US3883632A - Method of forming thermoplastic matrix including preheating of thermoplastic blanks - Google Patents

Abstract

A thermoplastic blank is preheated in a press with an insulation layer disposed between the blank and the original die form. The insulation layer is removed and the blank is heated and pressed against the original die form.

Description

United States Patent Petrochko et al.

[4 1 May 13, 1975 METHOD OF FORMING THERMOPLASTIC MATRIX INCLUDING PREHEATING OF THERMOPLASTIC BLANKS Inventors: Robert P. Petrochko, Somerville;

Thomas E. Hayes, High Bridge, both of NJ.

Union Carbide Corporation, New York, NY.

Filed: Oct. 29, 1973 Appl No.: 410,882

Assignee:

U.S. Cl. 264/322; 264/293; 264/331; 264/334 Int. Cl. 8411: 3/06; 829d 9/06; B291) 3/00 Field of Search 264/322, 284, 313, 316, 264/293, 294, 299, 107, 334, 331, 319, 119, 109, 120, 122, 220; 101/401.1, 401.2; 156/209 References Cited UNITED STATES PATENTS 3/1937 Browne 264/316 McGraw-Hill, N.Y., (1950), pp. 460 & 463 relied on.

Primary ExaminerDonald J. Arnold Assistant Examiner-willard E. Hoag Attorney, Agent, or Firm-Robert C. Brown [57] ABSTRACT A thermoplastic blank is preheated in a press with an insulation layer disposed between the blank and the original die form. The insulation layer is removed and the blank is heated and pressed against the original die form.

7 Claims, No Drawings METHOD OF FORMING THERMOPLASTIC MATRIX INCLUDING PREHEATING OF THERMOPLASTIC BLANKS BACKGROUND The basic concept of molding thermoplastic matrices from which to mold printing plates, sound records and the like has been disclosed in US. Pat. No. 3,380,878.

The basic process used for producing thermoplastic matrices is described in U.S. Pat. No. 3,380,878. improvements in this basic process, all intended to achieve higher accuracy and greater efficiency, primarily by careful regulation of molding conditions, are described in US. Pats. No. 3,622,659 and No. 3,743,463.

Newspapers having very large circulations, in the millions of copies, must operate many printing presses simultaneously in order to produce the necessary copies within a few hours time. To produce so many individual page press plates with the utmost speed requires that a number of matrices be made from each original type page, so that several plate makers may cast plates of the same page simultaneously. In addition these newspa pers typically make frequent edition changes. Normally a minor portion of the original type is replaced and the major portion remains unchanged in the typeform. In the largest circulation newspapers a single typeform may be used to make as many as thirty matrices. In some cases, as in the classified advertising pages, the lead type may even stand" or be held over to be used again on successive days. The need for large numbers of matrices is also true in the case of sound recordings. The master or original die form for newspapers and sound recordings is often of delicate construction due to a desire for highly accurate reproduction. This is particularly true of sound recording masters which tend to be structurally weak and is also true of printing originals which are commonly lead typeforms, lead stereotypes, fine engravings of copper, zinc or magnesium or delicate photopolymeric originals. When these masters are used for the production of matrices they tend to become blunted clue to structural weakness in some cases and due to the inherent softness of the materials used in the construction of the master especially at elevated molding temperatures. This naturally gives rise to a gradual but substantial loss in accuracy in applications where large numbers of matrices are required.

The conventional method for producing matrices from thermoplastic materials is to place a sheet or composite sheet ofa thermoplastic material in contact with an original plate or pattern followed by the application of sufficient heat and pressure to produce a matrix. Molding temperatures ranging from on the order of 445F. to 590F. are typical but vary depending upon the particular physical characteristics of the thermoplastic material used. Molding pressures vary from about 200 psi to 4000 psi and again are dependent upon the physical characteristics of the thermoplastic material and also the molding temperature selected. Loss of resolution due to successive matrix molding operations is a particularly difficult problem in newspaper printing applications where the original is usually a lead typeform having large areas ofwhite space". It is typical in molding matrices from lead typeform originals to find that only on the order of 7 or 8 matrices can be formed before reaching a point where the original lead typeform has been damaged to such an extent that it is no longer capable of being used to produce satisfactory matrices. The metals commonly used in these originals are very easily damaged at the high processing temperatures typically used even when relatively low molding pressures are employed. Because heat and pressure are applied simultaneously, it requires some time for the thermoplastic blank to soften. When the blank initially contacts the type form it is a relatively hard material and it must be remembered that molding pressures are not applied uniformly across the entire surface of the original but actually only to a small portion ofthe original represented by the raised surfaces of the type. In addition it must be remembered that due to variations in gauge of the original and minor differences in exact parallelism between the original and the thermoplastic sheet, which are virtually impossible to avoid, pressure is initially concentrated on a few localized areas that appear higher than the rest of the originalv This of course aggravates the situation, and, obviously, damage will first be experienced at these high points.

