US3284284A

US3284284A - Controlled deposition pulp molding method and apparatus

Info

Publication number: US3284284A
Application number: US351489A
Authority: US
Inventors: Wells Roger
Original assignee: Diamond International Corp
Current assignee: Diamond International Corp
Priority date: 1964-03-12
Filing date: 1964-03-12
Publication date: 1966-11-08
Anticipated expiration: 1983-11-08

Description

NOV. 8, 1966 R, WELLS; 3,284,284

CONTROLLED DEPOSITION PULP MOLDING METHOD AND APPARATUS Filed March 12, 1964 I?! M40 i l.' \42 a 4l; 3 f4 4`4 )I N I l 4a 42 44 INVENTOR ROGER WELLS Cc): L l BY @CMS ATTORNEY United States Patent O 3,284,284 CONTROLLED DEPOSETEON PULP MOLDHNG METHOD AND APPARATUS Roger Wells, Stamford, Conn., assignor to Diamond International Corporation, a corporation of Delaware Filed Mar. 12, 1964, der. No. 351,489 11 Claims. (Cl. 162-218) The present Iinvention relates to a method and apparatus for molding pulp articles, and more particularly a method and apparatus for molding pulp larticles to produce a pulp layer having a differential thickness along its length.

The concept of attempting to use different sized drainage hole-s in a pulp mold to control the thickness of pulp deposition is almost as old as pulp molding itself. So far, however, it has been impossible to control the thickness of pulp deposition on the mold wire, because immediately after deposition of a first thin layer of pulp fibers, such pulp layer becomes an effective barrier to the passage of water .and the drainage rate of the pulp itself, rather than the holes in the mold, becomes the controlling factor inthe deposition of further pulp.

It is clearly advantageous for many reasons to be able to control the deposition of pulp. One advantage resulting from such control is the manufacture of pulp articles having a first thickness of pulp deposit in one `area and a second thickness of pulp deposit in lanother area. For example, in the formation of a -pulp tray it may be desirable to form the side walls of a greater thickness than the flat bottom portion since the side walls are subject to stresses and strains and must be much stronger than the bottom portion. Until the present time, however, it has been difficult and expensive, requiring devious `and ingenious operations, to provide pulp molded articles having a layer of variable or differential thickness.

It has previously been proposed to form pulp molded articles having a variable ply thickness by separately pulp molding two distinct plys and then laminating the two plys together. Thus, in the case of a molded pulp tray, a first system would mold a conventional tray and a second system would mold just side walls; after the separate molding operations `the two pre-products would be taken to a laminating section where they would be bonded, such as with adhesive. The disadvantages of such a system are obvious, since not only is an expensive laminating system required but also twice as much molding facility is necessary. Such systems are very costly and the resultant articles produced are too expensive to be competitive.

Other systems have also been proposed such `as those which provide pulp molding of an entire layer for a specified unit of time Vand which then -shield lpart of the deposit from deposition while continuing molding for a second period of time on the Unshielded deposit. These systems have not proven feasible since they require a batch operation with complex reciprocating shields.

It is therefore an object of the present invention to provide a method and apparatus which will overcome the `above-mentioned difficulties inherent in the prior art.

It is another object of the present invention to provide .a pulp `molding method and apparatus for depositing a variable thickness ply, which method and apparatus are simple and inexpensive.

It is another object of the present invention to provide a method and apparatus for controlling the deposition of pulp on a mold by controlling the number and size of drainage holes in the -mold in relation to the drainage rate of the pulp used.

Other objects and the nature and advantages of the instant invention will be apparent from the following detailed description taken in conjunction with the accompanying drawings wherein:

3,284,284 Patented Nov. 8, 1966 ice FIG. 1 is a sectional view of a mold in accordance with the present invention;

FIG. 2 is a sectional detailed view of the mold in FIG. l showing a pulp deposit thereon; and

FIGS. 3 and 4 are schematic illustrations of the manner of deposition in accordance with the theory of the present invention.

A `mold itl, an example in accordance with the present invention, is shown in FIG. 1. The mold 10 is provided with a perforated facing plate 12 and is contoured complementary to the shape of the article desired to be produced in a `manner well known in the art. The perforated facing plate 12 may be contoured to produce an `article of any desirable shape, but is here shown in cross section to produce as an article, a rectangular shallow tray. As is conventional, the facing plate 12 is provided uniformly with perforations 14 which pass entirely through the plate 12 and serve as ya passageway for vacuum behind the mold to reach the pulp slurry in front of the mold. The perforations 1d also serve to draw :away water from the pulp `slurry immediately adjacent the face of the mold 12. A foraminous (straining) backing screen 16 is provided over the outer face of the facing plate 12 and is maintained in position by conventional means such as by welding, soldering or a, peripheral retaining ring (not shown).

