Veneer and Plywood
Veneer is a thin sheet of wood either sliced or peeled from a log or flitch.
If the log is peeled, as depicted in the bottom of the above Figure 14.4. taken from the Textbook on Page 359, the radial direction becomes the thickness and the tangential direction becomes the width of the sheet. Peeled veneer is used mainly for structural purposes since the principal axes now align with the geometric axes and engineered products can be produced. Although, some peeled veneers are used for decorative purposes.
Sliced veneers, as depicted in the upper portion of the above figure, are used for furniture, cabinets, paneling, and other decorative and architectural purposes. The logs are sawn to produce flitches with the correct grain orientation and the flitches are sliced.
By orienting the knife at different angles to the growth rings, different grain figures and appearances can be produced.
As hardwood veneer comes off of the slicer, it is stack in the order that it was cut. It is then put through the drier in the order it was cut and restacked. The customer buys the whole flitch of veneer. This way the customer can match grain patterns in their product.
These veneer matching figures were taken from the Architectural Woodworking Institute's Quality Standards.
Softwood Veneer and Plywood Grades
Softwood veneer grades are A, B, C, and D. A description of the grades is given in Table 14.4 on Page 375 in the Textbook.
Softwood plywood grades are the combination of the face veneer and the back veneer.
A-C Face is A grade and Back is C grade
C-D Face is C grade and Back is D grade
B-B both Face and Back are B grade.
What are the requirements of exterior plywood?
1. Exterior (water proof) adhesive.
2. ALL Veneers (face, back or in the middle) are C grade or better.
What are the requirements of marine plywood?
1. Water Proof adhesive
2. ALL Veneers are B grade or better.
In the above Figure 14.2. taken from the Textbook on Page 355, the upper plywood sections are typical of softwood plywood. In the old days if one found a sheet of plywood with an even number of plies it would be thrown out for being defective. Modern production techniques allows even plies when the middle two are going in the same direction and acting as one ply.
The lower sections are typical of hardwood plywood. Whereas, softwood plywood is produced as a commodity, hardwood plywood is usually customer specific. The customer will state what species to use and where, type of adhesive, construction arrangement, etc.
Softwood Plywood Plant Layout
Every plant is different but the following is a generic layout. The logs are usually brought to the plywood plant in tree length or multiple lengths. They are stored on the yard until they are needed. The following figure is of the log prep operation just outside the plant. The logs are cut to accurate lengths, debarked, checked for metal and placed in a soaking vat. The purpose of the soaking vat is to heat the interior of the log up to 180 degree F prior to peeling. The temperature helps plasticize the lignin, reduce lathe checks, and reduce power requirements.
After the bolts (logs) are warmed, they are brought into the plant onto the live deck. The lathe charger positions the bolt so that when the lathe starts to peel it, a minimum of wood is removed before there is a prefect cylinder. The veneer is then placed on stacking trays which act as surge bins since the lathe is working faster than the downstream equipment. But the lathe is not peeling 100% of the time due to dropping the core and charging a new bolt. Following the trays, the veneer go under an optical scanner which determines where the veneer should be cut. This information is sent electronically to the clipper which cuts the veneer. If there are no defects, the veneer is cut to 54-inch widths. Why? Following the clipper, the veneer strips are separated by size and stacked. The 54's go to the end and are automatically stacked. Veneer strips that are not of full length (fishtails) are stacked with the good end in the same direction. These are cut to half length and used as cross banding in the plywood.
At the dryer, green veneer stacks are placed on the infeed table. Individual sheets are sent into the dryer. There are several temperature zones inside the dryer which gradually raise the veneer temperature to above boiling. As the veneer comes out of the dryer, it passes under a moisture meter. The goal is between 5 and 8 percent MC and if it is over that a paint sprayer marks the veneer indicating it will have to be re-dried. As the veneer passes down the conveyor, it is hand pulled by grade and stacked accordingly.
Some veneers can be upgraded by replacing defects with patches. "Boat" patches are the most common but there are others such as oval patches and strips.
With the veneer separated by grade and stacked in storage, we are ready to make plywood. Again each plant is different but this is a generic example of the layup line. The back veneer is placed on the conveyor which goes under the second stage. At the second stage, cross banding is fed through a two side glue spreader and then placed on top of the back veneer and conveyed to the third stage where the middle sheet is place on top. As the material is conveyed on, a second cross banding is fed through the glue spreader and placed on top. At the final stage, the face veneer is placed on top. The new panels are stacked and moved to a cold press where pressure is applied to insure intimate contact of all veneers and adhesive.
After the cold press, the stack is conveyed to the infeed table of the hot press. This table works as an elevator raise the top of the stack even with each new opening in the hot press. As the operator slides the new panel into the press, it forces the pressed panel out the back onto the discharge table which also acts as an elevator. After the press is full, it closes to a predetermined thickness and holds for several minutes pressing the panels and heating them to well over 200 degrees. The discharged panels are cooled and placed in storage the let the adhesive finish curing.
From storage, the panels are taken to a skinner saw where the panels are conveyed between two saws that trim the sides and conveyed at right angle to a second set of saws that trim the ends. Some panels are sent to re-patch to repair surface defects. Some panels are sent to a sanding operation to produce smooth panel. And there are other specialty operations to produce such products as tongue and curve flooring, T-111 siding, etc.