Key principles.
Cylindrical interference
Shaft slightly larger than hole. Permanent assembly. Standard ISO 286 H7/p6 typical. Holding force from friction.
Plastic flexibility
Elastic deformation engages mating feature, then springs back. Plastic parts only. Permanent or removable per design.
Position datum
Dowel pins position mating parts precisely. H7 reamed holes, ±0.005 mm achievable.
Plastic threading
Heat-set or ultrasonic inserts provide metal threads in plastic. Stronger than self-tapping.
Fast assembly
Permanent magnets for fast positioning, alignment. Common for tooling, displays.
Quarter-turn
Twist-and-lock mounts. Fast assembly/disassembly. Lens mounts, quick-change tooling.
FAQ
Press fit: any material, requires force, generally permanent. Snap fit: plastic only, no special tools, can be removable. Choose by material and disassembly need.
Light press: H7/n6 (0.005-0.015mm interference per 10mm). Standard: H7/p6 (0.015-0.035mm). Heavy: H7/u6 (0.035-0.08mm). Choose per holding force needed.
Plastic creep over time loosens snap. Heat softens plastic, releases snap. Repeated cycles fatigue. Design for finite cycle life if removable.
Inserts handle higher torque, repeated assembly. Molded threads cheaper but limited cycles. Heat-set inserts standard for quality plastic assemblies.
Adhesives provide gap-filling bond, work with any clearance. Slower (cure time) but tolerant of dimensional variation. Reversible with heat.
For service-friendly products, design assembly for non-destructive disassembly. Threaded fasteners or removable snap fits. Avoid permanent press fits where service needed.
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