Printing Matrix or Mold Component Formed From an Aminoplast Resin-Polyvinyl Alcohol Reaction Product

1. Field of Use

While the invention is not limited to any particular field it has specific utility in the production of printing plates, pattern plates and other marking and shaping items.

2. Description of Prior Art

In the newspaper industry, for many years printing plates have been made of lead formed against a matrix of wood pulp, clay and water.

Recently, to expedite printing plate duplication, plastic printing plates have been tried on a limited scale in place of the lead plates. In one process, a magnesium metal master or pattern plate is prepared by a photographic and etching process. A negative (or matrix) is then formed by pressing a plastic against the metal to form a reproduction (negative) of the master plate. The plastic matrix is then used to produce a number of "positive" copies for the actual printing function when multiple press operation is employed. The copies ("positives") are formed by molding or pressing thermoplastics or rubber elastomers against the "negative" matrix.

In another process, instead of the metal "master" an organic plastic "master" or pattern plate is made using a liquid polyester polymer which is cured by transmitting ultra-violet light through a photographic negative or transparency of the printed format. In the areas struck by the ultra-violet light, the resin is hardened. In the areas "screened" from the ultra-violet light, the resin remains liquid and is washed away by an aqueous developing solution.

The polyester master or "positive" is then used as described above as a substitute for the metal master to make a "negative" matrix which in turn can be used to produce multiple "positive" copies. The copies are then used as the actual printing plates. (A process using the polyester concept is sold by Hercules Incorporated, Wilmington, Del. under the trademark "Merigraph".)

However, the above prior processes have inherent deficiences as the matrix plates (negative plates) have been susceptible to taking a compression-set on repeated uses. As a result, the printing plate copies or duplicates made from the negative matrix have letters of different heights. The printed copy is thus blurry, smudged, and hard to read. Certain areas might have too much ink and others too little.

One conventional matrix concept uses a non-dense core of an asbestos-cellulose composition which has a phenolic resin coating. However, when a moldable material such as polypropylene is molded against the matrix, the core gradually flows and compresses in thickness providing uneven print characters. Moreover, the phenolic resin of the matrix is susceptible to flex-cracking and peeling from the core material.

Other problems have been inherent with the prior phenolic-type matrix plates. When formed against the organic polyester "masters", the pressures required to form the phenolic "negative" matrix have caused the print or characters on the organic polyester to collapse. The phenolics also have a tendency to stick and bond to the polyester. In addition, the phenolic matrix plates cannot be bent easily to the curved surface of a mold platen (having substantially the same diameter as a printing press cylinder) without cracking.

The present invention overcomes the deficiences of the prior art by providing a matrix which can be used against either metal or plastic masters to receive very sharply defined impressions. The imprinted material then serves as a mold against which a plastic material is molded to form multiple "positive" duplicates for use as the actual printing plates.

For this purpose, the matrix must be capable of becoming dense when cured so that it will not take a compression-set when hot molten plastics under pressure are molded against it; it must be flexible so that it can be shaped to take the contour of a printing press drum or cylinder without cracking; it must have good release properties from other plastics, particularly the polyesters, without bonding, sticking or distorting; it must be porous during its initial forming to allow for the expulsion of air and gases; it must be capable of being molded against a "master" (such as the polyesters) under relatively low temperatures and pressures so as not to cause the images and characters of the master to collapse and flow laterally and it must be able to withstand the molding thereagainst of other plastics such as the polypropylenes without taking lateral and vertical flow compression-set. The present invention accomplishes the foregoing by means of a novel matrix which includes an amino-plast-polyvinyl alcohol product.

FIG. 1 is a plan view showing a matrix in sheet form prior to shaping or molding.

FIG. 2 is an exploded side view showing the matrix of FIG. 1 being placed between molding platens for forming against a "master" ("positive") printing plate.

FIG. 3 shows the matrix, after forming in FIG. 2, bent or deflected to a cylindrical curvature and used as a part of a mold against which a plastic material can be injected in the making of multiple "positive" printing plate copies.

FIG. 4 shows a "positive" duplicate printing plate after molding to the curvature in FIG. 3 mounted on a printing press cylinder.

While the invention has many applications, it will be specifically described with reference to its application in the forming of printing plates and similar articles.

As shown in FIG. 2, a matrix sheet 10 of the invention is shaped or formed under heat and pressure against a "master" ("positive") printing plate 11 mounted in a molding press shown generally at 12. The press is shown schematically to have a top platen 13 and a lower platen 14.

The master ("positive") printing plate 11 is used as a pattern and may be of metal such as magnesium or of plastic such as polyester. The sheet 10, which contains a moldable resin (as will be described in detail later) is then formed or molded by closing of the platens 13 and 14. Shaping is done at pressures of 20 to 200 psi and preferably in the range of 50 to 200 psi at temperatures generally between 225.degree. F. to 370.degree. F. depending on the material of the master 11.

