Question: I need to bond a Lexan Polycarbonate to a T6 Aluminum alloy and am wondering about the best bond gap for a UV adhesive. Are there any other issues to watch out for in bonding these materials?
Answer: Light-curable adhesives are great at filling gaps, but the ideal bond gap is 0.004-0.006 inches to get the best adhesion. If the bond gap or tolerances are too small, then the adhesive can be squeezed out of the bond line, resulting in a failure. If the bond gap is too big, you start to rely on the cohesive strength within the adhesive rather than the adhesive strength to the substrates, as well as the problem of the adhesive running out of the bond line during your dispensing and handling of the part. If you can incorporate a small tongue and groove design into the components, this will help by creating a mechanical lock, which will complement the chemical adhesion.
Since you are working with polycarbonate, test and evaluate the light penetration through the plastic. Many plastics, such as polycarbonate, incorporate a stabilizer into the plastic to avoid yellowing and brittleness as they age, but these can sometimes reduce the amount of UV light getting through the plastic to the adhesive. We typically see a 15-25% reduction in standard grades, but often see up to a 75-80% in some grades. These can affect how long it takes to cure the adhesive. The good news is that most lamps and adhesives these days are designed to work in both the UV and Visible light spectrum, and any visible range (ideally in the 395-450 nm range) frequencies that get through will help cure the material.
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