US3459108A - Apparatus and method for fabricating structures in contact with the earth's surface - Google Patents

Description

Aug. 5, 1969 W. J. PATTERSON APPARATUS AND METHOD FOR FABRICATING STRUCTURES IN'CONTACT WITH THE EARTH'S SURFACE Filed June so, 1967 2 Sheets-Sheet 1 WILLIAM J. PATTERSON,

INVENTOR BYMWW FIG 4 AGENT Aug. 5, 1969 w. J. PATTERSO 3,459,108

APPAR S AND METHOD FOR 'FABRICA STRUCTURES CONTACT WITH THE EARTE'S FACE Filed June so. 1967 2 Sheets-Sheet 2 WILLIAM J. PATTERSON, INVENTOR United States Patent 3,459,108 APPARATUS AND METHOD FOR FABRICATIN G STRUCTURES IN CONTACT WITH THE EARTHS SURFACE William J. Patterson, Dallas, Tern, assignor to LTV Aerospace Corporation, Dallas, Tex., a corporation of Delaware Filed June 30, 1967, Ser. No. 650,401 Int. Cl. E0lc 19/23, 19/12 US. CI. 9422 3 Claims ABSTRACT OF THE DISCLOSURE A roller assembly for forcing a pliable mat of fibrous material coated with an initially liquid bonding agent into intimate contact with a portion of the earths surface so that the mat, when rigid, conforms exactly to the earths surface, with the roller assembly including a plurality of narrow rollers, each having a central aperture much larger than the axle on which the rollers are mounted.

This invention relates generally to fiber-reinforced structures held together with a bonding agent and particularly to a compacting device for fabricating such a structure in contact with a portion of the earths surface.

The utility of mats comprising fiberglass reinforcements and curable polyester resins for protecting or covering a portion of the earths surface has become an established fact. The usefulness of such mats in making, for example, a landing mat for a VTOL aircraft or a helicopter, are described in US. Patent No. 3,311,035 to Hubert J. Poskey and Gerald F. Thomas entitled, Method of Making Heat-Resistant Mats. While the utility of the finished mat has been frequently demonstrated, there has remained the difiiculty of the speed with which such mats could be fabricated. Thus, when the area to be covered is relatively small, such as would be appropriate for only a single helicopter pad, a total elapsed time of two hours or so from bare ground to a completed and serviceable landing pad may seem favorable when compared with some other construction techniques. But it must be acknowledged that when the areas to be covered are measured in thousands and tens of thousands of square feet, etc., the process described in Patent No. 3,311,035 of walking over the ground to be covered and dispersing individual fibers of glass until the ground is adequately covered is too slow to be acceptable for large projects. While the process of dispersing individual fibers can be speeded by using several persons, with each dispersing individual strands of glass fiberrandomly on the ground, there is a practical limit of how many people and how much individually carried equipment can be assembled at a work site before confusion begins to detract from the efficiency of each man. Thus, in anticipation of making increasingly larger reinforced mats, there has arisen the objective of providing a means of suitably placing a large quantity of reinforcing fibers on a portion of the earths surface in a short period of time, and subsequently locking those fibers together with a binder.

At first blush, it might be assumed that the above objective could be easily achieved by simply laying a prefabricated mat of some fabric such as woven roving glass or the like on top of the ground. Given this technique as a proposed solution, however, it is soon recognized that the problem of getting a large quantity of fiber on the ground has been apparently solved, but that new problems have arisen to take its place. For example, a Woven mat which has the texture and form of coarse burlap will be suspended over depressions by those portions of the mat which are in contact with high spots in the ground. Similarly, grass will usually support a mat and keep it from achieving intimate contact with the ground. A heavy load applied to the finished mat directly over one of these unsupported areas will tend to fracture or pierce the mat unless the load can be efiiciently transmitted through the mat to those portions which are in contact with the ground. It is usually not wise to rely too heavily on the ability of an unsupported mat to carry point loads, and accordingly, it is desirable to press a freshly laid mat against the ground in such a Way that the mat will stretch or be pulled until it is in intimate contact with all or nearly all of the ground. If the flexible mat is kept in intimate contact with the ground until it hardens, i.e., cures, localized loads will be readily transferred to the ground over a wide area, such that the service life of the mat is extended and the ground can support greater loads than would otherwise be the case.

