Recycled content in engineering parts is increasingly expected by customers and regulators. What changes when you switch, from properties and consistency to colour and odour, and how to validate a recycled grade properly.
A manufacturer committed to 30% recycled content in a polypropylene housing to meet a major customer's sustainability requirements. The first trial parts looked fine. Then the problems surfaced: variation in melt flow from lot to lot, occasional black specks, a slightly different colour and, in some lots, lower impact strength at low temperatures. None was a reason to abandon recycled content. Each was a reason to manage it carefully, as we later documented in switching a housing to recycled polypropylene.
Recycled plastics are becoming a requirement rather than an option. Using them successfully in engineering parts depends on understanding what changes.
Types of recycled material
- Post-industrial recycled (PIR): scrap from manufacturing processes, such as runners and off-cuts. Usually cleaner and more consistent.
- Post-consumer recycled (PCR): material from used products and packaging. More variable, with more contamination, but it is what most sustainability claims and regulations focus on.
- Mechanically recycled: ground, washed and re-extruded.
- Chemically recycled: broken down to monomers or feedstock and re-polymerised, giving near-virgin properties, at a higher price.
What can change
- Mechanical properties: often lower impact strength and elongation, sometimes changed stiffness.
- Consistency: melt flow, properties and colour can vary between lots.
- Contamination: specks, gels and other polymers.
- Colour: often grey or off-colour, limiting light or bright colours.
- Odour, especially in PCR.
- Regulatory compliance: food contact, medical or restricted substance requirements may limit what can be used.
Designing for recycled content
- Allow more margin in the design for property variation.
- Prefer darker colours or textured surfaces that hide specks and colour variation.
- Avoid relying on recycled material in highly stressed or safety-critical features until validated.
- Consider blending recycled and virgin material, starting at lower percentages.
- Keep walls uniform and avoid sharp corners that magnify reduced toughness, applying the rules in why uniform wall thickness matters.
Validating a recycled grade
- Test multiple lots, not one, to understand variability.
- Test at the extremes of the use conditions, such as low-temperature impact.
- Run a process window study: recycled grades may need different settings, and the window may be narrower.
- Agree specifications with the supplier for melt flow, contamination, colour and key properties, with certificates for each lot.
- Consider incoming checks, such as melt flow testing, for critical applications.
Supply and documentation
- Choose suppliers with traceability and consistent sources.
- Document recycled content claims, with supplier certification, for customers and regulators.
- Plan for supply availability: demand for quality recycled resin is growing faster than supply.
Recycled plastics can work very well in engineering parts. The key is to treat a recycled grade as a new material, with design, testing and supply management to match.
