Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
I`his invention rclates to a process for molding stereotype
matrices. ~lore particularly this invention relates to a process for molding
thermosetting stereotype matrices from plastic pattern plates using a
cast film of propylene polymer as a release agent.
Recent developments in thc art of preparing plastic printing plates
have allowed the use of plastic master plates for molding stereotype matrices
which in turn are used to prepare copies of the plastic master. One major
drawback in this process has been the problem of adhesion of the plastic
master to the matrix. Various mold release agents described in the prior art
were tried in an attempt to prevent this problem. For example, various ;
polyolefin dispersions, such as disclosed in United States patents 3,~23,503
and 3,424,607, were tried. While these dispersions do tend to prevent
adhesion, they are difficult to prepare and often result in spotty or in~
complete coverage. Attempts were also made to use polyethylene terephthalate
release films as described in United States patent 3,887,579. However, the
type of film described in this patent would not deform sufficiently during
the matrix molding cycle and led to imperfectly formed elements in the matrix.
Now it has been discovered that thin cast films of propylene
polymer can be used as release films during the preparation of stereotype
matrices from plastic masters. The cast films of propylene are commercially ; "
available, inexpensive, assure complete coverage, yield readily (even at low
temperatures) to afford matrices which replicate the relief surface of the
plastic masters with excellent fidelity and prevent adhesion.
According to one aspect of the invention there is provided in the
process for molding thermosetting stereotype matrices from plastic pattern
plates the improvement of using as release films thin cast films of propylene
polymer c~ntaining at least 85% of ~mits derived from propylene to afford
matrices which replicate the relief surface of the plastic pattern plate
with excellent fidelity.
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According to cu~other aspect of tlle invention there is provided in
tlle process for molding polypropylene printing plates using thermosetting
stereotype matrices, the improvement of using a thermosetting stereotype
matrix prepared with the aid of a thin cast release film of propylene
polymer containing at least 85% of units derived from propylene so as to
afford matrices which replicate the relief surface of the plastic master
with excellent fidelity. -
Any thin cast film of propylene homopolymer or copolymer
containing at least 85% of units derived from propylene can be used in
the process of this invention. Propylene homopolymers and copolymers of
propylene with ethylene are most preferred. In general the cast film
will be from about 0.25 to about 2 mils in thickness.
In i~s broadest terms, the process of this invention is operable
when Inolding any thermosetting stereotype matrix from any
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i~;31~35~:~5
of the well-known photopolymer plates. Typical of the materials
which can be used to mold thermosetting stereotype matrices in ac-
cordance with the process of this invention are the phenolics, the
epoxy resins and the unsaturated polyesters. Typical of tha photo-
polymer plates operable in accordance with the process of this in-
vention are those based on prepolymers of unsaturated polyesters
~such as those disclosed in U.S. patent 3,79~,494), polyurethanes,
prepolymers of terminally unsaturated polybutadienes, crosslinked
polyamides (such as those disclosed in U.S. patent 3,625,696), un-
10 saturated polymers crosslinked by dithio compounds (such as thosedisclosed in U.S. patents 3,640,923 and 3,662,022), an~ polyvinyl
alcohol crosslinked with dimethacrylate compounds. Both shallow
~10~20 mils) and deep etch reliefs (up to 100 mils) can be faith~
fully reproduced in accordance with the process of this invention.
The instant invention is particularly useful when the matrix
is to be used for injection or compression molding of polypropylene
letterpress plates. Thus~ if any of the cast propylene polymer re-
lease film sticks to the matrix surface it melts and becomes an
integral part of the polypropylene letterpress plate produced us-
20 ing the matrix. Further, any of the film remaining attached to
the matrix does not result in a defect even in the initial molded
polypropylene plate,
The molding procedures and temperatures required in preparing
the various matrices and then their use in preparing letterpress
plates are well known to those skilled in the art and need not be
described herein.
The following examples are presented for purposes of illus-
tration, parts and percentages being by weight unless otherwise
specified.
~xample 1
A crosslinked polyester photorelief plate (40 mils thick) was
placed face up on the sliding tray of a Williamson Platemaster
Hydraulic Molding Press, heated to 310F., and equipped with 75~
r,il thickness control bearers. All but a small area of the plate
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l~BS~
was covered with a sheet of cas~ polypropylelle film (1.0 mil thick).
~n uncured phenolic matrix board (501cl b~ Rogers ~orp., 75 mils
thick) was placed over the plate, with the matrix coating down,
and the board was covered with a thin sheet of glass cloth
The assembly was subjected to liyht con-tac-t pressure and
preheated for a period of 30 seconds. The press was then closed
to the bearers over an interval of 40 seconds, achieving a pressure
of 135 tons over the 24-in. ram. The matrix wasm~lded under these
conditions (temperature of 305~315F.) for a period of 60 seconds,
10 then pressure was relieved. After removing the sandwich Erom the
press~ the matrix board separated cleanly from the plate wi-th the
exception of the area not covered with the polypropyl.ene film. In
this area, matrix and plate stuck together tenaciously. The molded
matrix board was returned to the press, and post-cured at the same
temperature bu-t without contact pressure or 60 seconds.
Inspection of the matrix hoard showed good fidelity of re~
production of the image in the plate, except in the area not
covered by the polypropylene release ilm. The matrix board was
used to prepare a polypropylene le-tterpress printing plate by com-
20 pression molding a 50-mil sheet of polypropylene against it at a
temperature of 430F. and a pressure of 300 psi. The plate was an ~.
accurate reproduction of the matrix pattern.
Example 2
A phenolic matrix board was molded according to the proce-
dure described in Example 1, with the exception that the molding
press was at a temperature of 340F. ~hen the press platens were
opened, the ma-trix and plate separated,but the polypropylene had
melted onto the surface of the matrix.
The post-cured matrix board was placed in the cavity of an
30 injection molding machine, the cavity was closed, and polypropyl-
ene was injected into the mold cavity at 420F. After cooling,
the cavity was opened, and a pol~propylene plate with good image
fidelity was ejected. The surace of the matrix was free o any
r olypropylene residues~ ;
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