Spoiler: They aren't the same as steel tools

This post was written by Applications Engineer, Ben MacDonald

Injection molding has become the go-to manufacturing solution for plastic components. In 2019, the global injection molded plastics market size valued at $258.2B billion. Injection molded parts are used in a variety of industries and the application space is only expected to continue to grow. Even in automotive, plastics are trending to replace metals and alloys with injection molded plastics. 

The economics behind injection molding parts is advantageous for molders, especially when the required part quantity is greater than 100,000. Though the dominant cost of injection molding is the high capital investment of a machined metal mold, once this mold is made, the costs of producing parts are minimal. Combine this with cycle times of 30 seconds or less and injection molding is a cost-effective solution for high-volume production. Even though the up-front cost of an injection mold is significant, molders can easily justify the expense based on the body of work that goes into completing a mold including:

  • Maintaining extremely tight tolerances ensuring proper fit of a mold and its components 
  • Accounting for the shrinkage of molded parts upon cooling so they are geometrically accurate 
  • Controlling the surface finish of a mold, specific to the desired material for the specific part

Today, this is done with a hardened steel as it is the material of choice to withstand the intense requirements of a production run greater than 100,000 parts. 

The Molders’ Dilemma

What happens when you aren’t producing hundreds of thousands of parts on a mold? With numerous industries (like electric vehicles) looking at low-volume production, how can molders justify the high cost of a tool when you are producing only hundreds of parts? Furthermore, because of the high cost of prototype tooling (sometimes called soft tooling or bridge tooling) designers find it impossible to rationalize properly prototyping parts. Rather than molding 10 iterations of a single part in their end-use plastic, designers are forced to take their best guess at which version will be the best for their specific application, relying on sub-par rapid prototyped parts in the interim for form, fit, and function assessment. After waiting 6-8 weeks for a tool, they either realize they selected the right design or, more often than not, have to accept the design being just good enough. In the worst case, the design is completely wrong wasting both time and money that was invested in the mold.

3D Printing as a Solution

This is exactly where 3D printing injection molds step up to the plate. The ability to rapidly print and mold parts is game changing for part designers. 3D printed tooling exhibits faster lead times (1-2 days) at a fraction of the cost of machined soft tooling, making them a viable candidate for molders who are looking at the economics behind a tool that is only used for small volumes. Additionally, 3D printed mold tools enable designers to print and mold multiple iterations of a part. This gives them freedom to explore many more designs and confidence that their final design will be the right design. Using a 3D printed mold tool from Fortify gives part designers flexibility to mold geometries in a variety of engineering-grade plastics so that their prototypes can match their final parts. Because Fortify’s tools are fiber-reinforced they are able to maintain their stiffness and withstand the high temperatures of injection molding.

Integrating 3D printed tools into your injection molding operation for prototyping and low volume production starts with understanding the differences in the tools and how that affects how you design, machine, and run a tool. 

The obvious fundamental difference between steel molds and printed molds is the material they are made out of. A good rule of thumb is to design 3D printed tools to be more “forgiving” than a steel tool. For instance:

  • Shut off angle: In a steel tool, it is recommended to have an angle of at least 3 degrees when designing shut off features. Any less and you run the risk of the features wearing out or breaking too quickly. For a 3D printed tool instead of 3 degrees, 5 degrees is recommended 
  • Draft angle: Draft angles of features on steel tools are typically 1 or 2 degrees, and can go down to 0.5 degrees or less. 3D printed tools follow that same trend, except 3 degrees is recommended, and certain features are able to be molded with much less.
  • Ejector pin: Determining the placement of ejector pins on a mold so a part doesn’t break during ejection is important. In steel tools this is pretty straightforward and usually minimizes the amount of ejector pins needed. In 3D printed tools, designs need to also consider small and delicate features, and have pins placed so that the parts won’t flex during ejection. This can be accomplished by placing a pin next to a high aspect ratio extrusion or at the base of a rib rather than next to a rib. The great thing about adding in all of these ejector pins is that they provide excellent opportunities for ventilation. 
  • Ventilation:  3D printed molds are unable to withstand the kind of pressures that steel molds can endure. One way to compensate for this is to add in more ventilation than there otherwise would be in a steel tool. This can take the form of surface vents, ejector pins, and vent holes to name a few. The goal with this ventilation is to always give the air in the cavity an easy path to escape so that the flow of the plastic is never restricted. 

By understanding and accepting these design differences, molders can realize significant success when molding parts. Additionally, 3D printing molds can give designers much more freedom. If a designer wanted to iterate on two features within a complete mold, they don’t have to print multiple iterations of the mold. They can simply design the features as inserts for one common mold. Furthermore, fine details and complex geometries don’t cost extra to print (unlike EDM for traditional tools), and designers are able to add in detail that would significantly drive up the cost of a steel tool. 

As 3D printed injection mold tooling continues to be adopted, it is important for both part and mold designers to learn how to best design for additive. A good design will drive success. There are plenty more tips on best practices for designing, machining, and running 3D printed injection mold tools. Join us and MMT for an upcoming webinar on  Best Practices for Use of Reinforced 3D Printed Injection Mold Tools this Thursday, February 20, 2020. Register today!