US 2452740 A
Description (OCR text may contain errors)
NOV. 2,1948. s,-
MOLDED STRUCTURE 'Original Filed Nov. 27, 1943 ',9 Th1-2- je Il. I..
l -r [NVENTORl i2 25T Hfs ATTaQA/Ey-- Patented Nov. 2, 1948 UNITED STATES PATENT OFFICE 2,452,740 MOLDED STRUCTURE.
Sherman M. Fairchild, New York, N. Y., assignor, by mesne assignments, ,to William B. Scarborough as trustee (C1. 2li-89) 3 Claims.
This invention pertains to the art of molding wood or the like materials, and is directed to a novel product or article of manufacture.
Under the usual practice of molding Wood or the like material, thin strips of the material are provided, and are laid side by side on a rigid form. One layer of strips of the material islaid on the form to comprise a face ply, and additional plies are usually laid successively on the face ply and on top of each other, until the desired thickness is attained The resultant layup is covered with a exible blanket or bag, usually consisting of rubber, to complete a package. The package is usually subjected tothe pressure of a fluid Which presses the layup against the surface of the die. An adhesive is applied to contacting surfaces between plies, and the pressure isfmaintained on the layup While the adhesive sets. Thisfabricates the assembly, formed to the contour of the die surace.
An example of the practice of this art is set forth in the patent to Potchen et al., No. 2,308,453, of January 1-2, 1943.
The usual prior art practice is to form butt joints between adjacent strips of a given ply. Th'e proximate edges of adjacent strips are tailored or fayed to exactly match each other, the prior art practice undertakes to adhere proximate buttv joint edges of adjacent strips together to close the joints. However, the prior art practice of closing butt joints is not very successful.
It is generally recognized that good adhesion between pieces cannot be attained unless they are subjected to pressure normal to the surface of contact between pieces. produce a thin glue line, which is requisite to good bonding, It will be appreciated that the pressure applied to layups, for example under practice oi the patent, No. 2,308,453, is not directed normal to surfaces of contact in butt joints between adjacent strips in a given ply. Accordingly the prior art practice of fabricating assemblies is not conducive to forming rrn bonds in butt joints between adjacent strips, e
In addition, if the moisture content of the material comprising the strips changes between the time they are cut to shape and the time they are molded, the dimensions of thev material will change sufficiently to give either an open joint, or one in which edges will overlap each other. It is not uncommon for Wood veneer to change 2% in dimension due to a change in moisture content, and in the case of strips 5" wide, this would inm Volve l/lO" dimension change. llhis, for obvious reasons, would give an unsatisfactory joint.
In addition to the problem of maintaining the moisture content of the strips, and their dimensional stability, between the time they are cut to shape and the time that they are placed in the form, there is another' ,phase of the operation Pressure is required to 1- which again affects their moisture content and their dimensions. It is usual practi-ce to apply heat for curing and setting the adhesive, and moisture is released by the curing operation incident to the application of heat. The presence of this moisture causes a variation of the moisture content of the ply material, which results in dimansion changes and incident bad joints.
It is also Well recognized in the art that it is difcult to produce layups in which adjacent strips `Contact each other at all points along their proxi mate edges, and this is especially true when assemblies are formed with compound curvature. The Work'of making layups, which includes fay ing the edges of adjacent strips to match each other accurately, requires a high degree of skill, and is time consuming and costly.
Finally, the layup is made with the strips positioned relatively lightly pressed againstthe surfacev of the die. When the layup is subjected to pressure, such as the fluid pressure applied under the practice -of the referred to patent', No. 2,308,453, the component strips of the layup shift into conforming engagement with the surface of the die. This shifting is inevitable, and it results in the edges of adjacent strips moving out of the relative positions they occupied after the layup operation. Regardless of how accurately the strips have been tailored and positioned in the layup' operation, it is almost inevitable that the strips'will shiftin the molding operation, and in some places the strips will separate, whereas it is not uncommon' that in other places they shift into overlapping relationship. l
When butt joints have been made with utmost care, and the appearance of the assembly indi- Cates they are Well bonded, it is frequently found that the joints are Weak, and with continued use of the article comprising the assembly, and its flexing in normal use, one or more points of fail-v ure may appear along butt joints. Failure may also-occur in butt joints after a time, even though the article of the assembly has not :been put to use, due probably to internal strains setup in the fabricating process, or to changes in atmospheric conditions affecting, points in butt joints which are under internal strain, Wherever a thick glue line exists, the space is occupied by resin, which is more dense and less flexible than the Wood Aof the veneer and at such points failure may occur.
