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Comparison of 3D printing plastic molds with traditional machining molds

update:01-11-2019
abstract:

Many 3D printing proponents claim that 3D printing is a […]

Many 3D printing proponents claim that 3D printing is a good alternative to injection molding that requires mold opening. Is this really true? Here we look at the relevant support and objections. Additive manufacturing (known as 3D printing) has had a huge impact on manufacturing. The hand pieces that used to cost a few hundred dollars and took a few weeks can now be designed in the morning, printed at night, and delivered to customers the next morning.
Some companies are already using the 3D printing process to make injection molds. It is no longer necessary to spend months waiting for the manufacture of the mold that can be used for production, or because of the downstream design changes that result in a large amount of money invested in modifying the mold, or uncertainty in the production layer. Whether it's mold validation or small batch production of injection molded parts, you can quickly print 3D molds. If there is a problem with the mold or if you need to modify the design, print one and repeat the verification or production. right?
These views have some truth. The plastic 3D printed injection mold is a bit like the plastic shed in our backyard. It is cheaper than the metal shed. The plastic shed is built quickly and performs well under low load. But if there is too much snow, they will break into pieces of a house.
3D printing molds have their own place, and some companies have been successful in the application of 3D printing molds. Proponents claim that 3D printing molds are up to 90% faster and 70% cheaper than traditional mold processing. In some cases this may be true, but it is important to understand the advantages/disadvantages of metal molds compared to 3D printed plastic molds.
Real mold, really fast
Rapid manufacturing company Proto Labs has been producing fast mold injection parts since 1999. It provides molds for the manufacture of parts such as engineering plastics, metals, liquid silicone (LSR) and other materials. The mold is mainly machined from aluminum (in some cases with steel), and can process several to 1000 parts, and the delivery time is 1-15 days.
Its industrial grade 3D printing services include photo-curing (SLA), selective laser sintering (SLS) and direct metal laser sintering (DMLS). Printable materials include thermoplastics such as polypropylene and ABS, industrial grade nylon and metals such as stainless steel, aluminum alloys and titanium alloys.
Since there is such a large range of processing capabilities, why not directly print the mold, but machine the mold?
Mould makers are wary
Proto Labs engineers have been considering printing molds, but after 16 years of rapid mold business, some reasons have forced them to adhere to a reliable rapid injection molding process:
1. Surface quality: 3D printing is processed layer by layer to obtain parts, which will result in a step effect on the surface of the product. There are similar problems with directly printed molds, which require machining or sandblasting to eliminate these small, toothed edges. In addition, holes smaller than 1 mm must be drilled, larger holes require reaming or drilling, and thread features require tapping or milling. These secondary treatments greatly reduce the speed advantage of 3D printing dies.
2, size factor: If you want to design a skateboard or plastic toolbox, 3D printing mold may be no problem. The part size is limited to 10 cubic inches (164 cubic centimeters), roughly the size of the grapefruit. And despite the high precision of current additive equipment, it is still not comparable to machining centers and EDM equipment. The latter's cavity is typically accurate to ±0.003" (0.076 mm) and parts are up to 59 cubic inches in size, roughly six times the volume of 3D printed parts.
3. High temperature environment: In order to ensure good material flow performance, the injection mold needs to be heated to a very high temperature. Aluminum and steel molds typically experience temperatures of 500 F (260 ° C) or higher, especially in high temperature plastics such as PEEK and PEI (Ultem) materials. It is easy to produce thousands of parts with these metal molds, and it can also be used as a transition mold before the final mass production mold comes out. Mold materials made using SLA or similar 3D printing processes are typically photosensitive or thermosetting resins that are cured by ultraviolet light or laser light. Although these plastic molds are relatively hard, they are destroyed very quickly under the thermal cycling conditions of injection molding. In fact, 3D printing dies typically fail within 100 uses in mild environments, such as high temperature plastics such as polyethylene and or styrene. For glass filled polycarbonate and high temperature resistant plastics, only a few parts can be produced.
4. Comparison Cost: The big reason for using 3D printing molds is because of their low cost. The cost of production-grade machining molds is typically $20,000 or more, which means that the same size as the $1,000 printing mold is comparable. However, this analogy is unfair, and the evaluation of the printing stencil usually only considers the material consumption, and does not consider labor, assembly and installation, injection systems and hardware. For example, ProtoLabsd's aluminum molds cost $1,500 for production. If you need to produce more parts? With 3D printing dies, you need to reprint and assemble new molds for every 50-100 products you produce. On the other hand, aluminum molds are still in good service in the production of 10,000 parts, regardless of the plastic used.
5, product design: the principle and practice of traditional injection mold manufacturing has been more than a century of history, the industry is relatively thorough research. The 3D printing mold is very new. For example, the draft angle must be greater than or equal to 5 degrees to meet most aluminum mold requirements. Plastic molds are challenging to mold plastic parts, and extra care is required for the number and mounting position of plastic mold thimbles.
Plastic molds (especially high injection temperatures) are somewhat more flexible in terms of increasing cavity wall thickness and reducing pressure. The design of the gate is also different, tunnel and dot gates should be avoided. Direct gates, fan gates, and wing gates should be increased to three times the normal size.
The flow direction of the polymer in the print mold should be consistent with the 3D print line to avoid high packing caused by stickiness and low pressure. The cooling system can increase the life of the mold to a certain extent, but it does not significantly reduce the number of cycles of the printing mold, because the heat dissipation capability of the plastic mold is not as good as that of the aluminum mold or the steel mold.
opportunity
Despite the many advantages of fast aluminum molds, in some cases 3D printing molds still play an important role. For manufacturers who have a 3D printer and have enough time to explore how the print mold works on the injection molding machine, they may think that the mold should be printed directly.
Of course, mold designers must understand how to make functional molds, and the cost of redesigning and manufacturing the molds. Relevant technicians and equipment are also necessary – mechanical workers for mold blasting, thimble installation, injection molding machine operators, etc., because the setting of these parameters is very different from traditional molds.
But wait - why not use DMLS? Why not directly print metal molds? DMLS uses lasers and precision optics to "paint" parts layer by layer on a small metal powder bed, producing fully compact commercial products for aerospace and medical applications. Some people predict that future molds for aluminum and die steel materials may be printed directly, providing ultra-efficient conformal cooling channels that will significantly reduce injection time and extend tool life. To some extent, DMLS direct printing molds are slow and expensive, usually only for very small, complex molds, or for mold inserts that are difficult to manufacture by conventional machining methods.
in conclusion
In general, Proto Labs believes it is best to use DMLS, SLA or other 3D printing processes to do what they are good at: printing parts without molds. However, 3D printing injection molds can be a reliable alternative if the following conditions are met.
1) For small batches and relatively simple parts, the product requires a relatively large draft angle.
2) The tool and mold design team is familiar with the design principles of 3D printing molds.
3) Personnel and equipment for processing and assembling plastic molds.
Final design considerations. If you need molds for long-term use, once the 3D printing molds verify the rationality of the design, the next step is to make molds with more permanent materials, such as aluminum or stainless steel, because plastic molds are mainly used in small batch production. Due to the different design of 3D printing molds and traditional molds, a certain number of mold redesigns and tests must be considered in the project time and budget.

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