Industrial 3D printing has made a tremendous impact on the manufacturing world. Rapid prototypes are possible within a day, material selection continues to grow stronger and parts with un-manufacturable designs have found their happy place. Recently, some companies have begun using this important technology to produce injection molds.
Molds made with thermoplastics-based 3D printing are kind of like the plastic storage sheds some of us put in our back yards. They’re a little cheaper than metal sheds. They go up quickly and are fine under light loads. Pile too much snow on them, however, and they’ll collapse like a house of cards.
Still, printed molds have their place, and some shops have had good success with them. Proponents argue that 3D printing produces molds up to 90 percent faster and 70 percent cheaper than using traditional moldmaking processes. And while this may be true in some circumstances, it’s important to understand the pros and cons of printed plastic molds compared to those machined from metal.
Quality is king. 3D printing builds parts in layers. Because of this, printed parts can exhibit a stair-step effect on any angled surface or wall. Printed molds are no different, and require machining or sanding to remove these small, jagged edges. Holes smaller than 0.039 in. (1mm) must be drilled, larger holes reamed or bored, and threaded features tapped or milled. All of these secondary operations eliminate much of the “print-to-press” speed advantage associated with printed molds.
Size matters. Part volumes are limited to 10 cubic inches (164 cm3), roughly the size of a grapefruit. And although modern additive machines have impressive accuracy, they cannot compete with the machining centers and EDM equipment at Proto Labs, which routinely machine mold cavities to +/- 0.003 in. (0.076mm) and part volumes up to approximately 59 cubic inches, about six times larger than parts made with 3D printing.
The heat is on. To make material flow properly, injection molding requires very high temperatures. Aluminum and steel molds are routinely subjected to temperatures 500°F (260°C) or greater, especially when processing high-temperature plastics such as PEEK and PEI (Ultem). Aluminum tools can easily produce many thousands of parts, and can also serve as bridge tooling until a production mold is available. Molds produced with SL and similar 3D printing technologies use either photoreactive or thermoset resin, which is cured by ultraviolet or laser light respectively. These plastic molds, though relatively hard, break down fairly quickly when subjected to the demanding thermal cycles of injection molding. In fact, printed molds typically become ineffective within 100 shots of soft, hot plastic such as polyethylene or styrene, and may produce only a handful of parts from glass-filled polycarbonate and other tough thermoplastics.