With the introduction in the 1980s of increasingly stricter fuel-permeation regulations, the automotive industry adapted by switching to the use of multilayer fuel systems, in which a barrier layer is integrated.
These multilayer tanks are produced in a process called multi-layer extrusion or coextrusion “COEX”. In this process, a tubular preform made of molten polymers – virgin HDPE, HDPE regranulate, an internal and external bonding layer, EVOH and again virgin HDPE – is transferred into a blow-mould where it conforms to the mould contours through the use of internal pressure.
In order to cure the tank, the component is usually cooled by using cold water in the cavity of the tool mould.
However, the process step is very cost-intensive and time-consuming: low cold water temperatures lead to the formation of condensation on the mould surface, which in turn results in unsteady product quality and increased scrap.
Moreover, due to the continuing heat exposure, the EVOH-layer in the tank can be damaged, limiting the functional capability of the tank.
“Cooling of the plastic product represents both one of the most critical and most lengthy sub-processes in the extrusion blow moulding process,” explained Aaron Farrag, deputy CEO of the FarragTech GmbH.
Particularly, according to Farrag, difficulties can occur due to the temperature drop between the exterior cooled by means of cold water and the still warm interior of the product.
The differences in temperature could lead to material stress as the tanks are complexly shaped, with different wall thicknesses in the range of 1.35 mm and 3.80 mm.
Until recently, this problem was addressed by interval blowing. This, however, resulted in an unsteady product quality, with products failing to pass the tightness tests, load tests and drop tests, as well as resulting more scrap.
According to Germany-based FarragTech, the combined use of two systems – an IACS and a MAP unit - offers an efficient solution. The company's MAP system, specifically designed to dehumidify moulds, can ensure sweat-free production through a constant supply of dry, filtered air. Via a filter, ambient air is sucked into the MAP unit, which encloses the mould area.
The air is cooled in two steps: first via a water-cooled heat exchanger, then in the heat exchanger of the integrated refrigeration circuit. In this process, the air is cooled to approximately 3 °C. For pre-cooling of the sucked ambient air during the process, cold water is used which also serves to cool the moulds. All condensate is collected and pumped out of the device.
The use of an IACS - Internal Air-Cooling System - in addition to conventional cooling, will enhance output and reduce material stresses. The IACS, equipped with an integrated Blow Air Chiller (BAC), allows the tank to be internally flushed with -35 °C cold air, cooling the plastic simultaneously inside and outside without stressing the material.
“For this type of plastics processing, as well as for further blow-moulded products, a combination of IACS and MAP system works well, because with optimal coordination of the two mechanisms, the cooling time can be shortened by up to 60%,” said Farrag.
Especially with thick-walled moulds, Farrag added, a production increase of up to 200% can be achieved.