Masking tapes range in composition from simple, low-cost paper tapes typically associated with painting, to expensive, heat-resistant polyester, polyimide and silicone used in high temperature surface finishing. Tapes are dispensed from a roll and are cut to fit the area to be masked. Some can be purchased in die-cut shapes, but this can be costly and is usually limited to high volume production of a single component configuration.
Pressure sensitive adhesive backing allows the tape to adhere to surfaces. Unfortunately, residue from the tape’s adhesive can remain on the surface after the tape is removed. A subsequent cleaning operation is often required to remove the residue. Tapes are durable enough for most masking applications involving one cycle of a dry process, such as shot peening or grit blasting. However, particulate often finds its way under the tape into inner core cavities, necessitating secondary removal and cleaning operations. Cleaning and plating baths also expose inadequacies in the manual tape application process. The liquid baths creep under the edge of the tape and flow quickly through creases and areas of low adhesion between the tape and component surface. Masking with tape is perhaps the most labor-intensive masking process. Tapes are applied by hand, and in most cases of multiple surface processing operations, reapplied in the same areas due to abrasion and wear. It is not uncommon for an operator to spend 2 to 4 hours masking a single intricate component. The taping process itself, in addition to being time consuming, can also create a hazardous work environment for employees. Despite the best protection, employees can sustain finger cuts from razor blades and from the tape itself. These injuries can result in lost time, workman’s compensation claims and higher insurance premiums.
Boots and molded caps, typically made of rubber or vinyl, are used in high volume processing of components with no dimensional variations. Because the cost of molds can be quite high, it is cost prohibitive to have boots and caps molded for small volumes or components with varying configurations or dimensions. Thus, molded boots and caps are not universal masking media. Rather, they are very product specific. Boots and caps rely on press fit tolerances and their elasticity to affix themselves to the components. However, this is not adequate protection for liquid processes like acid cleaning and plating. Boots and caps will eventually wear out after repeated use. They can also loosen and fall off during the process creating costly rework or scrap.
Wax is perhaps the least expensive material that can be used for masking. However, it is difficult to work with, as it must be heated above its melting point to apply (150°F or higher). Heating requires special handling equipment and thermal energy which can generate a surprisingly high utility cost. Operators also face the possibility of burns when repeatedly handling the hot wax. Typically, wax is poured into components to protect internal cavities. Parts with tight dimensions and serpentine pattern cavities must be pre-heated to allow proper flow of wax. Dipping is the only practical method of coating external surfaces, because hot wax cannot be applied in a controlled fashion. Removal of wax requires the same melting process as application of wax, with the exception that the component must now be heated with the wax. Again, operators must take precautions to avoid being burned by the wax and the components. Wax removal is usually conducted in a hot water bath. Care must be taken to ensure all wax is removed from the components, and the water bath must be well filtered or replaced frequently to keep the water clean. Melted wax creates a film in the water which can be redeposited on the components as they are removed from the tank. An additional cleaning operation is often required after wax removal. If any residual wax remains in core cavities, it may be necessary to burn it off in a furnace. The low melting point of wax limits or prevents its usage in high-temperature surface treatment processes such as acid cleaning, plating, and deposition coating. In addition, wax is soft. Precautions must be taken during handling to prevent accidental damage to the coating which could result in rework and possible scrap.
Typical masking lacquers are solvent-based and require a prescribed curing time to allow the solvent to evaporate and the remaining resin to cure, or harden, on the components. This type of curing process requires the masked components to be stacked or racked and left undisturbed for several minutes to several hours. Not only is the process bottle-necked, but valuable production space is consumed with curing parts. The solvents contained in many lacquers are the same solvents that are being restricted or banned in the workplace by governmental agencies for health and safety concerns. Evaporating solvents such as xylene, toluene, and other acetates have been linked to respiratory problems and other long-term ailments. Elaborate exhaust and air handling systems are mandatory when using solvents and solvent based products in the workplace. In addition, solvents can be extremely flammable, posing another health and safety risk.
Solvents, and products containing solvents, must be carefully monitored in the work environment. The U.S. Environmental Protection Agency mandates that consumption reports be filed for all solvent-based products. Solvent-based waste must be recognized as hazardous material and disposed in accordance with strict guidelines. In addition to higher waste disposal costs, insurance premiums can be considerably higher for manufacturers using solvents and solvent-based products. For these reasons, many corporations have mandated that solvents be phased out of their facilities as soon as possible. Some water-based lacquers have been introduced to overcome the solvent issues. However, they do not possess the same strength as solvent-based lacquers, and they require the same prolonged curing time, which can vary based on air temperature and humidity. Lacquers are predominantly watery, low-viscosity products. In order to provide adequate protection as a mask, it may be necessary to repeat dispense and cure cycles several times to build the proper mask thickness. Multiple masking cycles create delay and add cost.
Finally, lacquer removal can be a difficult process. Typically, lacquer-masked components are first warmed, then soaked in a solvent such as acetone. The majority of the lacquer slumps off the components, but some patches remain on surfaces and in cavities. Additional labor may be necessary to remove the lacquer remnants through subsequent processes involving grinding and burning.