We have found that these problems can be overcome and that as many as 30 matrices or more can be produced from a single lead type form without appreciable loss of accuracy in reproduction by following the spe cial molding procedure of this invention. Correspondingly higher quantities of matrices can be produced from other originals which are less subject to damage than lead, such as copper. by following the procedures of this invention.

GENERAL DESCRIPTION ln accordance with the process of this invention a thermoplastic matrix blank of proper gauge and config uration is placed in a conventional molding press. A steel or other metallic release sheet, preferably coated with an appropriate release agent. is placed on top of the matrix blank and the press closed for a preheating cycle. The upper and lower platens of the press are maintained at temperatures on the order offrom about 200F. to 750F. The press is closed and maintained at essentially contact pressure for a sufficient period of time, at the selected temperature, so that the matrix blank softens, but does not flow. The press is then opened and the steel release sheet removed. The matrix blank, now soft and relatively sticky, adheres to the steel release sheet. The steel release sheet with the still soft matrix blank adhering to it is placed over an original form in a press with the soft resin surface directly on the original form. The matrix blank is now in a ready condition for the normal matrix molding cycle. Be cause the matrix blank is in a soft but relatively nonflowable condition when the press is closed for the nor mal matrix molding cycle, damage to the original form is minimal. It will be appreciated that if desired the preheat operation may be conducted in the same molding press in which the matrix is to be formed. When this is done the temperature of the lower platen on which the type form or other original rests is maintained at a temperature significantly lower than that of the upper platen to minimize damage to the original form due to softening resulting from the application of heat. When the matrix molding press is to be used for the preheat step it is desirable to place an insulating material between the face of the original form and the matrix blank prior to the closing of the press for the preheat operation. This insulating material can be of any satis factory material which will not adhere to the softened matrix blank. A commercially available insulating material sold under the trade name SYNTHANE G-7 has been found to be very satisfactory for this purpose. It will also be understood that the use of some such material may also be desirable when the preheat step is conducted in a press different from the press to be used for actually molding the matrix, to avoid adhesion of the matrix blank to the lower platen. It will also be understood that the preheat step need not be conducted in a press since little if any pressure is required to be applied at this point in the process.

After preheating as described above the matrix blank may be molded in accordance with any one of the conventional molding procedures subject only to minor changes in the molding cycle times made possible by the preheating of the matrix blank. Any of the conventional processes may be used including those described in U.S. Pat. Nos. 3,380,878, 3,380,880, 3,408,437 and 3,743,463.

It will be appreciated that processing conditions usually vary only with respect to temperature and the actual pressing time and that these parameters are normally established by the physical properties of the particular thermoplastic selected for the matrix material.

Thermoplastic materials that can be formed into matrices for use in this invention include polyarylene poly ethers, polypropylene, acrylonitrile-butadiene-styrene (ABS) copolymers, polyhydroxyether, impact polystyrene, styrene-acrylonitrile copolymers, polycarbonates, poly-4-methylpentene-l, phenoxys, polyoxymethylenes, polymethacrylates, polyamides and the like.

The preferred class of thermoplastic polymers comprises those with a high degree of rigidity and little or no crystallinity, in which the softening temperature and the glass transition temperature coincide. These poly mers are amorphous polymers and include polyarylene ethers, polycarbonates, polystyrenes, polymethacrylates, polyhydroxy ethers, and the like.