Another factor is that paint will not adhere to 'resin which is set. For this reason, the rst'failf ures of finishes comprising paints are found above thick resin joints. A Y,
The failure of a joint, it will be readily understood, is the failure of the ply containing the joint, and this has the result of rendering the ply with a glue yjoint failure practically useless in the asmbly- .Y
The present invention provides an article` or structure which is free from the aforementioned disadvantages and which may be molded under a practice, for example, similar to that -disclosed in the patent, No. 2,308,453.
According to the practice of the present invention, the strips which comprise the plies 0f a layup are beveled or feathered along their edges. Adjacent strips in any given ply are positioned in the layupy with their proximate edges overlapping each other. Thus, when fluid pressure is applied to the layup normal to the surface of the die, the pressure at overlapping edges of adjacent strips in any given ply will be normal to surface of contact between the strips. Thus, the application of pressure under the normal molding operation will be in a direction conducive to making thin glue lines at joints. Furthermore, the surfaces of contact between overlapping strips is much wider, and the overlap joint is therefore stronger, than the maximum width attained by butt joints.
It is preferred that the edges of the strips be feathered or scarfed, the angle of the scarf being as sharp as is practicably possible, so that there is a minimum break in contour of the surfaces of the strips containing the scarf, and whereby both faces of the strip are virtually continuous from edge to edge.
It would appear to be preferable that the overlap at the edges of adjacent strips be equal to the f extent of the bevel, because this gives a thickness through the overlap most nearly equal to the thickness of each strip. However, it requires a high degree of skill to make the overlap exactly equal to the extent of bevel, and the work of making such a layup is tedious, time consuming and costly. This is especially true in structures with complex contours involving simple or compound curvatures.
Actually it is not necessary that the overlap f be equal to the extent of bevel because variations in thickness of the assembly do not affect the strength of the article adversely. Usually it is required that only one surface of the assembly be smooth and regular. Any irregularities of surface contour due to variations in thickness are, because of the nature of the molding process, confined to one surface of the article, and this is satisfactory in most cases, and does not detract from the utility, strength and appearance of the article of the assembly.
Accordingly, under recommended practice of the present invention, the overlap of contiguous edges of adjacent strips is made to only approximately equal the extent of bevel when the fabrication of the assembly is complete. The extent of overlap can be greater than, or less than, the extent of bevel, and can vary within wide limits. In the molding operation the material of the layup will be pressed to conform one of its surfaces to the desired contour predetermined by the contour of the die surface. There will be variations in thickness, and corresponding irregularities of surface contour, but such irregularities at overlap joints will be confined to the surface of the assembly opposite the one which contacted the rigid die surface. If required, such irregularities can be readily removed when both surfaces of the assembly must be regular.
It will be appreciated that, under practice of the present invention, layups can be made by less skilled operators, in less time and at less cost. The prior art practice of tailoring strips to an exact contour for precise abutment, usually including a cut and try method, is eliminated.
' prising outside face ply l2.
The present invention includes the practice of applying pressure, and usually heat, to set the adhesive to adhere together the various pieces comprising the assembly. The pressure will cause the strips to shift and adjust themselves to the contour of the die surface. Under prior art practice such shifting and adjustment of strips was feared as a cause of defective assemblies, and every eort was made to avoid it. Under practice of the present invention, it is desirable and advantageous that the strip adjustment take place, so that reproduction of the contour of the die surface in the assembly be accurate and complete, and in order that thin glue lines be attained.
