Factors influencing the decrease in cooling efficiency
The rate at which mineral deposits or corrosion byproducts are deposited depends on many factors, such as the chemical composition of the cooling medium and the operating temperature. Also, the way the cooling is designed can have a significant impact on the decrease in cooling efficiency. “Dead” areas that generate zero flows will be places where the deposits precipitated from the cooling medium will naturally settle. It is important to simulate the flow of the medium at the design stage of the mold cooling, so as to avoid fundamental errors that increase the risk of a drop in cooling performance. Unfortunately, at the stage of constructing the mold, the topic of the decrease in cooling efficiency due to calcification of the system seems so distant that it is rarely taken into account.
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The purpose of the efficient cooling system of the mold is to guarantee an even distribution of temperatures on individual molding cavities, which translates into appropriate dimensional tolerances and the quality of injected parts.
Any temperature deviations caused by flow disturbances and / or the inability of the channels to effectively take away the heat will, over time, translate into the deteriorated quality of the manufactured product, the circulating medium circulating in the mold cooling system receives heat from it, ensuring an even temperature distribution, which in turn translates into appropriate quality, compliance with dimensional tolerances or cycle time.
Thermovision picture of a mold with a clogged cooling channel
Temperature rise in the second cavity
Unstable injection molding conditions
Corrosion products, limescale and other deposits on the walls of the channels are the silent killer of the efficiency of our cooling system. Scale deposits reduce the efficiency of the cooling system by reducing the its diameter. Worse still, due to its very low thermal conductivity, even a thin layer of scale acts as an insulator, making it difficult to receive the heat from the molding cavity. The scale never precipitates evenly, which can quickly lead to disturbed mold thermal behaviour and quality problems. That is why it is so important to regularly monitor the cooling efficiency in our molds and act preventively.
It should be remembered that the issue of cleaning channels and maintaining high cooling efficiency throughout the life of the tool applies not only to those made in additive technologies, but also to conventional, drilled cooling channels. The only difference is that the complex geometry of the channels and the small diameters of the channels with conformal cooling make cleaning more difficult and requires the appropriate tools and methods. This does not mean, however, that classic structures with drilled channels are free from problems related to reduced cooling efficiency or clogging. It is a very common phenomenon and, worst of all, it happens imperceptibly.
Types of sediments and scale deposits
The mineral deposits are primarily composed of calcium and magnesium carbonates. Carbonates, which are generally insoluble, are precipitated by heating water containing soluble calcium and magnesium bicarbonates. Bicarbonate is thermally unstable and will decompose to form carbonates and thus limescale when heated.
Factors influencing scale deposition:
• The higher the (temporary) water hardness, the more scale will form.
• The higher the pH (alkaline pH) of the water, the greater the tendency to scale
• The higher the temperature to which the water is heated, the more scale build-up will be present.
Different scale types found in mold cooling channels
Precipitation and scaling on channel walls increases dramatically when the water temperature exceeds 60 degrees Celsius. It also depends on the hardness of water, so in some areas the problems caused by limescale are greater than in others. To counteract limescale problems, most injection molders deploy some form of water treatment to minimize the risk of mineral-based deposits such as calcium or magnesium.
The DS2 cleaner is recommended to remove limescale from calcium and magnesium deposits.
Another type of deposit is formed during the corrosion process. These can be solid, water-insoluble deposits (such as incrustations during the microbial corrosion process) or scale – a layer of solid corrosion products or hard iron oxides. Like other deposits, they are dangerous to the ducts, restrict the flow and reduce the efficiency of heat removal.
Rust-based scale in the channels of water-cooled molds will typically have a high concentration of iron oxides / corrosion by-products. This is mainly because companies use “closed-loop” water systems where the concentration of iron oxide is up to seven times higher than in regular tap water. Another reason for the formation of deposits from the corrosion process may be the water left in the channels after the cleaning process. The dissolved oxygen in the water reacts with steel causing corrosion.
The DS1 cleaner is recommended for descaling with a high concentration of iron oxides.
About parallel connections of channels…
An example of multiclamp type parallel connection
We see this type of connections most often in bigger molds that have a large number of cooling channels. It is impractical to connect cooling to each of the channels separately, so they are usually bundled into one manifold, the purpose of which is to supply the cooling medium to the individual cooling circuits in the mold. Of course, this type of solution is not perfect. We must remember that the parallel connection of channels will typically generate various pressure drops (these drops may result from large disproportions in diameters, lengths or distances of the channels from each other, forcing them to be connected to the manifold with hoses of different length) which inevitably translate into uneven distribution of the cooling liquid to individual circuits, even if a manifold with an appropriate pressure-balancing volume is used.
An example of parallel connection of channels
This solution should be treated as a compromise, as we should always try to connect channels in such a way that the risk of uneven distribution of the coolant is as low as possible. In a situation where one of the circuits is partially or completely blocked, a parallel connection will prevent the cleaning liquid from reaching this channel, because the solution will always try to find the path of least flow resistance.
However, it should be remembered that the process of cleaning the channels is not a process of cooling. This is a common misconception that people who bridge channels for cleaning use, arguing that the mold is connected in the same way during production. If we want to effectively clean the circuits connected to each other by means of a manifold, we must use a feed pump with a much higher capacity, that is able to overcome pressure drops generated by the cooling system, while maintaining the appropriate dynamics, which will guarantee the effectiveness of cleaning in the shortest possible time. The effectiveness of devices equipped with single pumps with a higher flow rate depends primarily on the number of liters of pumped liquid in a given time, which is never the most effective solution. That is why it is worth looking for solutions using hybrid systems, such as the patented, two-stage hydromechanical cleaning process in CoolingCare machines, in which two cooperating pumps for each of the cleaning sections are used. This approach gives much more flexibility and significantly reduces cleaning time.
