Uneven mould thickness traps air as molten plastic races ahead
A peer-reviewed simulation study links uneven mould thickness to faster plastic flow through thick sections and enclosed air pockets. In a circular cavity, outer streams joined before the thinner interior filled. The calculations varied thickness ratios and used earlier experimental images for comparison, describing where air becomes trapped while omitting its compression after enclosure.
Science··Morning
The flow bypasses the thin interior
A simulation of molten plastic filling a mould found that thickness differences changed the advancing flow front. In a circular cavity, plastic moved faster around the thicker outer region; those streams joined while the thinner interior was still filling, leaving air enclosed inside.[1]
The calculation treated plastic and air as two fluids and followed their interface, pressure and temperature. It used existing numerical methods rather than introducing a new solver. The study considered both rectangular and circular cavities with different ratios between thin and thick sections.[1]
Thickness and cooling act together
Equal-thickness cavities did not show the same fast outer route or enclosed air pocket. In the circular tests, trapping appeared when the thin-to-thick ratio reached 1/2.5, and further reductions in that ratio increased the trapped area. These are results for the modelled geometries and conditions.[1]
The model also represented viscosity changing with temperature. Plastic cooled more strongly in thinner regions, where the calculated resistance to flow increased. The mould wall had a fixed temperature; heat conduction through a solid mould was outside the calculation.[1]
Comparison locates the pocket
Earlier experimental images and commercial simulation results provided comparisons for the predicted filling patterns. The predicted trapped area was somewhat larger in the experimental comparison. The agreement concerns the location and development of trapped air, rather than a universal accuracy claim for all moulds.[1]
Air was assumed incompressible, so the calculation did not quantify its compression pressure after enclosure. Surface tension and contact angle were also omitted. Within those assumptions, the study maps how a faster outer stream can close the escape route before the interior finishes filling.[1]
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