After curing, the "molded" negative matrix (shown at 10' in FIG. 3) may then be bent or curved to approximate the curvature of a printing press cylinder and inserted in an injection molding press shown schematically at 15 (FIG. 3) having a top plate 16 and a bottom platen 17. In this view, the molded matrix negative 10' is shown curved and cradled in the bottom platen 17 with the imprinted image 18 thereon on the concave or upper side of the bend. The top platen 16 is then moved into clamping engagement relative to the bottom platen 17 as shown by the dotted line position.

After the top platen 16 is moved into the dotted-line position, a duplicate "positive" printing plate may be made by molding a plastic material against the curved negative matrix 10'. As shown in FIG. 3, polypropylene or the like may be injected through the sprue opening 19 into a relief cavity 20 in the top platen 16 overlying the molded matrix 10' to make a duplicate "positive" printing plate 21 (FIG. 4). The duplicate printing plate 21 upon molding has the curvature of the mold base 17 so as to substantially match the curvature of a printing cylinder 22. As will be noted in FIG. 4, the duplicate plate 21 has its printing images or characters 23 on the convex side of the plate and thereby correspond with those of the master plate 11.

Now in accordance with the invention, in order to provide multiple "positive" copies or duplicates without showing any creep or compressive deformation under successive moldings, the formed matrix 10' must be dense and homogeneous. In prior concepts, the matrix would compress and successive positive duplicates would vary in thickness. This resulted in uneven printing effects because of the variation in the relief depth of the print characters.

To accomplish these requisites, in its broadest aspect, the present invention includes the reaction of the components of an aminoplast system with polyvinyl alcohol in the range of from 0.75 to 2.6 parts by weight of the aminoplast to 1 part by weight of polyvinyl alcohol, with a preferred range being 1.0 to 1.5 parts of the aminoplast to one part of polyvinyl alcohol. As is well known, the aminoplasts are synthetic resins of the thermosetting type which are prepared by reacting an organic compound containing an amino group with an aldehyde or ketone. Typical of the amino group are urea, thiourea, cyclic ethylene urea, dicyandiamide, melamine, and alkyl and aryl melamines.

The molecular weight of the polyvinyl alcohol can vary between 2,000 to 120,000. However where lower molding pressures are required, a polyvinyl alcohol with a molecular weight range of about 2,000 to 14,000 has been found very desireable. The polyvinyl alcohol is available in various percentages of hydrolysis, and for some of the lower or medium pressure molding applications, a polyvinyl alcohol with a 60% to 70% hydrolysis and a molecular weight in the range of 2,000 to 4,000 produces optimum flexibility and release with good compression-set resistance.

As for the preparation, the aminoplast-polyvinyl alcohol product can be prepared with conventional equipment. A typical sequence involves mixing a constituent from the amino group and an aldehyde or ketone, adjusting the pH of the mixture in the range of about 7.5 to 9 and refluxing until a drop of the mixture hydrophobes (precipitates) in an excess of water at approximately 0.degree. C. The polyvinyl alcohol is then added and the reaction continued at reflux or at a suitable reaction temperature generally in the range of 180.degree. F. to 220.degree. F. for a suitable length of time. The reaction product may then be dehydrated to produce a fusible partially water soluble product. The dehydrated product may then be cured to an insoluble infusible form by heating at temperatures in the general range of 225.degree. F. to 390.degree. F.

As an ideal matrix, the sheet 10 may be made by applying the aminoplast-polyvinyl alcohol resin (combined with suitable well known fillers and lubricants as will be described later) to a non-woven or semi-porous carrier such as a glass fiber mat. The mat may then be handled easily and contain a predetermined amount of resin. Excellent results have been obtained using a resin content of at least 85% by weight to 15% by weight of the porous carrier with optimum results being approximately 90% by weight of the resin to 10% by weight of the carrier.

After the mat is cured in a mold 12 (FIG. 2), a matrix 10' is formed which is flexible and pliant and enables it to be bent or shaped for use as a mold insert 10'. In addition to being resilient and pliant, the matrix 10' in accordance with the invention has superior release properties from polypropylene and other printing plate materials which are molded against it. This is extremely important for print fidelity and definition since slight bonding or sticking to the matrix 10' can cause the print character to smudge or blurr.

In this context, for many newspaper printing operations the ability to produce many "positive" plate copies is extremely important because of the number of presses required to run the same format concurrently to meet high volume circulation requirements. The present invention thus provides a means of quickly making duplicate printing plates (positives) economically out of thermoplastics such as polypropylene and the like which may be recycled by regrinding and remelting after they are used.

Typical examples of the invention for purposes of illustration only and not for purposes of limitation follow.

