Conformal Cooling vs. Conventional Cooling - Pros and Cons

Injection molding technology is highly optimized for speed and consistency. The entire process from the initial plastic injection to the final ejection of a completed part is carefully tuned to strike a balance between production speed and part quality. One key optimization parameter is mold cooling. Improved cooling methods can dramatically reduce cycle time while also improving part consistency and quality. This article will compare conformal cooling vs. conventional cooling and describe their respective advantages. 

What is the Importance of Cooling a Mold?

Keeping the cost per unit low is one of the key drivers for injection molding. Apart from the cost of the mold and raw materials, one of the biggest determinants of cost is the injection molding cycle time. The cycle time refers to the overall time that elapses between the initial plastic injection and the final part ejection. However, before the part can be safely ejected, it needs to cool enough to properly crystallize (harden).

The ideal temperature for ejecting a plastic part is 80% of its HDT (Heat Deflection Temperature). HDT is simply the temperature at which plastic will deform under a predefined load. It’s fine for the part to cool in the mold beyond this point, but that only increases the injection molding cycle time and adds no value to the process. To cool a part, cooling channels inside the mold need to direct a heat transfer fluid (typically water) around the mold cavity to draw heat from the part. These mold cooling channels must be designed to cool the part to its ejectable temperature as quickly as possible but must do so in a way that cools the plastic evenly. On top of being strategically placed around the mold, these channels also need to be manufactured to generate turbulent flow. Turbulence increases heat transfer efficiency five-fold, and thus improves cycle times. This is easiest to achieve by rough milling the channels to create uneven inner surfaces. 

If the cooling channels are not positioned properly and the part cools unevenly, it will develop internal stresses, a poor surface quality, and may not maintain consistent dimensions. Additionally, if it is not sufficiently cooled before ejection, the part will likely warp or exhibit damage and will have to be scrapped. 

Mold cooling is subdivided into two categories: conventional cooling and conformal cooling.

What is Conventional Cooling?

To allow for post-injection cooling, an injection mold needs to have cooling channels machined into it. In a conventional cooling arrangement, the cooling channels are manufactured by straight-drilling channels such that they form cooling circuits at strategic places around the mold. These channels are often machined to have rough-machined finishes. The rough inner surfaces increase turbulence in the flow and thus improve heat transfer capacity. 

When is Conventional Cooling Used?

Conventional cooling has been used in the injection molding industry for many years and is good enough for most molding applications – especially those with relatively simple geometries. However, the simplicity of conventional cooling channels makes them inefficient on more complex shapes. Some features will not be reached by the cooling effect and will wind up as localized hot spots that don’t cool at the same rate as the rest of the part.

Relative Costs

The cooling channels used in conventionally cooled molds are easy to machine due to their simplistic geometry. Usually, they’re simply drilled into the mold, so no advanced machining techniques are needed. As such, conventionally cooled molds are cheaper than conformally cooled ones.

What is Conformal Cooling?

In a conformally cooled mold, the cooling channels follow the profile of the part as closely as possible. This means that the cooling channels are more effective at transferring heat, thus improving cooling efficiency. However, it is very difficult to manufacture these types of cooling channels with conventional machining techniques. Conformal cooling often demands the addition of cooling chambers with baffles or multi-part molds. However, metal additive manufacturing, specifically DMLS (Direct Metal Laser Sintering) has reached the point where it can be used to build reliable injection molds with these complex cooling channels printed in the most optimal positions, a feat that is simply not possible with CNC or EDM machines. There is almost no limit to the cooling channel complexity in a printed mold. For more information, see our guide on What is Conformal Cooling.

When is Conformal Cooling Used?

Conformal cooling is far more efficient than the conventional style, especially since the geometry of the cooling channels can be designed to induce extra turbulence. As such, conformal cooling design is used in cases where it is difficult to achieve mold temperature uniformity and fast cooling throughout the entire mold. This is a common problem with complex geometries and thin-walled parts. See the conformal spiral cooling channel in the image below as an example.

Relative Costs

Due to the manufacturing methods required, conformal cooling channels are more costly to create than conventional straight-drilled cooling channels. However, the reduced injection molding cycle time means they increase production rates, thereby improving the productivity of the mold. A careful cost analysis can determine whether conformal channels are worthwhile for your product.