Home made lip seals

[johnwm]

Having retired recently, and looking to take up a 'hands-on' sort of pastime, I have recently got involved with throwing pots and other types of clay building. These potters seem to use several machines which seem to be awfully poorly designed and I have been tempted to have a look at some alternatives.
I have been looking at an extruder. The clay to be extruded is very plastic and malleable. In the current design a wooden piston is used in a piece of 4" NB PVC soil pipe. Even with the wooden piston boiled in lube oil to reduce water absorption I am still seeing 10 to 20 thou variation in diameter with changes in ambient conditions. Wood is also anisotropic so the piston will not stay round. If the piston is tight enough to not leak past in it's small condition it becomes too tight to remove/replace for a fresh clay charge when expanded. If it is made slightly small then it lets past too much to be of use.
I immediately thought to use a pneumatic cylinder lip seal, but I can't find one that is a decent fit in the tube without having a special manufactured. Potters are a tight lot, so these devices really need to work, but at a very low cost, so specials from a manufacturer seem to be out of reach.

Qusetion: Does anyone have experience of home made lip seals - or any alternative ideas? I have been thinking along the lines of machining a lip-seal shaped groove on the face of an aluminium plate and then moulding my own. ?Meltable moulding rubber compound? ?Polyurethane casting compound (as used to take intermediate female moulds off male masters)?

This will never be a "commercial app" - just something to amuse me and our local potters group, but it may pose some engineering questions.

Ideas please?

Good Luck
johnwm
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[KENAT]

Would one of the stiffer O ring materials work?

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[zdas04]

For a Boy Scout project (bottle rockets) I designed a bicycle pump-looking contraption that had (I think, it has been a while) maybe 2-1/2 inch outer cylinder and 2 inch inner. Then I put a cap on the inner pipe and bolted a 2-inch thick stack of leather scraps cut with an arch punch to about 1/16 over the ID of the outer cylinder. A handle through the inner cylinder and a platform to hold it down and it stood up to hundreds of Boy Scouts over three summer camps.

For something as stiff as clay, you'd want to put a plywood stiffener in with the leather, and you'd want to work out a rigid mounting for the barrel and mechanical advantage for the ram, but might something like this work?

David

 

[Theophilus]

I've done this a few times in my career. Simple and inexpensive (for manufacturing these things) is using a piston designed for injection molding that has a molded lip around the front edge of the piston. Not great for high pressure, but a decent seal to push stuff.

For low-volume (home-made) applications, you can use some flat washers cut out of something like buna rubber (Viton is great, but a bit pricey) that are a bit larger (OD) than the ID of the tube/pipe. The piston has a concave-shaped chamfer around its outer edge, so the face of the piston sinks inward a bit. Then use a matching part (like a larger, solid washer) to sandwich your rubber flat washer in between the front washer and piston. Use screws (or whatever) to fasten the layer of parts together.

Getting the exact OD of the rubber washers to work within the pipe without crumpling or binding is the trick, and generally a matter of having a high degree of roundness and just the right OD--takes a bit of playing around with, in my experience.

If the text version doesn't make enough sense perhaps I can post an eDrawing after making some quick models in SolidWorks.



Jeff Mowry

http://www.industrialdesignhaus.com

A people governed by fear cannot value freedom.

 

[msquared48]

What kind of pressure and temperature ranges are you talking about?

Mike McCann
MMC Engineering
Motto: KISS
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[MintJulep]

The change in roundness with moisture can be controlled somewhat by making your pistons with the axis of the grain parallel with the axis of the piston. Even better, select a chunk from the middle of the tree.

Before rubber seals were made from leather.

 

[MintJulep]

Or rope.

 

[ivymike]

somewhere I might still have a 1940 article (Federal Mogul, maybe) on how to make your own iron piston rings. Otherwise, what about an o-ring?

