Aluminum fixture for cutting molds

[rcferguson]

So I got the new pneumatic switch in yesterday, got it hooked up today and did some experimenting.

Had mixed results. I can get a full stroke out of it (as far as the machine will let it travel - about 4-1/2"), but with my compressor maxed out at 125 lbs the stroke is slow, and as the pressure drops the stroke gets slower. In some cases was able to fill my molds, but not consistently, and way too slow. I could get a stronger compressor... I suspect that it would zip along at 150 lbs, but I don't really want to go that route. Other options are to redesign the adapter with a lever/fulcrum to get the full 8" of travel out of the cylinder. That should almost double the pressure I could exert on the plunger, but the hardware I'll need to get that on will be pretty big and ungainly. The other thing I can try is to buy a bigger bore cylinder. This one is an SC63 x 200, that is, 2-1/2" bore x 8" stroke. I can get an SC 100 x 200, which is 4" bore x 8" stroke. I'm no math whiz, but that's about 2-1/2 times the area for pressure to act upon, which I assume would give me 2-1/2 times more power.

[rcferguson]

Got the new pneumatic cylinder in. You don't realize the overall size difference until you see them next to each other. The old one was SC63 x 200, the new one is SC100 x 200, so went from 2-1/2" bore to 4" bore. Should give me 2-1/2 time the force. Time to design an adapter for my adapter to make this monster fit!

[Derek the Admin]

That's a good sized cylinder. What does something like that go for if you don't mind me asking?

[rcferguson]

Mar 12, 2019 1:15:07 GMT Derek the Admin said:

That's a good sized cylinder. What does something like that go for if you don't mind me asking?

This was from Amazon, $118.

Next size up is SC 125. That one is $169.

Not too bad, really.

[rcferguson]

OK, redesigned the adapter hardware. I will have to rework the vertical chassis parts to make room for the new linkage, and make 5 new parts:

     

[rcferguson]

Progress on molds...

I actually made this mold before last week. The parts come out OK, but the mold isn't going to work because it is just to hard to get them out of the mold without damaging them. I refined the design for the part, and re-did the mold. They should come right out now, can't wait to try them.

The parts themselves work great, just too hard to produce them with the first mold. Learning a little more about mold-making every day!

     

The black injection molded part replaces the white resin part. As you can see, it is a lot smaller, but works just as well. With the resin part, it takes 15 minutes at a minimum to mix, pour, cure, and demold to get 3 parts (the mold had 3 cavities). Also, most parts had imperfections (bubbles, cavities, etc.) and because the resin cures very hard, the parts were hard to press onto the potentiometer. With injection molding I can get 2 parts per minute, use less material, get perfect parts every time, and because I'm using polypropylene, which is soft, the part is much easier to press onto the potentiometer shaft. Also the action of the gears is much softer because of the material... the action just feels a lot smoother.

[Bruce]

Thanks for all the detail on your work process. Really interesting.

[rcferguson]

Almost there... I have to bore some holes in the top mounting brackets to bolt on the cylinder, and I'll be ready to try it out tomorrow.

[rcferguson]

It's alive!

This thing definitely generates a LOT more pressure than the previous cylinder. I'm able to fill the molds a lot quicker and more completely. Still a few problems though...the baseplate is bending significantly under the pressure. With the hand crank, you're pulling pressure down at the front of the machine working against the support, so not technically putting pressure on the base plate. Here there is no weight at the front of the machine to offset the pressure of the cylinder. I'm afraid the base plate or the flange holding the support pillar at the back of the machine are going to give out under the stress eventually. Also, I'm losing some of the force of the ram from the flexing, so not all the pressure of the cylinder is acting on the injection reservoir. Gotta rethink this design, figure out how to support this cylinder from the front as well as the back.

  

I'm really proud of how this latest mold turned out. Makes a really nice part, and the part is easy to remove from the mold. Not perfect... seems I cut the bottom mold where the shaft of the potentiometer goes a little to small, so the hole in the part is too small for the potentiometer shaft to fit. I can rework the mold, but i could only make the hole in the part smaller, not bigger. I'm going to have to re-cut that mold. Fortunately it is a fairly easy operation.

 

[rowiac]

Do you have a draft angle on that mold? Or do the parts shrink enough that it's easy to remove them from the mold?

[rcferguson]

Mar 19, 2019 17:44:08 GMT rowiac said:

Do you have a draft angle on that mold? Or do the parts shrink enough that it's easy to remove them from the mold?

The top of the mold (with the gear teeth) has only a small hole in the center, and I have a 5 degree draft on that hole, so the part shrinks immediately when you open the mold and it cools, and doesn't stick at all. I imagine the conical shape of the hole probably helps push the part off the mold as the part shrinks. The bottom of the mold is where the biggest cavities in the part reside, so the part tends to stay attached to the bottom. I have a 3 degree draft on those cavities, so all I have to do is grab the end of the sprue and tug, and the whole thing comes loose.

 

[Derek the Admin]

Really great work Rob. It makes me want to get one.

[rcferguson]

So I re-cut the one side of the gear mold, but I'm not going to test it out until I get a new frame built for this injection machine. The base plate was bending enough that I'm a little worried about something giving away. The flange that supports the main pillar on the machine looks like cast iron, so I'd be a little afraid if it fractures it could create a lot of shrapnel if it failed catastrophically. I've designed a new support framework, all 1/2" aluminum with a 12"x12"x1" thick base plate. Probably overkill, but better too much material than not enough!

I've ordered the material... should be here by the end of the week.Will take a while to cut all these pieces, though it's mostly just holes. The horizontal supports will have to be milled out a bit for the support arm to fit in, and the end pillars will have slots cut in them to hold the horizontals. I can re-use the base for the pneumatic cylinder, and the linkage parts. I haven't designed it in yet, but I'm going to put in a gravity feed system the feed plastic pellets into the barrel of the machine so I don't have to manually fill the hopper every time... I'm thinking maybe a 2-liter pop bottle with the bottom cut of that I can fill up with plastic, and the spout of the bottle will go into a hole in the horizontal supports. I've also ordered some extra aluminum to make a couple low profile vises that I can clamp to the Carve King base more or less permanently. This cheap vise I'm currently using is better than nothing, but it stands so high that I don't have much Z to play with on my parts.

[rowiac]

You could try the Simulation function in Fusion 360 to verify if your support design is strong enough. It should be able to predict deflection under load.

[Bruce]

ibohan came up with a low profile vice that might work for you.
millrightcnc.proboards.com/thread/1593/tiny-vise-syndrome