Even Plastisol has its sheltered corners
Everyone is familiar with snow drifts. Although the same amount of snow falls all around, small mounds form in some places, whilst in others only a thin layer of snow remains. The wind distributes the snow behind the house and under the canopy differently to how it does in open terrain.
Liquid PVC also tends to accumulate in certain places rather than others during production – though not because of the wind, but due to the geometry of the component. Under Accumulations of material This refers to areas of a PVC dip-moulded part where, due to the geometry, the wall thickness is greater than in the rest of the part’s geometry; this is a typical characteristic of the PVC dip-moulding process.
In the dipping process, a heated core is immersed in liquid PVC plastisol. A layer of PVC forms on its surface, which hardens during the subsequent gelation stage. However, depending on the shape of the component, the material does not distribute evenly throughout. In some places, more PVC adheres, whilst in others, correspondingly less does.
The cause lies in the heat distribution of the immersion core and in the flow behaviour of the PVC plastisol.
Areas of the diving core with a larger cross-sectional area store more thermal energy than delicate areas. They release this heat more slowly into the PVC plastisol. This allows the material to gel for longer in those areas and build up a thicker layer of PVC.
In addition, the shape of the component influences how the excess plastisol flows off after the dipping core has been removed. Transitions, curves, corners or larger cross-sections tend to cause greater localised accumulations of material. The viscosity of the PVC plastisol used, as well as the immersion and withdrawal speeds, also affect the subsequent wall thickness distribution.
For this reason, it is not technically possible to achieve exactly the same wall thickness throughout complex injection-moulded parts.
Material build-up is something every manufacturer of PVC moulded parts has encountered. At HAMCO, this is therefore taken into account right from the quotation stage for a component.
The wall thickness distribution can be controlled to a certain extent through the design of the dipping core, the selection of suitable radii, the orientation of the component during the dipping process, and the optimisation of the process parameters. However, the aim is not to completely avoid any build-up of material, but to minimise its extent. The crucial factor is that the material distribution meets the requirements for function, durability and the specific application.
Accumulations of material show that liquid PVC tends to accumulate slightly more precisely where its shape and physical properties allow it to do so.