Design for adhesive bonding

The load on the assembly can be applied in similar way to solvent bonding, see the figure below. Thin layers are advised in case of lap shear. Peel and split loads are best taken up by a thick layer of adhesive.

Lap shear, peel, split, tension and compression

General design guidelines are:
- design for lap shear loads
- maximize the bonding surface; for instance, use a scarfed or a dovetail joint
- avoid stress concentrations at thick-thin sections
- take care of sufficient venting on substrate.

Recommended joint designs are given in the figure below. Hermetic seals required for containers and bottles are achieved with the designs shown in figure A and B. Joint C is more universal.

To ensure successful joining with adhesives it is important to know the functional requirements of the assembly and the possibilities and limitations of the adhesive in combination with the substrate. The following checklist might prove useful:

- product: design joints specifically for adhesives
- mechanical load: lap shear, peel, split or tensile
- life of joint: use temperature, environment, relative humidity
- thermoplastic substrate: mechanical properties, wetting, moisture absorption
- adhesive: temperature and chemical resistance
- pretreatment: cleaning, etching, sanding, oxidation, primer
- safety: MSDS (Material Safety Data Sheet) chart.

The moisture content of polyamides does not strongly influence the bond strength. However it is advisable to do some bonding tests with conditioned parts prior to production.