A further problem to be faced is the possibility of trapped air pockets between the mat and the ground when the mat has been coated with liquid resin. That is, when a dry mat has been sprayed with resin, the resin will tend to form a film across the top of the mat which traps air in and under the mat. This trapped air quite naturally tends to inhibit the migration of the resin down into the dry mat. If the sprayed mat is not physically moved, agitated, subjected to vibrations, etc., a substantial part of the resin will harden on top of the mat where it does not produce as much mat strength as it would if it were dispersed throughout the mat. The trapped air pockets, therefore, result in relatively weak portions in the cured mat.

It is accordingly an object of this invention to provide a rapid method of fabricating a bonded fiber structure in intimate contact with a portion of the earths surface.

Another object is to provide a compacting device which is suitable for use on ground that has inherent and random irregularities in height.

A further object is to provide a rolling device which is particularly adapted for rolling uncured glass fiber mats having chopped fibers therein.

Yet another object is to provide a design for a roller which is useful in the production of a bonded, fiber-reinforced structure in contact with an irregular surface in sizes approximating that of a football field or the like.

Other objects and advantages will be apparent from the specification and claims and from the accompanying drawing illustrative of the invention.

In the drawing,

FIGURE 1 is a diagrammatic representation of an apparatus suitable for fabricating a structure on the ground in accordance with this invention;

FIGURE 2 is a sectional view of a portion of the rolling apparatus taken in the plane II-II in FIG. 1;

FIGURE 3 is a sectional view of an alternate embodiment of a roller similar to that shown in FIG. 2;

FIGURE 4 is a diagrammatic sectional view of a plurality of rollers in contact with a depression in the earths surface;

FIGURE 5 is a front view of a manually propelled rolling apparatus made according to the invention;

FIGURE 6 is a partially sectioned front view of an alternate form of rollers;

FIGURE 7 is a partially sectioned view of an alternate embodiment of the invention;

FIGURE 8 is a side view showing in a diagrammatic manner the relative movement between adjacent rollers in contact with portions of the earths surface having different elevations; and

FIGURE 9 is a front view of the rollers shown in FIG. 8.

With initial reference to FIG. 1, an apparatus is shown which comprises a portable frame 10 having mounted thereon a means 11 for dispersing a woven mat of fibrous material 12 such as woven glass fiber. The frame 10 is shown without reference to any supporting structure, since any means for moving it with respect to the ground 13 will suffice. For example, as a way of avoiding the custom fabrication of a piece of special-purpose equipment, it has been found expedient to employ a commercially available, light-duty farming tractor to support the frame 10 and move it over the ground 13. Attached to the frame 10 is one or more subframe members 14, with each having an axle 15 mounted for rotation with respect thereto. Mounted for rotation with respect to an axle 15 are a plurality of rollers 16 positioned substantially side-by-side.

With added reference to FIG. 2, two of said rollers 16 are shown in cross section, together with annular spacers 17 which are representative of the spacers between each pair of adjacent rollers. The rollers 16 have substantially equal outer diameters, and each roller has a central aperture 18 through which the axle 15 extends. The diameter of an aperture 18 is substantially larger than the diameter of the axle 15 such that a roller 16 may independently move radially with respect to the axle as well as rotate about it. Each roller has at least one peripheral surface 19 for rolling engagement with the material to be compacted, with each of the rollers in FIG. 2 having four such surfaces. As shown, the peripheral surfaces 19 are spaced from each other along the axle 15 so as to provide an air gap therebetween. A preferred spacing between respectively adjacent material contacting surfaces 19 is a space at least as great as the average width of the surfaces. When the material to be compacted contains filaments that are prone to adhere to the roller and therefore must be physically removed from the roller by brushing, washing or the like, relatively wide spacing between surfaces 19 may be desirable to facilitate cleaning. In such circumstances, it has been found practical to make the distance between adjacent surfaces 19 at least one-quarter inch.

