Written by Senior Applications Engineer, Benjamin MacDonald

Steel molds get to enjoy the cycle time benefits of water cooling channels, why can’t Fortify molds? Mold cooling is something that has plagued polymer-based molds since their inception and Fortify is certainly not the exception. Today, the most effective way to maintain tool temperature is to spray compressed air across the mold surface. This happens AFTER the part has gone through an elongated passive cooling phase and the mold opens. We know what benefits we might see with an active cooling phase before a mold opens, the real question is how. How would we overcome the thermal conductivity limitations of Fortify mold tools to actually see the benefits of water cooling?

The Problem

While printed molds are fast to produce, they are slow to process with cycle times between 2-3 minutes versus the 30-45 seconds for a traditional metal mold. This is acceptable when you’re doing low-volume work, but it can add up quickly the more shots you’re looking to achieve. Although our DT resin is the toughest on the market, it has low thermal conductivity. This makes it difficult to quickly remove heat and be ready for the next shot.

To counteract this, we implemented a two-phased (passive and active) cooling approach where we passively cool the part with the mold blocks while the mold is still closed. Once the part has solidified, we open the mold blocks and use compressed air to do the active cooling. This step which accounts for the majority of our heat transfer cools both the part and the mold blocks 

This approach has worked well enough for us but innovation doesn’t stop at “well enough”. We know that our molds have a low thermal conductivity so using the same cooling channels that metal molds use will yield significantly worse results. A quick look at the heat transfer equation for conduction tells us that in order to reduce the cooling time we need to either increase the surface area of our cooling channels or reduce the distance between the cooling channels and the part … or both.

The Design

Enter PTC. At Fortify we design all of our mold tools using Creo so a partnership with PTC was a no-brainer. A quick conversation around the capabilities of Creo 8 was all we needed, to know exactly how to attempt water cooling. In order to maximize cooling, we needed to minimize the distance between the water-cooling channel and the surface of the mold and maximize the surface area of the cooling channel. Creo’s lattice tool enabled us to achieve exactly that with a gyroid lattice infill for our cooling channels. This gyroid geometry enabled us to use a 5mm wall thickness between the mold surface and the cooling channel due to its structural nature.

image of redesigned mold tool

Redesigned mold tool with gyroid lattice infill with a threaded inlet and outlet for active water cooling.

Mold tool connected to a water source, actively cooling the tool.

Mold tool connected to a water source, actively cooling the tool.

The Result

Our mold tool was printed on a Fortify Series Flux One 3D printer in just 6 hours and was on the mold press the following day. Using active cooling we ran 50 shots of polypropylene with parameters:

  •   5 tons of clamping force
  •   2,000 psi injection pressure
  •   10mm/s injection speed
  •   1,500psi pack pressure

To ensure a solid comparison we used a control mold with no active water-cooling channels and the new mold with active cooling. With these two molds, we ran two tests to characterize exactly how effective these water cooling channels were.

Test 1

In the first test, we ran with a constant cycle time of 120 seconds and compared an actively cooled mold to a control mold that was cooled with compressed air. The actively cooled mold was at 55C after 10 shots whereas the controlled mold was at 70C, exhibiting 20% cooler (or 15 degrees less) than the controlled mold.

mold temperature per cycle graph

Test 2

In this second test, we brought both molds up to 50C to see how long it would take to bring them down to 30C. The actively cooled tool took half the time of the control mold.

cooling from 50c to 30c

Conclusion

Both show preliminary results that active cooling will allow us to shorten our cycle times by 1 to 1 ½ minutes which means our cycle times can be reduced from 2- 3 minutes to 1-2 minutes.

There’s still a significant amount of work to be done and our next step, which we are pursuing now, is a 200-part run test with active cooling. We will be looking to quantify exactly what kind of benefits this active cooling can produce in a real-world setting. In parallel, we will also be looking into refining our designs to increase cooling efficiency through flow simulations.

To learn more, watch our joint webinar with PTC, Lattices in Injection Mold Tools.