US2803047A

US2803047A - Method of making shell molds

Info

Publication number: US2803047A
Application number: US295668A
Authority: US
Inventors: Roy S Dahmer
Original assignee: Eaton Manufacturing Co
Current assignee: Eaton Corp
Priority date: 1952-06-26
Filing date: 1952-06-26
Publication date: 1957-08-20
Anticipated expiration: 1974-08-20

Description

I Aug. 20, 1957 I s, DAHMER 2,803,047

METHOD OF MAKING SHELL MOLDS Filed June 26, 1952 v 2 Sheets-Sheet 1' IN V EN TOR.

Aug. 20, 1957 R. s. DAHMER Q 2,803,047

METHOD OF MAKING SHELL MOLDS Filed Jane 26, 1952 .2 Sheets-Sheet 2 g C/XJ' I 1 A4; l 4

1!! A2 /dfl /2/ Y 7 4; o o /47 L INVENTOR. A? S, jd lrmert United States Patent 6 METHOD or MAKKNG SHELL MOLDS Roy S. Dahmer, Vassar, Mich, assignor to Eaton Manufacturing Company, Cleveland, Ohio, a corporation of Ohio Application .lune 26, 1952, Serial No. 295,668

Claims. (Cl. 22-193) The present invention relates to an improved method of casting metallic articles. More particularly the invention concerns a mold assembly for shell molds suitable forthe quantity production of metal articles and to a method and machine for making such assemblies and casting molten metal therein.

Shell molds which are in the broad sense relatively thin walled molds fabricated from conventional foundry sands and a resinous binder are known to require exterior support during the casting operation. The desirability of using shell molds in the quantity production of metal articles has emphasized the need for efficient and inexpensive means for producing such molds and for adequately supporting the same during use. The methods for fabricating shell molds heretofore used have been subject to a number of disadvantages including warpage of the resultant shell mold thereby producing flash in the subsequent castingoperation and inability to maintain uniformity of shell mold wall thickness. Certain disadvantages of the heretofore known methods of supporting shell molds which have been commonly experienced are the requirement of manual assembly into a suitable shell mold supporting means such as a box of loose shot or the like, or the requirement for relatively heavy wall thickness for casting large objects which necessitates the use of large quantities of relatively expensive resinous binders.

The primary object of the present invention is to provide a method for making shell molds which overcomes- A further object is to provide a method of casting metal articles utilizing the mold assemblies of this invention and to provide a machine for making such mold assemblies; and in general to provide a method for making shell mold assemblies that is relatively simple, inexpensive and which enables the casting of a wide variety of. metal articles of varying shapes and sizes and which is particularly well suited for the inexpensive mass production of metal articles to close tolerances and with good, surface finishes. Other objects of a related nature will be specifically pointed out or will appear 'inferentially as the description of the invention proceeds.

Heretofore, the method of making a shell mold half has comprised the steps of admixing a silica sand with a small quantity of a thermosetting resin, usually ofthe phenolaldehyde type, investing an excess of this sand-resin mixture on the formed surface of a heated pattern, maintaining the mixture on the heated pattern for a time sulficient to form a thin adherent layer of the sand-resin mixture in a partially polymerized state on the pattern surface, removing the excess sand-resin mixture, fully curing the partially polymerized layer, and removing the cured shell moldfrom the pattern surface.

The casting of metal in apair'of mating shell mold halves of this type- Patented Aug. 20, 1957 requires the positioning of two shell mold halves in faceto-face abutting relation to form a mold cavity therebetween and suitably supporting the rear surfaces of the halves to support the mold during the pouring of the metal in the cavity.

In accordance with this invention, the method of making a shell mold half is altered so that the steps of final curing of the partially polymerized layer and of providing support for the rear surfaces of the shell mold halves are combined and accomplished in one operation. Instead of final curing the partially polymerized layer with external heating means such as in an oven maintained at the desired temperature or with infrared rays, the method of this invention provides a contoured support plate which, when brought into contact with the rear surface of the partially polymerized layer, causes the resin to be further polymerized and to bond directly to the contoured surface of the plate. The contoured surface of the plate is left in a relatively rough condition so that the bonding thereto by the resinous components of the layer is sufliciently tenacious and adherent to transfer the layer to the plate surface and to retain the same in intimate contact therewith when the plate is subse quently separated from the pattern. The contacting surface of the support plate is contoured to conform to the recesses and projections on the surface of the pattern, due allowance therein being made for the anticipated thickness of the shell mold layer. The resinous constituents of this transferred layer are then fully polymerized or cured by heat which is supplied from the plate while in intimate bonded relation with the transferred layer. By securing together a pair of plates having mating shell mold halves bonded to their front faces, a mold assembly is formed which provides uniform and continuous support over the entire rear surface of the shell mold half. Because of the fact that the shell mold half is completelysupported during the casting operation, it is feasible to reduce the shell mold wall thickness by a considerable amount and to thereby lower the unit cost of the molds through the reduction of the quantity of resin required for molding. It will also be appreciated that warpage of the shell mold and the production of casting flash is entirely eliminated because of the intimate contact and backing by the supporting plates thereby effectively maintaining the planar relationship of the mating halves of the shell.

