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Causes Of Cracking And Deformation Of Solid Wood Furniture And Improvement Measures

wood curing

Table of Contents

Crafting quality solid wood furniture is a delicate process that involves precise material selection, drying, finger jointing, and splicing. If these processes are not correctly done, the furniture can crack, come apart, deform, and become unusable.

Solid wood furniture often experiences cracking and deformation due to its unique property – drying shrinkage and wet expansion. This property can lead to problems such as panel cracking, frame deformation, and tenon joint failure. 

To reduce the impact of the property, we need to control the design, wood drying, production and processing, and make rational use.

1. Reasons for cracking and deformation of solid wood furniture

1.1. Unreasonable furniture design

we can prevent solid wood furniture from cracking and deformation through a well-thought-out design. It includes a good size and splicing structure design. The design types should depend on the wood species.

Reasonable design can significantly extend the furniture’s lifespan.

1.2 Backward drying equipment

For furniture companies, selecting the proper drying equipment is essential for achieving quality wood drying.

Unfortunately, many small businesses need more funds to invest in advanced drying equipment, resulting in poor wood drying quality that can lead to furniture defects like cracking and deformation.

Investing in the proper drying equipment can decrease the wood’s internal stress and help create even drying.

1.3. Immature Drying Process 

The drying benchmark is an essential factor to consider when adjusting the temperature and humidity of the drying medium during the artificial drying process of wood.

The drying benchmark directly impacts the quality of the wood drying and the speed of drying. 

Unfortunately, many small furniture enterprises need a mature drying process due to backward technology. As a result, the wood may experience flaws such as cracking and deformation during the drying process, thus reducing the quality of the wood.

1.4. Incorrect Usage of Solid Wood Furniture 

Solid wood furniture will shrink in a drying environment and expand in a wet climate as its equilibrium moisture content changes due to the humidity levels of the surrounding environment.

In the winter, the indoors of northern China is with high temperatures but low humidity. As a result, if solid wood furniture is exposed to this environment for an extended period, its equilibrium moisture content will start to match that of the indoor, leading to shrinkage and cracking. In turn, the lifespan of the furniture will decrease.

2. Improvement measures

2.1. Reasonable design of solid wood furniture.

2.1.1. Tenon dimension design

To ensure the success of solid wood furniture, we must pay attention to rational design. In particular, tenon dimension design must be reasonable. 

The combination method used in solid wood furniture is mainly tenon joint, which generally takes the form of right-angle tenons. 

As wood is anisotropic, it shrinks and expands differently in different grain directions. We should allow a small clearance in the thickness direction of the tenon and an interference in the tenon width to prevent tenon joint failure during furniture use. 

The specific matching dimensions are as follows: 

mortise width(B) -tenon thickness(T)=0.1~0.2mm.

mortise width(B) -tenon thickness(T)=0.1~0.2mm.
mortise width(B) -tenon thickness(T)=0.1~0.2mm.

Tenon length(L)-mortise depth(D) =2~3mm.

Tenon length(L)-mortise depth(D) =2~3mm.
Tenon length(L)-mortise depth(D) =2~3mm.

2.1.2 Panel splicing structure design

The wood itself is anisotropic, meaning the shrinkage coefficient of the chord direction is greater than that of the radial direction (6%~12% in the chord direction and 3%~6% in the radial direction). Opting for radial boards or splicing panels of different shrinkage coefficiency is best to prevent cracking. 

In the past, we put the timber together side-by-side or by groove splicing.

panel splicing
panel splicing

However, if the board is thick enough, finger-joint splicing is especially effective in preventing cracks.

2.2. Reasonable board drying.

2.2.1. Wood drying benchmark selection

When it comes to drying wood, the selection of a suitable drying standard is critical. It depends on the species, thickness, and application of the wood.

If the standard chosen is not correct, it can have a significant harmful impact on the drying quality and output. 

We can divide the wood into two types: coniferous and broad-leaved. Their drying standards are different. 

Coniferous wood has a straight texture and good moisture conductivity. The wood’s moisture content gradient and internal stress are slight during drying. The wood does not tend to have defects and is easy to dry. So a harsher drying standard is generally used. 

On the other hand, broad-leaved wood has a slower moisture conductivity. During the drying process, the moisture content gradient and internal stress of the wood are significant. The wood tends to have defects. So a softer standard is common. It helps avoid problems that can happen during the drying process.

The thicker the wood, the more difficult it is to dry. We should choose a softer one for thick boards.

We can choose harsher drying standards for general-purpose wood to shorten the drying time. 

Important-purpose wood, such as aviation wood, should maintain good mechanical properties and strength during drying. It will need a softer drying standard.

Between the surface and core of a board, moisture content changes as you go down. For this reason, the surface layer is usually dry, while the center is very wet. Manufacturers start with the drier side (surface) for processing to compensate and create an even moisture content. It guarantees that, in the end, the moisture content average is higher than the final one.

To make the equilibrium moisture content for processed products compatible with the place of use, the value of final moisture content for the wood should be set 2% to 3% lower than the equilibrium of moisture content for the place of use, so the wood will not crack or warp.

2.2.2. Dried board curing

To ensure that the moisture content of the wood is consistently maintained along the thickness direction, it’s essential to preserve a reasonable drying standard when drying timber and do specific curing after drying.

Purpose of curing:

 ①: Balance the moisture content in the wood. 

When drying wood, the moisture content of the wood surface and core, the upper and lower parts of the wood pile, are different. Proper curing can make its internal moisture content tend to be consistent.