The glass transition temperature of a polymer as defined herein is taken as the center of a narrow temperature range where the polymer changes from a viscous or rubbery condition at temperatures above this region to a hard and brittle condition below it due to a lack of sufficient thermal energy to permit rotation of the segments of the polymer chain. This transition is equivalent to the solidification of a liquid to a glass. It is designated as the second order or glass transition temperature. At this temperature the polymer exhibits changes in specific volume, heat content, thermal conductivity, coefficient of thermal expansion and especially stiffness as measured by modulus of elasticity. The glass transition temperature of an amorphous polymer is, therefore, described herein as the softening temperature. A typical method of determining the glass transition temperature is to make measurements of specific volume as a function of increasing temperature. At the transition region the slope of this curve changes abruptly to a larger value. Examples of glass transition temperatures of typical hard, amorphous polymers, suitable for use in this invention are:

tam

MATERIAL TgF Polysulfone 375 Polyphenylene oxide 400 Bisphenol A polycarbonate 302 5 Acrylonitrile, butadiene styrene, terpolymer 230 Polystyrene 210 Polymethylmethacrylate 200 Crystalline polymers on the other hand, soften at their crystalline melting points, which are usually substantially higher than their glass transition temperatures.

Examples of crystalline polymers useful in this invention are poly-4-methylpentene-l, polyoxymethylenes, polyamides, and polypropylenes. The crystalline melting points of these polymers are usually determined by observing the temperature at which cloudiness in the polymer sample disappears as temperature increases, indicating complete dissolution of the ordered crystalline region of the solid polymer.

Poly-4-methylpentene-l melts at 465F and polypropylene melts at 335F.

As employed herein the term softening temperature means the glass transition temperature of amorphous polymers and the melting point of crystalline polymers and is intended to mean the point at which a polymer is capable of being embossed.

The preferred thermoplastic materials for use in molding matrices in accordance with our invention are polycarbonates, polyarylene polyethers and polyhydroxyethers.

Suitable polycarbonates have recurring structural units of the formula:

wherein B is a divalent aromatic radical of a dihydric phenol.

Suitable polyarylene polyether polymers have a basic structure which includes recurring units having the formula:

wherein E is the residuum of the dihydric phenol and E' is the residuum of the benzenoid compound having an inert electron withdrawing group in at least one of the positions ortho and para to the valence bonds, and where both of said residua are valently bonded to the ether oxygens through the aromatic carbon atoms. The foregoing polyarylene polyethers and their preparation are described in detail in U.S. Pat. No. 3,380,878.

Matrices can be formed from other thermoplastic polyarylene polyethers such as those composed of recurring units having the formula:

a n R Q- RI wherein the free valence of the terminal oxygen atom of one unit is connected to the free valence of the terminal benzene nucleus of the adjoining unit, b is an integer of from 0 to l, inclusive, R is a monovalent substituent selected from the group of hydrocarbon radicals, halohydrocarbon radicals having at least 2 carbon atoms, R and R" are the same as R and in addition hydrogen. The foregoing polyarylene polyethers and their preparations are described in U.S. Pat. No. 3,341,753.

Polyhydroxyethers useful in the practice of this invention are substantially linear polymers having the formula:

wherein D is the radical residuum ofa dihydric phenol, D' is a hydroxyl containing radical residuum of an epoxide and n represents the degree of polymerization and is at least 30 and is preferably 80 or more.

Thermoplastic polyhydroxyethers and their preparation are described in U.S. Pat. No. 3,245,865,

Generally the polysulfone thermoplastic materials described above require somewhat higher molding temperatures than are required by polycarbonates which can usually be molded at relatively lower temperatures. The use of low molding temperatures is highly desirable as it avoids an excessive buildup of heat in the original form and tends to minimize damage to the original.

The scope of the present invention is not limited to the particular matrix materials disclosed herein. A wide variety of thermoplastic materials can be used for producing matrices provided only that the physical properties of the finished matrix are such that printing plates, sound records, and the like can be molded from such matrices.

Matrix blanks can be formed from thermoplastic pellets or sheets. However, it is preferred that they be formed from sheets.

