Molding Techniques and Procedures - Rubber Stamp Matrix Board

Molding Techniques and Procedures

Over the course of the six years that Bedell-Kraus has been selling products direct to the Art Stamp manufacturers, many have questioned the how's and why's regarding Molding Matrix Molds and Rubber Stamps. "Where is the book written on the How to...?" is often questioned. Anderson and Vreeland has a book written regarding Rubber Printing Plates for the Flexographic printing industry. Much of that information is applicable for the Art Stamp manufacturer.

The following information comes from the above mentioned Anderson and Vreeland book, knowledge gained from our father and his 50+ years in the Flexo industry, our experience gained in Custom Molding, knowledge gained from our many customers, and good old trial and error. These situational experiences that often times cause one to ask "it worked yesterday, but doesn't today...why?"

There are certain supplies all Rubber Molders should have:

Micrometeres: It is always good to know the thickness of the matrix, the matrix floor, your original, the ruber to be molded, and that of the final stamp.
Thermometer/Pyrometer: Just to verify the platen temperature is what the molding press says it is. As will be seen, variation of temperature alters the rubber's cure drastically.
Mold Release Spray: usually a silicone type spray.
Release Paper: Again a silicone covered paper used to cover the rubber when molding so it does not stick to the platens. Usually .005" thick.
Matrix Board: The proper matrix board depends on the photoengraving. Type 18 engravings can use Bedell-Kraus' TM2-120. Either Tm2-120 or TM2-125 can be used for Type 11 deep-etched engravings.
Molding Rubber: Usually a natural rubber either brick red, pink, or gray in color. Other colors available. Thickness is usually .090" - .095" but also .075" to .110" is used. For more solids it may be best use thicker ruber, for fine detail type, a thinner gauge is possible.

Matrix Moldings
DEFINITIONS
Bearer Bars: Steel strips used to achieve proper molding depth of matrixes and stamps.
Originals: Either a magnesium photo-engraving or hard polymer reproduction of the artwork form
which a matrix can be reproduced.
Matrix Floor: Point of measurement from the back of the matrix to the lowest point of impression
in the matrix.
TM-120 Floor is recommended at .125"
DETERMINE THICKNESS OF BEARER BARS
1. Add the height of the original to the matrix floor.
2. Add the thickness of the release paper if placed between the bearer bars.
3. Be sure the bearers are the same height on each side.
STEPS FOR MOLDING
1. Cut the matrix board 3/4" - 1" larger than the original photo engraving.
2. Spray the original with a mold release spray.
3. Place the matrix board coated side up on the serving tray, the original is then
place on top face down. Cover with release paper.
4. Close press to a slight "daylight" opening and preheat TM 120 for 3 to 3 1/2
minutes.
5. Slowly close platens until bearers are tight, avoid speed of closure and excessive
pressure.
6. Cure for ten minutes total at 307 F, preheat time is included in this ten minutes.
7. Carefully remove original from matrix mold, let cool. A soft bristle brush may be
used to remove any foreign particles.
TROUBLESHOOTING & REMEDIAL STEPS
TROUBLE POSSIBLE CAUSE REMEDY
Cracks in the
coating surface.
Cause is probably too long of
preheat.
Close more slowly and reduce preheat
time.
Blistering. Caused by excessive moisture
in the board or too high of
press temperature.
Dry board, be sure to store in dry
location, check press temperature.
Matrix coating
dulls or sticks in
matrix.
Caused by insufficient cure,
too low of press temperature
or undercut originals.
Be sure to cure matrix at 307 F for 10
minutes. Be sure your engraver gives all
images "good" sloping shoulders.