mold.

The method of the invention will now be described in.

greater detail with reference to the accompanying drawings illustrating suitable apparatus for semi-automatically producing the mold assemblies of the invention.

In the drawings:

Figure 1 is a perspective view of a machine embodying the principal features of the invention in a partially dis-, assembled form;

Fig. 2 is a fragmentary elevation view of the machine shown in Fig. 1 illustrating the position of the rotating parts during the stage of investing the sand-resin mixture on the pattern surface;

Fig. 3 is a fragmentary view showing the mold assembly of the invention molded in an automatic casting machine; Fig. 4 is a view of the contoured surface of a support plate of this invention; and

Fig. 5 is a cross-sectional view of a mold assembly embodying the features of this invention.

Referring more specifically to the drawings, there is illustrated in Figs. 1 and 2, a machine having a hori zontally disposed base plate 25 and a pair of spaced vertically disposed, triangular shaped end frame sections generally designated 27, 29 secured to plate 25 by abut- .ting downwardly inclined flange portion 31 as'by welding or the like. End frame sections 27, 29. are substantially similar and are comprised, of a pair of upwardly extend:

ing enclosed channel members 33, 35 which are angularly inclined toward each other and the upper portions of which abut and are rigidly secured by a plate 37 by suitable attachment means such as welding or the like. End frame sections 27, 29 are provided with inwardly projecting toe flanges 39, 41, respectively, to enable the rigid securing of the machine to the base upon which it rests.

A horizontally disposed shaft 43 extends across the upper portions of end frames 27, 29 and is journaled for rotation in a pair of pillow block bearings 45, 47 which are carried on plates 37 and securely attached thereto as by means of screws 49 or other suitable attaching means. Shaft 43 may be a solid rod or a hollow pipe as desired, and for the purposes of this invention, it is preferred that shaft 43 be hollow in order to accommodate electrical connecting means for supplying power to the heating means carried by the rotating pattern as will hereinafter be more fully explained. Shaft 43 is provided with means for reversibly rotating it, such as spur gearing 51, fast on shaft 43 and mating spur gearing 53 carried by the output shaft'55 of a conventional reversible direction gear head motor 57. Motor 57 is supplied with power from switch box 58 through cable 59. The operation of motor 57 is controlled by push button switch 61 interconnected with cable 59 by cable 63.

A pattern cradle 65 is mounted on cradle shaft 43 by means of outwardly projecting flanges 67 positioned at either end of cradle 65 which are rigidly connected to shaft 43. Pattern cradle 65 is comprised of a rectangular shaped bed section having upwardly disposed flanged pattern attachment means 69 and 69 disposed at each of the four corners thereof for releasably securing thereto a pattern generally designated 71. Pattern attachment means 69 and 69 are provided with suitable releasable attachment means such as set screws 73 or the like, and this means enables the convenient assembly with and disassembly from the pattern cradle of a plurality of variform patterns.

Pattern 71 is comprised of a generally rectangular shaped bed section having a plurality of downwardly depending corner posts 75 which are adapted to cooperatively mate with and be releasably attached to cradle 65 by attachment means 69 and 69'. The upper surface 77 of pattern 71 which may best be seen in the broken away portion of Fig. 1, is shaped or configured by conventional methods to provide thereon the required projections or recesses to form the desired shape of the article to be produced. In addition, surface 77 is provided with a pouring basin, a downgate, runner, riser and preferably with a plurality of alignment bosses to assure the proper alignment of the shell mold halves produced thereon. Pattern 71 is additionally provided with a pair of bosses 79, 81 projecting outwardly from the sides thereof and disposed approximately medially in the fore and aft direction having integrally attached thereto an upstandingstud or pin 83, 85, respectively. Upstanding studs 83, 85 function as guides for the proper positioning of a contoured support plate 87 into juxtaposition with the upper surface 77 of pattern 71.