The width of each roller 16 is not critical, although a large number of narrow rollers will more accurately conform to the surface to be compacted throughout the breadth of an array of rollers than will a small number of relatively wide rollers. A roller width of about one inch has been found to be a suitable compromise between good conforming action by the roller assembly and a practical goal of keeping the number of components in the entire assembly at a moderately low level. Similarly, the periphery of the roller 16 is not too critical, as long as it has at least one rather narrow material-contacting surface. Also, the roller body between adjacent materialcontacting surfaces should be recessed from the roller periphery by about one-quarter inch or more, with this requirement being based upon the need to insure that only the appropriate surfaces contact the mat. In a given roller such as roller 16A in FIG. 3, the depth of the recess between adjacent surfaces 19A is represented by the dimension d which is easily established at a desired value during the fabrication of the roller. The dimension d however, is a dimension measured between two relatively movable parts, i.e., the roller 16A and a spacer 17A, and it, also, should be approximately one-quarter inch in order to provide desirable ground clearance. The necessary relationship between the dimensions of the parts 16A, 17A to achieve a quarter-inch differential can be defined by assuming that the spacer 17A is in contact with the top of the axle 15A; the spacer will then be extending or protruding from the axle by a maximum amount in the downward direction. The minimum value of the dimension d will be found, then, when the roller 16A protrudes from the axle 15A by a minimum amount in the downward direction, that is, when the roller is in contact with the bottom of the axle. In other words, if the maximum protrusion of the spacers 17A with respect to the axle 15A in a given direction is at least one-quarter inch less than the minimum protrusion of the rollers 16A in the same direction, there should be about one-quarter inch clearance between the spacers and the contacted material. Consequently, only the rollers with their special, narrow peripheral surfaces 19A will ever contact the structure being fabricated.

The size of the roller aperture 18A will, of course, determine how much radial movement during operation will be permitted between the axle 15A and the roller 16A. It has been determined, however, that if the aperture diameter is much less than one-half inch greater than the axle diameter, the compacting device will be limited to use with surfaces that are either substantially fiat in their original state or require an unreasonable amount of leveling effort to achieve a nearly flat surface. Accordingly, it is preferred that the aperture diameter be at least one-half inch greater than the axle diameter.

The weight of each roller such as roller 16 in FIG. 2. is established such that each of the material-contacting surfaces 19 applies a pressure of at least 1 psi. to the material being compacted, with said pressure being the result of roller weight alone. That is, during operation of the device, it will be found that the weight of the axle l5 and frame 14 causes the axle to ride on the bottom of many of the roller apertures 18. Thus, the weight of the axle 15 will be distributed to one or more rollers 16 such that it will add to the weight of a roller and will therefore increase the pressure on the material being compacted. When a depresion of less breadth than the axle length is encountered, such as is diagrammatically represented in FIG. 4, the rollers 16B in register with the depression will follow the terrain contour downwardly, while the axle 15B will be held up by those rollers that are still on relatively high ground. As a roller 16B breaks away downwardly from the axle, the roller weight alone serves to compress the material underneath. Thus, the structure being rolled will be subjected to locally varying pressures along the breadth of the compacting device, but a minimum pressure due to the weight of a roller will always be applied to the material. The maximum pressure that will normally be applied to the material will be that resulting from the weight of a roller plus about one-half the combined weight of the axle and, depending on the suspension system employed, some portion of the frames weight. It will be recognized that this predictable range of pressures is advantageous since, with care in design, it will be nearly impossible to apply too much or too little pressure which would tend to produce great variations in the density of the structure.

Referring still to the diagrammatic representation of a compacting device in FIG. 4, it will be obvious that those rollers 16B which are in contact with a sidewardly sloping surface will lean toward the low side if they are not restrained in some manner. Such leaning is to be avoided, for it is desirable that the rollers move over a surface in a straight path so that there is no tendency to push any material in a sidewise direction. If the rollers are permitted too much freedom to lean to one side or the other, such that they may wobble, the likelihood of causing separations between the material being worked and the ground thereunder is increased, and random fibers may be pushed upwardly from the mat to the side of the rollers. If chopped glass fibers are being used in the mat, and the bonding agent is allowed to solidify with the ends of many of such flibers protruding upwardly from the mat, the abrasive effeet on bodies subsequently coming in contact with the completed structure can be quite deleterious. Accordingly, a means for maintaining the rollers substantially perpendicular to the axle is provided in order to assure, as nearly as is possible, that the mat material will be subjected to roller forces applies downwardly and not outwardly. One such means, shown in FIG. 2, for maintaining the rollers 16 substantially perpendicular to the axle 15 is the plurality of annular spacers 17, one of which is located between each pair of adjacent rollers.

