CNC Mill for small run injection molds

[nebbian]

Hi,

I've been working on a product similar to Lego, which interlocks. There are a number of different bricks that I want to produce, and I've gotten them all interlocking nicely using a 3D printer and ABS plastic. The 3D printed product is nice for prototyping, but at over half an hour per brick to print, it's limited with regard to scaling up.

The next step is to machine some molds and go to injection molding. This is where you guys come in

The pieces are mostly about 30 x 15 x 10mm in size, with the largest about 90 x 15 x 10mm. I have around a dozen different designs, and require around 20,000 pieces made in total. The pieces have thin walls, and internal cross webs. Each wall is around 0.8mm width, and 6.2mm high. I'd need a bit that can reach down that far and be able to mill a slight taper into the wall (for ejection).

I'm looking at a 3 axis Taig mill at the moment, with a CNC conversion kit. I'm also looking at making a small benchtop injection molder to do prototype molding with. I built (and have designed many upgrades to) my 3D printers, so I'm no stranger to automation.

The problem I've got is that I know nothing about milling at all What sort of aluminium would you recommend for the molds? Is the Taig good enough to make injection molds? Is it possible to get milling bits that can create those 0.8mm slots that are 6.2mm deep? Am I looking at this the wrong way?

Any ideas welcomed.

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[tenaja]

For small runs, you can use anything...6061 is the cheapest, and will be fine for test runs or a few thousand parts. But it's gummy, so clearing the chips is critical. 7075 is harder (machines nicer), so it will last a lot longer, but it's 2-3 times the price. QC-10 transfers heat better so it's better for larger volumes.

You will likely be frustrated with a Taig. Get the heaviest machine you can afford, even if it's used. In machines, heavy typically means more rigidity, which is better overall.

[G59]

Well.............for what he's going to be doing, the Taig is perfect. It's rigid enough for aluminum even 7075. A big machine to mill out small parts in aluminum with super small cutters? For a beginner?

I would make sure you get a highspeed spindle especially if your going to use small cutters like 0.8mm. Just so you know, they make cutters that are much smaller than 0.8mm.

Shoot for a 20K RPM or better spindle, it wouldn't hurt. You didn't tell us how big of a mold you wanted to make other than small. What is small 150mmx 250mm? Then the Taig will be fine.

Remember spindle runout is super critical when playing around with small cutters.

Good luck.

[tenaja]

Taig perfect? No. Useable? Yes.

I've used a Sherline, and it was useable, and it's more machine than the Taig. The smile on my face when I went to a larger machine was priceless, because the larger machine removed so many barriers. It doesn't even matter how much bigger the machine is, it could be an EMCO, and it's miles better than either Sherline or Taig.

Even a G0703 or whatever it is would be way better than a taig. The taig is the bottom of the barrel, like deciding you are going to drive cross country, so you bought yourself a moped. If the adventure is the purpose, then go for it. If the result is the purpose, then anything else will get you there faster and better.

[G59]

The smile on my face when I went to a larger machine was priceless

What machine did you get??

If the OP did go larger, then he has to get a side mounted spindle for high RPM. A G0703 has what, a 4000 rpm spindle? It would take him days to mill a 0.8mm slot into anything.

He's making molds for small intricate parts with tiny/small cutters. How much power do you think he needs? How rigid of a machine does he have to get to handle a 1-4mm endmills.?
I wonder if my dentist has a huge Hurco in the back milling away at a tiny mold because bigger is better.
A Sherline is more machine than a Taig?? How??? Aluminum vs cast iron?? That makes no sense.

I know for a fact that larger doesn't always mean better. With your thinking all the little mills are a waste of time and there's no reason for them to be out there. But in reality, every machine has it's place.
Some to learn on, others to work with.

For you, they weren't big enough. That's all.

[tenaja]

If the taig mill he is referring to is cast iron, then I'll take back what I said about Sherline being more. I thought the taig was the other aluminum machine that was even smaller.

There is more to evaluate rigidity than the size of your cut. If you look at a lightweight machine wrong, or breath heavily on it, it bends out of shape. You certainly cannot accidentally bump them, or you will get a witness line in your mold. I'd take a bigger, more rigid machine with a slow spindle over a lightweight machine that deflects with 5lbs of pressure any day, especially for low volume mold making.

I did not say a small machine is a total waste of time, I said bigger is better. There is almost no situation where smaller is better, unless you are talking about your pocketbook or your work space is already overflowing.

[G59]

There is almost no situation where smaller is better, unless you are talking about your pocketbook or your work space is already overflowing.

I agree.

I just checked the Taig website. It's a steel construction with an Aluminum table and priced for well over $2200.

That for me is a big No No. Those are two disimilar metals that are known to suffer from galvanic corrosion between each other when wet. I don't recommend. Sorry Taig, but my engineering degree tells me, bad idea.

To the OP, spend the extra cash and get a 'bigger or better machine'.

[nebbian]

Thanks to all for the replies. I've been digesting them over the past few days.

Just so we're all on the same page, the mill I'm considering is this one:
Taig Tools - Desktop Milling Machines and Lathes.

I was a bit confused at some of the replies telling me I should choose a bigger mill, as I posted it in the desktop milling section. But then I figured out that a moderator moved it to the moldmaking section

When browsing this section, I came across this post:
http://www.cnczone.com/forums/bencht...tml#post265553

Funnily enough I've come across this guy before, when looking at what others in the same niche market are doing. He ended up getting a Taig So I guess they're not such a bad machine for what I'm after.


I'm still not convinced that I should go down the home injection molding route either, so I'll keep lurking and learning in this forum. Thank you to all who responded.

[hanermo]

I respectfully suggest that You are looking at this all wrong.