Sheets suitable as blanks in the process of the present invention can be fabricated by any known thermoplastic forming technique such as extruding, compression molding, injection molding, solution casting, and the like. The thickness of the sheets employed is not critical but is rather governed by practical considerations such as cost and ease of forming. in general, the most useful range of thickness for thermoplastic sheets is from about 0.030 inch to about 0.250 inch while the range of from about 0.070 inch to about 0.125 inch is preferred.

As noted above, the physical characteristics of the thermoplastic selected for forming the matrix, dictate the time and temperature parameters of the process.

When polycarbonate is used (the softening point of polycarbonate is 302F.) the temperature desired at the molding interface at the time of embossing is approximately 3023 30F. In the case of polysulfone an interface temperature of about 375-400F. is necessary.

It is desirable to maintain the lower platen at a relatively low temperature and to effect heating of the matrix blank primarily through tne upper platen in order to minimize damage to the original and to reduce the possibility of flow into deep crevices which may be present in the original. This is particularly important where lead typeforms are used as the original.

When polycarbonate is used as the matrix material, the lower platen is preferably maintained at a temperature of about 250F. while the upper platen is maintained at a temperature of 700F. Where polysulfone is used for the matrix material a higher lower platen temperature is necessary, about 325F., due to the higher softening point of this material.

Heat can be supplied by any conventional heating means, such as a heating plate. The heat is directed from the backside of the thermoplastic sheet. Preferably, the temperature of a hot plate approximates the temperature at which the polymer flows readily and is about 700F. in the instance where the thermoplastic material is polysulfone.

In the first or preheat step of our process the matrix blank is heated to a point at or slightly above the softening point. The amount of time required to achieve this level is governed by the platen temperatures and to a slight extent by the efficiency of the insulating material interposed between the original and the matrix blank. It should be noted that the particular insulating material selected is not critical in the practice of our invention so long as it lends itself to clean and rapid re moval from the original and the heated matrix blank, Where polycarbonate is selected as the matrix material a preheat time of approximately thirty seconds is adequate to raise the temperature of the blank to the softening point when the lower and upper platen temperatures are maintained at 250F. and 700F., respectively. It is desirable that a very slight pressure be applied dur ing this step, on the order of one or two pounds pressure to insure adherence of the hot matrix blank to the release sheet to facilitate handling.

It will be understood that the pre-heat step need not be conducted in the same press used for active molding of the matrix although this is customary to minimize capital expense. Preheating may be effected in a separate apparatus. When preheating is effected separately it may still be desirable to maintain a temperature differential between the upper and lower platens to avoid oversoftening of the embossing surface of the matrix blank. However, the use of an insulating material may be dispensed with as there will be no original present which requires protection against overheating. An efficient release agent or release sheet may be used to keep the matrix blank from adhering to the lower platen. Any conventional release agent or material may be used for this purposev It is essential in the practice of this invention that the time and temperature parameters selected for the preheat step be sufficient to raise the temperature of the thermoplastic matrix blank to or slightly above the softening point. This requirement is an important distinc tion between our process and prior art processes in which the matrix blank is heated to a point not higher than from about 25F. to about F. below the soft ening temperature of the matrix material during the preheat cycle in an effort to prevent deep plastic flow into the original and to avoid damage to the original due to overheatingv In the practice of our invention there is no need to alter the platen temperatures. Removal of the insulat ing material between the original and the heated matrix blank permits the lower platen to make a greater contribution to the heating of the matrix during the embossing step.

We have observed however that in the course of producing matrices by our method, unless cooling means or temperature regulating means are provided there tends to be a temperature rise on the order of from 5 to l0F. in the lower platen during each molding cycle. A temperature rise greater than about 10F. in the lower mold is not desirable and should be avoided. This can be accomplished by providing cooling or tempera- 7 ture regulating means or simply by leaving the press open with the heat off until the desired reduction in temperature is achieved.

Following the preheat step, the insulating material is removed and the hot matrix blank and release sheet inserted into the press. The handling of the matrix blank is quite rapid and convenient due to its adherence to the metallic, preferably steel, release sheet.

The steel or metallic blank used in the practice of our process has a second important function in addition to its contribution to the handling of the hot blank. The excellent adhesion obtained between the matrix blank and the release plate prevents the creation of voids on the backside of the matrix which commonly occurs in conventional processes as a result of sagging. The creation of such voids causes severe problems in maintain ing proper gauge when the finished matrix is ued for the production of positive replicates.