Contoured support plate 87 is a generally rectangular shaped plate having its lower or contacting surface 89 configured to cooperate with or to conform to the recesses and projections which occur on the surface 77 of the pattern 71. The rear surface of contoured plate 87 is preferably provided with a plurality of reinforcing ribs 91 extending in the fore and aft direction and a plurality of transversely extending, intersecting ribs 93 to provide sufiicient strength to the plate to enable it to be separately handled and secured to a mating plate to form an integral mold assembly such as is illustrated in Fig. 5.

The particular construction used on the rear surface of plate 87 is not critical to the successful functioning of the plate in the method of this invention, but in the interests of economy. and ease of handling, it is desirable to maintain the solid contoured portion as thin as possible. If desired, the reinforcing ribs may be eliminated and the thickness of the plate increased to provide the necessary strength. Alternatively contour support plates 87 may be fabricated in two parts, the first part being a reuseable reinforcing member having integral guiding ears and a second part being the contoured surface portion suitably releasably attachable to the reinforcing portion.

The surface condition of the contacting surface 89 of plate 87 is important and preferably is somewhat rough although the degree of roughness is not critical. The irregularities in the surface should be shallow and suitable surface finishes may, for example, be obtained by sand blasting, shot blasting, or the surface may be used in its as-formed condition, i. e., as machined, milled or ground.

it is preferable to provide the contacting surface of the plate 87 with a multiplicity of relatively small apertures extending through the surface 89 which are scattered over the surface 89 and preferably disposed in the areas adjacent to the contoured areas which define a portion of the mold cavity. Such cavities 9t) perform the function of venting the gases formed during the curing operation to the rearward side of the plate 87.

For the purpose of enabling contoured support plates 87 to be united into an integral mold assembly, there is provided a pair of outwardly extending

ear members

95, 97 positioned to correspond to the location of bosses 79, 81 on pattern 71.

Ear members

95, 97 are provided with substantially centrally disposed apertures 99, 101, respectively, which are adapted to cooperate with and receive upstanding guide pins 83, 85 to thereby guide plate 87 into mating relation with pattern 71 as it is lowered thereover.

Pattern 71 and contoured plates 87 may be formed from a variety of materials including copper alloys, steel, polished aluminum, polished gray iron, or the like. High carbon irons are particularly suitable and are preferred because of their tendency to maintain an even heat over wide temperature variations. Pattern 71 and plate 87 are provided with a plurality of transversely extending aperatures 103, 105, respectively, disposed parallel to and positioned just beneath the contacting surfaces thereof. Each aperture is provided with a suitable electrical resistance heating element for maintaining the desired temperature. The heating element may be of conventional type suitably connectable to a source of power. The heating elements carried by the apertures 103 of pattern 71 are connected to a wire 107 which is positioned within hollow shaft 43. The resistance elements positioned at apertures 105 are provided with a central connection or plug 106 suitable for allowing the intermittent supply of power thereto. It will be appreciated that contoured plates 87 may be heated by other suitable means such as the open application of flame, by positioning in an oven preliminary to use, etc. and that heating elements carried in apertures 105 are shown for exemplary purposes only.

An open top container 109 is mounted on a freely swinging frame having downwardly depending arms 111, 113 and a horizontally disposed cross-bar 115 attached to the end extremities of the arms 111, 113. The upper ends of arms 111, 113 are rigidly attached to flange-type bearings 117, 119 respectively, which are freely rotatable about cradle shaft 43. Container 109 is guided relative to arms 111, 113 at its upper. end by vertically

slideable brackets

121, 123, respectively. Bracket 123 is adapted to ride in a vertical slot 114 carried by arm 113, and bracket 121 is adapted to ride in a corresponding slot (not shown) on the opposite side of container 109. As shown in Figs. 1 and 2, container 109 is a generally rectangular box having an arcuate lower end portion, but it will be understood that the shape of container 109 is relatively unimportant with the exception that the open top portion should be shaped to correspond to the shape of pattern 71 so as to enable the nesting of container 109 on peripheral ledges or fiatszof pattern 71 to form an airtight integral unit therewith.

Vertical sliding motion of container 109 is obtained relative to arms 111, 113 by means of

air cylinders

125, 127 in conjunction with conncctingmembers as explained below. Air.

cylinders

125, 127 may be of conventional design having an outer wall cylinder and internally re ciprocative plungers not shown, which are integrally connected to

plunger rods

129, 131.

