Even a pancake isn’t the same thickness everywhere
Imagine pouring pancake batter into a hot frying pan. No matter how you go about it, the batter can’t cover the whole surface of the pan at once, so it won’t be an even thickness. Some areas will be slightly thicker, others slightly thinner – simply because the batter spreads with a time lag and the amount of batter left is gradually decreasing.
Liquid PVC also follows the same physical laws during its manufacture.
Differences in wall thickness In the PVC dip-coating process, these are unavoidable and are among the typical characteristics of this manufacturing process.
However, this does not in any way mean that the component is defective. Rather, the wall thickness is determined by the interplay of component geometry, heat transfer, material properties and the duration of immersion in the bath.
Whilst simple, rotationally symmetrical injection-moulded parts often have a relatively uniform wall thickness distribution from the closed end to the open end, complex geometries almost always feature areas with noticeably thicker and thinner layers of material.
The key factor is the Heat storage in the immersion core and the dwell time in the immersion bath.
Solid sections of a dip-coated core store more thermal energy than delicate sections. As a result, their surface remains hot for longer. The PVC plastisol gels there over a longer period and can build up a thicker layer of material whilst the workpiece is in the dip bath.
Added to this is the flow behaviour of the PVC, which has not yet set. Until the material has completely set in the oven after the mould core has been removed, it may still shift slightly. Radii, transitions, edges and cross-sections of varying sizes further influence this movement (see also Drain marks and drain drops).
The viscosity of the PVC plastisol, the temperature of the dipping core and the speeds at which the workpiece is immersed and removed also affect the final wall thickness.
That is why there is an important principle in the PVC dipping process:
The wall thickness of a rotationally symmetrical dip-moulded part, such as a round cap or a bellows, will be greatest at the point where the dip moulding core first comes into contact with the plastisol. This is also the point that is withdrawn from the dip bath last; consequently, it remains in the dip bath for the longest time and is referred to as the closed end of the dip cap, where there is also the additional thickening caused by the Drip from the drain forms.
It is not possible to achieve completely uniform wall thicknesses in die-cast parts without mechanical finishing – and in most applications this is not even necessary.
Consistent and reproducible production is far more important. This is precisely where the strength of a well-controlled immersion process lies: every component reliably meets the requirements for functionality, protection and service life.
The wall thickness of a die-cast part is similar to that of a pancake – it is not the same thickness or the same thinness throughout
The wall thickness increases continuously from the open end to the closed end. At the very top is the drain drop at the closed end of the bellows.