Cooling the molten plastic injected into the mold in a plastic injection molding machine is a critical process. The cooling system controls the temperature of the mold and impacts the quality and forming efficiency of the injection molded product. Uneven temperatures in the mold create temperature differentials in different sections of the product. The varying temperature across the product can cause temperature-induced stress, shrinkage, warping and deformation. Products with complex shapes and uneven wall thickness are particularly susceptible to these effects if the temperature distribution is uneven. An efficient cooling system that can maintain a uniform temperature distribution can minimize these defects in the molded parts. This article will present both options and recommendations for designing effective cooling channels in injection molds and explore strategies and considerations crucial for maximizing cooling efficiency and optimizing molding outcomes.
The mold and its cooling system
The mold itself can be considered as a heat exchanger in which the heat from the hot molten plastic is removed by circulating coolant, typically water. To achieve uniform cooling, the mold must have cooling channels or passageways machined through the mold. The cooling channels carry the coolant around the part as it solidifies. There are two types of cooling channels, straight-drilled and conformal channels. Figure 1 illustrates a cooling system with cooling channels running through the mold.1
Figure 1. Components of a plastic injection molded cooling system
Straight-drilled cooling channels
As the name describes, the channel is a straight line drilled or milled through the mold. If a part has a simple geometry, a straight-line cooling channel can uniformly cool the part. Adding bubblers, baffles and thermal pins to the straight-line cooling channels can enhance uniform cooling. Bubblers are small tubes connected to the straight-line channel and inserted in deeper parts of the mold that the channel cannot reach. The bubbler helps to ensure more uniform cooling in all parts of the molded part. Baffles are thin metal inserts that direct flow to bubblers to allow the bubbler to cool portions of the part further from the straight-line channels. Thermal pins are closed components filled with a fluid that cools by absorbing heat from the mold, vaporizing and then condensing when cooled by passing coolant.
Straight-line cooling channels can be arranged in the mold in two configurations, series and parallel. A series connection consists of multiple cooling channels connected end to end from the coolant inlet to the coolant outlet. The series configuration is the most common configuration designed into molds. If the channels have the same diameter, the coolant can maintain a constant, turbulent flow rate through the entire length of the channels. Turbulent flow transfers heat from the molded part to the coolant more effectively than laminar flow. Large molds with long cooling circuits may require more than one series channel configuration to ensure a uniform cooling of the molded part because the incoming and exit temperatures of the cooling medium, sometimes called “Delta T”, is too large and inconsistent temperature control is the result. Figure 2b shows series, straight-line cooling channels.1
The parallel arrangement of cooling channels is the alternative configuration. Multiple independent cooling channels allow coolant flow through the mold. Slight differences in flow resistance between the straight-line channels can cause varying flow rates. As a result, the heat transfer efficiency in each channel can be different and prevent uniform cooling at different parts in the tool which may distort the final product. Figure 2c illustrates the parallel cooling channels configuration. 1 To overcome some of the flow differences, some tools may have adjustable flow meters that can help equalize flow.
Figure 2. Straight line cooling channel in series and parallel configurations
Conformal cooling channels
Conformal cooling channels, as the name implies, conform to the surface of the molded part. Thus, the cooling path matches the mold cavity surface so that the cooling channel has a nearly constant distance to the mold cavity surface all along the surface of the part. Conformal cooling channels can have the same curvature as the molded part. This type of channel has a higher cooling efficiency than a straight-line channel which can reduce cooling time.
Machining conformal channels into a mold is extremely difficult. Techniques used to create a mold with conformal channels include 3D printing or laser sintering. Laser sintering uses a laser beam to build up layers of melted powder. The individual layers can be as thin as 40 µm. Figure 3 shows how the conformal channels closely track the shape of the mold cavity. 1
Figure 3. Example conformal cooling channels in a mold
Selection options for mold materials
The complexity of the part determines whether straight-line or conformal cooling channels provide the most uniform cooling. Also mold design and manufacturing costs factor into the choice of cooling channel type. For high volume production, steel is the preferred material. Steel has a high wear resistance and can withstand abrasive materials and high-pressure environments. Aluminum, the alternative mold material, has a thermal conductivity that is four to 10 times the thermal conductivity of steel, so aluminum can have a more uniform temperature distribution across the mold surface and dissipate heat more quickly. The higher heat transfer rate allows more rapid cool down of molds. Thus, an aluminum mold can have a shorter cooling cycle. However, aluminum does not have the durability of steel and is more suitable for low volume production.
Recommendations for Cooling channel design
Whether using straight-line or conformal cooling channels, the following recommendations will assist with uniform cooling:
- Use as many small channels as possible rather than fewer large channels.
- Maintain constant diameter channels to ensure a constant coolant flow rate.
- Ensure cooling channels are as close as possible to the molded part, especially the thickest dimension of the part.
- Design both halves of the mold for an identical cooling rate.
Obtaining uniform cooling in a mold involves is as much art as complex design. The recommendations offer valuable hints but do not guarantee uniform cooling of all part structures.
Simulation to optimize the cooling channel design
The complication of the heat transfer physics prevents using mathematical analysis methods to optimize cooling channel design. Analysis by simulation is the most effective technique for feasibly optimizing cooling channel design. Have your mold designer present documentation of simulation studies resulting in a cooling channel design that generates a uniform heat transfer across your injection molded part.
Benefits of effective injection mold cooling
The cooling stage consumes from 66 to 75% of total plastic injection molding cycle time. An efficient cooling channel design that keeps part temperature relatively uniform during the cooling stage and cools down quickly reduces cooling time. Since cooling consists of the majority time in the injection molding cycle, a shorter cooling time reduces the total injection molding cycle time. A shorter molding cycle time allows greater output/shift which lowers the cost/part.
Furthermore, uniform cooling improves the quality of the part by maintaining dimensional accuracy and stability. Re-work and repeat cooling cycles are avoided which also contributes to higher productivity and lower parts costs.
Effective mold cooling channel design is critical
The options and recommendations presented will enable the design of cooling channels in a mold that will cool with a uniform temperature profile around the plastic part. Of most importance is using simulation techniques to optimize the mold design. Uniform temperature cooling prevents part deformation, warping and shrinkage. Expensive re-work and material waste are avoided. With the cooling cycle over 2/3 of the total injection molding cycle, uniform cooling can potentially shorten the cooling cycle time and the total injection molding cycle time to increase productivity, part output and lower part cost.
For assistance with obtaining an effective mold design with optimized cooling channels, the AIM Processing team can provide assistance with mold specifications and guidance on selection of mold designers. Contact AIM Processing.
1. Design and Simulation-Based Optimization of Cooling Channels for Plastic Injection Mold, New Technologies - Trends, Innovations and Research, Prof. Constantin Volosencu (Ed.), ISBN: 978-953-51-0480-3, InTech
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