While the apparatus illustrated in FIG. 1 is particularly suitable for the production of large structures on top of the ground on what may be denominated as a mass-production scale, as, for example, structures of the size of a football field or the like, the basic theory of making compacting devices according to this invention is adaptable to various sizes, including relatively small hand-pushed models for smaller structures like floors for tents, etc. In FIG. 5, for example, a compacting device suitable for being pushed or pulled by hand over a surface is illustrated. A bifurcated frame 14C has bearings 20 in which the axle 15C is rotatably mounted. The bearings 20 are preferably made of a material having a very low coefiicient of friction such as tetrafiuoroethylene. Bearing surfaces commercially available under the tradenames Teflon, Kel-F, etc., have been found satisfactory from the standpoint of friction characteristics, as well as with regard to their immunity to the solvents (such as acetone) that are normally employed to clean the devices after use. Since usually it is preferable to submerge an entire roller assembly in a tank of solvent during cleaning, the possibility of using external lubricants to reduce friction forces within the device is practically precluded. Accordingly, it is advantageous if the coefiicients of friction between all relatively moving surfaces are as low as those normally achieved between, for example, tetrafluroethylene and dry aluminum. The preference for low friction forces is based in part upon a desire to insure that the device rolls over the surface being compacted and does not slide ,over it, with sliding action being offensive because it tends to cause ripples in the mat ahead of the rollers.

A further means for precluding ripples in the mat is to size the rollers such that they are very large with respect to the amount of mat depression under a roller. That is, since a roller in contact with a portion of the mat locally compresses that portion, while the mat material ahead of the roller is uncompressed, the roller in effect is constantly going uphill. If the roller is not relatively large, a severe snowplow effect is realized and the roller tends to push the hill ahead of it rather than rolling over it. A satisfactory roller size to diminish the snowplow effect has been found to be a diameter of about six inches when the normal mat depression under the weight of a roller is about Vs inch.

Referring still to FIG. 5, each roller 16C has three material compacting surfaces 19C, each of which is of less width than the roller. An alternate embodiment, however, shown in FIG. 6, has rollers 16D which have singular peripheral surfaces 19D of the same width as the rollers. The rollers 16D in this embodiment are easier to fabricate since they constitute a simple disk with a suitable aperture, but they must be made either larger or of a more dense material than other rollers in order to obtain the needed weight to provide optimum compacting pressures. The distance between adjacent peripheral surfaces 19D serves a beneficial purpose in that it leaves an air gap through which any excess bonding agent on top of the mat may pass in order that it will not be pushed ahead of the advancing rollers. Furthermore, the array of narrow peripheral surfaces 19D constitute an interrupted compressing means, and loose fibers on the mat do not adhere as readily to the spaced surfaces as they would to a continuous surface. A ratio of the distance between adjacent peripheral surfaces 19D to the width of the surfaces of at least two-toone has been found to produce good results even when the entire mat surface consists of chopped glass fibers. Accordingly, a ratio of two-to-one is a convenient rule-ofthumb to use in establishing sizes for rollers 19D and spacers 17D.

A further embodiment of the invention is shown in FIG. 7, in which a central frame member 14E carries a rotatable axle 15E about which rollers 16E are free to rotate. This embodiment employs no spacers, and the rollers 16E are constrained to move up or down or rotate with respect to the axle 15E by the close spacing between interior and exterior washers 21, 22.

Utilization of the compacting devices is best described with respect to FIGS. 1, 4, 7, and 8. In FIG. 1, the ground 13 represents any portion of the earth surface, which, almost without exception, will have certain inherent and random irregularities in height. Thus, even a sandy beach that has been meticulously leveled will usually be found to have an inadvertent footprint or the like, which keeps the surface from being smooth. As a general rule, however, either the timeis not available or the cost would be prohibitive to prepare a truly smooth surface on which to fabricate a ground-covering structure. Accordingly, this invention is directed to utilizing a relatively rough portion of the earths surface and forcing the structure of conform to that surface.

Since the surface of the ground 13 will usually be nonadhesive, a tack coat of bonding agent may be advantageously applied to the surface as by spraying through nozzles 23 before the fibrous material is placed thereon. It will usually be impossible, however, to retain all of the applied bonding agent within the mat 12, so that the small portion that does pass therethrough would probably serve to lock the mat to the ground in the absence of a tack coat. The tank 24 that carries the bonding agent may be mounted on the same vehicle that supports the frame 10, or it may be independently mounted on another vehicle, since there is nothing critical about the sequence in which the steps of thi process are arranged.

