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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[ScottyUK]

What about the rubber disc attachment from a drain rodding kit? If you put a slightly undersized stiffener behind it made from a disc of hardwood or plastic sheet I think you would get a pretty good piston and seal.


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[johnwm]

Thanks guys - problem is now solved. Compositepro, your take on the inadvisability of lip seals got me started, and I found that an old CD screwed to the face of an undersize wooden piston works well with little drag and a good clean seal. For anyone interested the piston is 2mm (80 thou) undersize on diameter in the cylinder, and the CD is 0.3mm (about 12 thou) oversize for the bore. I will experiment with different oversizes when I get the chance and I am also looking for a slightly softer material than old CDs. Note that CDs are quite difficult to machine as a tad too much feed and you have silvery confetti everywhere.

Good Luck
johnwm
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[Compositepro]

CD's! What a great idea. It's also another addition for that other thread on other uses for CD's.

One other parameter you can play with is the diameter of a disc between the wiper disc and the piston face. Another one or two CD's of smaller diameter than the piston will give the wiper the ability to flex more. Or, just shape the face of the wooden piston. A flat piston face with only 0.080" clearance does constrain the ability of the disc to dish.

 

[MikeHalloran]

To replace the CDs, with fewer machining difficulties, just buy a sheet of thin polycarbonate.



Mike Halloran
Pembroke Pines, FL, USA

 

[patprimmer]

If you are buying plastic sheet, Acetal or even PE or Acrylic will be better than PC for abrasion resistance. Nylon would probably expand to much from water absorbtion.

Regards
Pat
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