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Thorough cleaning, the fountain of youth for tools

The second essential and easy--to-place pillar that contributes to the reliable operation of injection moulding tools, as well as the production of quality parts, is preventive maintenance, which should always be incumbent upon the specialist department. The underlying measures, according to experts, should be timed tool-dependently and can vary depending on how complicated the manufacturing system is. All moving components like sliders or ejectors must be cleaned and conserved, this also applies to hot runners, if applicable. Some of the manufacturing parameters should be analysed as part of the preventive maintenance. The tool’s condition can be concluded based on the injection pressure and hydraulic pressure values, as well as the cooling medium's flow rate or individual hot runner control circuits.

Stefan Oberlies, authorised officer at Gesellschaft Wärme Kältetechnik (gwk), emphasised the importance of caring for the temperature control system as part of tool maintenance. Mr Oberlies pointed out: “Those who chance the calcification or corrosion of their cooling ducts when processing plastics risk a loss of competitiveness.” The tool, according to Mr Oberlies, is technically a specifically engineered heat exchanger. The effectiveness of the heat transfer is dictated by the condition of the cooling system, the coolant, as well as the placement of cooling ducts to the cavity. The latter cannot be influenced since they are part of the machine's construction. However, the production quality is affected if corrosion, sediments or foreign bodies impair the heat transfer. Fortunately, this can be prevented.

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Powerful micro motor system for tool and mould making

Neglected variable, coolant

Mr Oberlies stated that the coolant used, notably water, affects the heat exchange, but also the cycle period and production quality greatly. Water quality, in particular, does not receive enough attention. Ions that lead to hardened tap water can cause deposits or abet corrosion. According to Mr Oberlies, heat transfer efficiency, caused by unsuitable water, can decrease substantially over time; this leads to a fraction of the performance compared to new tools. In the worst-case scenario, tools might be destroyed irreparably by these deposits.

The issue was further illustrated with the help of a practical example and empirical data: The used tool is designed to produce a polypropylene part with 2-mm strong walls. A theoretical cooldown time of 10.14 seconds was calculated if the water has an inlet temperature of 15 °C and the tool wall is at approximately 40 °C, caused by the warm melt after the shot. As explained by Mr Oberlies, a lime-scale layer of around 1 mm formed and thermal conductivity decreased from 33.5 W/(m⋅K) to 13.5 W/(m⋅K). A 2 mm limescale layer decreased it to 8.5 W/(m⋅K). At the same time, the tool wall temperature increased to nearly 60 °C and at the end to more than 65 °C. The cooldown period increased initially to 16.6 seconds, the 2-mm deposit increased the time to 28.8 seconds, said Mr Oberlies. A 1-mm thick limescale layer increased the cooldown period by 30%, assuming a cooling time proportion of 70% for the entire cycle. Mr Oberlies pointed out that water temperatures of 40 °C cause the dissolved lime to turn into calcium carbonate and carbon dioxide. The carbonate blocks the channels. The carbon dioxide attacks the steel and dissolves ions; this leads to rough inner walls which, in turn, facilitates the accumulation of limestone. Mr Oberlies calculated the additional costs caused by a 1-mm sedimentation, such as the use of a second machine to reach the target figures, to be 52,000 euro per annum, assuming three shifts are working, and the cost for running the tool is 25 euro an hour. The utilisation of the second machine is not even included in this calculation, said Mr Oberlies.

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