Warping is one of the most common processing defects in engineered flooring manufacturing. After full-scale flooring planks have been cold or hot pressed, equilibrated and cut into flooring slats, warping can occur. Sometimes this is up-warping (where the middle of the board bends upwards) and sometimes bow warping (where the ends of the board bend upwards). This processing defect is ubiquitous in all flooring factories and unavoidable.
Warped flooring slats can seriously affect the subsequent processes. Various quality problems – both predictable and unforeseeable – can be attributed to flooring warping. For example, enforced grooving on the bent boards can induce a significant thickness difference and ragged chamfer at the edges. Painting on bent boards will also ruin the finishing where defects associated with warping have been created, such as uneven and embossed lacquer at the edges, wavy lines and sanding marks. It can also damage the finishing line.
In a flooring factory, the frequency of warping is generally 1% of the output (usually within 0.5% in our factory). A certain degree of warping can be fixed and straightened by technical methods. However, this takes a lot of time and labor, and there are still losses. The overall cost for straightening is about USD 8.00–15.00 per m2; therefore, the production process must be strictly controlled to reduce the secondary processing rate.
This article will introduce the causes of warping, straightening methods and potential quality risks from the manufacturer’s point of view. With a comprehensive analysis of the processing defects, customers will have a deeper understanding of the story behind the finished product and how to avoid potential risks. Hopefully this helps you in purchasing quality engineered flooring.
Causes of warping
Before getting into how to fix the problem of warping, we should first understand how it is generated. There is a variety of possible causes for flooring warping. Multi-layer engineered flooring, for example, is likely to be warped when the moisture content distributes unevenly in the slab.
Technically, full-scale flooring boards are sent to kilns to re-balance the moisture for 3 to 5 days after gluing and pasting. In this process, the adhesive is further cured, while moisture within the slab migrates until it distributes uniformly. However, there may still be moisture content gradients between the surface lamella and subfloor in some slabs. Warping usually occurs when the flooring boards are divided into their given widths. At this point, the slab loses the stress restraint of the stack, and any asymmetrical moisture content causes non-uniform swelling and shrinkage in different layers. When the moisture content of the upper lamella is higher than that of subfloor and bottom veneer, the middle of the floor arches upward. When the moisture content of the upper lamella is lower than that of subfloor and bottom veneer, the flooring bows upwards. The warping degree of the flooring can be measured by a wedge plug ruler.
- Mechanical straightening
A straightening machine is used to directly apply reverse stress on the warped flooring. A heating source can also be used to increase plastic deformation and thus aid straightening. This method is simple and coarse, so only suitable for warping – downward or upward – with small deformation amplitude (generally with no more than 3mm gap from flooring center-point to inspection platform). Mechanical treatment is fast and will not affect the delivery date, but there are some quality risks involved. When the warped flooring is pressed, a “click” is often heard: fibers on the tight side (dry shrinkage side) are stretched and broken quickly to release the tension stress, restoring the flooring to flatness. Moreover, delamination and peeling can occur because of the relative slippage between different layers.
- Hydrothermal treatment
This method readjusts moisture distribution and reshapes the warped boards. The flooring is firstly sprayed with water on the tight surface, then pressed by thermal press machine. With the help of thermal and pressure treatment, the moisture in the boards redistributes to create a new balance. Heating helps to reduce the elasticity of the wood fiber and improve plasticity, reshaping the wet layer dimensionally and avoiding the return of warping after cooling. The hot-pressing temperature is usually between 95 and 115°C, the pressure is about 5 tons, and the pressing lasts 15 minutes. Pressure and time vary according to product thickness and equipment.
After hydrothermal treatment, the flooring is then placed between reverse pre-stress strips to promote the plastic reshaping of the floor. After 3 days, the flatness of the flooring is checked. If the flooring is still flat, it is unlikely to warp again.
Quality Risk Control
After the above straightening treatment, the warped flooring will again be flat; however, there are still some potential quality risks: some of the treated flooring can warp again, while some can delaminate due to slippage between layers. Therefore, quality risk control after treatment is essential.
Standing for observation: To ensure the straightened flooring will not warp again after treatment, the flooring panels are separated and left in atmospheric conditions for 72 hours to simulate usage scenarios. No deformation should be detected before the next processing step.
Dip peel test: After straightening, our QC assistants take a large number of samples from the treated flooring to conduct dip peel tests to ensure the adhesion quality before proceeding to the next process.
Warping is an infrequent but inevitable processing defect which is widely present in all flooring factories. A small percentage of straightened flooring can still meet product quality requirements. However, it is important to note that the factory must enhance the quality risk control after the straightening process. 100% customer satisfaction is always the quality goal we pursue.