Over only the bottom portion of the rnold 10 in the area where only a thin layer is desired to be deposited, .a thin perforated shim plate 18 is provided. The thin shim plate 18 is provided with very small .holes 19 which pass through the plate 15 in the same manner that perforations 14 pass through facing plate 12. A facing screen 20 is provided over the rst screen 16 and also over the shim plate 18, and the screen Ztl serves as the outward facing wire upon which the pulp is deposited. The screen 2t) is anchored to the mold 1t) in the same manner as is screen 16. Shim plate 18, on the other hand, is maintained in position by either a frictional contact between the

screens

16 and 20 as shown in FIG. 2, or by a peripheral weld or solder if formed of metal, or suitable adhesive if formed of plastic, as shown at 21.

The holes 19 in shim plate 18 are spaced relatively far apart and are preferably so spaced and placed that `they lie directly above the perforations 14 in facing plate 12. The holes 19 are preferably spaced on 1/2 centers and it has been found that if placed on much smaller centers than 1A", it is difiioult to control deposition.

The shim plate 18 may be formed of any suitable material which can be easily perforated to form the very small holes 19. Metals, such as brass, may be utilized for plate 18. Noting FIG. 2, the shim plate 18 may be formed of plastic film,such as polyethylene, polypropylene, vinyl resins, polystyrene or polyesters. If the shim plate 1S is not adhesively secured, soldered, or welded as shown at Z1, itis desirable to fill the demarcation line at the edges of the shim plate with a substance to reduce any possibility of lateral flow between

screens

16 and 20 at the sides and shim plate 18 in the middle of the mold. Suitable materials are plastics, resins, grease, putty, rub` ber, etc. It has also been found useful to Wrap the edges of the shim plate with plastic electricians tape.

The mold 10 must have two distinct sizes of effective holes along its surface wherein at least the smaller holes are sufficiently small so as to provide a drainage rate which is lower than the drainage rate of the first thickness of pulp deposited across the face of the mold. This is so because the rate of build-up of pulp will be controlled by the lowest in a series of drainage rates. If the pulp itself were to have the lowest drainage rate, as is normally conventional in pulp molding, then the rate of build-up would be uniform across the entire mold regardless of the variation in hole size, since the rate of deposition would be controlled by the drainage rate of the first member of the series of pulp layer, screen and facing plate, i.e. the pulp deposit itself. Thus, taking the mold of FIG. l as an example without the shim plate 18, regardless of the hole size and/or spacings of holes 14 and regardless of the hole sizes of

screens

16 and 20, the rate Vof further pulp deposition on an initial layer of pulp deposited would depend on the effective hole size of the pulp itself, i.e. the drainage rate of the pulp, if the drainage rate of the pulp were lower than the drainage rate of the holes in any of the other members of the series of barriers through which the draining water must pass.

Therefore in order to develop a molding system which will permit, in a controlled manner, the deposition of varying weights of fiber in selected areas of a molding die, it is necessary to carefully regulate and .control the combined effects of (a) the size and spacing of the drainage holes in the forming die and (b) the drainage rate of the pulp used. In this regard, FIG. 1 is an example of the manner of controlling the size and spacing of the drainage holes in the formingv die. In the FIG. 1 embodiment this is accomplished by providing a perforated shim plate 18 which has a lower drainage rate than the drainage rate of the pulp used. By using such a shim plate 18 in conjunction with a pulp having a higher drainage rate than the shim plate, the rate of build-up on the bottom of the mold opposite the shim plate 18 will be controlled by the drainage rate through the shim plate while the build-up on the remainder of the molds, i.e. on the sides, will be controlled by the drainage rate of the pulp itself.

