Make Injection Molds from Plastic on a 3D Printer?

Is it possible to injection mold a plastic part using mold tooling that is also made of plastic? 3D printing technology provider Stratasys says this is not only possible, but preferable in some cases. The company has 3D-printed “digital ABS” plastic molds on display in its Booth N-6144, where it is also demonstrating this mold tooling by making parts on an injection molding machine.

The mold tooling material is produced on a Stratasys Connex 3D printer. This printer digitally creates combination materials by rapidly laying down tiny dots of different materials as it builds the part. To create the mold tooling, it combines a heat-resistant plastic with a matrix engineered for high strength. The result is a material that can withstand both the high pressure and high temperature of a molding cycle.

The mold tooling material is in fact one of the very strongest materials created on the Connex 3D printer, which is more frequently used to make multi-material prototype parts. Stratasys sales manager Nadav Sella has been involved in the development of this machine’s application to mold tooling ever since an end user of the machine first hit on the idea of making molds this way nearly 5 years ago.

Sella says the life of one of these digital ABS tools is heavily influenced by both the material being molded and the geometry of the part. But to give one illustration: On a six-cavity injection mold making ice cream spoons in polypropylene, the digital ABS mold delivered 600 spoons. For more complex geometries using reinforced nylon, the tool might deliver 20 injected parts, he says. He also notes that 3D printed molds are good for blow molding, where geometries are smooth and pressure is lower. In general, where tooling is needed for low quantities or for an initial run of parts, 3D printing a mold can both save cost and speed the time to market, he says.

There are limitations. Sella and others within Stratasys have worked through a number of applications of digital ABS molds, and this has allowed them to develop a set of best practices that they share with users. That set of best practices keeps improving as digital ABS tooling is applied to more geometries and materials, but the key, he says, is to respect the mechanical and physical properties of the tool. Heat conductivity is not like aluminum or tool steel, so this leads to tool design considerations aimed at avoiding heat concentration. One example concerns gate size and type, he says—point gates, cashew gates and banana gates should be avoided.

Precision is also a consideration. The 3D printer is precise, but not as precise as a CNC machine tool. Thus, it can’t produce molds with the finest features, such as the tight-tolerance details of some electronics-industry molds. Also, to ensure the accuracy needed for precise seating of ejector pins, these holes should be 3D printed undersize, then reamed to achieve an accurate diameter.

“This is a different material,” Sella says. Established moldmaking professionals are accustomed to molds being made from metal. Compared to this, 3D printed tooling requires slight design and process changes, he says. His advice to potential users is to expect to take some time getting used to what this option can do. However, “for the shop that does 100 molds per year—some in steel and some in aluminum—what if 10 or 20 of those molds could be 3D printed instead?” That portion is realistic, and could amount to considerable savings in cost and time.