High precision polymeric or silica based microbeads can be added to adhesives to provide consistent gap control between substrates.
The best way to set the target fill of spacer beads in an adhesive is a math problem based on desired quantity of beads per unit length or area of dispense (plus a safety factor) and since population density of bead changes quickly as the beads get smaller, it is a good idea to do this math to avoid significant over/under charge. Some bead suppliers provide approximate quantity beads per weight which helps with the approximation calculation, but it is actually based on volume of adhesive per part. E.g. if you cut the dispense volume per part in half you would need to double the bead quantity to have the same number of beads per part. On a weight percentage basis, the density of the liquid can also come into play, and for two-part materials you may need to account for one part of the adhesive not containing spacer beads.
In theory, only three beads are required to suspend/separate two rigid parts, but this is obviously extreme, and significantly more beads are required. For “perimeter bonds”, an initial target the 0.3% (by volume) is a good rule of thumb. To optimize further, customers can use microscopy to count beads / unit length of glue dispense and calculate mean and std. dev.
For hard substrates, spacer beads can be crushed during the assembly process if the assembly pressure is too high. For softer substrates, glass spacer beads have a tendency to push into the substrate and not hold the desired gap, or cause damage to the substrate if the assembly pressure is even moderately high. An example is glass spacer beads for phone cover glass bonding (ink and plastic as the substrates being bonded). When the assembly pressure is too high, small white defects appear on the cosmetic side of the glass, which is caused by the glass spacer beads pressing through the ink layer and causing the visual defect.
For dispensing processes, having a high quantity of beads can cause issues with blocking of fine needles and/or the operation of jetting valves. There are options for polymer spacer beads to alleviate this, and these are better for jetting valves, but the lower hardness will cause them to deform easier when parts are pressed together; production team will need tight control on assembly pressure.
Axion Adhesives can add custom spacer beads to any of our existing adhesives.
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