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Proper Application of Brush-On Rubber
Technical
Tip: When Making A Brush-On Rubber Mold, Do Not Over Apply Rubber
- Especially The First Coat
Our toll-free technical line receives hundreds of calls every day.
A complaint heard often from people making brush-on molds for the
first time is: “my mold rubber dripped off my model and when it
cured, I had tiny bubbles on the working surface of my mold that
are reflected in my casting. What am I doing wrong?”
Answer: You are probably trying to apply too much mold rubber to
the model surface.
More Is Not Better - People assume that brush-on mold rubber needs
to be applied in “quantity”. Applying globs of rubber to your
model surface entraps air and and wastes your valuable material.
Generally, it is better to apply 4 or 5 thin layers than 3 heavy
layers. Thin layers work best - especially for the critical first
layer or “detail coat”. The secret is to apply the first layer
thinly with dabbing strokes that will get rubber over and into all
of the surface detail. Applying rubber in this manner also lets
bubbles move away from the model surface and up through the rubber
where they can pop on the surface. Rubber applied too heavily will
trap air. After the first coat, you can apply heavier layers - but
not too heavy.
Let your first coat become tacky. Mix and apply the second coat - this time, add a small amount of liquid pigment or tint to the mixture. This will help you distinguish the second coat from the first coat (ever try brushing white on white? It is impossible to tell where you’ve been).
Adding color (pigment or tint) to every other layer will help avoid thin spots in the finished mold.
Smooth-On Brush-On Series mold rubbers were carefully formulated to address several problems experienced with brushable mold rubbers of the past. High tear strength and
abrasion resistance are but two attributes that have made these products so popular. How they perform when actually making the mold is equally as important.
Brush-On 35, 40, 50 & 60 all offer three
distinct advantages when applying:
First: these Brush-On’s hold vertical surfaces like no other
mold rubber on the market. However, if you apply too much mold
rubber to the model surface, the weight will cause it to slump
or drip off the model.
Second: They do not chemically thicken (like our EZ-Mix 50 or
similar competing products) and give the end user plenty of
working time.
Third: The Brush-On Series products wet out nicely onto the model
surface - the rubber will literally “melt” into itself. This
will eliminate brush strokes in the finished rubber, helps avoid
“thin spots” and helps achieve a uniform coating over the
model surface.
Fourth: Because these mold rubbers “melt’ into themselves,
they level off and de-air themselves if given the chance - meaning
that if the rubber is over applied in thick amounts, air will not
have a chance to escape and may become entrapped on the working
surface of the rubber. Your castings will reflect these air
bubbles.
We hope you’ve enjoyed this mini-lesson on Brush-On rubbers and
invite your comments.
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Smooth-On