By Paul Crumpler, Pollution Prevention Engineer
You will not find no-clean technology listed in the Yellow Pages. This is because there is no specific cleaning product to buy, and vendors sell products. No-clean methods generally require procedural or design changes that eliminate the need to clean products during production, and production equipment between production runs.
An example of converting to a no-clean method is to eliminate degreasing of a steel component manufactured in a lathe prior to painting. The degreasing step could be deleted by replacing or eliminating the use of cooling oil in the machining process. In this example, the cooling oil could be eliminated by some combination of a reduction of the machining rate, use of ceramic or carbide tool inserts, cooling with water only followed by drying, cooling with air or other compressed gas, or cooling with a fluid that is compatible with the paint.
Another example is to eliminate cleaning in mixing vats between production runs by using dedicated mixing vats for each product or product family. This would require the purchase of additional mixing vats, but the purchase may have a significant payback when waste costs are factored into the economic justification. Waste costs include lost raw material costs, lost profit on sales of the product, treatment costs, and disposal costs.
Degreasing in a vapor degreaser followed by shot blasting will usually provide a steel component with a very clean surface appropriate for painting. This is an expensive method to assure cleanliness. The component may have already been clean enough for painting prior to degreasing or blasting. Vapor degreasing cleans a dirty component just as well and for the same cost as a clean component. Excessive cleaning usually cannot be found by inspection or by function. The only evidence of over cleaning is in increased waste generation and increased cost. Before specifying a specific cleaning process, the level of allowable contamination should be known for the component.
Experiments can be designed to determine the level of cleanliness required; cleanliness inspection methods can also be devised. In the above example of a steel component manufactured on a lathe prior to painting, paint adhesion could be compared between several parts that were degreased and several that were not degreased. If adhesion levels do not vary between the two groups, degreasing is not necessary and can be eliminated. If adhesion does vary but only in specific locations on the component, it is important to determine the cause. Contamination such as cooling oil generally would cause paint to not adhere consistently over the component. Fingerprints left by workers during part handling commonly cause spotty paint adhesion. Instead of degreasing, lathe or paint booth operators may need to wear gloves. A cleaning and adhesion problem in one manufacturing plant was found to be hand cream used by several machine operators. Hand washing, not parts washing, solved the problem. Components with varying levels of contamination can be painted to determine the maximum contamination level and type of contamination that causes paint adhesion or other quality problems. This information should be communicated to machine operators or vendors in the form of cleanliness specifications. If the cleanliness specifications can be achieved without cleaning, reduced waste and cost can be achieved. Another method to achieve required paint adhesion without cleaning may be to switch to a different type of paint. Paint vendors may be able to supply paint that will provide the required performance without or with reduced surface cleaning.
Production scheduling may reduce or eliminate the need to clean processing equipment between production runs. Production runs should usually be scheduled so that residues left in processing equipment from the previous process do not interfere with the subsequent process. Processes such as painting, printing, or dyeing should be run with products that use the exact same colors and chemistries over the course of a day or shift to reduce cleaning to only the end of the shift. If color or chemistry changes are needed, colors should be run so that each production change will result in a darker color used. Run colors from white to black. Also, run compatible chemistries together. Avoid changes such as from water-based to oil-based back to water-based. Rescheduling to run the two oil-based chemistries back to back could eliminate one cleaning process. Be aware of temperature changes; processes that run at the same temperatures should be run consecutively to conserve energy.
Use of dedicated mixing vats was discussed previously as a method to reduce waste and save money. Assume that a manufacturer mixes a number of different cleaning agents. When designing the mixing process, the number of mixing vats will need to be determined. The manufacturer can determine the most efficient production schedule and optimize the number of dedicated mixing vats needed by comparing chemistries and determining which ones are compatible. Using one mixing vat and cleaning between each production run may be a waste of resources; using a dedicated tank for each product may also be a waste of resources. Each of the product chemistries should be analyzed and grouped for chemical compatibility. This information can be used to determine the most efficient production schedule that will minimize cleanup wastes in the minimum number of dedicated vats.
Production cleaning should be considered during product design. Blind holes should be avoided. Holes that are drilled and tapped into a component should be drilled completely through where the design allows. Not only are through holes easier to drill and tap, through holes are easier to clean. Blind holes are traps for machining residues. Hand wiping and most other cleaning methods are not effective in removing residues from blind holes. Typically, blind holes will hold liquid and machining chips. Machining chips left in internal oil passages will probably not cause painting or other processing problems. However, residual machining chips and cooling oil left in internal passages and other blind holes are among the leading causes of machine failure in new assemblies.
