The greatest pollution prevention (P2) opportunities in wood furniture manufacturing are achieved by improving transfer efficiency (TE) of coating to the work piece by training operators and using more efficient application equipment and using less toxic and/or hazardous coatings. P2 opportunities in this section are organized into the following categories:
Spray gun set up: Proper spray gun set up can improve finish quality on the first try, reducing the need for rework. It is always better to change the gun tip and nozzle size rather than increasing air and/or fluid pressure. Correct nozzle and tip size and shape can increase TE by up to 20 percent. The lower the viscosity of the fluid, the smaller the inner diameter of the fluid tip that should be used. Generally, five considerations are involved with selecting the correct gun nozzle and tip:
type of gun
size of object to be coated
desired line speed and finish quality
type and viscosity of coating
available air volume and pressure
More Efficient Spray Guns: High-volume low-pressure (HVLP) spray guns have a TE of 40 to 60 percent in practice. This means about half the amount of coating is needed to coat the same object when a conventional air spray gun is replaced by an HVLP gun. In addition to reducing overspray, HVLP guns generate less coating bounce-back, reducing operator exposure to potentially harmful solvents.
HVLP guns are portable and easy to clean. They provide good coverage and performance and are good at penetrating recessed areas. HVLP guns cost approximately $300 to $600 and can pay for themselves in just a few weeks or months through reduced coating use. There can be a significant variation in TE among different HVLP gun manufacturers, so it is important to test guns from several different manufacturers.
Spray gun operators control many of the factors that affect TE. Improving TE reduces overspray so spray booth filters will last longer and there will be less lacquer dust to clean up and manage, and the spray booths themselves will require less frequent cleaning. Proper operator technique can increase TE by up to 20 percent. Proper operator technique include the following:
- Hold the spray gun perpendicular to the surface of the part being sprayed.
- Trigger the gun after each pass.
- Overlap each stroke by 50 percent.
- Maintain a constant distance between the gun tip and the part.
- Spray with a suitable speed.
- Adjust the air and fluid pressures at the pressure tank, not by adjusting the gun.
- Select the correct tip size for the coating and gun used.
To prevent pollution, facilities can increase the solids content of traditional coatings or switch to an alternative coating material. Using less toxic or hazardous coatings to begin with ensures that workers will not be exposed to these materials nor will the general public through releases or when reconditioning work pieces during their useful life.
There are numerous benefits to increasing the solids content to 35 percent or more:
facilities that have increased solids content report that their final product is of higher quality and appeal to customers;
one application can place twice the solids on an item, so fewer finishing steps are needed to produce the same final product;
although on a per-gallon basis high-solids coatings might be more expensive, less is needed so overall costs are the same or lower. High-solids coatings can also reduce harmful air emissions by 50 percent or more and application technique, repair and cleanup requirements are similar to traditional low-solids coatings therefore little training is required.
There are many benefits associated with water-based coatings:
solids content is often higher so overall cost is the same or less than solvent-based coatings;
finish is more durable than solvent-based coatings;
cleanup uses soap and water;
low flammability means there are no restrictions on storage, no explosion hazards, and lower fire insurance premiums; and
water-based coatings emit substantially fewer toxins, so large ventilation air flows are not needed, saving money on utilities, especially in the winter.
Ultra Violet (UV) - Cured Coatings
UV-cured coatings are typically applied using an automated system and therefore are most applicable to finishing flat wood, such as furniture components, prior to assembly. There are many benefits to using a UV-cured coating system:
full curing occurs within seconds of exposure to UV lights, enabling fast production rates and eliminating the need for flashoff space;
VOC and hazardous air pollutant (HAP) emissions are virtually eliminated, reducing or eliminating regulatory burdens;
fire and explosion hazards are eliminated, reducing insurance costs;
ventilation requirements are lower, reducing utility costs;
the as-applied coating cost can be lower;
UV-cured coatings are extremely durable;
and UV-cured coatings will not cure unless exposed to UV light, reducing cleanup requirements.
Some of the best ways to handle materials and reduce waste generation were discussed above. In addition to these techniques, implementing an inventory control system and collecting and reusing cleaning solvents can provide additional benefits.
Inventory Control System
Controlling purchasing and handling of materials can reduce waste generation significantly. By developing a log-out system with one person assigned the responsibility of retrieving needed materials from the storage area, facilities can track where materials are used and in what quantities. In addition, operators become less wasteful in their use of materials when they don't have open access to the storage area and they know someone is recording material use. Since operators typically perform their own line, gun and spray booth cleaning, additional training can further empower them to reduce the use of materials.
Coatings that have passed their expiration date become a waste and should not be used. Therefore, coatings should be purchased in as small a quantity as possible to avoid exceeding expiration dates. Label incoming materials with shelf life dates and have a first-in/first-out policy. Lastly, facilities should work with suppliers to take back off-spec and empty containers, if possible.
When spray gun application is used, spent cleaning solvents are generated from gun and line cleaning, and spray booth cleaning. Solvents are also used for cleaning when coatings are applied by brushing and dipping. Collecting and reusing solvents as much as possible can lower virgin solvent purchases and hazardous waste disposal costs for spent solvents. A small distillation unit can produce recycled solvent appropriate for use in a variety of applications.