Meaning
Unwanted foreign materials present in recycled polymer feedstocks include paper, metal, adhesives, and polymers of different chemical families that alter the properties of the host resin. In the injection moulding of recycled plastics, recycled resin contaminants lead to cosmetic flaws, tool damage, and reduced mechanical strength in the finished parts. The impact of these contaminants is determined by their concentration, size, and thermal stability during melt processing.
Melt Filtration
When processing recycled resins, these foreign substances often fail to melt at the processing temperature of the primary polymer, causing blockages in extrusion and molding systems. To mitigate this risk, melt filters are installed in the extruder to trap solid contaminants before they reach the die or mold gates. However, fine contaminants or those with similar melting points can pass through these screens, resulting in weak spots in the molded parts.
This particulate contamination can lead to stress concentration points, which reduce the impact strength of the molded components.
Process Disruption
Some contaminants, such as PVC in a polyester melt, can degrade thermally and release corrosive gases like hydrochloric acid. These gases corrode the screw and barrel surfaces, leading to costly maintenance and downtime. Non-melting contaminants also cause localized flow disruptions, which produce surface blemishes like splay and silver streaks on the molded parts.
Contaminant Analysis
Identifying the type and quantity of contaminants in incoming resin shipments helps molders adjust their process settings or reject out-of-specification batches. Techniques such as infrared spectroscopy and ash-content testing are used to verify the purity of the recycled material. Keeping contaminant levels below specified limits ensures the long-term reliability of the molding process.
