Material selection for injection molds of thick-walled PP-R parts: experimental comparison of conventional 316L steel, CuBe alloy, and selective laser melting with conformal cooling: thermal performance, part quality, and financial viability

Imagine you are baking a giant, thick loaf of bread. If you put it in a standard oven, the outside might burn before the inside is even warm. Now, imagine if you could build a special oven with cooling pipes that weave inside the oven walls themselves, snaking right up to the surface of the dough to pull heat away instantly. That is the dream of "conformal cooling" in the world of plastic injection molding.

Injection molding is how we make millions of plastic things, from toy cars to water pipes. To make them, you melt plastic, squeeze it into a metal mold, and wait for it to cool down so you can pop the part out. The waiting part—the cooling—is the slowest, most expensive step. For thin plastic parts, engineers have found a way to speed this up using 3D printing to make molds with those fancy, winding internal pipes. But what happens when the plastic part is thick? Does the 3D-printed magic still work, or does it become too expensive to be worth it? This is the puzzle a team of researchers set out to solve, comparing three different ways to build the metal molds needed to make thick plastic pipes.

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The Great Mold Showdown: Steel, Copper, and 3D Magic

In this study, the researchers acted like race car pit crews, testing three different "chassis" (molds) to see which one could cool down a thick-walled plastic pipe fitting the fastest and cheapest. The star of the show was a specific type of plastic pipe called PP-R, used for hot water systems, with walls as thick as 8.8 mm. They tested three contenders:

1. The Classic (316L Steel): The old-school champion. It's a stainless steel mold made by a machine drilling straight holes for cooling. It's reliable, but the cooling pipes are far from the center of the thick plastic, like trying to cool a pizza with a fan that only blows on the crust.
2. The Speedster (CuBe Alloy): This is a special copper-beryllium metal. Think of it as a super-conductor for heat. It doesn't have fancy 3D-printed pipes; instead, it just conducts heat away from the plastic incredibly fast because the metal itself is so good at it. It's like swapping a wooden spoon for a metal one to stir a hot soup—the heat moves away much quicker.
3. The Futurist (SLM-CCC): This is the high-tech 3D-printed mold. Made using Selective Laser Melting (SLM), it has cooling channels that twist and turn to hug the shape of the plastic perfectly, just like the "dream oven" mentioned earlier. It's designed to be the ultimate cooling machine.

The Race Results: Who Won?

The researchers ran the molds through real-world tests, measuring how long it took to make a part, how much the parts cost, and whether the final pipes were strong enough to handle hot water pressure.

The Copper Speedster (CuBe) took the gold medal.
Because copper conducts heat so well, this mold cooled the plastic 36% faster than the classic steel mold. The total time to make a part dropped by 18%. Even better, because the mold was made using standard machining (just like the steel one), it didn't cost much more to build—only about 10% more. The result? A 12% drop in the cost of making each pipe. To make this extra investment pay off in three years, the factory would just need to grow its sales by 8 times. That's a big jump, but a very doable one.

The 3D Futurist (SLM-CCC) came in second, but with a heavy price tag.
The 3D-printed mold did cool the plastic faster than the steel one, shaving off 20% of the cooling time and 11% of the total cycle time. This saved 7% on the cost per part. However, there was a catch: making this mold took 142% longer (more than double the time) to fabricate than the others. The laser had to print the metal layer by layer, and then the mold still needed to be polished and finished by a machine. This made the mold 46% more expensive to buy. To make that huge extra cost back in three years, the factory would need to grow its sales by a staggering 18 times.

The Classic Steel (316L) was the baseline. It was the slowest and the cheapest to buy, but it produced the most expensive parts in the long run because it took so long to cool.

The Verdict: It's Not About Speed, It's About Scale

Here is the twist that the paper reveals: The 3D-printed mold didn't win.

While 3D printing with conformal cooling is famous for making thin plastic parts incredibly fast (sometimes cutting time in half), this study found that for thick parts, the magic fades. The thick walls of the plastic mean the cooling pipes can't get close enough to the center to make a huge difference, no matter how twisty they are, because the volume of the plastic is too large relative to the surface area the pipes can reach. Meanwhile, the time and money it takes to 3D print the mold are massive.

The researchers found that the Copper (CuBe) mold was the clear winner for this specific job. It offered a massive speed boost without the crazy manufacturing cost of 3D printing.

The paper explicitly rules out the idea that 3D-printed molds are automatically the best choice for thick-walled parts. In fact, the data suggests that unless a factory is planning to explode its production volume by 18 times, sticking with the 3D-printed mold is a financial trap. The "break-even" point for the 3D mold is so high that it's unlikely to be reached in the near future for this type of product.

The Takeaway

If you are making thick plastic pipes, don't just reach for the 3D printer because it sounds cool. The study shows that a mold made of special copper (CuBe) is the smarter, faster, and more profitable choice right now. It cools the plastic nearly as well as the high-tech 3D mold but costs a fraction of the price to build.

The researchers also confirmed that all three molds made pipes that were strong enough to handle hot water pressure for 1,000 hours, so no one had to sacrifice quality for speed. But when it comes to the wallet, the copper mold is the undisputed champion for thick-walled parts, while the 3D-printed mold remains a very expensive option that only makes sense if you are selling a truly astronomical number of parts.