If you think you don’t have the budget or know-how for cost-reducing conformal cooling, it’s time for an update on the technology.
Growing numbers of injection molders are discovering the advantages of using conformal cooling channels that follow the shape of the cavity and core, reach hot spots, and promote temperature uniformity in the plastic materials being molded. These molders are seeing striking results: shortened cycle times, improved plastic part quality, and—above all—cost reductions.
Even so, many manufacturers, some of them with long experience in the industry, still think conformal cooling is too difficult and too expensive. They think these things because both used to be true.
The costly milling and welding needed to create conformal cooling channels with conventional moldmaking has led many molders and moldmakers to avoid it.
Recent updates in conformal cooling involving direct metal laser sintering have made it easier to improve part quality with channels that follow the curves of the part.
Up until four or five years ago, cooling channels for molds were (and mostly still are) drilled in secondary machining operations, and they followed straight lines. If more cooling was needed than a simple channel could provide, toolmakers might create an insert that had channels with baffles or helix baffles. In rare cases where more intricate cooling channels were unavoidable, toolmakers split the mold into segments, milled matching half-channels into the segments, and soldered the segments together to produce channels that didn’t follow straight lines. The milling was costly, and the mold life was shortened because the solder often deteriorated over time.
But recently, new technology has made conformal cooling channels both easy and affordable to produce. Direct metal laser sintering (DMLS) can create parts with highly complex external and internal geometries, “manufacturing” the cooling channels along with the tooling inserts—in a single process.
A cool solution
DMLS is an additive manufacturing technology that produces parts from 3D CAD models by melting metal powder with a focused laser beam, layer by layer, in strata as thin as 40 µm. The first DMLS material available to moldmakers was a bronze alloy ideal for molds and inserts used in low- and medium-volume production runs. The addition of EOS MaragingSteel MS1 (a prealloyed ultrahigh-strength steel in fine powder form) has made it possible to create tooling for high-volume applications as well. Both these materials are readily machined to create the desired mold surface.
Not only does DMLS eliminate the need for secondary drilling, but it also can be used to make optimized cooling channels that closely conform to the surface of the mold cavity, or that reach crucial hot spots that conventional channels can’t. In addition, the channels can be formed in shapes that add to the volume of coolant flow or that generate turbulence to carry off more heat—making the channels far more efficient and mold temperatures more uniform.
Still, some toolmakers and injection molding experts will hesitate to adopt a new technology because they think they’re doing fine with the old ones. They’re already making profits using proven methods; why replace something they know so well with something they don’t?
There are several convincing reasons.
First off, laser-sintered conformal cooling doesn’t replace existing processes; it complements them, allowing the processor to take on production runs that would be too costly—or even impossible—to do the traditional way. It opens up possibilities that can stretch design creativity and bring in new business.
What’s more, conformal cooling is a proven technology. Over the past few years, benchmarks of conformal cooling against traditional processes have documented significant reductions in two of the most important cost drivers of injection molding: cooling times and scrap rates.
A striking example of success with conformal cooling comes from a project undertaken by a major laser-sintering supplier in Germany. The supplier created a tool insert that incorporated conformal channels to boost coolant flow at hot spots in a mold for manufacturing low-volume plastic parts (40,000/year). Here are the results of that project:
• The first savings was in the cost of the new insert: €3250 vs. €19,444 for a traditional insert.
• Then there was the cycle time cut of 55%, from 90 to 40 seconds.
• Finally, the two cost reductions together resulted in an amortization time for the insert of only two months.
The new insert, which carried off heat more uniformly and efficiently, also decreased scrap and promoted better part quality.
Today, millions of injection molded plastic products are produced with tool inserts manufactured with DMLS. Some of the inserts are in molds that have already produced more than 10 million shots.
This insert for cooling the injection point of a mold for sealing cap production was built in EOS MaragingSteel MS1. The integrated cooling decreases cycle time and increases product quality (fewer problems with distortions). Above right is a CAD image of the pin.