After insertion into the press. the embossing operation is conducted in the usual manner. A wide range of molding pressures may be used. Pressures may vary from 200 psi to 2000 psi but are preferably from about 200 to 500 psi, the lower pressures being desirable to minimize damage to the original. Pressure is generally applied over an interval of from about it) to about 120 seconds. lt is preferred to maintain a very slight pressure. on the order of 2 to about l psi for a short initial period of the order of it) to 20 seconds before applying high pressure to effect embossing. This low pressure interval or contact time facilitates temperature equaliza tion within the matrix blank and permits some flow of plastic which tends to nullify pressure buildup on high points resulting from irregularities in the original.

After completion of the embossing step the matrix is allowed to cool, under pressure to a point conveniently below the softening point of the thermopiastic. After cooling the press is opened and the matrix removed and separated from the release sheet. ln general the matrix separates readily from the original without the use of a mold release agent. Mold release agents, such as graphite, silicone oils, molybdenum disulfide and the like may be used, if desired, to facilitate separation. The finished matrix after such trimming as is necessary may now be used for production of duplicate originals, such as printing plates, sound recordings and the like. in the conventional manner.

The following specific example is provided to more clearly illustrate our invention.

EXAMPLE A one page tabloid size type form. measuring 10 k inch X 14 inch was locked in a standard steel newspaper chase. The typeform was placed on the lower platen of a molding press which was heated to 250F. for a period of about 15 minutes to raise the printing surface of the type to 225i 10F. A sheet of Synthane (3-7 insulating material of one-sixteenth-inch thickness was placed over the type form covering the entire surface. An extruded sheet of polycarbonate. having a glass transition temperature of 302F. and a 264 psi heat distortion temperature of 270F. measuring l6% inch X l 2 /9 inch X 0.080 inch was placed over the insulation and typeform. A steel release sheet lightly coated with graphite was then placed over the polycarbonate, covering the entire surface. The press was equipped with a 700 heated steel plate. having a sheet of high temperature insulation large enough to cover the ma- LII trix blank interposed between the upper surface of the heated steel plate and the lower surface of the upper platen to effect thermal isolation of the upper platen from the heated steel plate. The press was then closed on the sandwich for 30 seconds at contact pressure which raised the temperature of the polycarbonate matrix blank to a point slightly above its softening temperature of 302F. The press was then opened and returned to the starting position. The steel release sheet was lifted and removed carrying with it the now soft and sticky matrix blank. The insulating sheet was then removed and the steel release sheet and hot matrix blank reinserted into the press once again covering the original type form. The press was then closed and held at contact pressure for 20 seconds after which time pressure was increased to 200 psi (hydraulic pressure) and held for 60 seconds to achieve the desired embossing of the matrix blank by the original type form. The press was then opened, the heated steel plate retracted, and the press re-closed to cool the matrix against the relatively cold upper platen of the press for 25 seconds at 400 psi (hydraulic pressure).

Upon completion of the cooling cycle the press was opened and the matrix and release sheet removed. The matrix was easily separated from the original type form and steel release plate and was found to be an excellent reproduction of the type detail and had uniform floor thickness throughout the image areas and was without voids on the backside.

This complete cycle was repeated 30 times producing in each case a high quality matrix from the same typeform. It was found however that a temperature rise of from about 5 to l0F. was experienced in the type form temperature after each full cycle. To maintain the proper temperatures the type form was removed from the press and cooled between each cycle. After 30 pressings only minimal damage or blunting of the original typeform was detected and it could have been used, if desired, for the production of additional high quality matrices.

This experiment was repeated using Japanese mono type letters in combination with standard linotype letters. The Japanese monotype letters were found to be as much as 0.005 inch higher than the standard 0.918 inch linotype letters. Excellent results were obtained following the identical procedure described above for the production of 30 matrices. Damage to the typeform original was minimal despite the high concentration of initial pressure on the higher Japanese monotype letters.

