US2707801A - Mold - Google Patents

Description

G. E. GARD MOLD Filed Feb. 20, 1953 May 10, 1955 E .c im HO S INVENTOR GEORGE E. GARD ATTORNEY United States Patent O MOLD George E. Gard, East Hempfield Township, Lancaster County, Pa., assigner to Armstrong Cork Company, Lancaster, Pa., a corporation of Pennsylvania Application February 20, 1953, Serial No. `338,107

6 Claims. (Cl. 18-34) This invention relates to a mold for confining a charge of dielectric material during processing by dielectric heating, in which the charge of material and the mold are both dielectrically heated simultaneously. As disclosed in Scott Patent 2,526,698, it is desirable in the manufacture of cork compositions, for instance, where dielectric heating is employed to activate the binder, to employ a mold the side walls of which are made of a dielectric material such as hard wood impregnated with ceresin wax. As disclosed in the Scott patent, such walls have a dielectric loss factor between .l and .4 and are so disposed in the mold assembly with the electrodes connected to the source of high-frequency power supply that the walls are heated simultaneously with the charge of cork composition disposed within the mold.

In commercial practice under the Scott patent a problem has arisen when entirely different cork composition charges are run successively. For instance, after baking a glue, glycerine, and formaldehyde bound cork composition as disclosed in the Scott patent, followed by a run of cork composition bound with a phenol-aldehyde resin, difficulty is encountered on the next cycle of operation again employing the glue-bound composition. The reason for this is not clear. One possible explanation is that during the manufacture of the glue-bound composition a film of binder and moisture remains on the inner surface of the wooden mold walls. So long as a similar cork composition is being made in these molds, the binder and moisture being at a level in equilibrium with the composition being produced, no deleterious results are noted. However, when these molds are used to produce a diiferent cork composition such, for instance, as a mat for the production of cork tiles employing a binder of phenol-formaldehyde resin and no glycerine, the binder has a high vapor pressure; and, during repeated curing cycles in the molds in the manufacture of such cork tile compositions, the ilm of glue on the inner surface of the mold walls adsorbs water from the cork tile compositions. This does no harm so long as cork tile compositions continue to be run in the mold, for the dielectric properties of the thin film are not so essentially different from those of the cork tile composition being cured as to adversely affect the curing of the composition. This is not true of other cork compositions, however; and should cork tile composition be followed by a glue, glycerine, formaldehyde composition in the molds, the adsorbed moisture in the film on the interior of the mold walls is transferred to the outer area of the charge of cork, glue, and glycerine, rendering this area relatively higher in dielectric loss factor than the remaining portion of the mass, and nonuniform heating results. Regardless of the theory involved, it is known from actual cornmercial practice that the outer surface of the charge of cork, glue, glycerne, and formaldehyde will be charred and the product will be undercured internally on the first cycle following a run of cork tile composition having a binder of phenol and formaldehyde, making it impractical commercially to produce alternately cork tile and general 2,707,801 Patented May 10, 1955 ICC industrial cork compositions with glue binders in the same molds. The molds are large and expensive, and it is not economically feasible to have different molds for different cork compositions.

The principal object of the present invention, therefore, is to provide a mold for dielectric heating which may be uesd alternately for runs of different materials, such as the cork tile and the glue-bound industrial compositions mentioned.

Another object of the invention is to provide a mold which will be useful for confining charges of materials of various sorts for dielectric heating in which the charge and the mold walls are both dielectrically heated simultaneously, being equally satisfactory for use with phenolformaldehyde resin-bound cork tile composition and for use with glue-glycerine and formaldehyde cork composition, gl-ueand rubber-bound compositions, and the like.

Other objects of the invention will be apparent from the following detailed description of an embodiment of the invention.

According to the invention, the conventional wood mold as disclosed in the Scott patent referred to and in Lockwood Patent 2,625,710 is provided on its inner wall surfaces with a liner which possesses the following characteristics:

l. A dielectric loss factor between about .0002 and 0.1, at a frequency of 10 megacycles and at room temperature, 70 F., and in all events lower than the dielectric loss factor of the mold wall and the material to be heated in the mold. This provides for simultaneous heating of the charge in the mold and the walls of the moid, with the inner surface in contact with the charge heating at a rate less than the rate of heating of the mold walls, minimizing the danger of overcuring or charring of the composition in contact with the liner.

2. Good resistance to absorption of moisture, providing less than 1% absorption upon immersion in water for 24 hours. This characteristic is necessary to avoid the problem of substantial change in dielectric characteristics of the mold on runs of different compositions within the molds.

3. A smooth surface to which the cured binder will not adhere with greater strength than that of the mass so that upon removal of the mass from the mold, parting will be effected in the interface between mass and wall.

This will prevent excessive build-up of contaminating quantities of binder on the interior of the mold which would affect subsequent runs of different compositions as in the resin-bound cork tile composition and gluebound industrial composition mentioned above.