Having thus stated the nature of the invention, a better understanding thereof will be attained from the drawings, which show one embodiment of the invention as applied practically to one typical article of assembly. The method of making this article forms the subject matter of a copending application Serial No. 512,009 of November 27, 1943, of which this is a division. In the drawings- Fig. 1 shows, in perspective, a structural assembly embodying the present invention;
Fig. 2 illustrates the nature of the strips employed in the practice of the present invention, viewed in transverse cross-section;
Fig. 3 illustrates in cross-section a ply layup preparatory to being formed and adhered together to comprise a structural assembly;
Fig. 4 illustrates in cross-section, the structural assembly resulting from the layup of Fig. 3; and
Fig. 5 is a fragmentary schematic View in perspective, to illustrate possible -disposition of adjacent strips of a ply in the ply structure.
The illustrated example of Fig. l, constitutes a shell such, for example, as would be suitable for use as a member of an aircraft fuel tank. This member is chosen as a convenient medium for illustrating the principles of the invention involved, and it should not be construed as limiting the scope of the invention. The assembly of Fig.
1 presents a structure with compound curvature, but the invention is equally applicable to flat pieces and simple curvatures.
The structural assembly illustrated in Fig. 1 comprises a plurality of strips Il, laid side by side to form a face ply I2, comprising one surface i3 of the assembly.
Within the scope of the invention, the structure may comprise the face ply I2 cf strips il, either alone or together with one or more additional plies of the same or of different material. In the embodiment shown, the structure coinprises a plurality of plies, of which one face ply l2 presents the outside surface I3 of the shell, and the opposite face ply presents the opposite surface i4, which constitutes the inside surface of the shell. The inside face ply, in the embodiment shown, comprises strips l5, which are laid side by side in a manner similar to the ply com- There can be as many intermediate core plies as are desired, comprising strips i5 laid up on the face ply l2, and on each other, until the final inside face ply is laid up to comprise the surface lll. The disclosedY embodiment presents one intermediate core ply to comprise a three-ply assembly.
As shown in Fig. 2, each strip ll, is of a predetermined thickness, strips IE being of like construction, and the several plies are built up and adhered together to comprise a structure of the desired thickness. The thickness. of .stripsinany given ply are usually the same, butthe respective different plies may, and frequently do, comprise strips ofdifferent thicknesses.
The degree of thickness of the several plies depends to some extent on the degree of curvature ofthe structure, and a thinner Veneer is selected for sharp curvatures than is required for less severe curvatures. Also, the width of` the strip is vdetermined to some vextent by the .amount of curvature the piece is to. be subjected to. For example, in .places where the .curvature is greater .asat the ends it .and l'l of the structure shown infliig'. 1 the strips must v'be narrow, `and they ,can be Wider between `the ends It and H where they are exed to a lesser extent in a. plurality of directions.
As shown in Fig. 2, the side edges of the strip are` bevelled, as at i8 and IS. The amount of bevei can be varied within wide limits without departing from the scope of the invention, but in the preferred practice, the edges at I8 and I9 are feathered or scarfed. Thus, the bevels at IB and t9 present sharp angles to the surface 2|, andV continue into the opposite surface 22 practically without abrupt angular change. In this manner the surfaces of both faces 2! and 22 of the strip are virtually continuous from edge to edge.
The` edges 25 and 28 of the several strips may be parallel, or theymay be tailored or contoured to t, together with other strips, to present smooth coverage of the layup. In actual practice, especially in a layup of compound curvature as illustrated in Fig. 2, each strip is tailored to t a predetermined area of the layup, the several strips being numbered to insure that they be properly located in their predetermined position in the assembly. The edges of each strip are feathered'at i3 and i9, as a separate operation, after it has been tailored to nt the area it is to occupy in the assembly.
Each strip li and i5 may be made of a length to extend from edge to edge of the layup. .However, under one practice, it is preferred that some or all strips extend less than the full distance from edge to edge of the layup. Under this practice it is preferred that each strip extend from the edge of the layup to a position where it meets andoverlaps the edge of another strip extending toward it from the opposite edge of the layup. Thus at least one end of each strip may be cut cross-cut, or at an angle, to meet the proximate edge of another strip along the lines 29 of Fig. 1. Such overlapping ends of strips in a given band will be feathered along their overlapping edges 29 in a manner similar to the feathering i8 and l5 along the side edges. The object of this construction is yto permit the strips disposed end to end to adjust themselves longitudinally relative to each other, under the pressure of molding the layup to form an assembly.
