The purpose of this report is to explain the origin of needle stripes. Instead of “needle stripes, the term “scratch marks” frequently is used, but it is in fact misleading.
Needle stripes are elevated stripes on a panel, as shown in Figure 1. They are caused by absent rows of grit on a sanding belt.
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Figure 1: Typical appearance of needle stripes
At a feed rate of 40 m/min, it would take 0.0825 s for the foreign particle to cover 55 mm. At a belt speed of 22 m/s, 1.815 meters of belt pass over the particle within this window of time, 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.
Type II needle stripes – Sanding belt
The second potential cause of needle stripes is related to the condition of the sanding belts themselves. Figure 4 shows the sanding dust that typically collects on the floor of a sanding machine. Black grit is clearly visible.
Figure 4: Sanding dust collected from the floor of a sander
These black grains are silicon carbide particles that broke out of the belt the first time it was run.
Type I needle stripes – Foreign particles in the panel
Figure 2 shows the calibration sanding process, with a panel passing under the calibration head. The yellow curve indicates the contact zone between belt and panel. On Steinemann machines with a contact drum diameter of 455 mm and a sanding thickness of 0.10 mm, the contact zone measures 19 mm. A foreign particle (black triangle) is lodged in the panel. At an assumed feed rate of 40 m/min., it would take 0.0285 seconds for this particle to pass through the 19 mm contact zone. In the first K-head of a Satos system, the belt speed is 34 m/s (at 50 Hz). In 0.028 seconds, 0.969 meters of belt pass over the foreign particle. If the foreign particle is hard enough, it can break out a row of grit of just under a meter long in this case, or about a third of the total belt length. The slower the feed rate, the longer the stripes will be.
Figure 2: Diagram of the calibration sanding process
Taking a look at fine sanding in Figure 3, we see two, distinct differences compared with calibration sanding. First, the contact zone is significantly longer at 55 mm (width of the insert) and second, the belt speed is significantly slower at 22 m/s (at 50 Hz).
Figure 3: Diagram of the fine sanding process
Figure 5: Direction of sanding belts (blue lines) and flight path of dust (red dotted lines)
Figure 5 shows the direction in which the belts run and the flight path of the sanding dust. If a particle (or several) loosens from the belt the first time it is run, the particle may not necessarily be removed by the extraction system. In this case, it could fall onto the surface of the panel or the floor of the machine. If the particle falls on the surface of the panel, it can potentially come into contact with the belt and in turn cause a row of grains to break out of the belt (see Type I). The probability of a grain row breaking out in the last N-head belt is twice as high as for the other heads, because a particle can potentially fall from two heads onto the panel.
Needle stripes basically are caused by two scenarios:
1. Foreign particles in the panel
2. Grain break-out the first time a belt is run.
A distinction can be made between these two causes. If needle stripes occur, the bottom side of the panel should also be chalked. 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 at all, it is highly probable that the marks are Type II needle stripes. The flight paths in Figure 5 show that in the case of grain break-out, the particles can only fall onto the top of the panel. On the bottom of the panel, the particles would fall onto the floor of the machine. In this case, customers should attempt to reduce the load on the new sanding belts by retracting the heads somewhat after a belt change. If the machine 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.