Often referred to as scratch marks, needle stripes are elevated stripes on a panel. There are two possible causes of needle stripes.
The first is due to absent rows of grit on a sanding belt, typically caused by a foreign particle lodged in the panel. At a feed rate of 40 meters per minute, it will take approximately 0.0825 seconds for the foreign particle to cover 55mm; at a belt speed of 22 meters per second, 1.815 meters of belt will pass over the particle within this timeframe, or more than half of the belt’s total length.
The longer the flawed areas on the belt, the longer the needle stripes on the surface of the sanded panel. Consequently, needle stripes from calibration sanding are shorter than those from fine sanding.
The second potential cause of needle stripes is related to the condition of the sanding belts themselves, causing grain break-out when the belt is run.
A distinction can be made between these two causes. If needle stripes occur, the bottom side of the panel should be checked. If needle stripes are visible there, they are clearly caused by one or more foreign particles in the panel. Foreign particles in the mat fall inside the forming station towards the bottom of the panel. Therefore, in the case of Type I needle stripes, even more should be visible on the bottom of the panel than on the top (assuming the top of the panel coming from the sander is also the top when it comes from the heat press).
If no needle stripes are visible on the bottom, it is highly probable that the marks are Type II needle stripes. In this case, the machine operator should attempt to reduce the load on the new sanding belts by retracting the heads somewhat after a belt change.
If the sander operator forgets to retract a sanding head after changing belts, the load on the belts is so high that needle stripes are very likely to occur on account of the extensive grain break-out.
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