 

[ivymike]

or you could perhaps just take a 3-piece oil control ring from an engine of appropriate bore diameter (there are lots of 'em @ about 4") and use it.

Or get a slinky of slightly larger diameter, and clip a few coils. find an appropriate wire spring to expand your coils into the bore, and fit it all into a groove on your piston.

 

[KENAT]

Just looked and Mcmaster have some fairly cheap, fairly hard O-rings in around the size you're looking for.

Of course, you'll have to put a groove in the piston, but I'm guessing that could be worked out.

Another thing to over come temp issues would be to match the material of the cylinder to the bore. Maybe you can put a 'face' on the piston of the same material as the cylinder.

If memory servers, some ground pipes are made of UHMWPE, and so are some plastic cutting boards. UHMWPE also has fairly low friction (though I'm guessing the clay may make that almost irrelevant).

So if you can find suitable pipe, and at least put a face on the plunger, you may be onto a winner.

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[ivymike]

I wonder whether McMaster 9562K57 would work for you?

or, how about if you took a length of the rubber string used to hold window screens into their frames, and wrapped a spiral on your piston (drill holes for the ends of the string, and hot glue in place)?

 

[Eltron]

We use an extruder for ceramics at work. It has a square piston housing and the piston plunger is HDPE. The square plunger has no seals around it, it's just a snug fit. I'll see if I can find a part number when I get back to the office for reference.

Dan

http://www.eltronresearch.com

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[patprimmer]

I think David has it right. My old bicycle pump had a leather cup shaped washer and it worked fine. It should upscale OK.

The piston could be metal or HDPE. I see no need for the extra cost of UHMWPE. PE is nice and slippery and non stick and inert.

Regards
Pat
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[johnwm]

Thank you all for your very quick input. We're snowed in at present, but I will try a few of the suggestions and let you know what I find.

Thank you again.

Good Luck
johnwm
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[msquared48]

FYI... Drafting tape worked well for my kids when they were young.

Mike McCann
MMC Engineering
Motto: KISS
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[Compositepro]

I suggest that you really don't want a lip seal for extruding clay. You need to use a squeegee type of seal. With viscous materials it is very important to keep the seal contact area very small to minimize viscous drag. A lip or cup seal is self energizing so the pressure pushes the seal tighter and creates more drag and contact area. Squeegee seals are made from flat sheets of rubber, plastic, or metal shim stock which is slightly larger in diameter than the bore. When the piston pushes into the bore, the flat sheet will cup backward and contact with the bore is only with the sharp edge of the sheet. This minimizes contact area and drag while increasing contact pressure to "squeegee" clay off the bore.

This method is used for pumping high viscous materials from buckets and drums. The seals work well and are easy to make from flat sheet stock so the are cheap and easy to replace. One issue to consider is that to reverse the motion of the piston the seal has to be forced to invert. Often compressed air is used to do this, but you have to be careful of the dangers of compressed air. In your case maybe you could remove the piston from the opposite end that you inserted it.

 

[zdas04]

Or leather. Now we've come full circle.

David

 

[Compositepro]

Leather is more suitable for a cup shaped lip seal. It works well for bicycle air pumps and similar applications. This is quite different from what I suggested. Lip seals are cupped in the direction of motion so that pressure makes the seal tighter. A squeegee is flat but deflects elastically in the direction opposite the direction of motion because its diameter is larger than the bore. This elastic deformation creates the spring force to squeegee the bore wall. Leather does not make a good spring at all, particularly with wet clay.

 

[unclesyd]

I would give Woodex a call to see if you can get piece of their Maple Bearing Material.

http://www.woodex-meco.com/

zdas04,
Leather is an out standing seal material.
We used it for years on process designed for 10.000 psig, we operated at 6250 psig. Chevron leather packing was used on the piston pumps (7) and also as valve stem seals.
The material came for Chicago Rawhide Co.

 

[zdas04]

It worked well for me in a similar application, it helped that I already had an arch punch in the right size.

David