The facing screen 20 is an important feature of the present invention. If the shim plate 18 is used without a facing screen 20, the holes 19 have a tendency to plug up quickly with fines, and in addition, the pulp build-up is found to be not entirely uniform, there being a greater build-up immediately above the holes 19 than between the holes. This is believed to be due to the fact that the holes 19 in the shim plate 18 are not only very small, but are also spaced relatively far apart, preferably the same distance apart as the holes 14 in the facing plate 12. The use of the facing screen 20 permits horizontal drainage along the outer face of the shim plate 18 through the facing screen 20 which not only tends to eliminate the plugging of the holes 19 with lines, but also provides for a uniform build-up of pulp. In addition, utilization of the screen 20 provides a uniform surface finish to the resultant product, since without such screen, part of the article would be finished complementary to the shim plate 18 and have dimples corresponding to the holes 19.

FIGS. 3 and 4 illustrate the theory of the present invention wherein the size of the holes through the mold are varied and are maintained very small so that the drainage rate is controlled at least in part by the drainage rate rthrough the mold. Thus, in FIGS. 3 and 4, a mold 42 is shown having both large holes 44 corresponding to the holes 14 in FIG. 1 and small holes 48 corresponding to the holes 19 in FIG. l. The mold 42 is provided with a facing screen 46 having a drainage rate illustrated by holes 50. A pulp deposit 52 having a drainage rate schematically illustrated by holes 54 is shown on the screen 46.

Noting FIGS. 3 and 4, it is apparent that with an open die, i.e. no pad formed, the size and spacing of the

drainage holes

44 and 48 in the mold are the controlling factors on the volume of water which can be passed through the mold. It is also clear that if holes 44 have twice the cross sectional area of holes 48, assuming that in a selected area of given size the number of holes are equal, then the area with holes 44 would 'pass twice the volume of water as compared with the area having holes 48. Control can be exercised over such drainage rate by merely controlling the sizes of

hole

44 and 48 within a wide range, with the top limiting factor being the open area of the backing wire 46 as illustrated by holes 50.

When the open area of the molding die just exceeds the open area of the backing wire, then the backing wire becomes the` controlling factor and further increases in the open area of the forming die are without effect.

As soon as the mold 40 is placed in a pulp stock slurry of water and pulp, and initial layer of pulp 52 will be built up on the screen 46. If the pulp 52 has a lower drainage rate than the mold 40 in the area of holes 44, then it is readily apparent that the drainage rate of the pulp (as illustrated by holes S4) then becomes the controlling factor on the volume of water which can be drained through the mold 48. This is the condition which exists in normal molding operations. The freeness and -drainage rate-s of the pulps normally used are such that the effective open area of the pulp layer formed is considerably less than the open area of the molding die. Under this condition, increasing the drainage hole size or reducing the hole spacing is without effect on the fiber pick-up.

However, if very small holes 48 are used, such as in another area of the mold, and such second area of the mold as a result of such small holes 43 has a lower drainage rate than the drainage rate of th-e pulp 52, such holes 48 will still be the controlling factor in the rate of water drainage. Thus in FIG. 3, a thicker deposit of pulp will form adjacent holes 44 than will form yadjacent holes 48 since the drainage rate of the pulp adjacent holes 44 is greater than the drainage rate of holes 48 and more water will pass through the series of water barriers adjacent holes 44 (the pulp layer itself being the greatest barrier), than will pass through the series of barriers adjacent holes 48 (the holes 48 being the greatest barrier).

In FIG. 4, the mold 4i) is used in :connection with a pulp 52 which has a greater drainage rate than even the large holes 44. In this case after the initial deposit of pulp 52', the drainage rate will still be controlled by the

holes

44 and 48. Thus the

holes

44 and 48 will both have to be made extremely small and the pulp used will have to have a high drainage rate. Assuming that holes 48 are half the size of holes 44, then the pulp deposit adjacent holes 44 should be twice as thick as the deposit adjacent the holes 48. It is clear that if a pulp is selected that has a high freeness and drainage rate, a pulp deposit can be formed whichlwill have a greater effective open area than the open area of the molding drainage holes with the result that the drainage holes will be the controlling factor on the rate of liber pick-up.

In the schematic illustration in FIG. 3, the drainage rate of the screen as represented by holes 5t) is greater than the drainage rate of the mold as illustrated by holes 44 which is in turn greater than the drainage rate of the pulp as schematically illustrated by holes 54. In the other .section of the rnold in FIG. 3, the holes 50 in the screen are larger than the holes 54 in the pulp which are in turn larger than the holes 48 in the mold. In FIG. 4, the holes 50 are larger than the holes 54 which are in turn larger than the holes 44. In the other section of the mold in FIG. 4, the holes 50 are larger than the holes 54 which are in turn larger than the holes 48. Assuming that the

holes

50, 48 and 44 in both FIGS. 3 and 4 are the same size, then it will be clear that the difference in height between the two sections of the pulp deposit 52 will be greater than the difference in height between the two sections of the pulp deposit 52.

