3D Printed Molds vs. Traditional Molds

By now, 3D printing has made a huge impact on the manufacturing industry. Handmade parts that once cost hundreds of dollars and took weeks to make can now be designed in the morning, printed at night, and delivered to the customer the next morning.

Some companies are already using the 3D printing process to make injection molds. No longer is there a need to spend months waiting for a production-ready mold to be built, or a large monetary investment in modifying the mold due to downstream design changes, or uncertainty on the production floor. Whether it's for mold validation or small production runs of injection molded parts, molds can be 3D printed quickly. If there is a problem with the mold or the design needs to be modified, just print another one and repeat the validation or production. Right?

There is some truth to these points. Plastic 3D printed injection molds are a bit like the plastic sheds we have in our backyards; they're cheaper than metal sheds, and plastic sheds build quickly and perform well under low loads. But if there's too much snow, they'll break into a houseful of pieces.

3D-printed molds have their place, and some companies are having more success with them. Proponents claim that 3D printed molds are up to 90% faster and 70% cheaper than traditional mold processing methods. This may be true in some cases, but it's important to understand the advantages/disadvantages of metal molds over 3D printed plastic molds.

Real Molds, Real Fast

Rapid manufacturing company ProtoLabs has been producing rapid mold injection molded parts since 1999. It offers molds to make parts in engineering plastics, metals, liquid silicone rubber (LSR), and other materials. The molds, which are primarily machined from aluminum (and in some cases steel), can be machined from a few to 1,000 parts, with a shipping time of 1-15 days.

Its industrial-grade 3D printing services include light-curing (SLA), selective laser sintering (SLS) and direct metal laser sintering (DMLS). Printable materials include thermoplastic-like materials that mimic polypropylene and ABS, industrial-grade nylon and metals (such as stainless steel, aluminum and titanium alloys).

With such a wide range of processing capabilities, why not just print the molds, but machine them to make them?

Mold makers when alert

ProtoLabs engineers have been considering printing molds, but after 16 years in the rapid tooling business, a number of reasons still compel them to stick with a reliable rapid injection molding process:

01Surface quality

3D printing gets the part machined layer by layer, which can result in a product surface with a step grain effect. Directly printed molds suffer from a similar problem, requiring machining or sandblasting at a later stage to eliminate these fine, toothed edges. In addition, holes smaller than 1mm must be drilled, larger holes must be reamed or drilled, and threaded features must be tapped or milled. These secondary processes largely diminish the speed advantage of 3D-printed molds.

02 Size Factor

If you're designing skateboards or plastic toolboxes, 3D printed molds may be no problem. Part size is limited to 10 cubic inches (164 cubic centimeters), roughly the size of a grapefruit. And as accurate as current additive equipment is, it's still not comparable to machining centers and EDM equipment. The latter typically process cavities with an accuracy of ±0.003 inches (0.076mm) and parts up to 59 cubic inches in volume, roughly six times the volume of a 3D printed part.

03High-temperature environments

To ensure good material flow properties, injection molds need to be heated to very high temperatures. Aluminum and steel molds typically experience temperature environments of 500F (260°C) or higher, especially when processing high-temperature plastics such as PEEK and PEI (Ultem) materials. It's easy to produce thousands of parts with these metal molds, and they can also be used as transition molds until the final mass production molds come out. Mold materials made using SLA or similar 3D printing processes are typically photosensitive or thermosetting resins that are cured by UV light or laser. These plastic molds, despite being relatively hard, break down very quickly under the thermal cycling conditions of injection molding. In fact, 3D printed molds typically fail in less than 100 uses in mild environments with high temperature plastics such as polyethylene and or styrene. For glass filled polycarbonate and high temperature resistant plastics, only a few parts can even be produced.

04Compare costs

One of the great primitives of using 3D printed molds is because of their low cost. Production-grade machined molds typically cost $20,000 or more, meaning that a $1,000 printed mold is a like-for-like comparison. But the analogy isn't fair, as printed molds are often evaluated on the basis of material consumption alone, not labor, assembly and mounting, injection systems, and hardware. For example, ProtoLabsd's aluminum mold can be used for production at a cost of $1,500. If you need to produce more parts? With 3D printed molds, you'll need to reprint and assembly machine test new molds for every 50-100 products you produce. On the other hand, aluminum molds typically remain in good service for 10,000 parts produced, regardless of the plastic used.

05Product Design

The principles and practices of traditional injection mold manufacturing have been around for more than a century, and the industry has studied them more thoroughly. 3D printed molds are very new. For example, the draw angle must be greater than or equal to 5 degrees to meet most aluminum mold requirements. Plastic molds for injection molded plastic parts are challenging, and extra care needs to be taken with the number and location of ejector pins in plastic molds.

Plastic molds (especially with high injection temperatures) are somewhat more flexible when it comes to increasing cavity wall thickness and reducing pressure. Gates are also designed differently, and tunnel and spot gates should be avoided. Direct, fan and wing gates should be increased to three times their normal size.

The direction of polymer flow within the print mold should be aligned with the 3D print line to avoid sticking and high fill due to low pressure. Cooling systems can improve the life of the mold to some extent, but will not significantly reduce the number of cycles of the printed mold, since plastic molds do not dissipate heat as well as aluminum or steel molds.

Timing

Despite the many advantages of fast aluminum molds, there are some situations where 3D printed molds will still play an important role. For manufacturers who have 3D printers and have had enough time to explore how printed molds would work on an injection molding machine, perhaps they think they should just print the molds.

Of course, mold designers must understand how to make functional molds, which are costly to redesign and make. Relevant technicians and equipment are also necessary - mechanics for sandblasting the molds, ejector pin installations, injection molding machine operators, etc., because the parameters are set up very differently than in traditional molds.

But wait - why not use DMLS? Why not just print the metal molds? DMLS uses lasers and precision optics to "paint" parts layer by layer on a bed of fine metal powders, producing completely dense commercial products. DMLS is widely used in aerospace and medical applications. Some predict that in the future, molds made of aluminum and die steel may be directly printed, providing ultra-efficient follow-through cooling channels that will dramatically reduce injection time and extend mold life. To some extent, DMLS direct printing of molds is slow and expensive, and is typically used only for very small, complex molds, or for machining mold inserts that would be difficult to create through traditional machining methods.

Measured and reliable

Overall, ProtoLabs believes it's best to use DMLS, SLA, or other 3D printing processes to do what they're good at: printing parts rather than molds. However, 3D printing injection molds can be a solid alternative if the following conditions are met.

1) Small batches of relatively simple parts, where the product requires a relatively large extraction angle.

2) The tool and mold design team should be familiar with the design principles of 3D printed molds.

3) Have the personnel and equipment to machine and assemble the plastic molds.

Final design considerations. If you need the mold to last for a long time, once the 3D printed mold validates the design, the next step is to make the mold in a more permanent material, such as aluminum or stainless steel, since plastic molds are primarily used for small-volume product production. Because 3D printing molds and traditional molds are designed differently, the project time and budget to consider a certain number of times the mold redesign and testing.