Suggestions for Preventing or Minimizing Veneer Checking
Robert C. Gilmore
Superintendent, Hodges Wood Products Laboratory
North Carolina State University
Stephen J. Hanover
Associate Professor and Wood Products Extension Specialist
North Carolina State University
Several veneer checking studies have been conducted at the Hodges Wood Products Laboratory, North Carolina State University, Raleigh, NC. The first of these studies was initiated at the request of the Southern Furniture Manufacturer’s Association and a report was presented to that Association in 1970. Without going into the details of that and later studies at North Carolina State University, the following recommendations are presented to help the furniture manufacturer reduce or prevent the problem of veneer checking. These comments are aimed at the most critical veneer surfaces seen by the consumer, for example, case tops or table tops.
1. Veneer Quality – Select well manufactured tight-cut veneer. Tight cut veneer is defined as veneer with shallow lathe checks. The depth of lathe checks can vary within a sheet or flitch. Some manufacturers select only the tightest cut portions within the sheet or flitch for use in critical areas such as tops.
2. Veneer Moisture Content – The face veneer should be 6% at time of pressing. Decorative faces may require re-drying of the veneer to flatten it and reduce the moisture content to enable the faces to be properly made. Pressing a few sheets stacked in a pile in a hot platen press is the best way to flatten and reduce moisture content. Veneer stock in process, and completed faces should be kept at 6%. Veneer moisture content at time of pressing is the most important factor in controlling veneer checking.
3. Panel Components and Construction – Core components and crossband materials should be held at about the same moisture content as the face veneer although it is not as critical as the moisture content of the face veneer itself. Lumber bands should be stored in the same conditions as the core materials so as to prevent telegraphing problems. Three-ply construction using medium density fiberboard or particle board as a core can perform well if the face veneer moisture content is controlled to 6%. Five-ply construction is usually a little more forgiving than 3-ply but it is still necessary to control the face moisture content for best performance. Yorkite has performed well as a crossband material, also hardboard, such as 1/10″ Lebanite. It is important to note that if a wood crossband such as yellow poplar or lauan veneer is used, there should be no parallelism of the grain direction of the face veneer with that of the crossband, otherwise cracking and checking will certainly result.
4. Gluing Procedures – It is best to minimize the amount of water in the glue mix. Do not extend the glue mix too much with wheat flour and additional water.
5. Pressing Procedures – Work at North Carolina State University involved hot pressing only. Other researchers feel that hot pressing is less likely to cause veneer checking than cold pressing. Assembly time has been shown to have a significant effect on veneer checking – the longer the assembly time (the time that elapses between glue spreading and assembly of components, and pressing) the more checking that will subsequently occur. Remember, keep the moisture content of the face veneer at 6% right up to the time of pressing. Keep assembly time to a minimum – within minutes of spreading.
6. Panel Conditioning – Whether cold pressing or hot pressing is used, it is obvious that moisture or temperature balances have been upset. Many manufacturers have found it beneficial to stack panels coming from the hot press in bulk to complete glue curing. Some manufacturers will sticker panels that have been hot pressed to allow them to equalize and cool before sanding or other processing. Panels that have been cold pressed have generally absorbed a considerable amount of moisture. It is advisable to sticker such panels and allow excess moisture to escape. These panels should be stored in an area where conditions are kept around 6% equilibrium moisture content. It is advisable to have both temperature and humidity controls plus good air circulation in a conditioning room. The length of conditioning time should be at least two days, depending upon temperature.
7. Sanding – A rather extensive study has shown about 40% of the original thickness of decorative faces is removed in sanding. There is some interaction between quality of veneer and sanding. If we use good tight-cut veneer for making sketch or decorative faces which involves book-matching, then normal sanding will usually remove the lathe checks on the exposed loose face.
8. Finishing – Nitrocellulose lacquers have been the major type of finishing material for furniture. Finishing materials should not be viewed as barriers to moisture content changes – they are really only retarders. Moisture content changes are therefore possible.
9. Warehousing Conditions – The furniture manufacturers should be aware that reasonable temperature and humidity conditions should be provided for storing completed furniture. If humidity rises substantially during storage, the manufacturers will encounter not only sticking drawers and doors, but the possibility exists of creating veneer checking if that furniture is subsequently placed in a very dry atmosphere.
10. Conditions in Service – The manufacturers have no control over conditions in the customer’s home. However, there are areas in the U. S. where cold, dry, outside air brought indoors and heated to 700F will result in equilibrium moisture content of 3%, which is very low. These extreme dry conditions can induce veneer checking even in properly made panels.
The use of trade names in this note is for reader information only and does not imply endorsement by North Carolina State University.