In the embodiment of FIG. 2, the rate of deposition of pulp 22 adjacent the plastic shim plate 18 will be controlled by the low drainage rate throughgsmall holes 19 while the rate of deposition of pulp 24 adjacent the area where no shim plate is provided is controlled by the drainage rate of either the facing plate 12 or the deposit 24 itself, whichever is smaller.

Example l As a control, a conventional mold having an area of 65 square inches was utilized on a conventional molding machine operating at 60 steps per minute. A tray was formed during two steps in the vat, i.e. in two seconds. The stock in the vat was maintained at 1% consistency, i.e. 99 lbs. of Water per lb. of fiber. The tray leaving the Exampe 3 A mold similar to the mold of FIG. 1 was utilized. The total .surface area of the imold was 8" by 8". The facing plate 12 had 0.094 inch dia-meter holes 14 placed Vall comprised about COHSSHCY 0f 9 lbS- 0f Water 5 on 1/2" centers and was covered with a backing wire 16 Per .1b- Qf `lbff- ThefefOfe, duflllg the WO Second for' and a yforming wire 20. Various pulps .having different mation time 1t Was DeCeSSaYY OI" 90 1bS- 0f Water Q be drainage rates as well as different consistencies were used. Pulled hT'OUgh he mOd -PET Pound 0f bel" dePOSllelL Pieces of 0.010 thick brass shim stock Lil/2 X 51/2 with After drying, the tray weighed 0036 1b- ThllS, iiSSUmdifferent sized holes on one-half inch centers were posiing that the tray was completely dry `after drying, 1t Was 10 tioned under the forming wire 20 as shown in FIG. 1. necessary in order to forni the dish `to pull a tOal Of The edges of the shim stock were taped with plastic elec- ().036 lb. 90 lbs. of Water=-24 lbS. 0f Water through tricians tape to reduce the lateral drainage between the the mold 1n tWO Seconds. This amOunliS t0 1.62 lbS- per shim plate and the forming wire toward the open side second for 65 square inches, or 0.02? lb. per square inch walls of the mold. After molding and drying of the pulp per second, or 1.5 lbs. per square inch per minute, or 15 deposits, discs 2 in diameter were cut out of the 8 X 8 0.18 gallon per minute per square inch of mold surface. sample from the shim area .and from the normal molded By way of contrast, a bare mold With 3/32 inch diameter area. These samples were weighed on the imicro balance holes on 1/2 inch centers covered with a backing wire and and the difference in weight calculated. Table II shows a facing Wire under 18 inches of mercury vacuum will the results of this example.

TABLE 11 Weight iu grams Stock Hole .Molding 012 discs over- Percent Group Canadian Diameter, Consistency, Temp., Time, Shim Standard inch Icrcent 11. secs. vs. Freeiiess Reg. Shim Reg.

Meid Piate 180 .040 87 00 ca. 2 .703 .755 95. 3 is0 .030 87 60 ca. 2 .817 .702 as 180 022 87 66 ca. 2 .723 .03.9 s2 180 022 .s7 06 1. 02 .700 .478 08 180 022 .87 137 1. Lis .850 .56s 07 180 022 .55 104 1. 72 .500 .295 53 iso .040 87 70 ca. 2 .700 .727 as 5.50 .030 1 00 70 .s7i .310 ar 550 .022 t 70 1. 069 .281 24 pull 2.02 gallons or 17 lbs. of 70 F. water per square 35 As the hole size was reduced from Groups l to 3, the inch per minute. inasmuch as the mold will carry away article thickness was reduced. In Group 4, the molding more than 11 times the amount of water available during time was reduced which resulted in the effect of the smaller molding, it would seem that the hole size would have to holes being much more pronounced. In Group 5 the tembe reduced to 9% of its present size before it would reperatui'e of the stock was increased and the time decreased; duce the iiow of Water through it. Therefore, in order this resulted in a heavier pad due to the increased temto control the deposition of the pulp used in the preceding perature and a greater elTect of the smaller holes due to the paragraph, a hole would have a diameter of only 0.03 shorter time period. In Group 6, the consistency was reinch. However, a hole of this small size unless the screen duced which resulted in a greater effect due to the smaller size was very carefully selected, would tend `to plug up holes. In Groups 8 and 9 the freeness of the pulp was so quickly. In addition, a hole 0.030 inch in diameter great that water drained out of the pulp so fast that a large through a facing plate 0.375 inch thick would he similar difference in deposition resulted between the regular mold to a pipe whose length was 12 times its diameter and this and the shim plate. would cause pipe friction which is not taken into account Example 4 in the above calculations. For the above reasons, as a Y practical consideration, it is desirable to utilize a shim hMoidstlmlllfflr tol tthat'shOVZI/{I} Iggy; veo'me plate 18 rather than providing the .small holes in the fac- W. ere1.n e s um p a es were 2 X 2 X t 1C with diierent diameter holes on 1/2 centers. The following plate 14 itself.

