Establishing Rheological Dispersity Limits to Prevent Strain Hardening Failure in Regrind Blends
Establishing Trouton ratio floors via capillary extensional rheology prevents catastrophic web tear and parison sag in regrind polyolefin blends.
Thermoforming Web Tear
Meaning
A mechanical failure occurs when a polymer sheet stretches beyond its tensile limit during the forming process, causing a localized rupture. This thermoforming web tear manifests as a jagged separation in the plastic that ruins the integrity of the part. Vacuum or pressure assistance pulls the material into the mould with high force, and the sheet breaks if the thinning rate exceeds the elongation properties of the resin at specific temperatures.
Material Limits
High melt strength allows the sheet to support its own weight and resist excessive drawdown before the mould closes. Polypropylene grades with low branching often suffer from this instability because the material loses viscosity quickly once heated above the softening point. A virgin resin batch provides predictable performance, while regrind contents introduce varying molecular weight distributions that reduce the force required to create a hole.
Moulders mitigate the risk by increasing the heating time or adjusting the plug assist speed to distribute the load across a larger surface area.
Forming Dynamics
Process engineers monitor the sheet sag to anticipate thinning before the plug contacts the polymer surface. An aggressive vacuum setting pulls the material into tight corners where the radius is insufficient to allow for natural orientation. If the temperature profile across the sheet is uneven, the hotter regions stretch faster than the cooler surrounding sections.
The resulting stress concentration leads to failure where the plastic remains thinnest.
Economic Impact
Total part rejection follows every instance of this rupture because the air seal required for vacuum forming cannot be maintained. Production lines experience downtime when broken scraps accumulate in the trimming station or clog the discharge conveyors. Tooling damage occasionally results if the fragmented material fuses to the mould surface, requiring manual cleaning that halts the cycle for several hours.
Continuous monitoring of web tension prevents the waste of expensive barrier films that otherwise disappear into the scrap bin during high speed runs.