Dynamic Viscosity Curve Generation and Transfer Point Calibration in Technical Molding
Calibrating transfer points via press-side viscosity curves optimizes fill pressure, cuts part mass variation, and locks in DIN 16742 tolerance bands.
Residence Time Degradation
Meaning
Irreversible chemical chain scission or crosslinking occurs when molten resin remains inside a heated barrel or die channel for prolonged durations. Injection moulding and extrusion operations experience residence time degradation when thermal exposure exceeds material thermal stability thresholds. The process governs loss of molecular weight, color shifting, surface splay and reduced impact strength, stopping when the molten polymer exits the nozzle or thermal exposure drops below reaction threshold temperatures.
Datasheet melt stability parameters report short-term thermal tolerance, whereas actual production barrels contain stagnant boundary layers that experience extended exposure times.
Thermal Breakdown
Extended thermal exposure breaks covalent bonds along the polymer backbone, lowering average molecular weight and increasing melt flow rate. Processing heat sensitive polymers like polyoxymethylene or polyvinyl chloride with excessive residence time degradation yields gaseous reaction products that cause surface splay and structural voids in moulded parts. Scission mechanisms reduce impact strength and tensile performance, rendering regrind material unusable if thermal history is unmonitored.
Purging barrel contents during press interruptions prevents accumulated thermal degradation from contaminating subsequent production cycles.
Processing Window
Barrel sizing relative to shot weight dictates average residence time within the heating zones. Operating a large barrel with small shot volumes extends material heat history beyond safe boundaries, causing yellowing and loss of toughness. Optimal machine selection keeps barrel inventory within three to five injection cycles.
Mechanical Degradation
Reduced melt viscosity caused by polymer breakdown destabilizes injection pressure requirements and process repeatability. Drop in molecular weight alters mechanical compliance, turning tough engineering thermoplastics into brittle components susceptible to premature field failure.