Masking techniques for metal finishing

Masking is a mature finishing process, but there are some twists you can use to improve on masking techniques and reduce costs.

Masking tapes have new pressure sensitive adhesives that release easily for better edge protection and ease of removal. The backbone of a successful masking tape is not just the paper but the adhesive. The newer pressure sensitive adhesives allow the tape to be removed without leaving a residue or harming the surface being finished. The industrial tapes are rated on a 1–100 scale based on the strength of the adhesive; one rate at about works for most industrial masking applications.

Paper masking tapes have some very real drawbacks when it comes to use in harsh application environments. To cope such conditions, tape suppliers change the base and the adhesive to match the usage. By mixing and matching the adhesives with the material bases, masking tapes can be used for any masking purpose, not just for painting.

Polyester, metal foils and vinyl based tapes will standup to the chemistries found in the plating industry. Powder coating masking tapes include glass cloth-based and polyester-based types. Polyimide film-based tapes have good heat resistance and are used in electronics applications including hand and wave soldering.

The thousands of industrial adhesives fall into three main categories: rubber, acrylic and silicone. Rubber adhesives have the lowest temperature resistance but can be formulated to provide the highest adhesion. There are now new smooth mini-crepe paper masking tapes designed for automotive applications where aggressive room temperature and high temperature curing maybe required (163º C for 30 minutes).

Acrylic adhesives offer a wider temperature range from below freezing to about 150º C. The new acrylic materials have an advantage as they don’t age in the same way as rubber-based adhesives.  They don’t discolour significantly and don’t soak into the surface like rubber based adhesives. Silicone adhesives provide the highest temperature resistance, with some tapes (polyimide films and glass cloth tapes) allowing for intermittent (check the data sheets to match the tape with cure) use around 260°C.

Masking Machines

Masking machines help to control the dispensing of the tape and paper tape combinations. They come in all sizes and shapes and start with portable carts with several different sized tape/paper dispensers with shelves. Stationary models are wall-mounted units. Remember though, you get what you pay for and the less expensive models are not always the best value.

So how can masking increase the productivity and improve the quality of the product you are finishing? Walk the shop: do you see workers removing excess masking with razor blades? Workers removing excess masking with a razor blade could be doing it faster, safer and better with a die-cut tape or a molded mask.

Die-cut masks

When do die-cut tapes work out better than hand masking? “Peel and stick” die-cut masks usually become more economical than individual workers taping and cutting at fairly low production rates. The tipping point between hand and die-cut masking for most finishers needs a fairly complicated masking project that takes about 20 minutes using hand masking techniques. Just the same ten to twenty parts a day offers a good economic argument for die-cut masks.

If your customer is unhappy about your dimensional masking consistency, then you have another reason to look at die-cut masking. Die-cut masking is dimensionally stable when you choose the correct base and adhesive to within 0.127 mm. Widths as narrow as 0.77 mm are possible. Die-cut masking can help to minimize alignment difficulties by using guide holes or tabs, which allows for quick precise alignment of the mask onto the part surface. If you’re masking a serial number, new refinements such as transfer liners allow for quicky, easy and precise alignment of several separate masking elements like.

We’ve discussed applying the mask, but what about removing it? It’s easy to overlook the fact that time saved removing the mask is just as good as time saved applying the mask. Other factors in removing the mask should be taken into consideration. To eliminate finish marring, avoid using tools. Die-cut masks can be made to include tabs that speed up the mask removing process.

Custom Moulded Masks

Custom moulded masks represent a major step up from the die-cut tape masks. These masks can be placed quickly with better protection and are reusable. The more complex masking requirements, the more custom moulded masks make sense. An example of a great application would be wheel assemblies. You can mask the stud holes and the hub or bearing surfaces all at the same time, saving time with installation and removal. You need masking speed to take full advantage of today’s high speed coating lines and robotic coating equipment. Nothing is faster than custom moulded masks because you can mould ergonomic hand grips into the mould to rapidly apply and remove masking.

The materials that the moulds are made from can be varied to meet your line’s chemical and abrasive environments as well as your temperature requirements. But they are not indestructible. You may see up to 50 uses depending on conditions.

Competitive Pressures

Looking at your masking in a new light can be difficult without some new eyes. Invite a masking engineer from your supplier to have a look around. He may see a bottleneck you are missing. Or he may see concerns for worker safety that can easily be fixed.  The bottom line is yours to fix, and improved masking is a good place to start looking. CM

Fred Mueller is a finishing expert and corporate quality and safety manager with General Magnaplate Corp. Linden, NJ.