Load Introduction into Vat-Photopolymerised Parts: Inserts, Threads, Press Fits and Overmoulded Interfaces

Every printed part that has to carry a load carries it through a small number of discrete places: an insert, a thread, a press fit, a bolted hole. Those places are where the material is most highly stressed, least well characterised and most casually specified. This review takes local load introduction into vat-photopolymerised components as its subject. It located one vat-photopolymerisation insert study with accessible quantitative results, in which the insert was adhesively potted — placing its retention mechanism partly in ER-165 — and a second vat-photopolymerisation pull-out paper whose relevant content could not be obtained and to which no value is attributed. From that narrow base follows the result that governs everything downstream: beyond the single accessible study, the comparative quantitative installation literature located is thermoplastic, and the thermal installation mechanism on which it rests is unavailable in a vat-photopolymerised part. Heat-set, ultrasonic and thermal-staking installation work by softening a thermoplastic and allowing it to flow into an undercut. A vat-photopolymerised part is a crosslinked network — the digital-light-processing literature places its gel point at a degree of cure of 0.37 — and it does not re-flow. Those values therefore cannot be transferred as vat-photopolymerisation design values. This review further located no study of heat-set, ultrasonic or press-fit insert installation in a vat photopolymer; no press-fit retention data for any printed polymer; no primary source on a load-bearing overmoulded or captured interface in a vat photopolymer; no pin-bearing data for a vat photopolymer; and no insert pull-out or torque-out standard for thermoplastics or thermosets in the standards search performed here. The one pull-out standard located for a fastener in a non-metallic substrate is written for wood; the fastener-property standard most often imported by habit is written for carbon and alloy steel. What follows separates the two vat-photopolymerisation anchors that exist from the thermoplastic evidence used as declared contrast, states what a printed photopolymer’s post-cure and viscoelastic state does to a preloaded joint, and proposes a load-introduction qualification architecture in which the local interface is qualified as a configuration rather than inferred from a bulk material property.