If You have a hobby doing molds, and building diy stuff, by all means go ahead.

If you want to start getting pieces done, look at commercials costs .. then look at duplicating commercial capabilities yourself.
Look at Your costs for doing the same.

A tiny, obsolete, commercial molder might make 40 of the described pieces, per shot, about 2 secs / shot, so => 20 pieces/second.
Commercial cost == 0.01$ / piece.
(Much) less with new, volume, product.

So, if you want to actually make the "lego type pieces" yourself in qty you must then produce stuff cheaper and faster than the commercial one, as otherwise its much cheaper to buy them from the commercial makers, no ?

No question You can, technically, make a small molder.
And the molds.
And the machine to make the molds.

Look at tech speed.
A tiny, "home" injection molder, very slow say 30 secs / shot, at say 10 pieces per shot, = 2 / min => 1200 per hour.
Obsolete-old-commercial => 72.000 per hour.
60 fold difference.

If you as operator cost 8$/hr + your machine costs 2$/hr, it takes you 60 hours to make 72.000, => 600$.

The limiting basis is the commercial cost of product.
Unless you can make stuff better (as good) + cheaper (same price ) and (optionally) faster, why bother ?

[nebbian]

Thanks for your input. As per the first post, I only need ~20k pieces... Does this change the maths?

Complicating things is that the parts I'm designing have a small overhang, which I've come up with a solution for. It's not a conventional solution, and I need to make some prototypes to test this process.

[rowbare]

I think you need to rethink your mold somewhat. Slotting .8 mm that deep will cause you no end of grief. You should see if making that mold half in two parts is feasible i.e. a base plate to hold the pins and the actual part detail. That would save you a lot of very tricky machining. You would put the draft angle on the pins.

bob

[awerby]

I like Rowbare's suggestion. Making those deep slots with a very skinny tool isn't going to be much fun whatever mill you use (although one with a faster spindle speed will work better). You'd have to use a smaller tool than the width of the slot, and deflection would mean that the width would be smaller at the bottom than at the top. If you're really set on doing it that way, buy your endmills in the jumbo pack - you're going to be breaking a lot of them. If a mold like that was being made commercially, it would probably be done in reverse; cutting the part positive in graphite and using EDM to burn it into the steel mold blank.

As to the Taig mill, it's a bit bigger than the Sherline. (I sell both of them, and am pretty familiar with them.) It's not made from cast iron; as G59 points out, it's made from steel and aluminum. But the aluminum table rides on brass gibs and steel prismatic bars set in a massive aluminum block that's mounted to the square steel tubing frame underneath; I've never heard of galvanic corrosion being a problem with them, but if it was, it would attack the brass gibs, which are replaceable.

[tenaja]

Actually, the tool deflection at the top and bottom of a slot will be the same (with the same depth of cut), unless your coolant is deflecting the tool. A molding slot will require draft, and that will give clearance so the tool is not rubbing the full length. Alternatively, if it essential to cut a slot without draft, you can get a tool that has a relief, so it only rubs for a short distance.

For legos, you'd typically make the cavity on the hot side, and the core on the cold side, so there are no deep slots.

[nebbian]

Actually, the tool deflection at the top and bottom of a slot will be the same (with the same depth of cut), unless your coolant is deflecting the tool. A molding slot will require draft, and that will give clearance so the tool is not rubbing the full length. Alternatively, if it essential to cut a slot without draft, you can get a tool that has a relief, so it only rubs for a short distance.

For legos, you'd typically make the cavity on the hot side, and the core on the cold side, so there are no deep slots.

Thanks Tenaja.

Here is a picture of the underside of a typical piece. How would you go about milling a mold for this without deep slots? The depth of the webbing is about 6.2mm.

[G59]

Ok well since Tenaja hasn't answered you, so here is a self explanatory image of how this is done with minimal tooling.




You see, you have to think out of the box sometimes.

Of course the 2nd plate is closed and has the reverse image or pattern of whatever the top of your piece looks like. I left it open in this example so you get better look at how its done.

Have fun!

[nebbian]

Thanks for that. Yes, I was going to do exactly what you had shown. However there are still many deep slots that need to be cut in the grey base plate, unless I'm mistaken?

[G59]

That's OK. The image above shows open slots meaning the chips have a better chance of evacuating. Deep pockets or slots are not a problem if the chips can leave the slots. Wether you flush the slots with flood coolant or use pressure air stream, as long as your not constantly recutting the chips, you'll be OK.
Of course, there are other options out there. EDM being one of them.

There are no free lunchs out in the mold making industry. No matter how you approach this, it will cost you either in time or you can bite the bullet and have it done by someone else. And I'll warn you, having it done by EDM will be expensive.

It's just the nature of the business.

[G59]

Just out of curiosity I simulated the machining of the 1st plate in Featurecam and using 20000rpm spindle it took 7 hours 34 minutes in 6061 AL.

[nebbian]

Thanks for that G59, I understand now. I hadn't considered evacuation of the chips before.

There's a lot to learn with regard to mold making!

[tenaja]

Just out of curiosity I simulated the machining of the 1st plate in Featurecam and using 20000rpm spindle it took 7 hours 34 minutes in 6061 AL.

So, with a hobby machine at 4,000 rpm (some can't even get that), you are at over 40 hours. Still very possible, but I would make sure to have a reliable UPS to protect against power glitches.


Especially with aluminum, the ONLY purpose of coolant is chip evacuation. The tool simply does not get hot enough to require coolant...but if the chips stay at the tool, they give you a bad finish--if you are lucky--when your tool hits them. If you are not lucky, then the tool breaks, or worse, breaks and gets friction welded in place.