Plunger rods

129, 131 are actuated by the motion of the plungers in response to air supplied to the end extremities of the air cylinders through flexible connecting

air hoses

133, 135, the supply of air: being controlled by an air valve 137. Air valve 137, which is connected to a compressed air source 139, is manually controllable by the positioning of handle 141. The lower ends of

plunger rods

129, 131 are connected to a horizontally disposed cross-bar 143 upon which container 109 rests. Container 109 is integrally secured to bar 143 at a plurality of points 145.

Before describing the sequential step-wise operation of the machine, it may be well to refer in greater detail to the materials and proportions thereof. which may be used formaldehyde resins containing a slight excess of'unreactedphenol and a mild alkaline catalyst which have. been sufliciently reacted to be in solid and preferably powdered form have been used successfully. Such resins may be conventionally admixed with the foundry sand in conventional mixing equipment such as a tumbler, a paddle mixer or a muller mixer. When using powdered phenolaldehyde resins, it is desirable to prevent dusting by first treating the sand to be admixed therewith with a small quantity of a wetting agent such as a petroleum hydrocarbon, for example, kerosene, solvent naphtha or other high boiling petroleum hydrocarbon. The desired quantity of resin is preferably added to the sand after the wetting agent has been incorporated therewith. The proportion of thermosetting resin which is employed will vary with the particular application, but for the majority of applications, a resin content of about 3 /2% to by weight of the sand-resin mixture is satisfactory and for most applications, proportions of resin between about 3 /2% to 7% have been found to be adequate. Uniform mixing is actually obtained'in a mixing period of about to 60 minutes.

The upper surface 77 of the pattern 71 is provided with a thin film of a releaseagent or lubricant such as parafiin or other high temperature melting waxes,,silicon greases or water emulsions, thereof, or the like, to facilitate the separation of the shell mold from the pattern surface. The pattern is-then'preferably heated to a temperature within the range of- 350 to450 F., the particular temperature being dictated by the composition of the particular mixture being used. The resin-sand mixture is then dumped or otherwise positioned on the pattern surface preferably in. an. amount in excess! of that required to form the shell mold. The sand-resin mixture is maintained on the pattern surface for a time sulficient to soften the resin constituents of the mixture and to form an adherent layer thereon having the desired thickness. The

time required for the. formation of the desired thickness.

may be easily established by a few routine experiments under operating conditions, but for general purposes may be on the order of about, for example, 5 to seconds to form a inch thick coating. After the partially polymerized layer has been formed on the pattern surface, the excess sand-resin mixture is removed, for example, by inverting the pattern. A contoured support plate of the type above described which has been independently preliminarily heated to a temperature in the range of about 500 F. to 1000 F. is then-gently lowered into juxtaposi tion with pattern 71, the latter portion of the motion being guided by upstanding pins 83, cooperating with apertures 99, 101 in

ears

95, 97. The plate is gently urged in pressure contact with the rear surface of the resin-sand layer 147 on the pattern surface 77 and this pressure may be applied either manually or by mechanical means such as a screw propelled lowering mechanism or air cylinder actuated lowering mechanism. In any event, the pressure should be sufficiently light so that nothing more than a gentle urging of the partially polymerized resin into the corners of the interstices on the pattern surface is accomplished. It is undesirable to apply sufficient pressure to cause the forced migration of a sizeable quantity of the sand-resin mixture because of the tendency to create stress lines and points of weakness in the layer wall which appear subsequently during the casting operation. Due to the higher temperature of the support plate 87, the resinous constituents of the sand-resin mixture are further polymerized and form an integral bond with the relativelyrough contacting surface 89 of the'support plate 87 The contacting surface 89 of support plate 87 is not preliminarily treated with a parting agent such as surface 77 ofpattern 71 is coated with, and due to its surface roughness and the lack of parting agent forms a tenacious bond with the partially polymerized layer 147 so that the layer is-transferred to and remains in intimate contact with the support plate 87' when it is separatedfrom the pattern 71. After a short period of time, the support'plate may be separated from the pattern 71 with the partially polymerized layer 147 attached to its forwardcontacting surface 89. In the usual case, the stored heat in the support plate is suflicient to completely polymermize the bonded layer 147 into a hard, strong shell mold.

