WO1991009714A1

WO1991009714A1 - Composite assembly press

Info

Publication number: WO1991009714A1
Application number: PCT/CA1990/000459
Authority: WO
Inventors: Mark Trelawny Churchland
Original assignee: Macmillan Bloedel Limited
Priority date: 1989-12-29
Filing date: 1990-12-31
Publication date: 1991-07-11
Also published as: AU6973691A

Abstract

An apparatus for pressing composite assemblies including a continuous belt press having a pair of endless belts facing one another. The distance between the belts varies to define therebetween a compression zone wherein lock-up of the composite assemblies occurs, a parallel bed press zone, and a transfer zone therebetween. The curvature radius of the compression zone, which can be positive, negative or zero, is longer than that of the transfer zone.

Description

COMPOSITE ASSEMBLY PRESS

BACKGROUND OF THE INVENTION

The present invention relates to belt presses for pressing composite assemblies and is more particularly an improvement on the technologies disclosed in U.S. Patent 4,517,148. This and each of the other patents mentioned in this disclosure are hereby incorporated by reference in their entireties.

Structural wood products can be manufactured from long relatively thin strands of wood by coating the strands with an adhesive, arranging the strands side-by-side in a lengthwise dimension of the lumber product and subjecting the arranged strands to heat and compression. A high strength dimensional wood product is thereby formed, and this process is disclosed for example in U.S. Patent 4,061,819. Belt presses are typically used in processes for the manufacture of composite wood products, and examples thereof are shown in U.S. Patents 3,120,862; 3,723,230; 3,792,953; 3,851,685; 3,993,426; 4,043,732; 4,850,846; and 4,410,474. The belt presses include facing endless belts between which the material is compressed and platens and anti-friction devices which hold the belts in pressure engagement with the materials conveyed therebetween. The inlet end of the press belts and the platens over which they run converge towards one another to form a compressing zone. As the strands enter the compressing zone they are generally free to move with respect to one another. However, as the belts converge within this zone the strands have their positions set, in a "lock-up" position, with respect to one another. After lock-up further compression of the material results from the further convergence of the press belts. Since lock-up usually occurs in a curved area, the material is curved as it is being pressed. After the compressing zone the locked-up material is passed through a parallel belt zone.

Although the resulting material passing out of the parallel belt zone is planar, the curvature of the strands at lock-up is "remembered" as an internal stress therein. Where the end products are thin and planar these internal stresses do not present a problem. For relatively thick products, however, such as dimensioned lumber, these internal stresses present a problem when the thick product is cut longitudinally. When so cut the internal stresses are released and the two resulting halves bow in opposite directions. In other words, with single radius infeeds which are tangent to the parallel belt of the press prestresses in the wood, which are a function of the infeed radius, occur and when later resawn axially the prestresses are released and the final product curves. This problem has been addressed in the past by using a very large compression radius.

Increasing the entire infeed radius may not always be possible, however, as the large radius may not provide a large enough infeed opening for the projected point of lock-up. Additionally, the radius at times cannot be extended sufficiently to open the press opening where there are physical limitations on the length of the press bed. Also, a long press has been needed to obtain an opening large enough to "bite" on thick incoming mats. For example, when a twelve inch product is being compressed, a twenty-four to thirty-six inch opening is initially required. It thus takes a very long press bed in proportion to the extra press depth to gain a smooth even compression.

In one known press the required shape of the compression radius and the parallel bed was cut from a steel plate about one-and-a-half inches thick. Four shaped "ribs" were cut out of an about ten inch deep, one and a half inch wide and fifteen foot long plate in about thirty inch wide sections and were bent over the plate. The ribs acted as stiffeners to hold the shape of the plate. The assembly was welded together, and two such platen assemblies were mounted facing each other. Roller chains covered the platens and moving bearing plates covered the rollers. A tensioned and driven belt provided the traction force and faced directly on the product. This is a constant radius type of fabricated radiused platen.

An example of another known press, a stepped radius press, is the commercial press available from Eduard Kuesters of Kref eld, West Germany. It uses a flat platen bent at specific discrete points defined by machined slots in the back thereof. Each flat section, which is typically eighteen inches long, is supported with a wedge block and frame. In this press, roller chain assemblies separate the steel belt which is driven by drums from the platen and form the friction reducing medium. The effective radius of the platen area, which can be defined by the intersection of the perpendicular bisectors of adjacent platen segments, is for example for a three-quarters of a degree bend between two sections or segments one hundred and twelve feet. The prestressing can be characterized by the amount of bow in beams cut from the product. A forty-eight foot long beam produced by this press and cut from the face of the billet typically has an often unacceptably large bow of two-and-a-half to four inches. SUMMARY OF THE INVENTION

Accordingly, it is a principal object of the present invention to provide an improved composite press assembly which can form thick products without any significant prestress.

