the part from the cooling circuit?
- - - -> by: SomNath Sharma
Some of the grammar needs to be corrected to make some sections easier to understand, especially in the following statement:


'Part with thicker wall would need longer cooling time. Generally, the cooling time is approximately proportional to the square and the thickness of plastic parts. If possible, propose to the part designer to minimum the wall thickness.'
- - - -> by: Anne

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