The ground 13 having been selected and prepared as desired, a mat 12 of fibrous material such as woven roving glass fiber, cotton, rock wool, hemp, etc., is laid on top of the ground. The mat 12 is then coated with a bonding agent in a liquid state. Representative of suitable bonding agents are polymerizable resins such as, for example, the

curable polyester resins commonly employed in making glass-reinforced plastic products such as boats, etc. If the finished mat is intended for use as a landing pad for aircraft, it may be desirable to employ so-called high temperature resins such as those disclosed in Patent No. 2,779,701 to Robitschek et al. The successful fabrication of a ground-covering structure by use of one of the compacting devices described herein is not, however, dependent on the specific type of bonding agent employed nor the fiber reinforcement (as long as the same is wettable by the bonding agent). Accordingly, it will be understood that a person skilled in the art will be able to select with complete freedom a bonding agent and a fiber reinforcement to meet the individual requirements of the finished structure.

Coating of the mat 12 may be accomplished in any way desired, including, for example, passing the dry mat through a tray filled with bonding agent immediately before the mat is placed on the ground. A preferred method, however, is to place the dry mat 12 on the ground 13 followed by the step of spraying the bonding in agent onto the mat. For this purpose, nozzles 25 and tank 26 may be affixed to the frame 10 as represented by the broken line 27, or the required bonding agent may be obtained from source 24.

The coated mat 12 is then worked or kneaded by passing the array of rollers 16 over the mat. As shown in FIG. 4, 8 and 9, the rollers 16B follow the contour of the ground 13 and cause the mat to do the same. The rollers move independently and vertically with respect to the adjacent rollers so as to maintain contact between the material contacting surfaces and the mat in spite of irregularities in the earths surface height. The rollers also rotate independently of each other so a to maintain at all times rolling contact between their peripheral surfaces and the mat. This is, as shown in FIG. 8, the roller 16F which is forced to follow the arcuate path between points a and b will travel a greater distance than will the roller which traverses a straight line between the two points. By permitting relative rotation between the rollers, rolling-and not sliding-contact is provided for all rollers. While a small amount of localized sliding action would probably not be ruinous, it is less preferable than rolling contact and is to be avoided when possible.

The spaced material-contacting surfaces, by applying spaced loads to the coated mat, produce several beneficial results. For example, by increasing the pressure on any trapped air bubbles in thee mat 12, air is forced out of the mat enabling the bonding agent to achieve intimate contact with, i.e., to wet, the fibrous mat. The roller peripheral surfaces also counteract the surface tension of any bonding agent that may be standing on top of the mat, such that the escape of trapped air is expedited. Too, if any bonding agent has accumulated in a depression in the ground, the rollers in forcing the mat down into the depression will displace the collected bonding agent and cause it to be spread throughout a much wider area. This will obviously promote economy in the use of binder, since assurance will be available that a large portion of the originally applied binder will not drain off to low spots in the ground where it does little good. The rolling action also serves to bend over any glass fiber that might otherwise protrude from the rnats surface; the bonding agent usually holds such fibers in place once they have been bent down.

It should perhaps be emphasized that all of the above benefits resulting from the rolling action described herein are achieved uniformly and automatically, i.e., without the exercise of individual judgment on the part of the person actually fabricating the mat. That is, with the array of rollers 16 being as wide as the mat 12, there is no possibility of failing to cover some portion of the mat, such that a latent weak spot may exist which is undetectable by a mere inspection of the top surface of the mat. Too, the weight of each roller 16 is adequate to supplyby itselfthe minimum pressure deemed desirable, and no external force need be applied through the arle, back-up rollers, etc., by machine or by hand, to achieve a desired uniform distribution of binder within the structure.

The binder must, of course, be cured or solidified after the mat has been coated; assuming this is to be accomplished through the use of a liquid curing agent rather than with heat, etc., it is usually preferable to add the curing agent to the binder at an early stage in the process so that the rollers can facilitate mixing of the two fluids while they are working the fluids into the mat. If an initially dry mat is simply laid on top of the surface to be covered, providing at least two side-by-side nozzles directed at approximately the same spot on the mat, with one nozzle dispersing binder and the other spraying curing agent, is believed to be the most convenient method of coating the mat. The coated but uncured mat is then rolled as described before, once, twice or as many times as is directed before the binder solidifies. Arranging the rollers in gangs as shown in FIG. 1, with the tracks of front and rear rollers being respectively offset has been found to be an expeditious manner of achieving a desired 8 amount of roller action with only one pass of the frame 10.

