Flip-top bottle cap

A thin PP hinge lets the lid and cap body work as one moulded part.

Flip-top bottle cap, illustrative image

Material

Cap body and living hinge: PP

Material selection rationale

Repeated bending
A suitably designed thin hinge flexes when the cap is opened.
Moulded detail
The rim, opening and lid can be made together.
Localised bending
The thin hinge bends while the thicker lid and closure remain comparatively rigid, using geometry to create different functions in one PP part.

How it becomes a product

  1. Mould the open cap: Molten PP fills the cap, lid and thin connecting hinge in one tool. Mould filling and cooling set the thin connecting web; its dimensions and orientation need to suit repeated flexing.
  2. Finish the hinge: Controlled initial flexing conditions the hinge before the closure is fitted to a bottle.

Preparation routes

Propene route

Catalytic addition polymerisation. Propene molecules join into chains. A suitable catalyst controls chain growth and the arrangement of the methyl side groups. Catalyst-controlled arrangement of methyl groups can favour regular packing. The monomer formula alone does not describe this tacticity.

A coordination catalyst system is used; it is not counted as a monomer. A thermoplastic moulding or foam uses already-made PP, not propene reacting inside the final product.

  1. Poly(propene): manufacture and usesChemical Industry Education Centre, University of York

Sources

  1. Moplen HP501H: PP homopolymer for caps and hingesLyondellBasell
  2. A guide to polyolefin injection mouldingLyondellBasell
  3. Poly(propene): manufacture and usesChemical Industry Education Centre, University of York