Rubber/plastics people

[cranetruck]

The image below shows a plunger portion of the shut-off valve for the SW fuel burning heater. The rubber/plastic "plug" has been seated against the valve opening for twenty some years and now has an imprint, which causes the valve not to seat correctly at all times, it may or may not leak, depending on how it ends up on the valve opening.
The plunger is circled in second image. The offending plunger "button" is #735411...
Question: How can this be restored to original (flat surface)? Buying a replacement plunger or "button" is not an option ( rebuild kit runs close to a hundred and I believe SW has a minimum purchase of 3 for this).
Possible solutions:
1) Slicing it off with a sharp blade?
2) Heat?
3) Sanding/polishing?
4) Other, chemical?

Any suggestion is welcome.
TIA

 

[Carl_in_NH]

How complicated is the sealing / plastic part of the plunger? Can it be disassembled? Just wondering if it's possible to machine a replacement sealing part from an appropriate material.

 

[cranetruck]

The rubber "button" comes off as shown in the 2nd image above. Making a new part is an idea...

 

[Carl_in_NH]

Thanks for adding the pictures, Bjorn - that helps. It looks simple enough to make - I'd be happy to make a couple for you on the lathe if a suitable material can be located that's soft enough to seal, yet firm enough to machine. Let me know if I can help.

Edit: one other comment - if you're going to try sanding or trimming it back, grab the dial calipers and take a full set of measurements first to aid in recreating it later on.

-Carl

 

[mdlbldrmatt135]

How hot does it get? I've cast a bunch of resin parts, and my supplier has quite a few 2 component rubbers.... that could be cast into the correct shape, after making a new master.

If you know about how hard it is and the temp requirements... try contacting Smooth-On - Mold Making and Casting Materials for a World of Applications!

 

[runk]

My experience with old hydraulics is that the rubber has to deteriorate pretty bad for a seal to take that much of a set. I wouldn't expect it to seal even if you cleaned up the end.
What are the approximate dimensions ? That kind of rubber tip on the end of a piston or rod looks real familiar (old hydraulics ?, master cylinders ?, english carburators ? SCBA regulators ? ....) but I just can't place it. We've used
Injection Molding - Real Injection Molded Parts - Protomold
at work for obsolete seals, but I think they are a minimum of $100-$200 per order...

 

[cranetruck]

Thanks for the inputs!
Machining would be nice, but I think it may be too soft for that, I have trouble visualizing it. The hardness is probably close to that of an O-ring.
I have several of these fuel control valves and removing one of these plungers puts it out of commission, so to speak, but could mail you one for evaluation, Carl.
The spring pressure is in the neighborhood of 6 oz. The O.D. of the "button" is 0.20 inches.

Molding or casting is good except for the making of the mold, which would have to be a two-piece job (the button is cup shaped to fit on the plunger tip).

Now I'm thinking along the lines of simply coating the surface of the button with a silicone or something similar that will bond to it...a small "sock" would probably work too...
This is not a high temperature application, but it's constantly exposed to diesel fuel...

 

[mdlbldrmatt135]

Bjorn,

a 2 part mold is no problem. I've made several and some of the modern silicone molding rubbers are really durable, some have a 200% tearing threshold. I have a pressure chamber setup for mold making & casting (vacuum pumps are $$) in this case I use a paint tank and put the items in and pressurize to about 45psi (for casting) or 60 for molds. Vacuum pumps remove all air bubbles.... as where pressurizing makes them really really small. that coupled with mixing the resins & rubbers carefully minimizes the air bubbles.

HTH

Matt

 

[Carl_in_NH]

No need to mail one if I can get a set of numbers on the various critical dimentions, Bjorn. Let me know if you want me to try making a couple. Even if the rubber is a little soft, I suspect I can machine them with a nice sharp cutting tool and light cuts. McMaster / Carr is likely to have some stock that would work for the application in question - might just take a little time to figure out which material is 'best'.

 

[gringeltaube]

If that bad it did seal at times.... (!?), I would try restoring its face a bit... which isn't complicated at all with a lathe or drill press, fine sandpaper (start with 500 end 1500) and some patience. (Done that many times to restore air governor, horn solenoid, fuel shut-offs, etc.)
My vote: option 3)
Give it a try before going the complicated route!

G.