4. Resistance to deterioration under repeated cycles of heating and cooling. The charge may be heated to about 215 F. in cork tile composition manufacture and the mold reduced almost to room temperature before its next heating cycle. The mold walls adjacent to the charge will attain a similarly high temperature during the heating cycle and must, therefore, not be deleteriously affected by repeated heating and cooling conditions encountered in use. In some products curing may be eected at higher temperatures, in the order of 250 F. for some cork compositions, for instance.

5. Mechanical strength adequate to withstand the pressures encountered in molding without excessive deformation or ilow. While the liner is fully supported by the wooden mold walls, considerable pressure is exerted against the walls by the charge, particularly in cork compositions of high density, such as cork tile compositions where the mass may be reduced from five volumes to one volume as it is compressed in the mold for baking. Other cork compositions are more dense, and the mass is reduced from ten volumes to one volume prior to heating. Requisite mechanical strength to receive and hold fassponsor tening means is also necessary, for it is preferred to secure the liner to the mold walls by wooden screws er other mechanical fastening devices of dielectric material.

6. Good resistance to arcing. The liner must be adequately resistant to arcing to avoid damage to the li'ner duringservice. v

' 7. A thickness which is not more than about one-fourth of the total thickness of the mold wall'. This is necessary with a mold wal-l backing having a higher dielectric loss factor than the liner to provide the requisite heating of the mold liner by conduction of heat from the mold wall' backing, supplementedI by the dielectric heating of the mold liner.

There are a number of commercially available materials which possess the foregoing characteristics, any of which may be employed in my new combination with a wooden mold wall. One such material is made of continuous lament glass fabric impregnated and bonded with a silicone resin and sold by The. Formica Company, Cincinnati, Ohio, under the trade designation Formica Grade G-7'. This material in sheet form 1A" thick has the following properties:

1. Average loss factor at 10 megacycles and 70 F., .005.8.

2.. Maximum water absorption of l" x 3" specimen after 24 hours. immersion, 0.35%.

3. The surface provides excellent stripping properties for cork composition with resinous as well as glue binders, parting occurring at the interface.

4. No` deterioration observed after hundreds of heating and cooling cycles with temperatures in the range of 90 F. to 215 F.

5. Tensile strength, 14,000 pounds per sq. inch. Compression strength, atwise, 37,000 pounds per sq. inch. Shear strength (ASTM 13732-46), 18,000: pounds per sq. inch.

6. Arc resistance (ASTM D-495), 250.

7. A sheet im thick may be used with a ceresin impreg nated wood mold wall 2% thick, making a total wall thickness of 2%", the liner constituting substantially less than 25% of the total thickness of the mold wall.

It will be observed from the listed properties that this material possesses to an excellent degree all of the characteristics recited above for a suitable lining material..

The mold of this invention may in the form shown in the attached drawing which illustrates a typical embodiment of the invention and in which:

Figure l is a detailed sectional view of a portion of a mold suitable for the dielectric heating of a charge of cork composition in the formation of a rectangular mat;

and

Figure 2 is a diagrammatic view, partially in section, i1- lustrating the mold connected to a source of high-fre.- quency alternating voltage.

The mold shown includes the general combination of t the Lockwood patent mentioned earlier. It includes side walls 2 and 3. The walls are made up of panels of wood. 4. Maple wood impregnated with ceresin wax will serve admirably. Contining end plates 5 and 6 which. constitute electrodes and form the top and bottom walls of the mold are provided, and these are coupled to a high-frequency source, as diagrammatically illustrated in Figure 2. Peripheral conductors 7 and 8 surround the edges of the mold walls and serve to create a more intensive field in the mass ancl mold walls than in, the remainder of the body as disclosed and, claimed in my Patent 2,663,790 and in Scott Patent 2,526,697.

Mounted in, good face-tofface contact with eachof the side walls of the mold on the inner surface. thereof are liners 9 formed of silicone resin impregnated glassffabric or other materialA possessing the requisite characteristics, listed above. The liner may be about, 1/s thick. It, is drilledV and countersunk and wooden screws 10 pass. through the drilled. openings andv are received within tapped openings provided inthe mold walls.

Preferably iii) the screws are made of maple wood and impregnated with ceresin wax. They thus have the same dielectric properties as the mold walls. The liners are preferably cut so that a single sheet covers each of the four side wall sections.

While the silicone resin impregnated glass fabric is the preferred liner material, other materials possessing the essential physical qualities recited may be substituted. The polytetraiiuoroethylene material sold by E. I. du Pont de Nemours & Company, Wilmington, Delaware, and commonly referred to as Tellon is suited for this surface. Itv has a lower loss factor (about .001) than the silicone resin-bound glass fabric referred to above and is externally resistant to moisture pick-up. It provides excellent stripping properties. Where it is employed the liners will generally have to be somewhat thicker than 1/s because of the tendency for the material to ilow under pressure, making it somewhat more diicult to secure the T etlon.- liner in position with mechanical fastening devices, such as the wooden screws mentioned. A melamine resin impregnated glass cloth liner will serve well also. Heat-resistant nylon having a loss factor of about .072 is also suitable. The particular liner material usedl will depend upon the types of materials to be heated in. the mold, the pressures encountered in molding, the temperatures at which heating will be effected, and other variable factors. The present invention isl not directed to a liner of a particular chemical composition.

