The after effects of Global recession are still felt in business environments worldwide & furniture industry in gulf is no exception. But this crisis has provided an opportunity to re think the strategies, ways of working and the methods. It also compelled us, to device new techniques to make the industry more efficient in production & innovative to offer better products to the market.
The fierce competition has shrunk the margins, compelling the producer to reduce the costs. Unfortunately while reducing the costs the quality also went down. This aggravated the situation.
A smarter way of working is the best remedy for the situation.
We have seen in the earlier article that the assembly of solid wood furniture & components. In the following article we will see the painting and finishing of the Solid Wood Furniture.
Painting Finishing of Solid Wood furniture
The solid wood furniture components produced on woodworking machines will be ready for sanding and finishing. If the tools used for processing are sharp and that the woodworking machines are without vibrations then the components produced will have a smooth finish. Otherwise the surface will have rough marks of vibrations. These have to be smoothly sanded to get high quality finish. The difference between a high quality and poor quality is determined due to three criteria’s Timber quality, Joinery and surface finishing. Finishing & painting is the highest value adding process amongst all the processes involved in manufacture of any wooden product. The high quality of finishing puts the product to a certain level of beauty, in fact, keeps adding a big amount of value to their products.
One can notice that 80:20 formula is well applicable to any wooden product. This means that one performs 80% of the requirements & lacks in the balance 20% which is very vital. In other words the 20% of care at each step of operation will increase the value of the product by 80%. This is the difference the customer is ready to pay for Good & Bad quality
The most important points to be taken care while finishing any wooden products are:
• The moisture content of the wood.
• Surface Preparation.
• Painting & finishing process.
• Painting & finishing material.
Moisture content in the wooden components is very important for high quality surface finishing. Higher moisture content makes the wood non-stable & the variation in the atmospheric conditions affects the finishing. This is because the moisture trapped within the timber cells tries to come out causing deformation in the finishing. Hence a properly dried timber of 8 to 10% moisture can be finished nicely.
The procedure followed for the surface preparation for the solid wood & MDF components is almost same.
All the processed components of any wooden product are sand finished by Vertical Oscillating belt Sander, Flat belt sanders, Fladders, Profile Sanders, Brushing Wheels, Wide belt sanders or even manually in certain cases. The use of different machine is made depending up on the shape, profile & flatness of the components.
Use of Alloxide open Coat paper of 100/150 grits is made for the raw sanding. After that Sanding with 200 grit open coat Alloxide paper follows to make the surface smooth. Alloxide paper lasts more than the conventional sand papers. The extra cost of the material is compensated by higher output.
One should check that the entire surface is uniformly sanded to give a smooth finish to the whole component. Any defect left at this stage is very costly to repair.
RECTIFICATION OF DEFECTS
The dents and cracks are filled with Wood Filler. Different types of wood filler are used which are compatible with the type of finishing material planned for the further operations.
Following are few types of finishing material available like Nitrocellulose, Alkyds, Urea / Melamine, Acid curing coatings, Polyesters, Acrylics, Polyurethane etc.
The component is left to dry for overnight. It is sanded to give smooth surface. Rework at this stage is easier and cheaper than any other stage afterwards. So quality inspection is done at this stage.
One should have a reliable information that refers first of all to painting & finishing material, equipment and tools that are involved in finishing.
PAINTING & FINISHING PROCESS
Staining of the wooden surface is done to give uniform & desired colour to the component. Staining material can be water based or solvent based depending upon the requirement. Normally dipping of the object in the tank of water based stains for a short time is the method followed in European factories.
While using water based stains some moisture is absorbed by the surface layers of the wooden components, but it dries fast when exposed to atmospheric heat.
Use of solvent-based stains is also made in case of high value added products, as these are expensive.
One should notice that only staining is done with water based chemicals & not the grain filling.
ALL natural wood0en surfaces have open grain structure. These can be finished in open grain & filled grain effect. In case if open grain finish the smooth sanded surface is spray painted with thin coat of sealer in order to seal the surface from absorbing the final finish coat. The amount of “cured” solid residue or the thickness of the film determines the thinness of the sealer. The higher percentage of the residue gives higher thickness of film. The amount of coating can be determined by the method of application like i.e. roller, curtain, spraying.
Two or three coats of sealer are necessary to give a smooth finish. No intermediate sanding is done. This is allowed to dry overnight.
This surface is sanded smoothly with 220 grit Alloxide open coat paper. Here precaution should be taken that sealer is not removed completely by sanding. The surface is made uniform and smooth.
In case of manual sanding operation, a separate team of workers does this process with very light pressure on the sand paper. All the material sanded is checked for dents and scratches. If needed the same wood filler is used to fill the gaps. After thorough inspection a final light coat of sealer is applied in case any touch up is done. The material is kept overnight for drying.
This material is sanded by 600-grit silicon paper or fine steel wool with a light pressure to give silky finish to the product. All the surfaces are cleaned with old cotton cloth followed by pressurized air cleaning and kept for final painting, in a dust proof painting room.
The above material is ready for any type of painting i.e. Shaded, Opaque, Crackle finish, Pearl, Texture finish, Marble finish etc.
The painting can be done using Pigmented or Clear Matt or Glossy finish a needed. The paint is allowed to dry for overnight. Then the material is ready for dispatch.