 

[XM 2742]

OK .first things first.what is the part made of. Rubber-plastic is just not enough info.

Call thje manufacturer and get specifics. "Rubber" could be PVC or thermoplastic rubber even another plastic. "Plastic"if harder may be nylon or another plastic.....and is machineable.

As a former plastics research technologist I have more than the average knowledge of plastics and knowing the exact makeup of the material is step one.

 

[runk]

If the OD is ~.2" one "replacement" you might look for is the plunger seal off a syringe. They come in lots of diameters, and are available in various rubber compounds (I'd look for a nitrile like viton for the diesel application).

 

[cranetruck]

Thanks for all the advise!
I'll update this thread when I can report progress...

 

[NMC_EXP]

The image below shows a plunger portion of the shut-off valve for the SW fuel burning heater. The rubber/plastic "plug" has been seated against the valve opening for twenty some years and now has an imprint, which causes the valve not to seat correctly at all times, it may or may not leak, depending on how it ends up on the valve opening.
The plunger is circled in second image. The offending plunger "button" is #735411...
Question: How can this be restored to original (flat surface)? Buying a replacement plunger or "button" is not an option ( rebuild kit runs close to a hundred and I believe SW has a minimum purchase of 3 for this).
Possible solutions:
1) Slicing it off with a sharp blade?
2) Heat?
3) Sanding/polishing?
4) Other, chemical?

Any suggestion is welcome.
TIA

Bjorn

It's probably rubber rather than plastic. The indentations are likely due to permanent "set". Permanent is the operative word - heat or solvents/chemicals will not restore it to original dimensions.

Rubber is hard to machine because cutting tool pressure deforms the rubber during the cut making final dimensions anybody's guess.

If you have enough stock thickness, sanding is probably your best bet. Use a belt sander or tape a piece of sandpaper to a flat surface and rub the surface on it in a figure '8' (as though you were lapping a steel surface). In the R&D lab we sometimes cut chunks of rubber from tires & etc for testing and worked them down to final test sample dimensions with (as I recall) 80 grit carborundum paper.

You might be able to glue a piece of rubber sheet stock to the end of the plunger and give it a fresh surface that way.

McMaster Carr and other outfits sell sheet rubber of various types, durometer hardnesses and thicknesses. For diesel fuel in an environment less than 250 deg F nitrile (a.k.a. Buna N) rubber is OK. Probably want a medium hard material, say 50 to 60 durometer.

In the lab, Loctite Black Max adhesive was our 1st choice for sticking rubber to rubber. This is a cyanoacrylate 'super glue' material which is modified so it is a little flexible and won't break the first time the rubber flexes. Never used it in diesel fuel but I do not think fuel will affect it.

THere's been some talk of using castable rubber to make a new plunger. Most of the castable rubber out there is silicone. Diesel fuel will cause silicone to swell to maybe twice it's original volume. Silicone if probably not a good choice for this app.

Remember:
(1) Rubber is evil.
(2) Don't rubber the wrong way.

Jim

 

[cranetruck]

Thanks Jim!
Sliced a 1/2 mm section off the tip (didn't come out as well as I had hoped, using sawing motion).
Sanded the new surface with 2,000 grit paper, kind of like using an eraser on rough paper.
After testing, valve still leaks...probably didn't get the surface perfectly level...
Will try the rubber sheet as suggested, next, perhaps the rubber has hardened over time too (also mentioned above).

 

[NMC_EXP]

Bjorn

For cutting rubber we used an Olfa brand razor knife (type with snap-off blades). Rubber dulls edges fast so it's necessary to change blades often. Dull blade = high force to cut = rubber deforms under cutting pressure = final surface not flat/square.

Single edge razor blades work well also. Red Devil made a single edge blade holder for this kind of work.

The real secret is to lubricate the blade and rubber - a soap water solution works great. Use a sawing motion as you did. The soapy water lube allows pressure on the knife to be kept at a minimum so the rubber does not distort as much and blades keep an edge longer.

Depending on rubber type and service conditions (operating temp and fluid type) rubber can harden with age. Even without permanent set hard rubber will not provide a good seal in low pressure situations. The exception to this is o-rings used for very high pressure (1000 psi and up). 90 durometer is typically used for these apps.

Regards

Jim

 

[cranetruck]

Okay, I'll try again with your suggested method of cutting...