In the practice of. the invention, the strips H or i5 of any given ply are laid up side by side on the surface of the diel 24 or on the surface of the previously laid up ply, with proximate edges 25 and 25 of adjacent strips overlapping. The position of the strip in the assembly is fixed by any suitable means, one practice being to attach the end of the strip to the rigid surface of the die at the border of the assembly, for example by tacking.
In kthe case of strips extending from border to border of the assembly, each strip is tacked at its opposite ends to fix it in position in the assembly. In the practice shown inA Fig.. l, two
` strips are disposed end to end to extend from border to border vof the assembly, the two strips being overlapped at 2Q, where their proximate `ends meet.
unrestricted, and affords liberal freedom for the several stri-ps to adjust themselves to the con tour of the die, under action of the molding pressure. The position of the overlap 29 can be at any point between borders, but it is preferred that the overlap in adjacent strips be staggered as shown in Fig. 1.
The extent of overlap at proximate side and end edges of adjacent strips may vary within wide limits and can be the same as, greater than, or less than, the extent of the bevels I 8 and i9. v2in making the layup, under the preferred practice of the present invention, the operator simply lays tWo end to end strips beside the next previously laid strips, and at two or three points along con-- tiguous edges provides an overlap-which will ap` proximately equal the amount of bevel When the layup is subjected to molding pressure. Likewise the amount of overlap at 2d Where strips meet end to end is made to approximately equal the extent of bevel after molding. Under this practice, it will be noted that the extent of overlap may deviate from the extent of bevel at various points along the lengths of proximate side edges, and at proximate end edges.
As noted in Fig. 3, when the layup is made, the strips will not lie flat against the surface of the die 24 at all points, but will lie spaced away from the surface thereof to a greater or lesser extent as at 21, due to the feathering of the edges and also due to the pieces being not naturally curved to the contour of the die. In the layup shown in Fig. 3, additional plies comprising the strips i5 are laid on top of the ply comprising the strips li and are overlapped in a similar manner, the extent of overlap approximating the extent of bevel but deviating along the lengths of the contiguous edges. The additional ply presents additional spaces 2% due to the scarfing and the nature of the layup. The embodiment shown in Figs. 3 and 4 presents a threeply construction, but the principles of the invention apply to an assembly of any desired number of plies.
On top of the uppermost ply, a covering 3l, comprising a blanket or bag of rubber or like material, is laid in the manner illustrated in Fig. 3. Now, under preferred practice, a vacuum is drawn, and the air between the blanket 3i and the die 2A is exhausted in a manner described more fully in the patent to Potchen et al., No. 2,308,453, previously referred to. This subjects the outer surface of the blanket 3l to atmosF u pheric pressure which is transmitted through the layup against the surface of the die 24. This pressure, being a static fluid pressure, is normalto the surface of the die 2Q at all points in its area.
Under the application of iuidpressure, the several strips H and l5 Will shif.l and adjust them selves to the position they occupy in the assembly. In the case of butt joints between strips, 'it will be understood, in some places the contiguous edges of i adjacent strips are spread apart and form open seams, whereas in other places, adjacent strips may be pressed together and caused to overlap. Under the practice of the present invention., this shifting of the several strips of the ply has the effect of changing the extent of overlapping slightly, to a greater or less amount from that which was originally applied in the layup. However, when the assembly is complete there will be an overlap between adjacent strips at joints, and this overlap, at 32V and 33 in Fig. 4, presents an extensive surface of juncture between adjacent strips in a given ply, which is continuous with surfaces of juncture between plies.
Under the applied atmospheric pressure, and preferably with the application of additional pressure in the process disclosed in Patent No. 2,308,- 453, the adjacent strips are adhered together along their surfaces of contact at the overlap between contiguous edges 25 and 2E. Also the several plies are adhered together along their contacting surfaces. The structure having been compressed in the manner described, the open spaces 21 and 28 are removed, and any air contained in these spaces is exhausted by the applied vacuum, together with any gases generated in the adhesive curing process.