②: release its internal stress. 

When drying wood, changes in moisture can lead to changes in size and volume in all directions, which will cause internal stress. Sufficient curing time will reduce internal stress.

Curing methods: Correctly stack the boards in the warehouse.

Dried board curing
Dried board curing

we should store hardwood for furniture used in the warehouse for at least 20 days. 2-7days in the national standards are the lowest request. 

However, it is imperative that humidity levels are at 40-50% and the temperature is between 35-40°C.

2.2.3. Forced to balance the moisture content of the board

If the wood moisture content gradient along the thickness direction is prominent after the panel is dried, the wood must be in a kiln for forced moisture balance, which decreases warpage and cracking in the finished product. 

The dry bulb temperature in the kiln should be at 40-50°C (104-122F). The wet bulb temperature depends on the moisture content of the wood.

Circulating wind speed should be about 0.5m/s (19 inches/second), and equilibrium time depends on the wood’s thickness and moisture level.

2.3. Processing technology control

2.3.1. Glulam process control

① Adhesive control.

The dose of adhesive should not exceed one hour. And the manual stirring time should not exceed five minutes after the primary and curing agents are combined.

If you’re using a machine, don’t allow it to mix for more than 0.5-1 minute.

The amount of adhesive applied should be 150-180g/㎡.

 With bonding pressure depending on the hardness of the wood you’re working with: Coniferous wood at approximately 0.5-1 MPa; broadleaf wood around 1-1.5 MPa. General pressure depends on the hardness of the wood’s species.

②Preheat the boards.

 Preheat the boards before splicing. It will prevent your board’s adhesive strength from decreasing due to its lower temperature. 

Usually, it is best to bond panels when the ambient temperature and the panel temperature are between 20 and 30°C. If you’re working with a low-temperature board, remember that it will take longer to hold it together because the bonding effect isn’t as strong when temperatures are below 10°C.

③Laying aside finger-joints board.

The moisture content difference between the timbers spliced together should be no more than 2% to prevent warping and deformation.

The workspace temperature should be at 20-30°C.

The curing time should preferably be more than 24 hours.

④ Glulam aging.

After the pressure is relieved, the glulam board needs to be moved to a place with a heat source for curing. During the curing period, the glulam should be stacked correctly. 

Place horizontal bars at the bottom of the stack, and leave space between the vertical division bars. 

To prevent cracking of the adhesive layer at the end of the board, ensure the two ends of the division bars are flush with the ends of the panels.

Of course, plastic wrapping can also be used to prevent cracking of the adhesive layer at the end. 

The temperature of the curing environment is generally set at about 20°C, and the curing time depends on the adhesive.

2.3.2. Production environment requirements

The temperature and humidity of the production environment greatly influence the processing of glulam and directly affect the quality of glulam.

 Therefore, if enterprises want to improve production and quality, they must control the temperature and humidity of the environment.

First of all, the ambient temperature of the adhesive is generally 10-30°C. To ensure the processing quality, it is better to control the ambient temperature above 10°C in winter and below 30°C in summer. 

The ambient humidity is controlled below 12%, which can prevent the furniture products from absorbing moisture in the natural environment.

Secondly, to prevent the moisture content of solid wood furniture from rising, the moisture content of packaging cartons also needs to be controlled appropriately. Generally, the moisture content of packaging cartons is 14% to 18%, and it needs to be dehumidified before use to reach about 10%.

2.3.3. Parts processing requirements

The most important thing for furniture parts in machining and woodworking workshops is reducing the moisture absorption speed. The best way is to install dehumidification equipment in machining and woodworking workshops. 

But to save costs, the general method is to improve the processing efficiency of parts and shorten the processing cycle, thereby shortening the moisture absorption time of parts. 

For example, parts carved and surface curve processing can be processed first due to their long processing time.

 For those that require secondary processing (referring to furniture parts that have been assembled by woodworking, they must be returned to the machining workshop for size correction and sanding) (waiting for reprocessing), parts are processed first to shorten the processing cycle.

 For detachable furniture, methods such as painting first and then storage can be used.

Whether the process design is reasonable also directly affects the length of the wood processing cycle. 

For example, multi-batch processing methods can be adopted for mass production tasks to shorten the processing time. Due to the long production cycle of solid wood furniture, enterprises can implement specialized production of parts and components to achieve the purpose of shortening the production cycle.

2.3.4. Paint application requirements

The paint film has the function of protecting wood and improving the aesthetics of solid wood furniture. Using better quality paint and the correct method for finishing is also very important to ensure the quality of solid wood furniture. 

Generally, the moisture content of polished white blank parts should be controlled to 10%. The moisture content of non-polished furniture white blank parts should be held within 12% before painting. Remedial measures should be taken for parts whose moisture content is out of control, such as secondary drying or surface dehumidification, to make it reach the required moisture content.

2.4. Requirements for the use of solid wood furniture

Solid wood furniture is used mainly to prevent cracking and deformation defects. 

The reason for the defect is mainly caused by the change in the moisture content of the wood, so pay attention when using it.

The furniture should not be exposed to the sun for a long time: the furniture should not be in an environment that is too cold or too hot; the furniture should not be in an environment that is too dry or too humid.

3. Summary

The improvement measures for the cracking and deformation of solid wood furniture need to be further studied. To improve the problem, advanced equipment and facilities should be introduced, as well as an improved production process. Finally, high-quality and high-skilled talents are required.

For the better development of solid wood furniture in the future, scientific research People should work together with furniture companies.

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