In this article, I’ll talk about why 3D printing is so cool and why it probably won’t ever replace injection molding. You might think the title of this post is a contradiction, and you’d be right, but it’s true, and I promise this is not a clickbait hobby. One of my customers recently pointed out that a budget 3D printer on sale is only a little bit more expensive than a video game. This is the first time in history that this has been true. If you have an idea, it’s likely that you can make it on your own. So this is a great time to be a maker or think about becoming one. So why won’t 3D printing become the standard way to make consumer goods? Well, the most common way to do things now is called “injection molding”.
Injection Molding Machine:
Almost 20 years ago, injection molding machines could make a huge number of parts very quickly. Now, when I graduated from high school, I went to college for industrial design, and a big part of what I learned there was how to design parts for injection molding. Even though I haven’t done it in a while, let’s try to figure out why it’s still better than 3D printing.
Let’s start with something very easy. It’s just a case for some electronic equipment. To injection mold this, it’s basically just a square prism that’s hollow on the inside. We need some molds, and here’s a simplified version of what they would look like. The simplest molds will have two halves, usually called a male half and a female half, and they might have some pins to keep everything in place.
How it works
As you know, one half has a hole and the other has a protrusion. When we put these two halves together, we have a mold. Now, if we put everything together and look inside, we can see that the plastic will be injected into real molds through a runner that runs along the inside. It’s common to have more than one place where the plastic is injected, but for this example, we’ll keep it simple. The molten plastic is put into the cavity and forced through with enough pressure to fill the extent before the mold pieces are actively cooled.
When the plastic inside hardens, we’re ready to push out our part. At this point, the machine would split in half. The part would be pushed out of the mold by the ejector pins, which would then retract. So the part was free to come out. The only problem is that this part, as we’ve shown it so far, would have a hard time coming out because its vertical sides would rub against the mold’s vertical sides.
So, we need a small difference, which we can make by giving each of our vertical surfaces an angle. We call this angle a “draught.” The change in angle is only a couple of degrees, but after the plastic is injected, it will make all the difference. This tiny angle will now help the parts come out nicely. Now, the slope in my diagrams is only a few degrees, but we can better understand this idea by doing it with our hands. We can fill the friction between two parallel surfaces by sliding them back and forth. But if we change that to a triangle, we’ll see that as soon as they come apart, there’s no friction because they’re no longer touching the mold. The ejector pins can do their job, and our part will be free. The problem is that the part we’ve been looking at so far is much too simple.
It’s almost useless, and if you wanted to make something like that, you’d probably use vacuum forming. Let’s make it more like a real product by adding some bosses with holes for self-tapping screws and ribs to support them. If we look at the back, we can see that this type of product usually has a logo that is pressed into the surface. Even though this is a complicated mold, we can still keep a two-piece mold on one half of it. We need a protrusion to match the raised logo, and we need cutouts to match the geometry on the other side.
Less time is needed to make a mold:
If we look inside the mold again, we can see that the outer surfaces still have draught, and if we move our section back to where the ribs are, you should be able to see that the ribs, bosses, and holes all have draught as well. So, what does this mean? It means that we’ve reached the holy grail of designing for injection molding, which is a two-part mold. So, the mold design needs to take as little time as possible.
When you can make a part so that it only needs a mold with two halves, you’ve done a good job. This is a fairly simple two-part mold. By making more than one part at a time, we can do a lot more with our time. Instead of using the mold to make just one part, we use it to make a grid where many parts are made next to each other. Now, for a part this size, you probably wouldn’t make as many at once as I’ve shown, because the mold would be too big. Also, keep in mind that all the water cooling channels, all the sprues, and runners to feed the plastic, and mount it to the actual machine add to the complexity.
Everything has to be able to be broken down into smaller, simpler pieces. But we can shape a lot at once, which is a ridiculously efficient way to work. As an example, the fact that dozens of bootle caps are made every few seconds is very impressive. The machines and shows are more realistic, but it’s probably still too easy. So what if we make it slightly more complicated, just like the real thing, by adding some holes on the side, just like you would find on real products where you need to plug in cables or mount LEDs or switches on the side of the part? This seems like a small, harmless change.
There are huge repercussions, and one of them is that we can no longer use molds with two parts. Imagine that the mold is now over the edge of this opening. When we try to take the part out, it will get caught and break into pieces. Because of this, we need to add a piece to our mold that comes in from the side and goes back out as the first step of the process. Let’s put everything back together and look at how ejecting the part works.
3D Printing technology:
3d Printing is the modern version of injection molding production. If you have an idea in your head, you can probably make it happen without putting in too much money, which is really exciting. The fact that we don’t need mold is another benefit of 3D printing. This saves us a lot of money. But it also means we can make things for which there is no way to make a mold since things like this chain mail are printed in place and already put together.
There’s no way you could fit all of the parts of an injection molding tool around all of these little details. The same goes for this print-in-place dagger. Even parts that don’t move, like this lattice cube, are too small for the tools to fit in between them. When we don’t need molds anymore, we won’t be able to do injection molding. We save money and time, and we only need to know much less about cad. We don’t need to know everything about injection molding. We just need to know simple things like making sure the base is flat and avoiding overhangs. 3D printing is great, right? Well, there are some bad things about it.
What’s bad about 3D printing:
When compared to injection molding, the main cost is time. We know how fast high-volume injection molding is, and if you’ve ever used 3D printing, you know that some parts will take hours or even days to make instead of seconds. When it comes to business, money talks, so injection molding will be the winner.
Not to say that 3D printing can’t be useful. We saw earlier in 2020 that when there wasn’t enough safety gear, makers from all over the world could work together and feel a shortage. Since I didn’t have to buy any tools in advance to make the parts I needed, injection molding is completely out of reach for consumers.
Home 3D printing of resin is getting better, and high-temperature resins make it possible to make a good mold and get this thing going. You can expect another article soon, so my goal with this one was to give you ideas. If you have a 3D printer, you have a lot of options, even if you don’t design your own parts. For example, it would be impossible to make something like this print in place, moving the clamp with injection molding.