Another object of the present invention is to provide a composite press assembly having lower production costs and consistent product output.

A further object of the present invention is to provide a composite press assembly design allowing, for the same amount of prestress, a shorter press bed thereby saving machinery cost, maintenance cost and floor space.

Directed to achieving these objects, a continuous press for processing composite assemblies, including a pair of facing endless forming belts symmetrical about the transport axis, is herein provided. The space between the belts varies as the composite assemblies are conveyed therebetween through gathering, compression, transfer and parallel bed press zones. Lock-up of the composite assemblies occurs in the compression zone, and the radius of curvature (positive, negative or zero) of the compression zone is great enough to minimize pre-stressing, especially for thick products. The curvature radius of the transfer zone is significantly shorter than that of the compression zone, allowing for a sufficiently large in-flow opening and for a significantly shorter press bed, but not so short a radius as to damage the composite product. Although the present invention relates primarily to presses for long strand composite materials, its concept can also be applied to any composites which go through lock-up on compression, such as thick section fiberboard and particle board.

Other objects and advantages of the present invention will become more apparent to those persons having ordinary skill in the art to which the present invention pertains from the foregoing description taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a diagrammatic side view of a first belt press of the present invention.

Figure 2 is a diagrammatic side view of a second belt press of the present invention.

Figure 3 is a diagrammatic side view of a third belt press of the present invention.

Figure 4 is a diagrammatic side view of a fourth belt press of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

Referring to the drawings, a first belt press assembly of the present invention is illustrated generally at 20 in Figure 1. Belt press assembly 20 and the other belt presses illustrated in Figures 2-4 are shown diagrammatically since they are of conventional construction except for the arrangement of their belts and platens. Examples of conventional presses are discussed and referred to hereabove in the "Background of the Invention" section of this disclosure.

Belt press 20 includes an upper continuous press belt 22 trained about a pair of rotating drums, one of drum 24 of which is illustrated, and a lower continuous press belt 26 trained about a pair of rotary drums including drum 28. Upper and lower platens 30, 32 apply compressive pressure on the strand material 34 being moved between and with the belts 22 and 26. The press 20 can incorporate a heating device (not shown) to heat the material 34 during its passage through the press. An example of a known microwave heating device used in conjunction with continuous presses is shown in U.S. Patent 4,456,498.

The strands 34 are conveyed to the continuous press such that their ends overlap. The strands in the lay-up mat are thus in a "card-decking" orientation, as shown for example in U.S. Patent 3,493,021, on the infeed conveyor 36. An improved method thereon is disclosed in U.S. Patent 4,563,237 wherein the elongate members are deposited on the carrier over a carrier length that is at least as long as about one-and-a-half times the length of the members and is at least as long as about thirty times the final thickness of the composite product.

The belts 22, 26 define by the variances in the distances between them a compression zone, a transfer zone and a parallel bed press zone along the flow of the strands 34. A key feature of this invention is that the curvature radius Re of the compression zone is larger than the radius R_T of the transfer zone. The inner smaller radius R_T does the final changing of direction of the final compression after lock-up.

The compression zone wherein lock-up occurs can have any large curvature radius Re including infinitely large, that is a flat platen (Figure 3), or even a concave platen (Figure 4) if it is desired to induce prestresses for whatever reason. As long as the radius R_T of the final compressing area, that is the transfer zone, is gentle enough to avoid damaging the product, the desired prestress can be obtained. In other words, once compression of the strand mat has progressed to a point of lock-up, prestresses have been defined and as long as the geometry of further compression (in the transfer zone) is gentle enough to avoid damage, the final product will not be affected. The point at which the product is damaged depends upon the properties of the product and would be apparent to one skilled in the art. This damage is by overstressing or straining the product by bending it too far and then straightening it. The product is bent curved as it is being compressed to the curvature of the platen, and then it is bent straight as it passes through the parallel portion of the press bed resulting in the "remembered" stresses.

What is critical is that there be a flat or nearly flat platen at the point of lock-up. It is difficult to define exactly when lock-up occurs so the compression zone must have an extensive area volume. An extensive volume is also needed if the press height is changed as the height of the mat changes with changing press height since a different amount is being fed into the press. In order to have the same press radius effect the same results of compressing to lock-up under the larger radius, a long section of larger radius is needed because lock-up occurs at different places for different press depths.

