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G R A P H I C S - P R O. C O M J U N E 2 0 2 3 • G R A P H I C S P R O 2 3 To premask or not to premask? When CNC or laser cutting we have to decide whether premask should be applied or not. For starters, premask should be applied to vinyl graphics before they are mounted to the substrate. Premask will prevent stretching and distortion of the vinyl. If the vinyl is stretched while being mounted, it will affect the final size of the cut piece, which could be rejected by your customer. Another reason to use premask is for clean cuts of the ink, vinyl, and sub- strate in certain scenarios. ere are other advanced ways to achieve this without using premask. Upcut, down- cut and compression end mills, oscillat- ing blades and drag knifes could be used. is article is not to overwhelm a begin- ner in print and cut workflows, so we will focus on using premask to aid in the cut- ting process. Here are a few general rules when premask should be applied to your graphics. CNC routing Cleaner cuts with ink printed on top. When premask is applied to the top of the print, we will kiss cut and remove premask from the areas the fiducials have to be recognized. When printing directly to harder substrates like Dibond, the UV ink can shear or tear instead of cutting cleanly. A premask on top would allow the ink to cut clean and have the premask tear. It will also aid in the protection of the graphic while shipping. Onion skin cutting. e vacuum of the CNC can only do so much. At a cer- tain point the force of the endmill is stronger than the vacuum holding down the surface area of the finished piece. Small graphics lose vacuum and tend to kick out of place. Once they hit the end mill unintentionally, a chip of material will be taken out, causing the piece to be ruined. Performing an onion skin cut will avoid this. is is typically done with smaller pieces like museum placards or 8.5" × 11" prints on Sintra. Premask is applied to both the top and bottom of the sub- strate. e premask on top allows the ink or vinyl to cut clean. e bottom (onion skin) premask is used to hold the piece in place after it is routed. To perform this, a few tests are needed to find the proper depth of cut. e idea is to fully cut through the material, but not cut through the premask on the bottom. When routing is complete, the prints can be removed from the premask with clean edges and corners. Optically clear SAV. You cannot put premask on optically clear SAV. It will distort the clarity of the unprinted areas and cause the vinyl to look cloudy when installed on glass. If you have jobs that are being printed on optically clear vinyl use a loose perimeter die cut to avoid premask. is is more cost-effective than cutting all the letters and graphics individually where premask would be necessary. An example of the loose perimeter die-cut would be the Tweety job we produced. Laser cutting e first thing that has to be noted is if the graphic was printed first or second sur- face. ere should always be premask on the bottom to protect from flareups that could damage the print if unprotected. is would be with or without a print on the bottom. We remove the premask from the top; it is not required and could potentially stain the edges of the acrylic when cut. e only exception is if you have a first-sur- face print. Premask should be applied to protect the ink or vinyl. ere should be a low-powered cut to get through the premask and ink or vinyl first. en a second full power cut to get through the substrate. If you skip this step and try to cut in one pass, the laser can stain the edges of the acrylic with the ink, vinyl, or premask being used (espe- cially if there is watermark printing on the premask like 3M SCPM44-x) GP