In case of Shaded Design, use of pigmented paint is made to do the shading. This is followed by one coat of clear coating to give either matt or glossy finish.
The final painting is done in a dust free spray room. Any dust setting on the wet material makes the finish rough and a re-work is necessary.
Hence, a separate spray room with filtered air inlet is used. For best results pressurized spray room is recommended. In pressurized room a positive pressure is maintained. Hence no outside dust enters the room and the doors of the spray room are always kept open.
Roller Coating is used for flat panels only. Roller Coater can be used for application of stainer, sealer & final coat. This machine is very good for thin & uniform coating.
Reverse Roll coater is used for completer filling of grains. The amount of coating can be controlled by the gap between main & doctor roller. The work piece is passed two or three times, with intermediate drying, depending up on the quality of surface finish required.
Curtain coater is used for coating of flat panels only, by slightly embossed surfaces can also be coated. The principle of formation of curtain can be over flow or pressurized chamber. This process applies thick coats of sealer & is used for Polyurethane & Polyester finishes. This process is advised for higher quality productions only. Multiple passage, with intermediate drying is advised for high quality finish.
Spraying is the most common process used in the finishing process. The spraying Guns used can be of following types – Gravity cup type, Pressure feed System, Electrostatic,
Air-less, Robotic etc.
Gravity cup Guns are commonly used as they are low priced, easy to operate & maintain. Different sizes of nozzles are available for different type of paints. This gives the optimum usage of the paint, which in turn imparts high quality finish to the work surfaces.
Pressure Feed System In this system the painting material is feed under pressure from a storage tank of 10, 20 Liters capacity. This gives uniform painting without any break while painting large surfaces. The consumption of thinner is less as thicker paint can be sprayed without any difficulty. Three to four spray guns can be connected to one system. As the weight of the spray gun used in pressure feed system is much less hence it causes less fatigue to spray operator. This system is suitable for medium & high Quality or Quantity production.
In any type of compressed air spray painting 55% to 65% of the paints actually reaches the work piece & balance of 35% to 45% of the paint is lost in the over spray.
In this kind of painting the paint is electrostatically loaded through contact or induction by high voltage. The ionized drop of material (negative charge) repulses each other, while they are attracted by the part to be painted (positive charge) which is electrically connected to ground. The paint particle, which do not collide directly with the part but which stay inside the electrostatic Field, are also attracted & go to cover the already covered & the less accessible parts. Therefore the material is evenly distributed over the part.
This painting system does not use air but the paint is compressed with the help of airless pump. This system can use high viscosity paints.
These are microprocessor controlled systems & need high investments.
Various drying systems like Hot air Drying, UV Drying, InfraRed Drying, can be utilized for drying of the coating applied. The type of drying does not affect the finish of the product.
HOT AIR DRYING
The air is heated either by electric resistance or by Thermic fluid heaters and is forced by a blower fan over the components through various vents. The system can be conveyerised in case of higher production volumes.In this type use of Ultra Violet lamps radiation is made to dry the paints. The system is quite expensive. The drying is very fast, in few seconds. Hence the product can be packed for dispatch immediately. Use of Infra Red radiation is made for drying of the paints & finishes. Infra red lamps are kept at a certain distance from the component to be dried. The system can be converised. This system is much economic & takes longer time as compared to UV.
Some other main guidelines to follow
We can say that, Irrespective of the type of the finishing like open or closed grain, clear or pigmented, stained or natural, Irrespective of the type of raw material to be finished being it solid wood, veneer, particle board, MDF Irrespective of the design of manufactured good, being it a flat panel. A carved / raised panel Irrespective of the type of coating equipment utilized.
Following operations will be required in sequence
01 Sending of raw material Compulsory
02 Staining Optional
03 Base coat application /s Compulsory
04 Base coat drying / s Compulsory
05 Base coat standing /s Compulsory
06 Final coat application Compulsory
07 Final coat drying Compulsory
Last but not least, we have to be fully aware of the fact that every type of timber has a different reaction to chemical coatings due to his chemical and anatomical peculiarities. This means that the results obtained on a certain type of timber not necessarily can be transferred to another species with the same result.
Finishing systems & procedure can be designed independently for Profiles And Picture Frame Mouldings, Raised / Curved Panels, Flush Doors, Kitchen cabinet doors Parquet flooring etc.
Robotic arms like shown in the following picture are employed to get uniform finish. The arm is fitted with spraying head with pressure fed / electrostatic / diaphragm pump system. The movement of the arm is controlled by CNC.
The programming have become very easy with the use of motion tracing & programming / Scanning camera. The use of this system is becoming popular in custom-made furniture due to the complex nature of the product. Multi stationed positioning of the wooden articles is also done in order optimize the loading – unloading time. Special training sessions are available in few painting institutes. They also have e- training lessons / webinars.
The painting of solid wood table tops, panels which are flat can be done with simple machines with moving / oscillating arm with pair of spray guns.
The picture above shows the simple machine to spray flat surfaces with couple of spray guns. The coating thickness is adjusted by controlling the speed of the conveyer and the speed of the oscillations of the spray guns. The over spray is controlled by efficient spray system. High end machines offer paint recovery system as a package.