Instead of being a continuous radius the press can comprise a series of discrete bends which are, for example, approximately eighteen inches apart. Thus similar to the previously-discussed Kuesters press, the present press can be constructed as a stepped radius press. An example thereof is illustrated in Figure 2 generally at 40, and its dimensions and geometry are further defined in Table I.

It is seen in Table I that the first two increments are .75° each, which is a much smaller radius than that of the next increments which are .333° . A nominal two hundred and fifty-three foot radius results, as shown in the bottom line of Table I. The angle increments are not consistent across because the platen lengths are not consistent. The places at which the bends were placed in the platen could not carry on in a four hundred and forty-eight millimeter increment for vestigial structure within the press that could not be changed. However, a radius of generally two hundred and fifty feet as shown on the bottom line was maintained. This radius was calculated by taking the perpendicular bisector of each of the segments, the four hundred and forty-eight millimeter segment or six hundred and seventy-two millimeter segment of the press bed for example, and determining the intersections of the adjacent perpendicular bisectors. This is a close estimate of the true radius. A one hundred and twelve foot radius R_T is provided because this is gentle enough bend to not hurt the product, and the critical compression is done on the larger radius R_C, the two hundred and fifty foot plus radius.

Referring to the alternative stepped radius design of Table II, the angle increment is .75° four times followed by two 2° increments and there are then no 0 through -5 sections of this press. The .75° increments through the first four sections provide a one hundred and twelve foot radius throughout and then a fifty foot radius follows on the very large opening of the press where the strands are fed in. The press design of Table I is preferred over that of Table II, however, because the Table I design has the bottom line effective radius. The designs of both Tables I and II are for product depths of 11.4 inches.

A gathering zone can also be provided at the inlet of the press. Similar to the gathering zone shown in the '148 patent it has a small radius R_G on the order of twenty, forty or fifty feet, gathers the strands into the horn of the press and does no real compression on the strands. In other words, the gathering radius allows the press to open up and gather the product and then take it gently to the lock-up point. As an example, the gathering radius R_G can be only about forty feet while the compression radius R_C is about two hundred and fifty feet and the transfer radius R_T about one hundred and twelve feet. The gathering zone will have a length of around two feet, the compression zone of eleven feet and the transfer zone of three feet.

Compared with the two-and-a-half to four inch bow resulting from the Kuesters stepped radius press as previously described, the press of Figure 2 having a two hundred and fifty-four foot effective radius reduces the bow to between one-half and one inch and more typically one-half inch.

A one hundred and twelve foot radius R_T is a gentle enough bend or curve to not hurt a wood strand composite product. It also opens the press up much faster. The critical compression must, however, be at a larger radius R_C, such as greater than two hundred and fifty feet. When a single radius over the same press bed length was used, only a one hundred and fifty foot radius out of the press could be obtained. In theory, if the press at the lock-up point has an infinite radius, that is it is a flat platen, the product produced is free from bending stresses, though there may be other induced stresses from other properties of the material or the process. A bed press with a flat platen compression zone is illustrated in Figure 3 generally at 50. The platens thereof have transfer radiuses R_T of around one hundred feet.

A concave compression zone type of bed press is shown in Figure 4 generally at 60. It is seen therein that the compression radius R_C is three hundred feet (or, more appropriately, minus three hundred feet) and the transfer radius R_T is one hundred and twelve feet. The tangent point(s) between these radiuses marks the end of the compression or lock-up zone.

This concept works well for strand mats and continuous presses and also for any composite which goes through "lock-up" on compression. The present invention works for example for fiber and particle boards made in thick sections, such as those greater than two inches. For a one-half inch thick product press bed lengths may not present a constraint because only a three or four foot of press bed length is needed to obtain the gentle large radius compression. It is especially when the product is twelve inches thick that twelve times that length of compression zone is needed for the same general compression. So practically speaking, the concept of this invention is more useful in thicker products though it still has utility in thin products if the structure of the press needs to be changed to obtain a smooth compression.

This invention can be used for example in making rubber belts by compressing fiber and a vulcanizable material in a hot press. A relatively thick, stress-free belt can be produced by compressing with totally flat platens converging towards the parallel region of the press and then connected thereto with a single radius of perhaps only a few feet — similar to going around a corner. Since rubber is still flexible it would not be hurt and could tolerate such a small radius.