How to explore conformal cooling
So how do you determine whether conformal cooling is right for your application? In general, molders who want to explore conformal cooling are already encountering problems using traditional cooling channels:
• Often, they are modifying an existing plastic product and suddenly the past process won’t work as well—for instance, the scrap rate is higher, or design modification requires a completely new (and very expensive) tooling insert, or there are new hot spots in the mold. This last challenge can increase warpage (scrap) or lengthen cycle times; 80%-90% of the plastic part may have cooled, but it sits in the mold waiting for the remaining 10% to cool sufficiently in a critical area.
• On the other hand, molders may be looking at a proposed design that they would like to manufacture using injection molding—but the shape of the part, or the presence of excess heat in difficult-to-reach points in the mold, make IM impractical with traditional cooling channels.
For many manufacturers, investigating conformal cooling is similar to costing out any other process. First, they submit a CAD model or physical sample to a toolmaking supplier with laser-sintering expertise, or to a DMLS supplier, along with background information—what the mold or insert is supposed to do, what coolant is used, what plastic material is being molded, and the estimated production volume. There is no need to share any proprietary information about current manufacturing processes.
When considering paths for cooling channels based on the amount and location of the heat that needs to be removed from the mold, it is important to remember that there are almost no limits restricting channel paths created with DMLS, meaning the mold can be created with the most efficient cooling possible. During this step, it is important to use digital simulation to confirm, before any physical prototyping, that the coolant is removing enough heat and that there are no problems with the design. For instance, the simulation can establish that the pressure and flow are the same in two diverging channels, and that both will remove the same amount of heat. It can also provide an estimate of cycle time.
Fortunately, the physics of heat transfer and fluid flow for conformal channels are no different from straight channels. It is fairly easy to establish what the cooling rate will be across a mold. In the instance of channels with geometrically complex features—for instance, an elliptical cross section instead of a circle, or internal ribs that raise the Reynolds number and create turbulence—a simpler simulation of channels with circular cross sections will prove out a design, and modeling laminar flow with no turbulence gives an accurate enough measure of coolant flow and heat transfer.
This way of simulating gives reassuringly conservative figures—a kind of design safety margin. Often the physical channels prove to be even more efficient than the simulation shows. If the analysis suggests that conformal cooling will reduce the expected cycle time in a traditional mold by 30% or more, then clearly it is a worthwhile option to consider.
All that remains is to manufacture and test a physical prototype. At this stage, some manufacturers elect to outsource the entire plastics manufacturing process to an experienced supplier for the initial production cycle. Others purchase the mold from a tooling supplier and do the part manufacturing themselves. A few (more than 20 companies with tooling expertise so far) purchase a DMLS system themselves and make it part of their standard production technology. This is the option that provides the biggest competitive advantage, the greatest freedom to innovate, and the largest business rewards.
The bottom line
What cost benefits should you expect from using conformal cooling? That depends to some degree on your production levels. In general, low-volume manufacturers get their greatest savings from the reduced tooling costs of DMLS. Midrange volumes—say, up to a million parts—incur savings from shortened cycle times that increase productivity. High-end producers of plastic parts (in the millions) also reap the rewards of faster cooling times, of course, but at these quantities, the elimination of scrap caused by uneven temperature distribution becomes an important source of savings as well.
Familiarity with conformal cooling leads to other benefits as well. Companies using DMLS are able to take on molding jobs their competition can’t touch. Production standards for quality and waste elimination can be much higher. Designers, freed from cooling constraints and thinking in terms of design-driven manufacturing rather than manufacturing-driven design, are able to create free-form shapes that would have previously been prohibitive for injection molding.
Of course, there are a great many other factors that will still affect processing time and quality. Clean, filtered coolant liquid is always desirable. Shorter cooling times won’t change your material insertion and part extraction times. And plastic part quality is dependent on a great many peripheral factors: heaters, sprue and nozzle diameters, and so forth.
But if cooling times are creating a production bottleneck, or uneven cooling is affecting part quality, optimized conformal cooling channels built with DMLS can save you plenty.
Siegfried Mayer is application engineer and Augustin Niavas is business development manager tooling for laser sintering systems manufacturer EOS GmbH (Munich, Germany).