Wax piston sprue extruder?

Yep that's the plan!

The thrust bearing is just to prevent the bacon pressure from damaging the gearbox.

 

Mark, the wax we are using is anything but slick. It's the stickiest stuff you've ever seen. The warmer it gets, the more sticky it becomes. You should see my work surface... All skid marks. The only way to get rid of it is with a razor blade or a bunch of heat. It wipes up then with a rag.

 

Wax no mater what the consistency will still be a lubricant when crushed between 2 mating surfaces. Bronze on steel is not a bad bearing. The wax would only improve lubrication. The problems with lubricants happens when all the lubricant is displaced by pressure or destroyed by heat.
In the extrude Zapin is building. It will be interesting to see how much pressure is exerted on the back of the screw. It will all depend on the viscosity of the wax at temperature.
I have a feeling this extrude will far exceed any needs he may ever have even if he sells his final product. As a wax extrude. It is going to be very capable machine.
Joe

 

From what I can see the axial thrust is in the opposite direction away from the wax nozzle so the bearing at the nozzle end just needs to keep the screw in position against sideways radial forces so it doesn't hit the wall. Waxes have long been used as dry lubricants and if the wax is at all paraffin based, then it's just a longer chain molecule version of mineral oil. The pressures involved are going to force it into any small crevices and hopefully keep it warm and more fluid. It's a very low RPM application so a bronze bushing is ideal compared to a sealed rolling element bearing getting filled with wax and not functioning properly. If it did somehow fail the shaft hole could be readily modified to take a rolling element bearing, just my two cents worth.

Edit: I just reread your post about having a thrust bearing to take the load off the gearbox. I misunderstood your question as the screw photo shows what I imagine is the nozzle end. At the gearbox end a flange on the screw driving shaft with an off the shelf roller thrust bearing would easily take the screw thrust load, it could be outside of the screw assembly well away from the wax.

 

How could I mount the bearung outside the case?

My plan was to cut the back off the hydraulic cylinder and weld a thick steel plate on the back to take the pressure from the screw. Then on the inside mount a thrust bearing.

I'll need to search for the right bearing so the shaft can pass through the middle of it.

Do you think I need a bearing or could the back of the bronze screw just push up against the plate?

The wax is 98% paraffin if I remember right. With some additives and recycled wax from burnouts.

I'm hoping no matter how I mount the screw and bearing that no wax squirts backwards. I don't think it should but I guess you never know.

What kind of tolerance do you think is reasonable to shoot for between the ID of the cylinder and the screw? I read on another forum that 1 to 2 mm gap would be fine, but this seems fairly large to me. Wouldn't such a large gap reduce the pressure and let wax go backwards?

 

After a bit of drawing and thinking about the application, you could dispense with the radial bearing on the screw near the nozzle and just rely on the screw being a snug fit for the barrel. I think you should aim for just enough clearance to allow it to turn freely when a bit of oil is used but not rattling around (ten thou?): the expansion coefficients of bronze and steel are within a few microinches per degree F of each other so it would have to be a very close fit for binding to occur when hot. I would opt for a flat roller bearing or maybe a shallow angle tapered roller that can handle high thrust forces: some Landcruiser 4WD's have a 45 degree cone tapered roller in the front diff swivel hubs that is cheap $15.

 

Ohh I see. Yes I like that design better! It allows me access to the guts when I need to do maintenance instead of having to cut the back off or figure out how convince the home depot geniuses to thread a 3.5" OD hydraulic cylinder for me.

Do you think it will leak out the back if left open like that?

10 thou hmm. I'll have to convert that to mm I have no concept of thousandths haha. Is that 10 thou per side (20 total?) Or 10 total?

I was also thinking of a tapered cone thrust bearing. I'll get one with a 7/8" bore so my 7/8" shaft can go through it.

Looks like I'll need a spacer collar between the screw and the bearing to transmit the force to the bearing.

I printed the screw at 80 mm. To allow for shrinkage and machining. The bronze cast is 78 to 79 mm currently. I think the ID of the cylinder is 3" (76.2 mm) so there should be enough to spare.

 

Where's the heater? You can't extrude this stuff cold. It will be crumbly.

 

The heaters are band heaters that strap around the outside of the hydraulic cylinder. There's definitely going to be a lot of fiddling with temperature to figure out the best temps for extruding without liquefying the wax too much or trying to extrude a solid block.

I've got 2 PID controllers to help control 2 different zones. Basically main barrel/preheat zone temp and final extruding nozzle temp.

 

There ya go.

 

There ya go. Pic for you.

The far right smaller 2" tube is on it's own independent PID.

I'll probably end up chopping the left 3 or 4 inches off the cylinder and removing the piston. I can't seem to get it out any other way. The bastard won't budge. The tube needs to be shorter anyway.

