Thread: What's a good material for making round knobs?

Originally Posted by Ueee

Hi Mark,
Where did you buy the resin from? and what do you class as filler, the black oxide, or did you add chips as well? I only ask as i got 500gm of diggers resin from bunnings today for my manifold and it was $19 and the catalyst was $5. The 500gm tin was the biggest they had.
And what is "pre promoted"?..ok i just took the time to google, ha i should know i used to spray polyester undercoat! Pre promoted is no fun though as you cannot mix A+C together and get a room fule of chemical smoke.........For others it means the accelerator is mixed in already. This makes it harder to poison yourself but shortens the shelf life of the product greatly.

Hi Ueee, I went to Nuplex Composites which was Fibreglass International a few months ago and bought the GP LSE resin ($20.41 with catalyst), they also have casting resin too for about $6 more. The pigment/filler is just Diggers black cement pigment from the local hardware store, was going to go to another branch as they were out of stock of the bright red pigment and Bunnings did not have it. Nothing else was added to the resin, I have some nylon cloth similar to stockings I bought from the fabric shop and was thinking it would be good for straining and breaking up lumps of the pigment before mixing. Shelf life of the polyester is guaranteed for 3 months before it eventually goes hard in the tin.

Showed the balls to my machining guru friend this arvo, he suggested half filling the ping pong mould with one colour, levelling it up and letting it partially cure before topping up with another colour. The other tip he had which I really like is using a nail to puncture the bottom of the rim groove on the resin can and also for paint cans, that way when you pour resin or paint into a container the excess liquid that runs into the bottom of the groove can drain back into the can instead of squirting all over the place when you put the lid back on and hit it, of course doing this to an old crud filled paint can is a waste.

Last question, i promise

What ratio of pigment/resin did you use or was it just "about that should do"?

Originally Posted by Ueee

Last question, i promise

What ratio of pigment/resin did you use or was it just "about that should do"?

No problem: if you mix too much pigment in it becomes a grease-like solid so you then mix more resin back in until it's a thick liquid that won't stand up: touching it with a paddle pop stick with cause a spike on the surface that collapses back into the liquid. I found that rapping the filled ping pong ball made bubbles come to the surface and pop.

Originally Posted by Graziano

No problem: if you mix too much pigment in it becomes a grease-like solid so you then mix more resin back in until it's a thick liquid that won't stand up: touching it with a paddle pop stick with cause a spike on the surface that collapses back into the liquid. I found that rapping the filled ping pong ball made bubbles come to the surface and pop.

When i poured the manifold i tapped the container i mixed in on the bench to get the bubbles out before i poured the resin. Only the big bubbles came out though, and i figured it didn't really matter if there was some small bubbles in the cast. When i came back after about an hour though all the bubbles had made there way out and not a single one remained. I guess it may be different with the pigment in and the mix being thicker though

It's about a honey like consistency, a lot of rapping was involved. I also forgot to mention that I put the resin/pigment container on a 2000g digital scale to keep track of the various weights of the ingredients I add so I know how much catalyst to use.

Not one like this i hope....Portable 40KGx20g OZ lb Electronics Digital Balance Hanging Hook Weight Scale | eBay I think they have the wrong pictures......

They are about $15 delivered on Ebay and run off 2xAAA cells, when I was epoxying threaded rod anchors into the wall. I was using them to batch resin and they are awesome for that.

What's the impact resistance of cast resin like Mark? The phenolic ball knobs I have bounce on concrete. Would a resin ball bounce or more likely shatter?

Bob.

Originally Posted by Anorak Bob

What's the impact resistance of cast resin like Mark? The phenolic ball knobs I have bounce on concrete. Would a resin ball bounce or more likely shatter?

Bob.

ok lessee.....a 250mm diameter buff wheel doing 2800 rpm gives a peripheral speed of about 132 kilometres per hour: 0.25 metres x 3.14159 x 2800 rpm x60 minutes. I found the second ball in the backyard behind the shed today, intact and not even chipped, so one split in half after hitting on the moulding dimple and one survived untouched after both hit a wood benchtop at a significant fraction of 130 odd kph.......I'd say they're fairly tough. One second you're buffing and the next your hands are empty with a huge crack of the impact sound in your ears . I still don't know how the ball got around the milling machine unless it bounced off the steel I beam in the ceiling.

Edit: Bounced a ball off the concrete floor about a dozen times hard enough to get to chest it leaves minor marking that would buff out fairly easily.

Tough enough.

Brings back memories of my student days at WAIT ( Curtin ). In the three dimensional design workshops there was a buffing and polishing room. There were a pair of twin wheel pedestal buffers, either GMF or Waldown, connected to a serious exhaust fan via sheet metal ducting. Often when the machines were in use, you'd hear a bang and clatter as some poor would be jeweller's handiwork was snatched out of their hands and hurled into the ducting.

Bob.

Just finished a batch of balls with bronze filings and brass needles from the milling cutter and a smaller amount of the black iron oxide pigment. Dropping one of these on the floor gives no bounce at all, looks like the hardness and elastic bounce are due to the iron oxide pigment content.

Bob: I was buffing a home made silver ring with a 6mm green synthetic stone that looks like emerald, when the buffer flicked it off the bench (on the back patio) and into a pile of leaves, 2 hours of searching later I found the ring.

Originally Posted by Graziano

Just finished a batch of balls with bronze filings and brass needles from the milling cutter and a smaller amount of the black iron oxide pigment.

Graziano, I would like to see a photo of these, It sounds like what I was saying in a earlier post I would like to try.

Russell

Originally Posted by rusel

Graziano, I would like to see a photo of these, It sounds like what I was saying in a earlier post I would like to try.

Russell

Hi Rusel, I was about to make a batch when I spotted the containers of bronze grinder filings: Hmmm Rusel mentioned brass...... I mixed in the bronze but the particle size meant it was too fine, so I chucked the brass needles in as well. Right now before sanding, it looks like a very dark brown lump, I expect with a bit of a sand it'll expose the metal and then a cleap epoxy dip will seal them off from tarnish like you mentioned.


Here the ball after some 120 grit sanding to remove the equator line, then a sand with 240 grit before buffing: I rigged a cardboard box to catch the flicked balls, used it three times!.





This image shows the epoxy and brass patch needed because I didn't tap the ball enough. The main fill is a mix of LG2 bronze and brass, LG2 is leaded gunmetal, so the next time someone talks about "Gunmetal Grey", point out to them politely that gunmetal is a bronze alloy.



Even though the ball looks like a rough sand cast object that's had a polish, you can see that it's actually smooth enough to show the reflections of my index finger, thumb and face and camera in the image.

Just a note for anyone who's going to cast some balls: it seems there's different plastics for table tennis balls and ping pong balls. First time round I got lucky with choice of ping pong ball by using cheap dollar store balls that are a translucent semi clear plastic which the polyester resin doesn't stick to. This last batch I bought table tennis balls from a sports store: it's a opaque rough textured plastic and appears to be a bit porous. The resin sticks to it like the proverbial s*** to a blanket.

I got the bright idea of warming the ball shell with a flame from a gas stove lighter to try and soften the plastic and it ignited like a Mission Impossible tape recording, flared up and burnt the entire table tennis ball off the resin inside leaving a black residue on the resin ball. A little bit of sanding cleaned the ball up nicely ready to polish although the equator moulding seam is deeper in this type of ball, I may have to tap and thread the balls on the lathe then use a ball turner of some kind to clean them up quickly.