Injection molding is the most widely used method for mass manufacturing plastic parts. Interestingly injection molding has its origins with the game of billiards. The balls used in the game were made from the ivory tusks of African elephants. To avoid killing the majestic animals, a billiard ball manufacturer decided to announce a $10,000 prize, which was a lot of money back then, to anyone who could come up with a replacement for ivory billiard balls. This resulted in the development of one of the first plastics and an apparatus to mold it into shape.
The concept of injection molding is simple. Inject molten plastic into a mold, allow it to solidify as it cools and open the mold to pop out the part. The part of the process that takes the longest time and most amount of work is development and machining of the metal tooling for the mold. This can take weeks. Also since the mold has to be opened to pop out the part, you need to follow certain design rules when designing the part. You can’t design a part for which a mold cannot be built.
Injection molding works best when you need a large number of parts – thousands or hundreds of thousands. It makes sense to spend all that time and energy to design the mold and machine it. But if you need a small number of parts, say a few hundred, then vacuum casting offers a better solution. If you are new to vacuum casting, watch this video to understand the process.
Vacuum casting needs a master pattern from which the silicon mold is created. For many years now 3D printing has been used to create the pattern. The 3D printing technology mostly used is Stereolithography (SLA) mainly due to its superior surface finish. As you can see in the video above, vacuum casting is quite a labor intensive process. So there have been attempts made in the past to use 3D printing to directly print the parts instead of vaccum casting or injection molding them.
Here are a couple of examples. Back in 2010 we printed 1,000 20 x 10 x 5 mm parts in 5 hours on a SLS 3D printer. In the end, it turned out to be 18 seconds per part.
In 2013, we printed 640 80 x 80 x 100 mm parts in 12 hours on a SLA 3D printer. That worked out to be 68 seconds per part.
Up until now adoption of 3D printing as an alternative to vacuum casting or injection molding has been limited mainly due to two reasons – time and materials. It takes just too much time to 3D print parts, especially if you want a production grade surface finish. Even if we are not restricted by time, most materials have not been production grade materials. This means they either don’t have the necessary mechanical properties to handle a production environment or they aren’t environmentally stable to last long enough without deteriorating. Since most of the materials are UV curable resins, they tend to deform or weaken over time with prolonged exposure to direct sunlight. So these parts had to be used indoors and/or for a short amount of time. This is why plastic 3D printing didn’t really find a seat at the plastic mass manufacturing table and it restricted itself mainly to prototyping.
In the next part of this series I will talk about how the 3D Systems Figure 4 Direct Digital Molding is changing the game. How it has the potential to completely replace vacuum casting while becoming a viable alternative to injection molding for short and medium run production of plastic parts.
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