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Sandpaper – Virtual Sanding

Spring is coming and it is time to get the tools out of the shed to build, repair, or fix things. One tool that many of us use is sandpaper. Whether that old bench outside needs sanding or some copper pipes need cleaning before adding fittings, sandpaper is the answer.

We did some virtual sanding ourselves by using micro-CT to slice a piece of 80 grit sandpaper every three microns to analyze the surface area. The surface area for sandpaper is important because, as the paper wears, it impacts how much material is removed from the object being sanded. Understanding this helps in the design of the sanding materials and improves its efficiency.

About Sandpaper

There is a lot of thought that goes into the creation of sandpaper and its composition is adapted to the application for which it is to be utilized. One of the key factors is the abrasive materials. Two natural abrasives include Garnet and Emery. Garnet is meant for hand sanding and uses particles that break down with use. This type of sandpaper wears quickly.  The other version is Emery sandpaper, which is good for both hand sanding and power sanding of metals1.

Manmade abrasives are also available, such as: aluminum oxide, silicon carbide, zirconia alumina, and ceramic alumina. Aluminum oxide is a common sandpaper and works on plastic, wood, drywall, and metal. The particles do break down but are strong and will last a long time. Silicon carbide is tougher and will remove material faster than aluminum oxide, but it is not as durable as aluminum oxide. It works well with plastic, wood, and metal and is often a good choice for rough sanding. Zirconia alumina is best for wood, fiberglass, metal, and painted surfaces. This type is mainly found in belts and discs for power sanding. Ceramic alumina is ideal for wood power sanding because of its ability to last longer than aluminum oxide1.

Out of all these types of sandpaper, the manufacturers lists whether they are closed-coated or open-coated. Closed-coated means the abrasive material is applied such that the back coating is completely covered by abrasive material. Open-coated, means the abrasive layer does not fully coat the backing. In our Micro-CT scan of the sandpaper, we have an open-coated, aluminum oxide abrasive1.

Micro-CT of Sandpaper

The goal of micro-CT scanning the sandpaper was to clearly measure the surface area of the abrasive material at each slice, therefore we selected a small pixel size while maintaining a large viewing area. Using a 6mm by 15mm section of sandpaper for the scan, at a pixel size of three microns, we were able to easily threshold and analyze the amount of abrasive material. The final result is surface area measurements over 292 images slices that are graphed to show a uniform virtual sanding (Figure 3).

Virtual Sanding

After the sample was scanned and the images reconstructed, the axial slice images were loaded into CTAnalyzer for surface area measurements. The first step involved finding a threshold to select the abrasive material. The threshold creates a white mask to represent all the abrasive particles above a certain grayscale level at a give slice. In this case, anything brighter the medium gray level of the abrasives (a threshold of 90/255) shown in the left axial slice image of Figure 1 was selected. After this, some of the smaller white specks were removed with a despeckle function as they were part of the glue and not considered a contributing factor to the sanding material (right image). Each slice was then ready for analysis.

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Figure 1. Raw Axial Slice Image (left), Thesholded abrasive material (right)

Using a built-in 2-D surface area algorithm, everything thresholded in the 292 slices was analyzed. The results were then populated automatically into an Excel sheet for further interpretation. Each slice number and the surface area represented were graphed, going from the top (beginning in air), abrasive side of the sandpaper all the way through the paper backing (Figure 2). The result is a nice bell curve which corresponds to the 3-D visual representation of the sandpaper (as shown on the cover page). Initially, the area of abrasive material is zero as the slices pass through air. The slices then reach the aluminum oxide particles which have sharp points at the extremes, both the top and bottom. As the sandpaper is worn by the virtual slicing, the surface area calculations increase because the particles have a larger diameter towards the center. The abrasives begin to decrease in surface area as the slices move toward the sharper abrasive portions that are in the glue. Finally, the area reaches zero after passing through the paper backing and back into air.

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Figure 2. Distribution of the surface area based on the virtual slicing of the sample

Analysis of the virtual slicing of the sandpaper is important because it shows a uniform orientation of the abrasive material to the backing. This creates even wear of the sandpaper and ensures it will have the maximum life possible with proper abrasive action. Based on our surface area analysis of the sandpaper, we can conclude the abrasive material is uniformly oriented and should perform well under sanding conditions.

This example is one of many where Micro Photonics has provided MicroCT testing to solve industries’ toughest problems. If you have an application you would like investigated or are interested in providing a sample for the next image of the month feel free to e-mail: brandon@microphotonics.com.

Works Cited

  1. 1. Lowe’s (2016). Sandpaper Buying Guide. Retrieved from: http://www.lowes.com/projects/woodworking-and-crafts/sandpaper-buying-guide/project

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