When manufacturing fuel tanks for the automotive industry, the multi-layer or co-extrusion process “Coex” is used. However, the process step was very cost-intensive and time-consuming.
“Cooling the plastic product represents both one of the most critical and most lengthy sub-processes in the extrusion blow moulding process,” explains Aaron Farrag, deputy CEO of Farragtech GmbH. “Especially during production of multi-layer products such as modern fuel tanks, a lot of time can be saved and the Evoh-layer protected against damages from heat exposure.” During the process, difficulties can occur due to the temperature drop between the exterior, which is cooled by means of cold water, and the still warm interior of the product.
These temperature differences often led to significant material stress – above all, because the large-scale tanks are complexly shaped, featuring wall thicknesses ranging between 1.35 mm and 3.80 mm. Until recently, this was counteracted by means of interval blowing. As a rule, the results were unsteady product quality as well as a failure to pass the subsequently carried out tightness tests, load tests as well as drop tests. Because of the relatively high scrap rate, the costs of production increased. “In order to avoid that, our customers moved on to reducing the temperature of cold water once more. However, apart from reduced product quality, this also resulted in an increase in energy costs,” Farrag further explained. “Instead, we recommended to carry out cooling of the interior by means of compressed air, in addition to heat reduction with 6 °C cold water when manufacturing petrol tanks consisting of several layers of plastic.” In order to achieve this, we selected the so-called Internal Air Cooling System (IACS) with integrated Blow Air Chiller (BAC). Modularly designable blow mandrels as well as suitable blow valve blocks were supplied. For blowing applications, the BAC provides air temperatures of up to – 35 °C.
Increase of productivity thanks to cooling from the inside and outside
The BAC is available in five different construction sizes, with the design always being linked to the air throughput for the specific application. In comparison to cooling by means of ram air, an increase in productivity from 25 to 200% can be achieved. For this purpose, the compressed air is brought to a dew point of < – 40 °C and then cooled in the integrated heat exchanger.
In order to ensure that the device operates practically maintenance-free, a previously defined, good compressed air quality with a pressure dew point of 5 °C at 7 bar as well as a residual oil content of max. 0.01 mg/m³ is absolutely necessary. In many blow factories, this is already a standard.
Foam-insulated cold air pipes ensure that the air temperature on the way from the BAC device to the blow tools can be kept low and does not freeze or that the condensation water does not drop into the production hall. Control of the BAC devices takes place via the Farrag Intelligent Terminal (FIT). By means of the IACS system, material stress could be efficiently prevented and a higher quality of the produced plastic parts could be achieved in total.
An unwanted side effect of the mould cooling using water, the temperature of which is below the dew point of the ambient air, was the formation of condensation water on the mould, which negatively influences both the product and the mould. Moreover, the crystallisation rate in the moulded plastic increased in many cases so that the product quality significantly suffered.
In order to avoid this, an attempt was made to air-condition the production halls accordingly – but this did not present a sufficient solution for the problem, all the more due to the fact that the profit was significantly reduced due to increased operating costs. An alternative to keeping the mould surface free from condensation water was the usage of dehumidification systems, which ensure that dry air can be produced by means of adsorption dryers.
This way, a very low dew point is achieved. However, this is associated with a high maintenance and energy requirement, because the molecular sieve must be exchanged regularly. In case of a defect, however, the complicated structure of the system causes significantly higher costs. For the regeneration of the molecular sieve, an additional energy requirement is necessary, which is why this did not come into question. The MAP system turned out to be a solution to this problem.
“Both with blow moulding and injection moulding processes with cold forming, it ensures optimal protection against condensation formation on the mould surface and this way contributes to a consistently good product quality,” explains Farrag.
Ambient air for cooling
Simple (ambient) air dryers are employed in the use of MAP systems. The mould area of the machine is separated from the ambient air and supplied directly by the MAP with filtered, dry air. With this process, a continuous use of cold water up to a temperature of 6 °C is made possible without condensation on the mould surface forming.
Ambient air is drawn via a filter and cooled in two steps: Firstly, via a water-cooled heat exchanger, then in the heat exchanger of the integrated refrigeration circuit. The air is cooled to approximately 3 °C.
For pre-cooling of the ambient air during the process, cold water is used, which also serves as cooling for the moulds. The humidity, which is eliminated as a result of condensation, gathers in a trough and is removed from the device using a pump.
Thanks to the isolated machine environment, a trouble-free daily work routine is now possible, also in the summer with its higher air humidity: Often, the cold water had to be heated again in order to ensure trouble-free production. As a result, the production process took more time.
“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%,” says Farrag. “Especially with thick-walled moulds, a production increase of up to 200 per cent can be achieved.”