Mix 125 parts by weight of melamine with 200 parts of Formalin (37% CH.sub.2 O--formaldehyde) and adjust the pH of mixture to 8-9. Heat the mixture, maintaining this pH at reflux until a drop of mixture hydrophobes (precipitates) in an excess of 0.degree. C. water. At this point 150 parts by weight of polyvinyl alcohol with a commercial molecular weight range of 2000 to 4000 is added and reaction continued for 40 minutes at reflux. The reaction product is then dehydrated, preferably with vacuum to produce a white fusible, partially water soluble product. From this product, thirty-five parts (by weight) thereof are mixed with 5 parts of Carbowax (as a hot melt processing aid), 1 part of zinc stearate (lubricant), 1 part of pigment for color and 51 parts of Wollastonite filler (calcium silicate), and 5 parts of water as a processing aid. This resin filler mixture is then applied by hot melt processing to a glass mat so as to result in an uncured matrix prepreg of 90% resin mixture (by weight) and 10% glass mat approximately 0.075" thick. The resultant uncured matrix is then utilized to make a cured matrix (negative) plate approximately 15".times.24".times.0.075" by molding and curing at 325.degree. F. and at 100 psi against a magnesium or polyester "master" plate.

The resultant matrix produced by pressing and curing the melamine-polyvinyl alcohol prepreg against the polyester "master" plate showed exceptional release qualities from the polyester as well as excellent flexibility. (If a conventional matrix of the phenolic coated asbestos-cellulose core is used against a polyester master, excessive streaking results making an unacceptable matrix.) The cured matrix plate of the invention was utilized as a mold insert in an injection molding machine and bent to an arc generated by an approximate 15" diameter as used on printing press cylinder. Polypropylene was then injection molded against the matrix to produce duplicate "positive" printing plates. The reproduction of the print from the polypropylene duplicate plates was excellent.

React 120 parts of urea (2.6 moles approx.) and 325 parts (4 moles approx.) of Formalin (37% CH.sub.2 O formaldehyde) at reflux temperature for 30 minutes a pH of 7. At this point add 125 parts of polyvinyl alcohol and continue heating for 30 minutes. This product is dehydrated to a fusible powder and then mixed with 50 parts silica filler, one part glycerol monostearate lubricant, 5 parts of Carbowax, 1 part pigment and 4 parts water. The mixture is applied to a glass mat carrier in the ratio of 25 parts resin mixture to 15 parts glass to give a matrix prepreg. The matrix is utilized to make a plastic "negative" printing plate by molding against a Merigraph polyester master at 100 psi and 300.degree. F.

Mix 125 parts by weight of melamine with 200 parts of Formalin (37% CH.sub.2 O--formaldehyde) and adjust the pH of mixture to 8-9. Heat the mixture, maintaining this pH at reflux until a drop of mixture hydrophobes (precipates) in an excess of 0.degree. C. water. At this point 75 parts by weight of polyvinyl alcohol are added and reaction continued for 40 minutes at reflux. The reaction product is then dehydrated, preferably with vacuum to produce a white fusible, partially water soluble product. From this dehydrated product, thirty-five parts (by weight) thereof are mixed with 5 parts of Carbowax (as a hot melt processing aid), 1 part of zinc stearate (lubricant), 1 part of pigment for color and 51 parts of Wollastonite filler (calcium silicate), and 5 parts of water as a processing aid. This resin filler mixture was then applied by hot melt processing to a glass mat so as to result in an uncured matrix prepreg of 90% resin mixture (by weight) and 10% glass mat.

The resultant matrix was then utilized to make a plastic "negative" printing plate by molding and curing at 325.degree. F. and 100 psi against a polyester "master" or "positive" plate (as in Example I).

This formulation was generally unsatisfactory for the production of duplicate printing plates as cracking occurred when the cured matrix was bent to the approximate 15" diameter to be used for a printing plate mold insert (against which it was desired to mold a polypropylene duplicate "positive" printing plates).

Mix 125 parts by weight of melamine with 200 parts of Formalin (37% CH.sub.2 O--formaldehyde) and adjust the pH of mixture to 8-9. Heat the mixture, maintaining this pH, at reflux until a drop of mixture hydrophobes (precipates) in an excess of 0.degree. C. water. At this point 280 parts by weight of polyvinyl alcohol are added and reaction continued for 40 minutes at reflux. The reaction product is then dehydrated, preferably with vacuum to produce a white fluxible, partially water soluble product. From this product, thirty-five parts (by weight) thereof are mixed with 5 parts of Carbowax (as a hot melt processing aid), 1 part of zinc stearate (lubricant), 1 part of pigment for color and 51 parts of Wollastonite filler (calcium silicate), and 5 parts of water as a processing aid. This resin filler mixture is then applied by hot melt processing to a glass mat so as to result in an uncured matrix prepreg of 90% resin mixture (by weight) and 10% glass mat. The resultant matrix was then utilized to make a plastic "negative" printing plate by molding and curing at 325.degree. F. and 100 psi against a magnesium or polyester "master" plate.

When used as a mold insert, the resultant matrix printing plate was generally unsatisfactory as it showed high compressive-set characteristics. The letters of the matrix compressed to different heights when polypropylene printing plates were molded against it thereby providing uneven and blurred printed copies.

While, the invention has been described in conjunction with the use of a glass fiber mat, other forms of mats can be used such as those with the common vegetable and mineral fibers. Likewise, while Wollastonite is used as a filler, others can be used as can other lubricants and hot melt processing aids which are well known. Also, while the cured matrix has been described in conjunction with its use as a "negative" to form duplicate positive plates, the cured and formed mat may be used directly as a "positive" printing plate for some applications.