The various embodiments of the rollers shown in the several figures is not meant to be an exhaustive showing of possible roller configurations; for the width of the rollers and the width of the material contacting surfaces may be varied as desired, depending on the roughness of the ground, the preferred density of the finished mat, the nature of the mat fabric, etc. For example, the rollers 16E in FIG. 7 will compress a mat more than the rollers 16D in FIG. 6, if the roller material is identical in both instances, since there is more roller weight per contacting surface in the rollers 16E. If the material being compacted has chopped fibers thereon, the relatively wide space between the rollers 16 of FIG. 2 will be more serviceable than the rollers 16E of FIG. 7, since an unduly protruding fiber may occasionally get caught between the confronting sidewalls of the rollers 16E, whereas there is little risk of such an occurrence with the rollers 16. Accordingly, the roller geometry may be varied within wide limits, as long as the essential features of the rollers are preserved, i.e., as long as the ability to move up and down in following terrain elevations is not impeded, and the necessary pressure criterion is met, etc.

It will be evident, too, that various further modifications are possible in the manner of practicing the invention and in the arrangement and construction of compacting devices. Accordingly, it should be understood that the forms of the present invention described above and shown in the accompanying drawing are illustrative only and not intended to limit the scope of the invention.

What is claimed is:

1. A device for compacting a mat of pliable material by pressing the same against a portion of the earths surface, comprising:

a portable frame;

an axle mounted for rotation with respect to the frame;

a plurality of rollers mounted for rotation about the axle, said rollers having substantially equal outer diameters, said outer diameter of each roller being at least six inches and including an increment of about six inches per As-inch of normal mat depression under the weight of the roller, and each roller having a central aperture through which the axle extends, the aperture diameter being substantially larger than the diameter of the axle such that each roller may move radially with respect to the axle, and each roller having at least one peripheral surface for rolling engagement with the material to be compacted, with said peripheral surfaces being spaced from each other along said axle by a distance of at least as great as the average width of said peripheral surfaces, and the coefiicients of friction between all surfaces of relatively moving parts are equivalent to those normally achieved between tetrafiuoroethylene and dry aluminum, whereby sliding motion is avoided and rolling motion of the rollers with respect to the mat is assured when the device is propelled across the mat.

2. The compacting device claimed in claim 1 wherein the coefficients of friction between all relatively moving surfaces are equivalent to those normally achieved between tetrafluoroethylene and dry aluminum.

3. The method of fabricating a bonded, glass fiberreinforced structure in contact with a portion of the earth: surface having inherent and random irregularities in height, comprising the steps of:

loosely placing on the surface to be covered a mat of woven glass fibers coated with a liquid polymerizable resin:

pressing the coated mat against the earths surface to insure intimate contact of the mat with the earth and to promote uniform dispersion of the liquid resin throughout the mat, with said pressing being effected by moving an array of narrow, axially spaced rollers over the mat, each of which rollers is suspended to move independently and vertically with respect to the 1,749,647 3/ 1930 Poujaud 94--50 adjacent rollers so as to maintain contact between the 1, 31 11 11 1931 Huntley 5 mat and the earth in spite of irregularities in the 2,354 536 7/1944 Fischer earths surface height, with the coeflicient of friction 3 192,838 7/1965 cook 94 5O between all relatively moving surfaces being as low T as those normally achieved between tetrafiuoroethyl- 5 3230843 1/1966 Santuccl 94*46 X ene and dry aluminum, such that motion of each of 3,283,676 11/1966 Sumte" 9439 X the rollers with respect to the wet mat is confined to 3,311,035 3/ 1967 Poskey et X rolling and such that sliding of the rollers is essen- 3,346,219 10/1967 Salyer et a1 94-7 X tially precluded, whereby ripples in the mat ahead of 3,3 1,043 1 1953 Beeson 9 5 X the rollers are avoided; and 10 subsequently causing said polymerizable resin to harden. NILE BYERS, JR" Primary Examiner References Cited US. Cl. X.R. UNITED STATES PATENTS 15 9440 1,190,257 7/1916 Henderson 94-5O 1,637,993 8/1927 Finley 94-50

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