 

This bearing looks decent. Let's me lock the cone onto the keyed shaft and I can use a set screw to secure the bearing outer collar in place rather than press fitting it into a tightly machined socket.

I don't know what 34,900 newton max load is but I'm sure that's enough to snap a thick tree in half.

https://www.ebay.com/itm/BRI142616-...110578?hash=item1e79f4e1b2:g:itQAAOxyvSVREhsV

 

In answer to you questions: 0.010" is about 0.04 millimetres which should allow for expansion differences and the fact that the tube bore is not going to be perfectly round or super straight. You just want the bare minimum to allow the screw to fit easily but not flop around. I think the leakage will be minimal as the screw will tend to move the wax back into the bore not to mention that end will be cooler than the warm end. The bearings you have selected have no axial thrust capability, but have strong radial capability like you'd need for a wheel on an axle. These are more like what I had in mind:





This bearing has barrel shaped rollers so will be self aligning to some degree to tolerate a degree of misalignment of the shaft flange and fixed base.

 

The end of the cylinder looks to be threaded. It seems to me you'll be money ahead to just use the current packing gland to support a bearing. You can put a sleeve inside to take up the extra 3" instead of cutting and welding something on which may not be that straight after it's welded.

Don't cut the cylinder just to get the piston out, it will come out, you can take the nut off the piston and remove it from the shaft and use that shaft as part of your drive assembly. The hopper depicted above is what I was saying weeks ago about cutting a window into the cylinder to use the piston to drive the wax. The same window can feed the screw.

 

This project is coming together nicely and Marks design for the back thrust is a good one.
On the tolerance of the screw. It does not have to be particularly tight to the side walls.
The wax will be compressed and churned causing heat from friction. This will then be smashed outward filing the cylinders cavity space between the auger and the cylinder where it will adhere to the side walls just to be sheared off and drug down the cylinder walls by the spiral screw causing further friction and heat.
This churning mixing can be both beneficial and detrimental in that it will heat the wax very evenly while mixing it to an extent. The process of pushing the wax down the auger will produce some if not all the heat needed for extrusion.
If it is to tight. One of the problem that a screw auger can encounter is inclusion of air. The wax becomes a plastic when compressed at pressure and the air is churned in where it is either compressed out along the sides of the auger if there is sufficient, pressure and clearance for the air to move or it is trapped in the plastic mix. Some parts of his problem can be handled in the feeding stage although air removal during compression is usual.
This type of auger is not a positive displacement pump although It will be able to produce considerable pressure. Slip should be a consideration.
In any event the wax will probably never reach the liquid stage so the auger action will keep the wax moving forward from the feed point. It will have no chance of a back flow causing leakage.

Joe

 

Interesting.

Yep churning air in isn't ideal. I wonder how to minimize that. I'm guessing something about the architecture of the screw helps keep air out as it is loading and being churned? Perhaps the less dense and more fluid air gets forced out the loading shoot as the wax is heated and squeezed? The speed of rotation probably also plays a role I'd guess. I see a lot of references for plastic extrusion that mention 100 rpm is a good number. Maybe that gives the air time to work it's way backwards out the device?

I'm not sure on the clearances. Other forums were sayin 1 to 2 mm gaps. That seems huge to me. Wouldn't the smaller the gap the more efficient the screw become? What benefit would a large gap give? Unless they were saying it will *work* with a large gap but didn't mean it was ideal.

*Edit*
I checked the forum I was thinking of and he said 0.15 mm tolerance between the pipe and screw. Which is 5.9 thousandths. They were using DOM pipe so I'm guessing that's as close as they could get. So it seems the smaller the gap the better?

Forum I got it from:
https://davehakkens.nl/community/forums/topic/the-quest-for-the-ultimate-extrusion-screw/

Also that's a good intro document. It helped straighten out some terms I have seen bit didn't understand. Thanks.

 

Well if I start with 1 thou I can always take off more if it doesn't work right haha!

I'll have to resort to experimenting to figure out the optimal process for sure.

One thing I do know after dealing with molten wax and using large metal syringes to make small spaghetti rods for many years is that liquid wax is a bitch to mold into shapes and shrinks a lot which pulls voids and tears if slightly too cool or goops out in large blobs if extruded at slightly the wrong speed or angle. It also takes about an hour to solidify from liquid state to hard. It is extremely difficult to get it to do what you want in one fluid motion and very slow. That's why I abandoned the giant syringe idea half way back through the thread because it would need long periods of time to liquefy and cool to the right consistency.

An extruder offers very constant conditions. Perfect temperature control, a gummy but not liquid state, constant production and good mixing of temperature as long as the hopper is full (I could also make infinitely long sprues instead of a certain length). It also offers the ability to easily make hollow sprues or different diameters. Assuming I can get the speed and heat settings figured out.

 

You got this one Zapins!
Joe