How to choose sand for sandblasting process before thermal spraying

       The effect of sandblasting is dependent on the type and size of the grit. Sharp, hard and angular grits provide the best roughening effect. Spherical or round grits are less effective for roughening and are not recommended. All grit should be clean, dry and free from oil, feldspar or other debris. The most common types include angular chilled steel grit, chilled iron grit, alumina grit, carborundum and silicon carbide grits.



The choice of sand types is mainly related to the hardness of the matrix. Refractory metal oxides with sharp edges and corners, such as alumina, can be used for hard substrates of martensitic steels. If used for soft substrates such as magnesium and its alloys, aluminum and its alloys, particles may be embedded in the substrate. The surface should be treated with pure compressed air after sandblasting to remove any sand particles that may be embedded. For most substrates with hardness below 40~45RC, chilled iron grit is best, which dulls but does not shatter on impact. Chilled iron sand is usually more likely to cause greater stress to the substrate than alumina. Therefore, in order to avoid deformation of the workpiece, it is not recommended to use chilled iron sand when blasting and roughening the substrate of thin-walled parts. Silicon carbide grains have a greater tendency to embed in the matrix and are more prone to breakage than alumina.



Since the roughness of the roughened workpiece surface is directly related to the size of the sandblasting particles, the sand particles are supplied in different particle size distributions. When a larger substrate surface is required to be treated per unit time, sand with smaller particles can be used. The larger particle size of the grit removes unwanted material from the substrate surface more quickly and results in a rougher surface. For various metal substrates, grit size 1 to 60 is recommended, while for most plastic substrates, grit size 60 to 100 should be used. For thin coatings, especially for thin substrates, sand with finer grain size (grain number 25~120) should be used; for thick coatings (greater than 0.25mm), in order to obtain the best bonding strength, it should be selected Coarse-grained grit (18-25 grit number) in order to produce a rougher surface.