These experiments are considered particularly significant due to the use of one tabloid sized page type form as opposed to two tabloid pages together or one standard size page, which is customary, resulting in the application of essentially twice the normal pressure to the ty e form surface during the molding cycle. The excellent results achieved indicate that with careful control of conditions and careful handling, a virtually unlimited number of matrices can be produced from a single type form.

What is claimed is:

l. A process for forming a highly accurate thermoplastic matrix which comprises:

a. placing an original form on the lower platen of a press,

b. covering the exposed surface of said original form with a thin layer of insulating material;

c. placing a thermoplastic matrix blank over said insulating material;

d. covering said matrix blank with a metallic release plate;

e. closing said press and applying pressure to said matrix blank not greater than essentially contact pressure while applying heat sufficient to raise the temperature of said thermoplastic blank to a point above the softening point, but below the flow point of said thermoplastic while maintaining the said 10 original form at a temperature below the point at which sufficient softening of the original form occurs to cause significant deformation of said original form when said pressure is applied;

f. opening said press and removing said insulation material, matrix blank and release plate;

g. stripping said insulation material from the surface of said matrix blank;

h. reinserting said matrix blank and release plate into said press so as to place the exposed surface of said matrix blank in contact with said original form;

i. closing said press and supplying sufficient heat to said matrix blank to raise the temperature of at least a portion of said thermoplastic matrix blank to a point above the flow point of said thermoplastic;

j. applying sufficient pressure so that the thermoplastic blank is embossed at the molding interface so as to replicate the surface details of the original form; and

k. cooling the resulting matrix and separating the said matrix from the original form.

2. A method in accordance with claim 1 in which a release agent is interposed between said matrix blank and said release plate prior to heating.

3. A method in accordance with claim 1 in which the lower platen of said press is maintained at a temperature of about 250F. and the upper platen of said press is maintained at a temperature of about 700F.

4. A method in accordance with claim 1 in which the molding pressure applied in step (j) is from about 200 to about 2000 psi.

5. A method in accordance with claim 1 in which the molding pressure applied in stepj is from about 250 psi to about 500 psi.

6. A method in accordance with claim 1 in which the lower platen of said press and said original form are maintained at a temperature below about 300F.

7. A method in accordance with claim 6 in which the upper platen of said press is maintained at a temperature above about 650F.

* i l l

Claims (7)

1. A process for forming a highly accurate thermoplastic matrix which comprises: a. placing an original form on the lower platen of a press; b. covering the exposed surface of said original form with a thin layer of insulating Material; c. placing a thermoplastic matrix blank over said insulating material; d. covering said matrix blank with a metallic release plate; e. closing said press and applying pressure to said matrix blank not greater than essentially contact pressure while applying heat sufficient to raise the temperature of said thermoplastic blank to a point above the softening point, but below the flow point of said thermoplastic while maintaining the said original form at a temperature below the point at which sufficient softening of the original form occurs to cause significant deformation of said original form when said pressure is applied; f. opening said press and removing said insulation material, matrix blank and release plate; g. stripping said insulation material from the surface of said matrix blank; h. reinserting said matrix blank and release plate into said press so as to place the exposed surface of said matrix blank in contact with said original form; i. closing said press and supplying sufficient heat to said matrix blank to raise the temperature of at least a portion of said thermoplastic matrix blank to a point above the flow point of said thermoplastic; j. applying sufficient pressure so that the thermoplastic blank is embossed at the molding interface so as to replicate the surface details of the original form; and k. cooling the resulting matrix and separating the said matrix from the original form.

2. A method in accordance with claim 1 in which a release agent is interposed between said matrix blank and said release plate prior to heating.

3. A method in accordance with claim 1 in which the lower platen of said press is maintained at a temperature of about 250*F. and the upper platen of said press is maintained at a temperature of about 700*F.

4. A method in accordance with claim 1 in which the molding pressure applied in step (j) is from about 200 to about 2000 psi.

5. A method in accordance with claim 1 in which the molding pressure applied in step j is from about 250 psi to about 500 psi.

6. A method in accordance with claim 1 in which the lower platen of said press and said original form are maintained at a temperature below about 300*F.

7. A method in accordance with claim 6 in which the upper platen of said press is maintained at a temperature above about 650*F.