Example 2 ing table shows the results obtained during molding with l conventional pulps.

A mold similar to the mold of FIG. l utilizing brass rr TABLE m shim plates 18, each having a thickness of 0.010 inch with 0 different hole diameters, all on one half inch centers were l,

. ercent Shim utilized between a fine forming wire 20 and a coarse back- 2146/ vs. Regular ing wire 16. The imold was placed in a vat filled with sample gljt Gimpet clear water at 70 F. The vacuum was quickly turned gratins' B y ny ori and the time to drain was measured with an electric Weight' mhp clock. The sides of the molds, free of the shim plate, l n H r were blocked off. The vacuum behind the wire was /ogills? Blassshlm Stock jgg gg ggfs 71 measured with an Esterline Angus Instrument and varied AV 51 from 171/2" to 20 of mercury. The results are shown 65 g .010 brass shim Stock, .030 .824 5F t Yo on the followmg tableiioieewcenrers. i 9.10 60 dgrs i 81 80 TABLE I Avg .867 5s Hole size diameter, Percent Vacuum Flowgals. Flowlbs. .10" brass shim Stock, .040 .915 G0 three 90 88 inch open behind per mili., per min., holes, 1/2 centers. 1.011 65 dots mold, inch per sq.in. per sq.in. 70

Avg .963 63 2. "f is 2.02 10.8 1.051 72 0.55 171/5 .as 3.16 Regular 1.093 72 i 0. 2s 20 .270 2. 30 0.15 20 .152 1. 26 Avg 1.072 72 When the formed trays were held up to a bright light, it could be seen by the increased light transmission that a sharp demarcation in thickness occurred at the area of the tray corresponding to the edges of the shim plate. During this test it was found that when the holes 19 in the shim plate 18 were not lined directly over the holes 14, the backing wire 16 had a tendency to plug up with pulp fines. However, when such holes were directly lined over the holes 14, the plugging problem was greatly reduced. During drying of the trays formed according to the above example, it was found that the trays had a tendency to dry much faster than conventional trays of the same size.

It will be obvious to those skilled in the art that various changes may be made without departing from the scope of the invention and therefore the invention is not limited to what is shown in the drawings and described in the specification but only as indicated in the appended claims.

I claim:

1. A method of forming a variable thickness pulp deposit comprising: placing a selectively perforated mold having small effective holes at one part and large effective holes at another part in Contact with a pulp stock having a water draining capacity greater than at least the water draining capacity of the small holes, said mold having a shim plate thereon to provide said small effective holes and being covered with a screen, said low water draining capacity of the small holes corresponding to holes on 1/2 centers of no greater than 0.040l diameter; and applying differential pressure to said mold in contact with said pulp to effect deposition of said pulp on said mold.

Z. A method in accordance with claim 1 wherein the water draining capacity of `said pulp is less than the water draining capacity of said large holes.

3. A method of controlling the deposition of pulp on a mold comprising contacting a mold having relatively very few very small holes providing a low water draining capacity with a pulp slurry having a higher water draining capacity than that provided by said holes for no more than about 2 seconds, and withdrawing the mold from the slurry, said mold having a shim plate thereon to provide said few very small holes and being covered with a screen, said low draining capacity corresponding to holes of no greater than 0.040 inch diameter on 1/2 centers.

4. A mold for forming pulp articles comprising a body portion shaped complementary to the shape of the article desired to be produced, said body portion having walls defining relatively large holes extending therethrough from one side of the body to the other side thereof, a backing screen contacting the front face of said body portion, a thin perforated plate contacting at least part of the front face of said backing screen, and a facing screen contacting the front face of said perforated plate, the holes in said perforated plate being sufficiently small to control the drainage capacity through said mold and being spaced apart a minimum of about Mt.