However, for laminating wood strands into thick lumber products a radius below one hundred feet risks overstressing the product when it is straightened out. In other words, the radius is a function of the material and the thickness of the product being compressed. For one-quarter or one-half inch thick wood board it would only be a few feet. This is because thin pieces of wood can be bent around a rather small radius without overstressing the wood. However, for thick sections a gentle larger radius must be used.

An advantage of this invention is thus that any degree of pre- stressing -- positive, negative or zero -- can be obtained in a finite continuous press. This invention also allows for a much shorter press to be used thereby saving machinery and maintenance costs and floor space. Since very thick products can be made without inducing any significant prestress, large cross-section, high value products can be made. Thus, a larger volume of products at the same process speed can be pressed, product costs are lower, and a straight even density consistent product can be produced. As previously discussed and as illustrated in the drawings, the concept of this invention works equally well with true continuous radiuses as well as actual segmented platens forming a pseudo or effective radius from a series of flats.

From the foregoing detailed description, it will be evident that there are a number of changes, adaptations and modifications of the present invention which come within the province of those skilled in the art. However, it is intended that all such variations not departing from the spirit of the invention be considered as within the scope thereof as limited solely by the claims appended hereto.

Claims

WHAT IS CLAIMED IS:

1. A composite pressing apparatus comprising: a continuous press for pressing composite assemblies; wherein said continuous press includes defining means for defining a parallel bed press zone, a compression zone wherein lock-up of the composite assemblies occurs, and a transfer zone therebetween, said transfer zone having a transfer zone curvature radius, and said compression zone having a compression zone curvature radius; and wherein said compression zone curvature radius is longer than said transfer zone curvature radius.

2. The apparatus of claim 1 wherein said compression zone curvature radius is relatively large whereby said compression zone is relatively flat.

3. The apparatus of claim 1 wherein said defining means includes a gathering zone upstream of said compression zone.

4. The apparatus of claim 3 wherein said gathering zone has a gathering zone curvature radius, and said gathering zone curvature radius is shorter than said compression zone curvature radius.

5. The apparatus of claim 4 wherein said gathering zone curvature radius is shorter than said transfer zone curvature radius.

6. The apparatus of claim 5 wherein said gathering zone curvature radius is generally forty feet, said transfer zone curvature radius is generally one hundred and twelve feet, and said compression zone curvature radius is generally two hundred and fifty-four feet.

7. The apparatus of claim 3 wherein said gathering zone has a length of about two feet, said compression zone has a length of eleven feet, and said transfer zone has a length of three feet.

8. The apparatus of claim 1 wherein said compression zone is eleven feet long and said transfer zone is three feet long.

9. The apparatus of claim 1 wherein said transfer zone curvature radius is large enough to avoid damage to the composite assemblies after lock-up thereof.

10. The apparatus of claim 1 wherein said compression zone is more than twice as long as said transfer zone.

11. The apparatus of claim 1 wherein said transfer zone at one end thereof is directly adjacent to said parallel bed zone and at the other end thereof is directly adjacent the said compression zone.

12. The apparatus of claim 1 wherein said transfer zone curvature radius at one end thereof is tangent to said parallel bed press zone and at the other end thereof is tangent to said compression zone curvature radius.

13. The apparatus of claim 1 wherein said compression zone curvature radius is an infinite radius.

14. The apparatus of claim 13 wherein said transfer zone curvature radius is a generally one hundred feet.

15. The apparatus of claim 1 wherein said compression zone includes at least one flat platen.

16. The apparatus of claim 1 wherein said continuous press includes a pair of continuous belts facing one another through said zones and driven towards said parallel bed press zone.

17. The apparatus of claim 1 further comprising said continuous press having a press inlet, and conveying means for conveying the composite assemblies in a mat to said press inlet.

18. The apparatus of claim 17 wherein said press has a gathering zone directly adjacent to said compression zone and at said press inlet.

19. The apparatus of claim 18 wherein said gathering zone has a gathering zone curvature radius and said gathering zone curvature radius is generally less than one-quarter of said compression zone curvature radius.

20. The apparatus of claim 1 wherein each said curvature radius is formed by a series of flats adjacent and at slight angles relative to one another.

21. The apparatus of claim 1 wherein each said curvature radius is formed by a smooth continuous curve.

22. The apparatus of claim 1 wherein said compression zone has a concave curvature.

23. A method for compressing composite assemblies, comprising the steps of: conveying composite assemblies through a continuous belt press compression lock-up region and thereby causing lock-up of the composite assemblies; thereafter, conveying the locked-up composite assemblies through a continuous belt press transfer region whose radius of curvature is shorter than that of the compression lock-up region; and thereafter, conveying the composite assemblies through a parallel bed press region.