Alternative materials may be a solution to cleaning. Steel components may need to be machined, degreased, and painted in order to meet functional and appearance specifications. Replacing steel with a colored plastic may eliminate cleaning and painting. An example can be found on most office desktops. Traditionally, staple gun exteriors were made of formed steel components that were painted either beige or black. Today, most staple gun exteriors are formed of injection molded plastic of the correct color, thus eliminating the need to form, clean, or paint.
Production equipment can also be designed to reduce the need to clean. Guards should be placed around processes to catch drips and splatters that would have to be cleaned from the floor. Funnels and guides can be added to equipment to allow operators to add dry or wet chemicals without spillage. Internal passages can be designed so that production chemicals drain into the process instead of becoming trapped inside the equipment. Internal passages and external piping should be designed to be as short as possible to reduce the amount of surface area that may need to be cleaned between production runs. Drip trays, pans, and drain boards can be designed as part of equipment to channel spills back into the process.
Paint spray or glue application guns used to apply two-part chemicals are often designed so that mixing of the resin and hardener takes place only in the gun tip. Resin and hardener are fed to the application gun tip in two tubes instead of being mixed in a pressure tank or gun cup. This allows the gun tip to be removed and cleaned at the end of the shift instead of flushing the hoses and pressure tank. In addition to eliminating the need to clean a large amount of equipment, this method reduces the amount of glue or paint wasted to only that very small volume in the gun tip. A very simple example of this process is the two-part epoxy syringe packages available in retail stores.
Non-stick surfaces, such as Teflon or polyethylene, can be added to production equipment to aid in removal of residues. Powder coating paint booths typically have very smooth plastic interior surfaces to prevent the powder from adhering. When cleanup is required, it can usually be achieved with vacuuming. Scraper and automated knives in the plastics industry are often coated with Teflon to prevent adhesion of the resins to the knives. Paint booths are often coated with paper or strippable chemical coatings to eliminate the need of using harsh stripping chemicals during paint cleanup. A very simple example of a strippable coating is applying a light coating of hydraulic oil to paint roll coating processing equipment exteriors to prevent paint splatters from sticking. When dry, the paint falls to the floor and can be swept into a dust bin.
No-clean methods also apply to process chemicals such as water. Typically, water is used one time for rinsing or cooling prior to disposal. Often, single use of water is a serious waste of money and natural resources. Rinse water is often clean enough to reuse many times for rinsing or for cleaning bath makeup water. Water usage can be reduced by up to 80% in rinse tanks by using counter flow techniques. In counter flow rinsing or washing, water flows from one rinse tank to the next opposite of the direction that the components being washed are moving. In this way, the dirtiest part comes in contact with the dirtiest water first. After getting the most out of water for washing or rinsing, contaminants can be filtered from the water and the water reused. Some plating and anodizing processors have reduced or eliminated wastewater discharges by combining water conservation methods such as counter flow rinsing and filtration. Common Sense Methods
No-clean methods can be summed up as common sense methods. If a component is not dirty and it is prevented from becoming contaminated, then it does not need to be cleaned. Processes and components should be designed to eliminate or reduce the need in order to clean to reduce cost and pollution.
Before cleaning, evaluate the item to be cleaned to determine the nature of the contamination. Determine what will happen if contaminants are not removed. Eliminate cleaning if the contaminants are compatible with the process. If contaminants are detrimental to the next process or will damage the component, determine the source of the contamination and correct instead of cleaning. If detrimental contaminants are residues or chemicals from a previous process, determine if there are substitute chemicals available that will not harm the process. Adjust production schedules so that residues left on production machinery and products are compatible with next process. If detrimental contaminants are residues from a previous process, determine if that process can be eliminated. If contaminants are detrimental to the next process, determine if the next process can be altered or eliminated so that contaminants are compatible. Determine just how clean components need to be in order to function. Publish cleanliness specifications for each component or process; do not specify a particular cleaning process. Evaluate current product design to determine if alternate designs and materials can meet functional requirements with reduced waste without cleaning. Remember that no-clean methods also apply to process chemicals such as water. Use and reuse water efficiently. Avoid spills and cleanup by installing guards, drip trays, and proper material handling devices on production equipment. Reduce the need to clean by using non-stick surfaces and strippable coatings on production equipment.