5. A mold in accordance with claim 4 wherein said backing screen has a coarse mesh and said facing screen has a fine mesh.

6. A mold in accordance with claim 4 wherein said perforated plate covers only a selected surface area of said body portion.

7. A mold in accordance with claim 4 wherein the size of said holes in said perforated plate on 1/2 centers are no greater than 0.040l in diameter.

8. A mold in accordance with claim 4 wherein said perforated plate is formed of materials selected from the group consisting of metal and plastic.

9. A mold in accordance with claim 6 further comprising means to inhibit lateral iiow of slurry fluid past the edges of said perforated plate, said means comprising a solid filler material `sealing the edges of said perforated plate between said two screens.

10. The combination of a suction mold and a pulp deposition controlling element, comprising:

a suction mold having a body portion shaped complementary to the shape of the article desired to be produced, said body portion having walls dening relatively large holes extending therethrough from one side of the body to the other side thereof;

a backing screen contacting the front face of said body portion;

means to control the thickness of pulp deposition comprising a thin perforated plate contacting a portion of the front face of said backing screen, the perforations in said thin plate having an area no greater than that corresponding to holes on 1/2" centers each having a diameter of 0.040; and

a facing screen contacting the front face of said perforated plate.

11. In a mold for forming pulp articles from a pulp slurry comprising a body portion shaped complementary to the shape of the article desired to be produced, said body portion having Walls defining relatively large holes extending therethrough from one side of the body to the other side thereof, and a backing screen contacting and covering the front face of said body portion, the improvements comprising:

thin perforated plate means to control slurry liquid drainage through a portion of said body portion comprising a thin plate having few, small perforations passing therethrough, said plate contacting a portion of the front face of said backing screen, each small perforation being spaced a minimum of about 1A apart;

a facing screen contacting the front face of said thin perforated plate; and

means to inhibit lateral flow of slurry liquid past the edges of said thin perforated plate.

References Cited by the Examiner UNITED STATES PATENTS 3,001,582 9/1961 Kindseth et al 162-411 DONALL H. SYLVESTER, Primary Examiner.

J, H. NEWSOME, Assistant Examiner.

Claims

1. A METHOD OF FORMING A VARIABLE THICKNESS PULP DEPOSIT COMPRISING: PLACING A SELECTIVELY PERFORATED MOLD HAVING SMALL EFFECTIVE HOLES AT ONE PART AND LARGE EFFECTIVE HOLES AT ANOTHER PART IN CONTACT WITH A PULP STOCK HAVING A WATER DRAINING CAPACITY GREATER THAN AT LEAST EFFECDRAINING CAPACITY OF THE SMALL HOLES, SAID MOLD HAVING A SHIM PLATE THEREON TO PROVIDE SAID SMALL EFFECTIVE HOLES AND BEING COVERED WITH A SCREEN, SAID LOW WATER DRAINING CAPACITY OF THE SMALL HOLES CORRESPONDING TO HOLES 1/2" CENTERS OF NO GREATER THAN 0.040" DIAMETER; AND APPLYING DIFFERENTIAL PRESSURE TO SAID MOLD IN CONTACT WITH SAID PULP TO EFFECT DEPOSITION OF SAID PULP ON SAID MOLD.

4. A MOLD FOR FORMING PULP ARTICLES COMPRISING A BODY PORTION SHAPED COMPLEMTNARY TO THE SHAPE OF THE ARTICLE MESIRED TO BE PRODUCED, SAID BODY PORTION HAVING WALLS DEFINING RELATIVELY LARGE HOLES EXTENDING THERETHROUGH FROM ONE SIDE OF THE BODY TO THE OTHER SIDE THEREOF, A BACKING SCREEN CONTACTING THE FRONT FACE OF SAID BODY PORTION, A THIN PERFORATED PLATE CONTACTING AT LEAST PART OF THE FRONT FACE OF SAID BACKING SCREEN, AND A FACING SCREEN CONTACTING THE FRONT FACE OF SAID PERFORATED PLATE, THE HOLES IN SAID PERFORATED PLATE BEING SUFFICIENTLY SMALL TO CONTROL THE DRAINAGE CAPACITY THROUGH SAID MOLD AND BEING SPACED APART A MINIMUM OF ABOUT 1/4".