I'claim:

l. I'n a mold for confining a charge of dielectric materi'al during processing by dielectric heating in which the charge of material and the mold are both dielectrically heated simultaneously, the combination of a mold side wall formed of a backing of hard wood impregnated with a waterproofingy impregnant and having a dielectric loss factor between .l and .4 and a liner of dielectric material, the ratio of the thickness of the liner to the total thickness of the mold wall of backing and liner being not more than 1:4, said liner constituting the exposed inner side wall` surface of said mold for contact with said charge of material and comprising a layer of sheet material selected from the group consisting of polytetrafluoroethylene,

silicone, melamine, and nylon and having a dielectric lossV factor between .001 and 0.1.

2. In a mold for confining a charge of dielectric material during processing by dielectric heating in which the charge of material and the mold are both dielectrically heated` simultaneously, the combination of a moldV side wall formed of a backing of dielectric material and a liner of dielectric material, the ratio of the thickness of the liner to the total thickness of the mold wall of backing and liner beingy not more thanL 1:4, said liner constituting the exposed inner side wall surface of said mold for contact with said charge of material and comprising a layer of sheet material having a dielectric loss factor less than that of said backing layer and dielectric fastening means passing through said liner and received within said backing and mechanically securing said liner and said backing together.

3. In a mold for confining a charge of dielectric material during processing by dielectric heating in which the charge of material and the mold are both dielectrically heated simultaneously, the combination of a mold side wall formed of a backing of dielectric material and a liner of dielectric material, the ratio of the thickness of the liner to-the total thickness of the mold wall of backingand liner being not more than 1:4, said liner constituting the exposed inner side wall surface of said mold for contact with said charge of material and comprising a layer of sheet material having a dielectric loss factor less than that of said backing layer and wooden screws having; essentially the same. dielectric properties as said backing passing throughv said liner and receivedy within said backingl and mechanically securing said liner and backing together.

4. in a mold for contining a charge of dielectric material during `processing by dielectric heating in which the charge of material and the mold are both dielectricall;I heated simultaneously, the combination of a mold side Wall formed of a backing of hard wood impregnated with a water-proofing impregnant and having a dielectric loss factor between .l and .4 and a liner of dielectric material, the ratio of the thickness of the liner to the total thickness of the mold wall of backing and liner being not more than 1:4, said liner constituting the exposed inner side wall surface of said mold for contact with said charge of material and comprising a layer of sheet mate* rial having a dielectric loss factor more than .00()2 and less than that of said backing and having a moisture absorption factor of less than l% upon total immersion in water for 24 hours at 70 F.

5. in a mold for confining a charge of dielectric mate rial during processing by dielectric heating in which the charge of material and the mold are both dielectrically heated simultaneously, the combination of a mold side wall formed of a backing of dielectric material having a dielectric loss factor between .l and .4 and a liner of dielectric material, said liner constituting the exposed inner 6 side wall suface of said mold for contact with said charge of material and comprising a layer of sheet material having a dielectric loss factor more than .0002 and less than that of said backing and having a moisture absorption factor of less than 1% upon total immersion in water for 24 hours at 70 F.

6. In a meid for confining a charge of dielectric material during processing by dielectric heating in which the charge of material and the mold are both dielectrically heated simultaneously, the combination of a mold side wall formed of a backing of dielectric material and a liner of dielectric material, said liner constituting the exposed inner side wail surface of said mold for Contact with said charge of material and comprising a layer of Sheet material having a dielectric loss factor less than that of said backing layer, and dielectric fastening means passing through said liner and received within said hacking and mechanically securing said liner and said backing together.

References Cited in the file of this patent UNITED STATES PATENTS 2,581,939 Deist et al Jan. 8, 1952

Claims (1)

1. IN A MOLD FOR CONFINING A CHARGE OF DIELECTRIC MATERIAL DURING PROCESSING BY DIELECTRIC HEATING IN WHICH THE CHARGE OF MATERIAL AND THE MOLD ARE BOTH DIELECTRICALLY HEATED SIMULTANEOUSLY, THE COMBINATION OF A MOLD SIDE WALL FORMED OF A BACKING OF HARD WOOD IMPREGNATED WITH A WATERPROOFING IMPREGNANT AND HAVING A DIELECTRIC LOSS FACTOR BETWEEN .1 AND .4 AND A LINER OF DIELECTRIC MATERIAL, THE RATIO OF THE THICKNESS OF THE LINER TO THE TOTAL THICKNESS OF THE MOLD WALL OF BACKING AND LINER BEING NOT MORE THAN 1:4, SAID LINER CONSTITUTING THE EXPOSED INNER SIDE WALL SURFACE OF SAID MOLD FOR CONTACT WITH SAID CHARGE OF MATERIAL AND COMPRISING A LAYER OF SHEET MATERIAL SELECTED FROM THE GROUP CONSISTING OF POLYTETRAFLUOROETHYLENE, SILICONE, MELAMINE, AND NYLON AND HAVING A DIELECTRIC LOSS FACTOR BETWEEN .001 AND 0.1.

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