In the operation of the above described machine to form shell molds, the pattern 71' is initially in its upward position as shown in Fig. 1 and the first operation is to actuate motor 57 to invert pattern 71 to its downward position which is obtained by rotation of 180. Air valve 137 is then opened, sending air under pressure into lines 133 to the lower end of

air cylinders

125, 127, thereby contracting

plunger rods

129, 131 and raising container 109 until' the. perimetral surface of container 109 abuts the upper surface 77 of pattern 71 to form an integral unit therewith which is rotatable with cradle shaft 43. Motor 57 is then energized to rotate container 109 and pattern 71- through an arc of 180 until upward position shown in Fig. 2 is attained. The resin-sand mixture which is carried :by container 109 is thus dumped upon the upper surface 77 of the pattern 71. After the lapse of the previously established time to form the desired thickness of partially polymerized resin-sand mixture in the form of layer 147, shaft 43 is rotated in the reverse direction back to itsoriginal. downwardly disposed initial position, the container 109 remaining in integral contact with the surfacev77 of the pattern 71. '

Air'cylinders

125, 127. are then actuated byradmitting air to the upper ends of the cylinders through hoses therebycausing container 109 to break its: seal with the upper surface 77 of pattern 71 and to assume its initial fully downwardly extended position. Pattern 71 is then rotated so that its surface 77 with the partially polymerized resin-sand layer 147 thereon faces upwardly. Contoured support plate 87 is then; gradually lowered into position in contact with layer 147 and-maintained in. pressure contact therewith until layer 147: is. integrally bonded thereto; Support plate 87 is then separated from pattern 71 and the final polymerization of layer.147 is completed while in intimate contact with plate 87.

The mold assembly of this invention, illustrated in Fig. 5 results from placing a:

pair of'support plates

87, 88* having integrally bonded fully polymerized shell molds on the forward face thereof, in face-to-face abutting relation and securing the same into a unit by suitable means such as spring clamps 155 which span

cars

97, 98 .carried by

plates

87, 88, respectively. Such mold. assemblies may be utilized in the casting of metal articles in a variety of ways. If desired, the mold assembly of Fig. may be used for batchwise production in which case molten metal may be poured directly into the cavity defined by the shell mold halves 147, 148. Such mold assemblies may also be used in mass production equipment, one illustration of which is shown in Fig. 3. As there shown, a mold assembly generally designated 157 is supported on a plurality of horizontally disposed supporting pins 159 carried by the outer stationary section 160 of a multi-station permanent mold casting machine of conventional type. Mold assembly 157 is maintained between stationary portion 160 and movable portion 162 of the permanent mold casting machine by means of a plurality of spring loaded pins 164, 165 which abut against and exert pressure on the rear surface of plate 88. Movable portion 162 of the permanent mold casting machine is mounted for reciprocative movement on an upstanding flange 167 of a carriage including a pair of spaced parallel arms 169, the ends of,

which are provided with a pair of grooved rollers 171, 172 adapted to reciprocate on a rail 173 suitably supported by frame members 175.

As the molten metal cools in the cavity of the

shell molds

147, 148, the resinous constituents of the shell molds become burned or charred and assume a Weak, crumbly condition or state such that it is relatively easy to separate the portions of the shell mold halves which remain in contact with the surfaces of

support plates

87, 89 after the removal of the cast metal article from the assembly. After suitable cleaning, the support plates may be again re-used for making additional shell mold halves and the casting operations repeated.

What is claimed is:

1. In a process for making shell molds wherein a thin layer of a mixture of sand and a thermosetting resin is formed on the surface of a pattern, the resin component of said mixture being in a partially polymerized state, the improvement which comprises the steps of positioning a heated support plate on the rear surface of said layer on said pattern and effecting a transfer of said layer from the pattern surface to said heated support plate while maintaining the contours in said layer imparted thereto by said pattern, and thereafter curing the resinous components in said layer with heat supplied by said. heated plate to an insoluble, infusible state while said layer and said plate are in intimate contact. f

2. In a process for making shell molds comprising the steps of investing a mixture of sand and a thermosetting resin on the surface of a heated pattern and removing the portion of the mixture which is non-adherent to the pattern after a short time, the improvement which comprises contacting the back surface of the adherent layer on the said pattern surface with a heated support plate having recesses and projections in its contacting surface cor-:

responding to the recesses and projections in the pattern surface, and while the resin in said layer is in an incomplete cured state removing the said plate from said pattern with the said layer adhering firmly to said plate.

