(For beginner) Plywood knowledge& Manufacturing Experience Part 5: Common veneer delamination defects

 Common veneer delamination defects.

 

Defect Cause Solution
Veneer blow in the hot press •     Too high moisture content of the veneer and/or substrate panel

•     Adhesive application rate too high

•     Temperature of the press too high

•     Press time too long

•     Reduce moisture content to between 4 and 8%

•     Reduce the quantity of water in the glue line

•     Follow recommended gluing parameters

•     Close and open the press after 30 seconds to let the steam escape

Telegraphing of the glue onto the veneer surface •     Too much adhesive applied

•     Glue bleeding through the veneer to the surface

•     Adhesive viscosity too low

•     Voids in the core material

•     Surface irregularities

 

•     Glue spread rate to be from 5 to 8 mils in thickness

•     Often visible only after finishing

Veneer delamination •     Too high or too low adhesive application

•     Glue line dry out

•     Follow recommended gluing parameters
Spotty erratic bonds  

•     Not enough glue transferred to both surfaces

•     Non uniform platen temperature

•     Increase the glue spread rate

•     Lamination pressure should be between 100 and 125 psi

•     Assembly time too long

•     Pressing time too short

Poor adhesive transfer to the back face of the veneer  

 

 

 

•     Veneer surface inactivated

•     Proceed with the water drop test; put a drop of water on the surface to be glued and another drop adjacent on a freshly sanded portion of the same surface. The water drop should start to be absorbed in the wood within a minute and be totally absorbed in less than 60 minutes. If not, this surface is likely to be resistant to the adhesive

•     Wetting and integrity of the joint is compromised

•     If surface is inactivated, sand the surface with 150 grit paper or wash with alcohol to remove the extractives from the surface to be glued

 

Storage

  • Veneer, either supplied rolled or flat, should be stored horizon- tally flat, in a dark (especially protected against sunlight),

dry (30 to 55% MC) and cool area (15 to 30°C), and off the ground on a rigid 4×8 panel.

  • Avoid extreme temperature and

relative humidity swings.

  • The corners should be prevented from rolling back by adding a light weight close to the corners until the veneer is conditioned and
  • Upon receiving the veneer, loosen the bundles to prevent condensation and the formation of mold – allow the material to

 

 

Table 2: Veneer trouble shooting.

Using Veneer

Wood veneer is seldom used alone; it is usually laminated with an adhesive onto a substrate panel. The choice of the substrate is guided by the structural properties needed, the edge appearance desired, and the cost and surface smoothness requirements. The most popular  substrates

used are: plywood, particleboard, medium density fibreboard (MDF), combination core and solid lumber.

 

The adhesives used for laminating the veneer over the substrate panels are mainly from the Urea Formaldehyde (UF) or Polyvinyl Acetate (PVA) adhesive families. There are two methods of veneer lamination: applied with a cold press at room temperature and applied with a hot press.

 

To ensure a good bond between the veneer and the substrate, it is recommended that:

  • The moisture content of the veneer and substrate panel should be from 4 to 14%, preferably in the range of 6 to 10%
Defect Cause Solution
Barber pole effect •     Tight and loose sides of veneer spliced together •     Natural book matching effect of veneer
 

Veneer checking, small splits in the face veneer

•     Sudden changes in moisture content create stress. Usually found after the pressing operation

•     The finish coat is absorbed differently by the loose and tight sides of the veneer

•     Sometimes created by the application of a heavy surface finish

•     Difficult to repair and easier to identify and reject before lamination

•     Apply thinner finish coating

 

Contamination

•     Evaporated water or solvent that made the wood grain rise (e.g., perspiration, oil, any foreign material) •     Wipe to clean first then sand to the finishing sanding grit
 

Core overlapping

•     Overlapping of the inner plies

•     Uneven absorption of the finishing material on the high density spots of the core material

•     Sand the high density area
Telegraphing •     Irregularities in the core (mostly in plywood core material) •     Fill the voids then sand to the finishing sanding grit
Delamination •     Separation of plies because of adhesive bond failure •     Re-gluing and repressing when possible
Glue bleed through, glue spill up •     Adhesive showing through the surface via the pores of open core material

•     Too generous glue spread

•     Detect the surface glue before processing or sand the glue contaminated area
Iron stain, blue stain •     Natural defect (i.e., tannic acid comes in contact with iron) •     Sand or remove chemically
 

Knife marks, sanding marks

•     Marks done by the veneer knife

•     Sanding marks are produced by a too coarse finishing grit

•     Sand to proper finishing grit
 

Panel warping

•     Grain orientation of veneer not consistent

•     Different species on face and back

•     Unbalanced layup

•     Ensure balanced layup
Water spots •     Dried water spots •     Sand
Ring shake   •     Natural defect

Let’s  talking about delamination within theplywood or delamination of the decorative veneer from the plywood core

Check Also

The Bending Strength of Medium Density Fibreboard (MDF) from

The Bending Strength of Medium Density Fibreboard (MDF) from … https://www.scientific.net/KEM.334-335.77 by MA Jamaludin – …

Leave a Reply

Your email address will not be published. Required fields are marked *