What is the correct procedure to ‘top off’ a simple open mold?
I am slightly over filling the RTV mold with resin (polyurethane) then placing a flat plate on top to give a flat surface to what will be the bottom of the part I am making. However this process is somehow trapping the odd air bubble which then remains in the finished part. The problem is the part is very thin (2.5mm) and these air bubbles are visible as a blemish in what will be the top visible surface of the finished part.
I have tried placing the top off plate straight down, down on an angle, wriggling side to side etc all to no clear advantage.
When I make an open mold I usually raise the part slighty with a scrap piece of plastic sheet. Then when I pour the resin I sand the bottom flat. In this case if you don’t want to make a new mold I would over fill the mold let it cure then sand it smooth.
When you pour your resin place the mold on a hard surface. Take an appliance that has some sort of vibration to it and place it on the table next to your mold. The vibration will help the air bubbles move to the surface and help the resin settle into all parts of the mold. “Laying” your resin initially into the mold will help with the bubble issue as it makes less bubbles. Also when you mix up the resin and hardner, go easy. You can make alot of bubbles just by doing that. Pouring makes bubbles simply by the physical action of trapping air as the liquid pours or as the resin covers the small voids of the mold. Open faced molds are easy to pour into by pouring down the sides of the mold and letting it creep along the surface or by gently brusing the resin on the surface and allowing it to fill all areas that way. Then you can top off by pouring if you like but again, pour along something and let it creep along the surface to fill the mold.
Ideally if its a large project or you are going to be doing alot of resin casting, I’d invest or build a vacum table and that will just about eliminate any bubbles.
A simple, slight over pour and let the resin sit proud of your mold by the action of suface tension will serve the purpose and sanding the excess off works great.
Talc/baby powder, a natural dessicant draws liquid (resin)thereby displacing air, the bubbles may remain only now hidden beneath a matrix of resin. Shake a liberal coat of powder into mold, upend mold and slap it a couple times,(This does nothing to advance the process,but it lets the mold know who’s boss!)to dislodge klingers. This is the simplest, cheapest way to improve casting quality, note I said improve so don’t expect a flawless anaerobic part. You may also try introducing the resin with an irragation syringe, some have a fine curved nozzle perfect for reaching into little negative spaces.
A couple of my molds have a slight undercut which can trap bubbles so I clear these with a toothpick or carefully “burp” them. Also if your resin is old or has moisture in it you can get bubbles(or a casting which won’t cure). All of the responses you’ve gotten so far are right on the money, from my experience. My biggest problem when I first started was mixing too vigorously creating bubbles.
Baby powder! Really!!! I’ve never tried that, but I guess I could try it. Does the powder make the cast come out rough, or do you get a decently smooth surface?
The baby powder works well to aid in releasing surface tension which traps bubbles in the details of the molds. However it will do nothing for the back of the part if you are flat plating the open mold as I suspect you are doing. What are you using to flat plate with? I used to have the same problem with trapped air between the part and the plate as well. I cured it by using a piece of 1/8" thick plexiglass. I flex the plexi and start placing it at one end of the mold and slowly 'roll" it out onto the mold. This will allow any air that might want to be trapped to escape out along the “wave” front. To keep the resin from sticking to the plexi I coat it with a mold release. cooking spray will work for this.
sometimes…the whole, plexiglass on top idea isnt always practical. an open pour in a pressure pot (powder is always good),will give great results.dont overfill…( no pot… a vibrating sander can helped work out bubbles as well.( a sandable ,planned out riser is smart, if theres a tolerance issue) hope it works out…
On all my urethane castings into rtv or silicone I usually burn the mold, by squeezing the halves till lots of resin comes out. Usually this forces lots of air bubbles with it. Then I simply pour more resin in to fill it back up. I find I get better molds this way. Of course you have a 1 part mold so with this setup I tend to slightly shake it and even tap it down on the desk. This usually works pretty well freeing up the bubbles. If at the end you have a decent part but it still has a bubble or two in it, no loss, simply use Tamiya Putty to fill in the bubble. It works really well.