29 August 2018 | Article

Expanded Foams like polypropylene and polystyrene offer versatility in design across a myriad of applications. To increase EPP’s functionality further you can incorporate living hinges, interference fits, plastic fasteners and textured finishes.

Expanded Polypropylene Interference Fits

Rather than using any other means of fastening like clips, glue or Velcro an EPP interference fit, also known as a resistance fit, press fit or friction fit uses friction to securely hold two pieces of expanded foam together. The right amount of interference is vital to the fit, so special attention is required around mould tolerance and shrinkage to ensure the precision required.

The tension at the joint is determined by level of resistance, this can be varied to change the strength of the fit from a loose fit to a tight fit. The amount of give on these types of fits also depends on the material density used, part size and what scale of friction is needed.

  • Easy assembly (parts are pushed together and pulled apart)
  • Reliable means of permanent or temporarily joining parts
  • No additional fittings or adhesives are required 

In moulding fasteners and fittings

The shift in the use of EPS and EPP from a packaging material to a high value internal component material means that there is more demand for securing EPP to other materials. If the second material cannot be integrated as part of the manufacturing process then fasteners can be moulded in or attached as a secondary operation.

These precision fasteners are now common place in the Automotive, HVAC and Consumer Electronics Industries, offering reliability and ease of use:

  • Fastenings are suited for tolerance independent assembly
  • Screwed into foam with no need for a pilot hole
  • Weight savings due to used plastic material
  • Manual, semi-automatic and fully automatic assembly possible
  • High load capacity in the foam
  • Recyclability

Fastenings are specifically developed to secure parts to foam components, particularly Expanded Polypropylene. Fasteners can be insert-moulded and fused with the moulded part during the manufacturing process or manually screwed into a highlighted area in the foam.  One of the most popular fixtures is the Snap-Fit which combines all advantages above with the addition of subsequent component attachment. Snap-fit hooks allow for easy and secure fastening of thin-walled components to the Expanded Polypropylene part. This process works by fitting screws into the expanded polypropylene while the snap-fit end locks and secures the plastic cover and panels with a wall that varies in thickness.

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