3. In a process for making shell molds wherein a mixture of sand and a phenol-aldehyde resin is invested on a heated pattern surface and maintained thereon for a time sufiicient to form a thin adherent layer on the pattern surface and the excess sand-resin is removed, the improvement of simultaneously effecting the final cure of the resinous components in said adherent layer and providing a supporting plate for the resultant shell mold half which comprises the steps of positioning in abutment with the rear surface of said layer, a heated support plate'having recesses and projections in the surface thereof which contacts the rear surface of said adherent layer, said recesses and projections eores'ponding to the recesses and projections in the pattern surface, pressing said plate against said layer and maintaining said, plate in contact therewith until the resinous components in said layer bond the layer to said support plate, withdrawing said plate having the adherent layer thereon, and maintaining the heat in said plate until the resinous components in said transferred layer are fully cured.

4. A process for casting metal articles from molten metal which comprises the steps of forming a thin layer of a sand-resin mixture on the contoured surface of a pattern, positioning a heated first support plate on the rear surface of said layer on said pattern and transferring the said layer from said pattern to said heated first support plate, the contacting surface of which is contoured to mate with the contours on said pattern, and curing the resin in said layer with heat supplied by said heated first support plate to a hard, strong infusible condition to thereby form a shell mold half in intimate contact with the said first support plate, forming a second shell mold half in contact with a second support plate in a similar manner, placing the said shell mold halves in abutting relation to form a mold cavity therebetween and pouring molten metal in the said cavity.

5. A process for casting metal articles from molten metal which comprises the steps of investing an excess of a mixture of sand and a phenol-aldehyde resin on a heated pattern surface, maintaining the mixture thereon for a time sufficient to form a thin adherent layer on the pattern surface, removing the excess mixture, positioning a heated support plate in face-to-face relation with said pattern and in abutment with the rear surface of said layer, the surface of said plate in contact with said layer having recesses and projections corresponding to the recesses and projections in the pattern surface, maintaining said plate in contact with said layer until the resinous components therein bond the layer to said plate, separating said pattern and-said plate, curing the said layer while in intimate contact with said plate to :thereby form a first fully supported shell mold half, forming a second shell mold half in similar manner, securing the plates of each said half together so that the shell mold halves define a casting cavity, and pouring molten metal in said cavity.

References Cited in the file of this patent UNITED STATES PATENTS 98,895 Tarr Jan. 18, 1870 1,467,112 Lucier Sept. 4, 1923 1,649,312 Laukel Nov. 15, 1927 2,311,680 Moravecky Feb. 23, 1943 2,568,364 Duesbury et al Sept. 18, 1951 2,659,944 Valyi Nov. 24, 1953 2,691,196 Banister Oct. 12, 1954 2,705,822 Vennerhohn Apr. 12, 1955 2,720,687 Shaw Oct. 18, 1955 2,724,158 Davis et al Nov. 22, 1955 FOREIGN PATENTS 522,168 Great Britain June 11, 1940 832,934 Germany Mar. 3, 1952 OTHER REFERENCES Fiat Final Report No. 1168, The '0; Process by McCulloch. Published by Ofiice of Technical Services, Dept. of Commerce, Washington, D. 0., May 30, 1947. Entire report 10 pages.

Modern Metals, Oct. 1950, pages 22, 23 and 27.

The Foundry, Oct. 1950,

pages

162, 164, 168.

The Iron Age, April 19, 1951, pages 81-85.

Claims

1. IN A PROCESS FOR MAKING SHELL MOLDS WHEREIN A THIN LAYER OF A MIXTURE OF SAND AND A THERMOSETTING RESIN IS FORMED ON THE SURFACE OF A PATTERN, THE RESIN COMPONENT OF SAID MIXTURE BEING IN A PARTIALLY POLYMERIZED STATE, THE IMPROVEMENT WHICH COMPRISES THE STEPS OF POSITIONING A HEATED SUPPORT PLATE ON THE REAR SURFACE OF SAID LAYER ON SAID PATTERN AND EFFECTING A TRANSFER OF SAID LAYER FROM THE PATTERN SURFACE TO SAID HEATED SUPPORT PLATE WHILE MAINTAINING THE CONTOURS IN SAID LAYER IMPARTED BY SAID PATTERN, AND THEREAFTER CURING THE RESINOUS COMPONENTS IN SAID LAYER WITH HEAT SUPPLIED BY SAID HEATED PLATE TO AN INSOLUBLE, INFUSIBLE STATE WIHILE SAID LAYER AND SAID PLATE ARE IN INTIMATE CONTACT.