" " wisno wood furniture finishing: FInishing room for Polyurethane

Friday, April 2, 2010

FInishing room for Polyurethane

Polyurethane Finish

Polyurethane or PU finish is very popular in wood furniture finishing industry. Finishing with PU material is widely used in the finishing for furniture products that require finishing layer with a maximum strength. Furniture products such as: dinning table, kitchen cabinet, bar table or furniture for the hotel is the furniture that need PU clear coating. The PU finishing material is also widely used for furniture finishing that requires a thick layer of film such as Italian or Japanese furniture. The Asian peoples like the PU finishing better then other finishing material since it is thick and considered stronger than other finishing materials.
Finishing process using PU is similar with the finishing process with other material. The basic principle of the wood finishing process are: the application of stain and glaze to make color and the application of clear coating (sealer and top coat) to build the film layer. In the PU finishing system we use PU coating as the sealer and top coat. The PU finishing system is more difficult then the NC system because the PU has longer drying time. The longer drying time makes the finishing process need to be done carefully and take more time.

We have to be more patience and prepare more time process when we are working with PU material. The PU layer also cannot be washed off with thinner after it is dry. We have to work more carefully. Any mistake should be corrected before the PU layer is dry. The revision when the PU layer is dry will need a hard work to strip off the finish layer. 
Since it needs longer drying time, then the wider finishing room setting is needed. The PU layer is also sensitive to the dust and humidity. The room designed for the PU should be cleaner and more spacious than the room used for the NC. For the same production capacity,  the finishing room for PU requires approximately 2.x room for the finishing with NC system. We still can work in a simple room such as a garage or even in the yard to do the finishing with NC, acrylic, shellac or varnish or even water based finishing. But for PU system we have to prepare a real dust free finishing room.

Room for spraying and drying the PU top coat

PU clear coating requires longer drying time than the NC or any other material. For outdoor PU then it is required drying time any longer. Since that he need larger place. To reach the touch dry condition the PU also need more time.  The PU is very sensitive to dust in the drying process. PU also has a disadvantage character in the drying, if there is any dust fall on the wet coat, the dust is bound and attached to the film layer. When it dries the film result will be rough and dirty. For finishing with white or black color the dirt and dust will more visible and obviously. Because of this reason, the drying room for PU top coat should be designed to bee completely dust free. 

There are some ways to make the room clean and free from dust, one of the most simple way is to always wet the floor of the room. By keep the floor wet, the dust that fell to the floor will be bound in a wet floor and will not fly in the room. This simple way is already greatly reduce the dust in the room very significant. Many furniture companies use this method with very good results. Another way to reduce dust is to install air conditioning in the room. The installation conditioning room in the drying room has other disadvantage that is to maintain the air humidity air is low; this will directly reduce the problems arising from the air humidity.  

In the spraying room, the air inside is suck and withdrawn by the spray booth. To keep it happen there must be air from outside the entry to replace the air inside. Outside air is usually a dirty air with contain dust that will cause problem in the PU finish. To maintain the inside air in the spraying room keep clean, the air that come in the room should be filtered. The filter that is used to filter the air inlet is the same filter with the air filter in the spray booth.


Room for spraying and sanding the PU sealer 

PU need longer drying time, since that the room for sealer should also be made with more room, big enough to accommodate the products to be finished in accordance with the desired production targets. The drying room for sealer has not really need a clean room as the room for top coat. The sealer will always be sanded. The dirt or roughness in the sealer will be cleaned with a good sanding. But however a clean room for sealer will be very helpful to do better finishing process.
The PU coating needs a good sanding, much better than NC finish. We should provide bigger  space for sanding the PU sealer. PU is the material with is dry with chemical reactions; the film layer formed will not melt by application of the next coat on it. The existence of sanding marks or scratches in the sealer sanding will appear at the top coat. If the sanding is done not perfect, it also easy to cause the orange peel. The sanding process in PU sealer must be done in the proper way. We need finer grade sandpaper to sanding the PU sealer. Mostly PU sealer needs to be sanded with 400 or 600 grade US standard sandpaper (see it in my previous article sandpaper for wood finish). For some case we may need to do wet sanding with finer grade sandpaper to make the smooth final finishing film. 
 


The room for storage PU material 
PU is the drying of materials using chemical reactions, he has expire date shorter than1 component finishing materials. NC finishing materials are still in good condition after for 1 year or even more, but PU should not be used after its cane is opened for more than 6 months. The PU hardener is the material that easily damaged. PU hardener is isocyanate, a chemical that very sensitive to water vapor in the air. Therefore, if the isocyanate is tangent to the outside air especially the humid air, he will react and harden. If this hardener had hardened then it can not be used anymore. Therefore we need to make sure that the PU hardener cans that have been opened should re closed tightly. It is safer if the hardener is stored in many smaller size can in than in one big can in large volumes. A little leakage in the can will make the hardener is hardened in just few days  and then cannot be used.  It is recommended to place the hardener in the conditioning room to keep it is in the dry air to have longer lifetime.

1 comment:

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