Stress cracks occur when the short-term mechanical strength of a plastic is subjected to adverse forces. Careful design of compatible substrates and consistent manufacturing and assembly processes reduce or prevent stress cracks.
Stress Cracking Causes
There are several areas to consider when looking for the cause of stress cracks in plastic components.
Thermal co-efficient compatibility of substrates
Adhesive chemistry & application process
Tips to Eliminate Stress Cracking
First – Examine plastic components for existing stress and, where possible, eliminate causes due to machining, molding, extruding, etc.
Second – Review the substrates for differences in thermal coefficients. A wide difference indicates a need for flexible and resilient adhesives that will move with the substrates as they undergo temperature changes.
Third – Select an adhesive recommended for the type(s) of plastic to be bonded and test thoroughly before beginning full-scale production.
To further minimize or reduce stress cracks while bonding plastic substrates with adhesives:
Avoid solvents and solvent-based adhesives.
If using cyanoacrylate apply the absolute minimum amount of adhesive to the bond area and remove any squeeze-out from the surrounding area.
Optimize relative humidity (40% – 60%) when using cyanoacrylates to accelerate cure time.
Use an accelerator to speed up the cure time of cyanoacrylates.
Minimize or reduce the time the uncured adhesive remains on the plastic for “cure-on-demand” adhesives, such as UV and light-curable, or bead-on-bead two-component formulas. The faster you cure the adhesive, the less likely stress cracking will occur.
Last – Review the component assembly process and compare it to design specifications.
Tapered fittings are subjected to hoop stress; for example, over-insertion will exacerbate existing cracks in the substrate. Adhesives and solvents lubricate tapered fittings making it easy to over-insert one part into the other. To avoid adding stress, control the distance that the tapered piece is inserted into the cylinder and use solvent-free adhesives.
In summary, carefully selecting and applying adhesives to plastics can produce component assemblies that meet design specifications. Consult the manufacturer for assistance in selecting the optimum combination of plastic and adhesive to meet the application requirements.
For further help and advice, please contact Permabond.