A commonly occurring and costly problem in wood furniture manufacturing is the development of small cracks in the furniture's finish called veneer checks. Veneer checks usually appear as uniformly spaced hairline cracks in the finish or, in severe cases, cracks with accompanying ridges on the wood surface which actually can be detected by touch. In any case, veneer checks always run parallel with the grain of the wood (Figure 1), a characteristic that can be used to distinguish veneer checks from other defects. Random orientation of cracks in the finish (not parallel to the wood grain) generally indicates non-wood-related causes.
Veneer checks are formed when stress failures occur in the face
veneer, caused by differential shrinkage or swelling between the
face veneer and the panel substrate to which it is applied.
As the relative humidity of the environment (in which a panel is used)
changes, so does the moisture content of the panel. With wood,
changes in moisture content mean shrinkage and swelling. Unfortunately,
when a veneered panel shrinks or swells, the veneer does not
"move" at the same rate as the substrate. This creates
considerable stresses within the panel which, if great enough,
result in wood failure. Failure will occur at the weakest part
of the wood which is generally over deep lathe checks, large pores,
or other weakened areas on the face veneer. Such failures in
the face veneer then create stress concentrations in the finish
which result in the visible cracks we call veneer checks (Figure
Figure 2. Veneer checks are the result of wood failure in
the face veneer caused by stresses created from shrinking and swelling of the wood.
These failures create stress concentrations in the finish which result in
cracks in the finish.
There are many factors which contribute to the formation and severity
of veneer checks. For example, veneer checking can result from
improper manufacturing practices, poor warehousing conditions,
or harsh environments in the consumers' homes. It is usually
very difficult, if not impossible, to determine the exact cause
of checking for any given incident. However, experience and research
have taught us some of the most common and severe influences of
veneer checking. These will be discussed below.
The quality, species, and cut of the face veneer all influence the tendency for veneer checking to occur. Tight-cut veneer (veneer with shallow lathe checks or knife marks), has been shown to perform better than loose-cut veneer (deep lathe checks). Likewise, species of wood with fine pores check less than wood with large pores. This is because deep lathe checks and large pores create weak spots on the face veneer which provide less resistance to failure when the face veneer is under stress (Figure 2).
Veneer with straight grain (radial face) has been shown to check
less than veneer with a cathedral grain (tangential face). This
is because shrinkage in wood is much greater tangentially than
radially. Greater shrinkage creates greater stresses and thus,
a higher chance of wood failure in the face veneer.
The most critical factor in preventing veneer checks is control of moisture content, not only of the panel components, but of the finished product as well. Checking only occurs when there is shrinking or swelling of the wood. Likewise, the severity of checking is related to the degree of shrinking or swelling. By controlling the shrinking and swelling of the wood (i.e., controlling the moisture content), the propensity of checking is reduced.
Before panel assembly, the face veneer and substrate should be dried to a moisture content appropriate for the conditions in which the final product will be used (6-8% is most common). It is imperative that the veneer and substrate be at the appropriate moisture content at the time of pressing. The phenomenon of veneer checking is exaggerated when the face veneer and substrate possess different moisture contents (not dried for the same final atmospheric conditions) at the time of panel pressing (or more specifically, at the time of bond formation of the glue line). In such a case, the dimensional change of the face veneer relative to the substrate is more severe, creating greater stresses, and thus, increased checking.
Manufacturers attempting to carefully control moisture content often overlook certain practices which adversely alter the moisture content of once properly dried panel components. For instance, veneer and panels are often stored in areas with no environmental control. In uncontrolled conditions, wood changes moisture content at a surprising rate. Even in environmentally controlled plants, certain areas may be inadequate for wood storage. For example, veneer stored next to hot presses may lose moisture driven off by the radiating heat. The manufacturer often does not realize that moisture content has changed and assembles the panel, only to have problems later. The solution is to store wood components in an environmentally controlled atmosphere, and check moisture content before panel assembly.
Another consideration often overlooked by manufacturers is the effect of the glue on veneer moisture content. If a high water content adhesive is applied to the veneer, especially with a long assembly time, the veneer can pick up considerable moisture before pressing. Avoiding high water content adhesives, thick spreads, and long assembly times will reduce the likelihood of veneer checks. Manufacturers should consult with their adhesive suppliers, however, before making such alterations.
Once panels have been assembled, it is important that they be conditioned in an environmentally controlled area to allow for the temperature and moisture balances to reach equilibrium. Two days is a common minimum conditioning time.
Finally, the finished product must be stored in an environmentally
controlled area. Too often, properly manufactured furniture is
stored in a warehouse with no environmental control. Some believe
that the finish protects the furniture from moisture changes,
so storage conditions are not important. This is not true. Although
the finish helps to slow moisture movement, it does not prevent
it. Furniture stored in adverse environments will surely check.
Plastic wrap around the finished product prior to warehousing
will help prevent moisture from damaging the furniture. Manufactures
should realize that veneer checks may also occur if the final
product is used by the consumer in a different or harsh environment,
even if checks were not visible when the furniture left the plant.
There is also a relationship between panel construction and veneer
checking. Panels should be constructed with the tight-side of
the face veneer up (lathe checks down) when possible. Tight-cut
veneer should be used when veneer matching makes it impossible
for the tight side to be laid up. The panels should not be over-sanded,
especially to the point of exposing lathe checks. Poor glue bonding
has also been correlated with veneer checking. Poor glue bonds
may be caused by inappropriate moisture content and temperature
of panel components; improper glue mix, glue spread, assembly
time, and press time; thick and thin veneer or panel substrate;
among other factors. Some research suggests that five-ply construction
is less prone to checking than three-ply, and hot pressed panels
are less susceptible to checks than cold pressed panels. Evidence
also exists that a water rinse after bleaching may also increase
the chance of veneer checks.
This report presents a brief explanation of the causes of veneer
checks, and discusses some of the major factors which have been
shown to correlate with veneer checking. By no means, does this
report attempt to address all causes of veneer checking. However,
it does provide information which, when combined with logical
reasoning, can help manufacturers better control veneer checking
problems. By better understanding veneer checking, and taking
steps to prevent it, the conscientious manufacturer is sure to
reduce problems associated with this expensive defect.