Better Masking with Die Cuts
Caplug’s John Gill explains how die-cut masks can be better and faster than masking with a knife and tape.
Q: We use a lot of tape for masking shaped components. I’m aware you can use pre-cut die cuts to do this, but what features make die cuts faster and better than a knife and tape?
A: Masking with tape and then trimming can be fast. Masking with pre-cut shapes, however, can be a lot faster and, more importantly, more consistent. There are a number of features that can be built into die-cut masks which make them better than masking with tape and then trimming. A few of these are explained here:
Selective adhesive exposure. This is a style of die-cut mask also known as the aircraft-window mask style where, instead of exposing all the adhesive on the die cut, only the outer perimeter is exposed. This is particularly useful for larger die-cut masks that are difficult to apply.
Only exposing the adhesive around the edge of the mask means the die cut can be positioned quickly and accurately without the problem of the adhesive sticking in the center of the component and making it difficult to align. It also means that this style of die-cut mask can be removed quickly. The adhesive border is enough to hold the die cut in position through the finishing process, but it can be removed much faster than when all of the adhesive is exposed and stuck to the component.
This style of die cut is also good for curved surfaces. The non-adhesive center portion allows the die cut to flex and crease if necessary, while the adhesive around the edge holds the mask in place.
Die cuts with this feature can also be made with patches of exposed adhesive in the inner portion of the mask, to help with additional grip or to seal a feature.
Masking kits. For components that need multiple areas masked, such as the inside of network cabinets or industrial enclosures, a kit of masking die cuts can dramatically improve production speed and quality. Each kit contains all the die cuts for a particular component. The masker can tell he or she has completely masked the component once all of the die cuts in the kit for that component have been used.
Kits can be supplied in sheet form, with one kit per sheet, or on a roll, with lines separating each kit. They can even be supplied loose in bags with other masking products such as caps or plugs for a particular component.
Masking kits can be marked with the part number of the target component. Each mask in the kit also can be marked with a number that matches with a work instruction for the masker. Kits with a large number of loose die cuts particularly benefit from numbering of each piece. Not only does numbering each piece help with the masking process, but it also helps with inspection during the kit manufacturing process. In addition to numbering each mask in the kit, instructions or markings can be added on the die cuts themselves to help with their alignment.
For kits in which multiple masks are contained on the same sheet, the masks can be laid out in a specific order that helps orientate the masks with the component.
Transfer die cuts. Transfer die cuts are extremely useful for masking components that have a number of unconnected elements, such as pads on a circuit board. They can also be used when masks need to be positioned relative to points on the component.
Transfer die cuts consist of three layers: the liner, the die cut itself and a top transfer layer. Typically, the masking material around the die cuts has been removed, and the transfer layer is clear to allow the masker to see through the top layer.
To position the die cut, the liner is removed to expose the adhesive, and the die cut is then positioned and pressed into position on the component. The top layer can then be removed. Transfer die cuts can be aligned to elements that need masking, or they can be set up to have registration areas on them so that they can be aligned with a registration mark or feature on the component that is being masked.
Tabs. A tab is an additional adhesive or non-adhesive part of the die cut mask that is added to help handle the mask when it is applied or removed. They can help the user remove the mask from the liner before it is applied to the component by providing an area to hold that is not a part of the main mask. The adhesive on the mask can therefore remain untouched during the mask application process, which leads to maximum adhesive bond between the mask and the component.
Tabs can also help when removing the mask from the coated product. If the user is having problems seeing and removing masks after coating is complete, a tab will help him or her find it and provide something to pull on to remove the mask. Tabs are particularly useful for pulling masks off quickly while a component is still warm.
Slit-back liners. Die-cut masks are manufactured on a liner that is usually a complete piece, but it can be split into strips. A liner that is in strips is called a slit-back liner.
Having masks on a slit-back liner means that, instead of removing the complete mask from the liner, a smaller section can be removed to expose the adhesive in one area. Exposing only a section of the adhesive allows the user to align the mask with the component and then stick down that section of the mask. Once that section of the mask is applied to the component, he or she then can remove the remainder of the liner.
Slit-back liners are also useful when the mask is of an unusual or delicate shape. The liner can be peeled away from the mask instead of the mask being pulled off the liner.
John Gill is an engineer with Caplugs. Visit caplugs.com.