A Little (or big) Context…
The global market for injection-molded plastics is in excess of US$325 billion and is forecast to grow at a pace of 5.7% per annum over the next 5 years. Significant growth will be fueled by the automotive, construction, consumer goods and medical equipment industry sectors, particularly in emerging markets like China, Brazil and India.
The basic concept of injection-molding is simple. A thermoplastic (such as PVC, ABS or polystyrene) is fed into a chamber and exposed to increasing temperature that gradually melts the plastic. The molten material is then injected into a mold with the desired shape and allowed to cool.
Since its introduction over 150 years ago, the process has evolved. Today, calculations accounting for the thermal properties of the metal mold, the plastic and the part, and of fluid dynamics, are all factored into the process. But certainly when compared to many alternatives, injection-molding really shines. It is is fast and efficient, and allows for high parts complexity and materials versatility.
Why This is Important…
Still, for all its advantages there is considerable room for improvement, and as we’ll see, that’s where additive manufacturing comes in.
Plastic cooling can often account for 70% of cycle time. For this reason, when designing the mold careful consideration is given not only to the thermal conductivity of the metal but also to the creation of channels throughout the mold. Coolant can be pumped through those channels to speed up the cooling process.
Cooling channels are traditionally machined into the mold using conventional methods, which typically results in straight shafts with uneven wall thicknesses between coolant and plastic. Thus, uneven cooling can result in a weak end product. One way to mitigate this issue has been to adopt “conformal cooling” design (where coolant channels conform to the shape of the mold). In the graphic to the left, a comparison between conventional and conformal cooling channels illustrates increased thermal control achievable with conformal channels.
The benefits of adopting conformal cooling can be substantial. In the 2013 Plastics Industry Survey Report released by Plante Moran, it was noted that properly implemented, conformal cooling could in some cases result in an increase in profitability by as much as 55%.
However given all the curves and the changes in channel thicknesses required for effective conformity, it is often not possible to attain effective conformity using conventional machining. Even where possible, the complex machining required to achieve conformity increases production costs.
How AM Can Help…
But additive manufacturing is a superhero that can create conformal channels with his little finger. The layer-by-layer production of 3D printing allows for the design of molds that directly integrate effective conformal channels into the structure of the mold without the need for post-production machining.
You don’t have to take our word for it. There is a growing body of evidence to support the marriage of SLM/powder bed fusion 3D printing and conformal mold creation.
For instance, designers at Bastech reported a savings of over 40 hours of shop and programming time in the creation of conformal cooled insert core for a net savings of almost $1,800 over conventional methods. More importantly, the conformal cooling mold resulted in 22% reduction in cycle time. These results have borne out in subsequent designs produced by Bastech.
Similarly, a cupping manufacturer worked closely with Shining 3D in order to produce metal molds with conformal cooling channels. The manufacturer had been producing the required polystyrene cupping using traditional injection molding with 20mm vertical CNC cooling channels. The combination resulted in uneven and longer cooling periods, and a relatively low transparency. The client had three identifiable goals:
to increase the transparency of the cups
reduce the net weight of the cups
to reduce the efficiency of the molding process, here defined as reduced production cycle.
Additive design was used to recreate the cupping mold, and selective laser melting (SLM) to produce the new molds with conformal cooling in place that lined the circular contour of the mold.
The results of the effort were excellent:
The cooldown time of 3D printed metal molds decreased from an average of 22.97 seconds for the conventional mold, to 16.63 second for the conformal mold, a net change of 26%.
The temperature difference of conformal cooling channels between mold inlet and outlet through 3D printed metal molds is at most 5℃, which met the design requirements of channels.
The pressure of 0.3Mpa equally met requirements of general mold temperature controllers without stagnation, eddy current, backflow and so on.
The School of Technology and Management at the Polytechnic Institute of Leiria, Portugal, researchers analysed a support structure for pipette tips used in medical industry: essentially a stackable rectangular rack with housings for the tips and divided by thin walls. Because of the extra weight they would bear, the outer walls were designed to be thicker. However, prototypes created with conventional manufacturing techniques had marks and warping on the thinner inner walls due to uneven cooling. Researchers identified the long cycle time needed to cool the thickest areas of the part as the culprit.
What the researchers did next illustrates the benefits of using Selective Laser Melting to build and test conformal cooling channels in injection-molding tools. The mold was redesigned so it could be created by additive manufacturing with conformal channels already intact. The object was to reduce cycle time and thereby prevent warping.
Results were dramatic. Using conformal cooling channels enabled them to reduce the cooling time by nearly 50% – from 35.5 seconds to 18 seconds.
“The second but not less important goal is to reduce temperature difference in order to prevent warpage…Numerical results show that, with this design approach, temperature difference is significantly lower. The comparison between several nodal temperatures on different areas of the part shows that the highest temperature difference is now 10.6ºC.”
Not only had the new SLM printed mold resolved the issue of warping, but overall cycle time was reduced by 34.2%, allowing for increased productivity. Researchers from the Institute also identified several other benefits to using additive manufacturing to create injection molding tools, including energy savings, scrap reduction, and overall efficiency.
In a nutshell: Additive design and 3D printing has helped make conformal cooling a viable, cost-effective option to add to your tool box.
Chalk up a point for additive manufacturing.
i3D works closely with Shining 3D to help bring you solutions. The EP-M line of SLM powder-bed fusion printers is ideally suited to help take your manufacturing efforts to the next level.
Let us know how we can help.