Meaning
Cumulative exposure to high temperatures during multiple processing cycles affects the molecular structure of a polymer. Regrind thermal history tracks how many times a material has been melted, cooled and granulated. Each pass through an extruder or moulding machine can cause chain scission, oxidation or cross-linking.
This degradation limits the amount of recycled material that can be blended with virgin resin without compromising the final properties.
Molecular Weight
Breakage of the long polymer chains leads to a reduction in the average size of the molecules. An extensive regrind thermal history results in a lower molecular weight, which manifests as an increase in the melt flow rate. This change in viscosity affects how the material fills the mould and can lead to issues like flash or short shots.
Monitoring the flow properties is a standard way to assess the extent of the material degradation.
Property Loss
Mechanical characteristics such as impact strength and elongation at break are highly sensitive to the integrity of the polymer chains. As the regrind thermal history increases, the material becomes more brittle and less able to withstand stress. This loss of performance is often more pronounced than the change in tensile strength.
Engineers must test the properties of the final blend to ensure it still meets the requirements of the application.
Blending Ratio
Determining the maximum percentage of recycled material allowed in a part requires a careful balance of cost and quality. A heavy regrind thermal history forces the manufacturer to use a lower ratio of regrind to virgin resin. Some specifications for safety-critical parts forbid the use of regrind entirely to ensure maximum reliability.
Clear labeling and tracking of material batches prevent the unintended accumulation of heat-damaged resin in the production stream. Thermal history management is essential for maintaining the integrity of recycled plastics.
