First time resin casting question

[Sledsel]

I bought myself a resin casting kit to do some small parts like valve covers and other things I can do with a single mold cast. My question is can I do a single mold cast of a set of wheels that are flat on the back?

[Aaronw]

It will largely depend on how much of an under cut there is for the rim. If there isn't a deep undercut, maybe up to an 1/8" you will probably be fine, particularly if you are only going to make a few. Silicone is pretty stretchy, but the mold will tear more easily the greater the under cut.

I'm assuming you are not pressure casting, so bubbles will be an issue but with some practice and "thinking like a bubble" you can eliminate a lot of them. Squeezing the mold and poking recesses with a tooth pick to help fill them with resin and allow bubbles to escape will help.

[ColonelKrypton]

6 hours ago, Aaronw said:

Squeezing the mold and poking recesses with a tooth pick to help fill them with resin and allow bubbles to escape will help.

I cast some small parts from time to time and do not use pressure casting. I have been using a syringe ( 5ml size is large enough for anything I do i.e engine blocks ) and use a 14g tip like these:

https://www.amazon.ca/gp/product/B00G9IUVN8/     

to "inject" the resin into the mold. You can point the tip into the bottom recesses and the mold will fill from bottom to top and you can move the tip around a bit to insure that the mold is filled. Before using the syringe I would often get bubbles in about half of my castings, since using the syringe my success rate is much better. Like everything, it took a bit of practice to get the feel for it.

The syringes can be re-used several times. The cured resin will or should just peel off the plastic syringe . After injecting I "pump" the syringe several times to remove as much of the resin as possible and then remove the plunger and wipe of what resin I can from the rubber bit. I frequently get a half dozen +/- uses from each syringe. and tip.  You could try a larger tip ( i.e. 12g ) or smaller tip ( ie. 16g ) but I found the 14g about right for my use. 

As for making molds for something like the wheels in your photo, I found that if I attach a smaller piece to the back side ( i.e. a disc a bit smaller in diameter than the part ), this will provide a nice defining line for that edge of the cast part. It will make for a small undercut but as @Aaronw noted, the silicon rubber is flexible enough that removing the part from the mold is not an issue. There is another benefit to this as well, if you slightly underfill the mold ( i.e.not quite to the top ) then a combination of surface tension and whatever shrinkage there is in the resin curing will leave a slight curved indentation in the center portion of the cast part leaving much less resin to remove and making it easier to clean up and make a nice level surface. Again, takes some practice to get a feel for it.

I make my mold boxes from bits of 0.040 or 0.060 polystyrene sized to suit my piece. Easy to glue together and take apart and the silicon won't stick to the plastic and the plastic is reusable.

If you have not already seen Robert Tolone's videos on youtube, you should have a look. Lots ( and lots ) of good advice and technique:

https://www.youtube.com/@RobertTolone

cheers, Graham

[Aaronw]

I have wondered about syringes, but thought they would be one use only, so never tried it. I can reuse the little plastic cups I mix the resin in, and since syringes seem to be made of a similar plastic, it makes sense that the resin doesn't stick well. 

Investing in a pressure pot makes sense for anybody casting in volume, but there are lots of little tricks to keep things simple for the just cast a part once in a while caster.