DIY Polyurethane molding

[Ianb]

Have no idea about this, but after making some cups for transmission bushes to reduce their movement and then finding they are still far softer than originals, I had another idea.

The current bushes are kind of like an egg timer with a narrow centre, I have aluminium cups that will fit them, but due to the shape they will not work, I am considering fitting the bushes into the cups and then filling the gap around th waisted section with polyurethane.

I have seen manufactuers produce polyurethane inserts to increase stock rubber bush stifness in a similar way.

Anyone have experience of using polyurethane for similar things?

Photos off web but show bush shape and cup they will sit in

 

[bigegg]

Have no idea about this, but after making some cups for transmission bushes to reduce their movement and then finding they are still far softer than originals, I had another idea.

The current bushes are kind of like an egg timer with a narrow centre, I have aluminium cups that will fit them, but due to the shape they will not work, I am considering fitting the bushes into the cups and then filling the gap around th waisted section with polyurethane.

I have seen manufactuers produce polyurethane inserts to increase stock rubber bush stifness in a similar way.

Anyone have experience of using polyurethane for similar things?

I looked into doing some for a LARPer / re-enactment friend of mine.

There are several different flexibilities, from almost solid, to very soft.

You'd have to make sure you get the right Shore rating.

Other than that, it should be just a matter of mixing it in the correct ratio, debubbling, and casting.
Then possibly putting in a pressure pot to shrink any remaining air pockets.

There are a LOT of youtube videos on using various PU foams for film prop making and cosplay etc.

 

[Ianb]

Thanks, the debubbling maybe an issue, also got to figure a way of getting it in there, unless I put the bush in some kind of tube and fill, then scrap the tube and place in aluminium cup as it doesn't need to be a tight fit/stuck to cup.

I will watch some videos, but does it expand or shrink when setting?

 

[Dcal]

It's very easy to do.
If you need to, you can make a mould out of silicone rubber to cast the bushes in, then they are very easy to strip.

The biggest problem I've found is getting the polyurethane to stick to the alloy. Apparently shellac is a good primer but I haven't tried it yet.

I have tried casting polyurethane to sand blasted alloy and it just peeled off after curing.
I put it down to the alloy getting contaminated by the silicone in the mold but I'm not sure if that was the problem.
More testing required, but if it's just rubber bushes it works very well.