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Masking

When a manufactured part is coated there may be certain areas of the surface that must be free of coating. For example, threaded areas may not fit properly if they are coated. Bearing surfaces with close tolerance cannot be coated. Some parts may require a bare spot for electrical-grounding contact. It is important to select a mask that is effective, easy to use and reasonable in cost. Masking may be done by caps, plugs, tapes, stoppers etc.

Masking parts will add additional labor to the coating process. It is important to understand the different types of masking products that can be used. Masking can be as simple as placing a plug in a single hole or it may require a specially made mask or fixture. One large hole in a large part will require very little labor while some smaller parts may have numerous spots that need masking and require a lot of labor. Selection of the right masking procedure will affect the cost of the part. The mask must be reasonable in cost and it must do an effective job of keeping the coating off the masked surface.

There is a full range of masking products to cover the finishing needs, masking caps, plugs and hooks to tapes and masking dots.

Masking tape is a type of pressure sensitive tape made of a thin and easy-to-tear paper, and an easily released pressure sensitive adhesive. It is available in a variety of widths. It is used mainly in painting, to mask off areas that should not be painted. The adhesive is the key element to its usefulness, as it allows the tape to be easily removed without leaving residue or damaging the surface to which it is applied.

When constructed with polymer films instead of paper, masking tapes can be used for more rigorous applications. Polyester based tapes are used to mask off during etching, plating, and in particular, powder coating. Tapes based on polyimide films can resist molten solder in electronics applications. Glass cloth tapes are often used in powder coating and sandblasting operations. Foil or vinyl tapes are often used in plating. Layered tapes made from multiple materials laminated together can be used for masking flame spray, thermal spray and HVOF.

The adhesive applied to a tape is often a critical determining factor for a given masking situation. There are three broad types of adhesives (with many chemical variations of each): rubber-based, acrylic-based, and silicone-based. Rubber based adhesives generally provide the greatest adhesion, but the lowest temperature resistance. Acrylic-based adhesives offer a wide temperature range, providing adhesion from sub-freezing temperatures up to 275–325° Fahrenheit (about 150° Celsius). Silicone based adhesives provide the highest temperature resistance, with some tapes (such as some polyimide films and glass cloth tapes) allowing for intermittent use up to 500°F (260°C).