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Comment by ipsum2 | Hacker News Reader

Maybe this is too off topic, but I've always wondered why injection molding couldn't be done cheaper (albeit slower) by just milling aluminum to be used as molds.

Why is there such a large gap between 3d printing something and setting up an expensive ($50k+) and time consuming (months) injection molding station?

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Aluminum is used for injection molds:

https://www.protolabs.com/resources/blog/injection-molding-a...

https://www.moldmakingtechnology.com/articles/lets-be-clear-...

tl;dr: the molds aren't as durable, and can't do certain things as well as steel molds can. But for small volumes and simpler designs that doesn't matter, so they are quite commonly used.

Next logical step is 3d-print those Aluminum molds.

Why? CNC machining provides better quality and is faster. You can CNC a simple mold in minutes with precision that exceeds any 3D printing technology.

One possible reason would be to allow geometry which isn't easily milled. Perhaps cooling jackets, for example, could be incorporated into the mold for example.

Beyond the cooling jackets there isn’t really going to be anything hard to mill. If the geometry is hard to mill, then it’s almost certainly inappropriate for injection moulding (assuming you actually want to extract your injection moulded object from the mould).

Minutes is a stretch. It will likely take multiple tools and setups to make a mold, and even more if you're starting from a billet that needs facing etc.

I agree it is definitely a stretch for a full mold, but was thinking along the lines of an mold pocket insert, like those which can be 3D printed [1].

You could make this from reasonably sized stock with 2 setups. First you rough the shape and cut the mold cavity, then you remove the stock and face the backside. I'm used to an automatic tool changer, which would be needed if you want to go fast

https://www.pressebox.com/pressrelease/apium-additive-techno...

Is there actual 3D printing of metals? Last time I checked there was a very difficult process of 3d printing resin-on-dust, then pre-baking the dust-resin form, then soaking it in molten metal. Also it wasn't aluminium.

Yes there is 3D printing of metals. I’ve seen it predominantly in aerospace parts or rocketry

Did you read the article?? They printed a metal propeller for the boat.

High temperature resins can be 3d-printed and used for molds. These can be used for low-run injection molding or vacuumforming. Formlabs have some content on the topic: https://formlabs.com/blog/braskem-3d-printing-injection-mold...

At one point aluminum tools were a good trade off in time vs quality, but these days they only save maybe 10-20% of the tooling schedule and cost over steel. Tool life and robustness isn’t great so they’re not so popular any more. Usually when someone says “soft tooling” these days, they just mean softer steel which is slightly cheaper and faster to machine, at some trade off in tool life.

There are already 3D printed steel core and cavity inserts for injection molds. They laser sinter powdered metal and then run a very tiny ball end mill around each layer to clean up the surface finish. Traditional CNC and EDM is still how most molds are made, but occasionally it makes sense to 3D print. You can get cooling lines in patterns that are impossible to machine and you can make tools faster sometimes. The costs for the machines are very high, and the size of the parts are limited.

Why? You just 3D print steel ones.

You use the metal sintering printer to deposit the metal. Then you use an in-envelope milling head to polish it.

The cooling can be conformal since the shape isn't limited by milling technology. And, since molds have a lead time of at least 6 weeks (and generally worse because something always goes wrong), speed isn't the issue.

There was a good talk at Molding 2018 by the CEO of a Silicon Valley medical prototyping company about how his folks simply won't deal with conventional molds anymore and he has bought 3 of the 3D metal printers with in-envelope milling heads.

Unfortunately, the Molding conference website is a dumpster fire for useful technical information. If I can remember the name of the company, I'll reply.

It isn't that expensive for low volume molding. There are commercial vendors that provide molded parts from CAD in less 5 days for about $5,000. The molds are aluminum, so they have limited lifetime, but are great for volumes in the 10s of thousands.

When I want high tolerance parts, or hundreds of parts, I find it is faster and cheaper to go with contract molding despite having 50k of 3D printing equipment in my lab

Molds can be really complex tools, with moving parts and coolant ducts. Designing one isn't easy, and machining definitely also isn't.

Probably the material cost isn't the problem.

In addition, injection molding machines are huge and complex. They need to melt the plastic and move it into the mold without clogging up and with a repeatable high pressure.

Here's[1] a video with some discussion around the process of designing a mold for a firearm part, from someone who's trying to bring an injection molded product to the market. Here's[2] a video from the actual molding process.

This is obviously for mass production, so I guess you can cut some corners for low-volume production, but I imagine designing a fairly proper mold is still required.

I'm not into firearms but it was interesting to hear the complexities involved, was a lot of things I hadn't thought about.

[1]: https://youtu.be/QmhYMRNzFu8?t=256

[2]: https://www.bitchute.com/video/EMOOqo9G2pPd/ (loud!)

>injection molding machines are huge and complex

Or they can be small (benchtop), simple and inexpensive ($1000). https://www.minijector.com/

Early in my career I ran one of these machines to mold parts for implantable medical devices. I've also designed tools for them which can be easily machined yourself for a few hundred dollars.

Thermoforming (aka. vacuum / pressure forming) is the faster way to achieve large format shapes. You begin with the skin which can arrive either pre-cut or in rolls. You then heat it in a large scale oven usually based on ceramic heating modules. Once pliable at a polymer-specific temperature use your aluminium mould to obtain a desired shape in combination with positive and/or negative air pressure. The alumnium's conductivity assists with cooling the part to set the new shape. Then withdraw the mould.

For super high speed boat building, you could use a combination of this and maybe ultrasonic welding to rapidly fuse multiple sheets and build up a higher body. The challenge would be fusing to a superstructure while maintaining integrity.

Currently, this process is only used for dinghy-scale vessels.

I have read an article on using 3d printing in metal for the inner mould (plug) for alpine ski boots. I think this was just an experiment that the manufacturer was doing with some of their top athletes.

There are houses that will do small runs of plastics using cheap molds made out of Aluminum.

https://www.protolabs.com/services/injection-molding/plastic...

Far as I can tell if you're making a few hundred/thousand parts a year that's totally reasonable.

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