Detailed Introduction to 3D Printed Molds

Injection molding is the most popular plastic parts manufacturing process worldwide. Injection molding uses durable metal molds that can produce hundreds of thousands of parts in a short period of time. And molding is not expensive. At least, it is not expensive to inject a dose of material into the mold: the process is very fast, and the pellets are a cheap plastic. But the molds themselves are expensive, and most molds are made of high-quality metal and manufactured by hand or CNC machining.

For prototypes and short-term manufacturing, professional molds can be far beyond a company’s budget. Even the cheapest aluminum molds (made by rapid tooling) are still too expensive for some projects. One way to make injection molded parts at a very low cost is to simply 3D print the mold. These plastic molds don’t last very long, and they certainly can’t be used for mass production, but they work well for some rapid prototypes. This article explains how and why to use 3D printing to make plastic molds for injection molding.

1.Why use 3D printing to make molds?

3D printing is not widely used as a mold making tool. Instead, it is most often used for rapid prototyping and small batch production of custom parts. However, if the budget is limited, 3D printing of injection molded parts offers some possibilities. Additive manufacturing is very low-cost, has minimal setup costs, and can be used to make one-off items such as molds.

3D printed molds are far less sturdy and durable than the most fragile machined metal molds, but they are adequate for certain prototypes made from certain materials. For the vast majority of cases, Xavier recommends using CNC machining to create injection molds.

2.What industries use 3D printed molds?

Some industries frequently use 3D printing for mold making, but rarely for injection molding. For example, jewelers often use stereolithography (SLA) 3D printers to assist in the investment casting process. After 3D printing a model of a piece of jewelry, jewelers can use the model as a positive mold to create a mold. The 3D printed model can then be burned away, leaving only the mold, which itself can be used to create metal jewelry.

The dental industry also uses 3D printers in a similar way. Dental professionals can now generate 3D models of a patient’s teeth and then use 3D printers to create reverse models of those teeth. The printed model can be used as a mold and can be used to make patient-specific dental aligners and retainers using soft, medically safe materials.

3.What kind of 3D printer is best for making molds?

FDM 3D printers (the most common desktop 3D printers) are not particularly suitable for making molds. This is because FDM-printed parts usually have rough surfaces and layer lines. Better AM technologies for mold making include material jetting 3D printing and stereolithography 3D printing. Both technologies have a high level of detail and accuracy, as well as a good surface finish that facilitates smooth flow of material during injection.

4.3D Printing Materials for Molds

A range of 3D printing plastics can be used to make injection molds. However, the material must have a high enough temperature resistance to still maintain its shape when the molten plastic is injected. The higher the melting temperature of the injected plastic, the higher the heat deflection temperature of the mold needs to be. For material jetting, a widely used mold making material is Stratasys Digital ABS, which has a heat deflection temperature of up to 95°C. Stratasys says the material is “ideal for rapid prototyping of high or low temperature snap-in parts, functional designs with multi-material versatility and flexibility, molds, manufacturing tools, electrical components, and more.”

For SLA, a common mold-making material is Formlabs High Temperature Resin. Formlabs says, “Both clear and high temperature resins can be used to print small functional molds, with high temperature resins being compatible with a wider range of thermoplastic melt temperatures… The relatively high stiffness of high temperature resins means the mold will not deform when the part is removed.”

5.Designing a 3D Printed Mold

When designing a mold for 3D printing, there are two things to consider: whether the design will print well, and whether the printed mold will function properly when molten plastic is injected into the printed mold.

For a mold to print well:

1. Place support structures outside the mold, away from the mold cavity. This ensures that defects caused by removing supports do not appear on the molded part.
2. Print at the highest resolution and lowest layer height to reduce step effects on surface features.
3. Include vents to reduce the chance of trapped air and bubbles.

For effective molding:

1. Keep walls thin and thickness consistent throughout.
2. Add draft angles to facilitate part ejection.
3. Include radii; no sharp edges or corners.

6.Framed or frameless?

A great way to extend the life of a 3D printed mold is to use an aluminum frame. The printed mold is placed inside a metal frame, which protects the more fragile plastic mold from the pressure and heat generated by the injection molding machine nozzle. Because the mold fits snugly inside the aluminum frame, it also does not warp after multiple injections. In addition, the frame contains a channel that connects the injection nozzle and the gate of the printed mold, eliminating direct contact between the nozzle and the mold.

An aluminum frame is more durable than the 3D printed mold placed in it. This means that manufacturers can use a single aluminum frame to make multiple different molds, which in turn can be used to produce multiple molded units.
It is possible to use a printed mold without a frame, but this requires more 3D printing material and makes the mold more susceptible to warping.

7.Alternative Methods

For businesses with access to SLA or material jetting 3D printers, injection molding using 3D printed molds may be a good option. However, low-volume molded plastic parts can also be achieved with vacuum casting. Vacuum casting uses inexpensive silicone molds to quickly create plastic prototypes. Like 3D printed molds, silicone molds have a short lifespan. Vacuum casting may be a better option because it produces molded parts with an extremely smooth surface finish.