When manufacturing fuel tanks for the automotive industry, the multi-layer or co-extrusion process “COEX” is mostly used these days: 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 and adjusted to mould contours due to interior pressure. In order to cure the tank, it has so far been cooled only by means of cold water in the cavity of the tool mould. However, the process step was very cost-intensive and time-consuming, because low cold water temperature led to formation of condensation on the mould surface, which in turn resulted in unsteady product quality and increased scrap. Moreover, due to the continuing heat exposure, the EVOH-layer in the tank was damaged and thus the functional capability limited.
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Simple (ambient) air dryers are employed for the use of MAP systems. The system functions in that the mould area of the machine is separated from the ambient air and supplied directly by the MAP with filtered, dry air. In this way, a continuous use of cold water up to a temperature of 6 °C is made possible without condensation on the mould surface forming. |
This is where the Mould Area Protection (MAP) system of FarragTech GmbH comes in: Due to the supply of dry air, the mould is prevented from sweating. For further increase of output, the Internal Air Cooling System (IACS) from FarragTech is also suitable: During the process, in addition to conventional cooling, the tank is internally flushed with -35 °C cold air, whereby heat in the plastic is simultaneously taken away inside and outside, with material stress being prevented. Thus during production, time and money can be effectively saved.
“Cooling of 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 the 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 due to heat exposure.” During the process, 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. These temperature differences so far often led to significant material stress - above all, because the large-scale tanks are complexly shaped, showing so different wall thicknesses in the range of 1.35 mm and 3.80 mm. Until recently, this was counteracted by means of interval blowing. But this process was less effective. As a rule, the results were unsteady product quality as well as failure to pass the subsequently carried out tightness tests, load tests as well as drop tests. By reason 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.
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Ambient air is sucked via a filter and cooled in two steps: first via a water-cooled heat exchanger, then in the heat exchanger of the integrated refrigeration circuit. |
Significant increase of productivity due to cooling from the inside and outside
The BAC is available in a total of five different construction sizes, with the design always being linked to the air throughput for the specific application. In direct comparison to cooling by means of ram air, an increase of productivity from 25 to 200 per cent can be achieved by using the BAC. 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 currently a standard already.
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 the condensation water does not drop into the production hall. Control of the BAC devices takes place via the specially developed Farrag Intelligent Terminal (FIT). In this way 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.
Combination with condensation water protection for optimal results
An unwanted side effect of the mould cooling using water, the temperature of which is below the dew point on the ambient air, was the formation of condensation water on the mould, which negatively influences both the product and the mould. Moreover, the crystallization rate in the moulded plastic increases in many cases so that the product quality significantly suffers. In order to avoid that, it was tried to air-condition the production halls accordingly - but this did not represent a sufficient solution for this problem, the more so as the profit was significantly reduced due to the increased operating costs. An alternative to keep 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. By means of this, a very low dew point is achieved. However, this is connected with a high maintenance and energy requirement, because the molecular sieve must be regularly exchanged. 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, too. What provided a relief here is the MAP system: “Both with blow moulding and injection moulding processes with cold forming, it ensures optimal protection against condensate formation on the mould surface and this way contributes to a consistently good product quality,” explains Farrag.
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Temperature differences so far often led to significant material stress - above all, because the large-scale tanks are complexly shape |
Simple (ambient) air dryers are employed for the use of MAP systems. The system functions in that the mould area of the machine is separated from the ambient air and supplied directly by the MAP with filtered, dry air. In this way, 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 sucked via a filter and cooled in two steps: first via a water-cooled heat exchanger, then in the heat exchanger of the integrated refrigeration circuit. In this, 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. The humidity, which is eliminated as a result of condensation, gathers in a trough and is conveyed out of 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 a 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 per cent,” says Farrag. “Especially with thick-walled moulds, a production increase of up to 200 per cent can be achieved.”