The resultant ply structure is illustrated in Fig. 4, and as shown therein, strips l I of the face ply are adhered together along contacting surfaces of the overlap 32. Also strips l of the core and opposite face plies are adhered together along contacting surfaces of the overlaps 33. Also, the strips H and I5 of the respective plies are adhered together at the contacting surfaces 34 between plies, making a composite ply structure thoroughly adhered together.
The surface I3 which was formed against the surface of the die 24 has a regular curvature which conforms with the curvature of the surface of the die. The surface I4 of the opposite face ply comprising strips i5 will, in general, be parallel with the curvature of the surface I3, but it will usually deviate from parallelism to a greater or lesser extent opposite overlapping joints 32 and 33. Figs. 3 and 4 illustrate the case of a plurality of overlaps 32 and 33 being opposite each other, producing the high spot 36, this being the situation where two lines of juncture in different plies intersect.
It will be noted that the structure of Fig. 4 is shown on an exaggerated scale to illustrate the nature of the invention. Usually, the high spot 36 is much less relative to the thickness of the assembly than is indicated in Fig. 4.
5 illustrates schematically what might be expected to take place along overlapping joints in any given ply. Along the length of a juncture, the distance between contiguous edges 25 and 25 will vary, and the extent of overlap will equal, be greater than, or less than, the extent of bevel. The edges 25 and 26 may cross the lines 39 and 38, which mark the juncture of the bevels with the surface 22, Fig. 2. Accordingly, the thickness of the joint may vary from-less than to more than the thickness of the ply. In actual practice the thickness of the ply structure will be virtually the same throughout its area, but there will be slight irregularities in the surface IA, Fig. 3. Under the practice of the invention, the irregularities will be conned to one surface I4, the opposite surface I3 being regular, and conforming with the contour of the surface of the die.
In those cases in which it is desirable to have regular curvature on both surfaces of a shell structure, irregularities in curvature can be readily removed by any suitable finishing operation.
Having thus fully disclosed the nature of the invention, it will be noted that it is not limited to the embodiment shown and described. The scope of the invention is determined by the accompanying claims.
1. A structural assembly, comprising strips having substantially straight edges, the edges of the strips being bevelled, the several strips being arranged in pairs, each pair extending from border to border of the assembly, the strips of each pair being arranged end to end, with proximate ends overlapping, the pair being arranged side to side with proximate sides overlapping, the overlapping ends and sides forming joints adhesively secured together, the thickness of the assembly at joints being irregular and comprising the additive thickness of the several overlapping edges, the structure having one surface of regular smooth contour, irregularities resulting from variations in thickness of the material at joints being additive towards the opposite surface,
2. A structural assembly, comprising elongated strips having substantially straight edges, side and end edges of the strips being bevelled, the several strips being arranged in pairs and positioned adjacent to each other with proximate side edges joined, the two strips of each pair extending from opposite borders of the assembly toward each other and meeting with proximate end edges joined end to end, the joints of proximate side edges of adjacent strips and proximate end edges of strips overlapping each other with surfaces of contact adhered together, the thickness of the assembly at joints being irergular and comprising the additive thickness of the several overlapping edges, the assembly having one surface of smooth regular contour, irregularities resulting from variations in thickness of material at joints being additive towards the opposite surface,
3. A ply structure having a contour of predetermined compound curvature and comprising two or more plies adhered together, each ply comprising strips having substantially straight edges, the edges of the strips being bevelled, and the strips of the structure being arranged in pairs, each pair extending from border to border of the assembly, the strips of each pair lying end to end with proximate ends overlapping, the pairs being arranged side to side with proximate sides overlapping, the strips being individually flexed to a curvature required to attain the desired contour of assembly, the overlapping edges as well as the contacting surfaces of the plies being adhesively secured together, the thickness of a ply at joints being irregular and determined by the extent of overlap, one surface of the structure being smoothly curved to the predetermined desired contour, the surface irregularities resulting from variations in thickness at joints being confined to the opposite surface.
SHERMAN M.Y FAIRCHILD.
REFERENCES CITED UNITED STATES PATENTS Number Name Date Fairchild Jan. 15, 1946