Static mixers, if applied in extrusion, can improve product qualities, as well as increase through puts and diminish start-up times in some cases. This leads to higher performance, less waste and higher productivity.
Many extrusion processes cannot be operated at the maximum output rate specified by the producer. The maximum production capacity is rather determined by the quality requirements of the extrude. High output rates then lead to unacceptable variations in layer thicknesses, to uneven or rough surfaces, sink marks or other quality problems. With quality requirements constantly increasing, and with polymer processing growing more difficult, start-up and adjusting the optimum operating point takes more and more time. Both factors diminish productivity, cause expenses, and can sometimesbe quite unnerving, too.
Quality Problems Due to Temperature Differences
Extruders are supposed to melt the polymer and convey it under high pressure at a constant rate. The extruder itself should be as compact as possible as well as generate high output rates. To meet these requirements, the polymer needs to melt within very short time, with driving temperature differences at a high level. Due to the nature of the process, high temperature differences within the melt are obtained. The difference between highest and lowest melt temperature measured in the extruder outlet may well be above 15 °C. And downstream devices such as melt pumps or screen changers also generate temperature differences. The variation in temperatures within the melt may cause in many cases quality problems. This is due to the fact that a temperature gradient in the melt means flow and cooling behaviors will vary, too. The flow profile in the case of laminar plastic melt flow is parabolic - meaning the melt flows much faster in the center than in the boundary section. This leads to significant differences in melt residence times. High residence times and temperature peaks may then yield the undesired effects of polymer degrading and building of deposits.
These are the challenges the static mixer tackles.
The Effects of a Melt Blender
A well laid out melt blender is able to equalize temperature differences effectively. A melt blender by Promix Solutions AG, Winterthur, Switzerland, can diminish temperature differences by a factor of 5 to 12, subject to layout. Correspondingly, it reduces a 15 °C temperature variation over the channel cross section down to 1.25 to 3 °C. Figure 1 presents typical temperaturedistributions in the melt at the extruder outlet with and without a Promix meltblender. The melt blender's geometry decides on the question whether themaximum benefit is achieved. The Title figure shows a static mixer of the latest generation. In an extremely efficient way, this melt blender continuously exchanges central and boundary flows, thus yielding high mixing performance at short installed length. What is more, the parabolic flow profile approaching the extruder outlet is converted effectively into a plug flow.
Correspondingly, the melt discharged from the melt blender flows at perfectly constant speed and temperature. This is the basic prerequisite for optimum process control. Thanks to the process window considerably enhanced, setting process parameters is much easierand faster. Even with high extruder output rates, layer thickness tolerances, surface qualities and other quality characteristicscan be adhered to within the ranges defined.
Additionally, distribution of additives and color pigments within in the melt is far more homogeneous. This can, for example, improve the protective effect of an UV stabilizer added as a masterbatch, or may allow for lower dosage. To provide for optimum benefit of the melt blender, it should sit at the end of the process line, if possible, i.e., right in front of the die or feed block.
Good Cleaning Behavior and High Strength
When choosing a melt blender, it is crucial for the unit to have outstanding self-cleaning properties, in addition to good mixing performance. This is tomake sure the flow passes homogeneously through the entire blender, in orderto prevent deposits that will increase pressure loss and require additional cleaning. The monolithic structure of the patented SMB plus melt blender raises the bar in this respect. Extrusion trials have shown that a red colored polypropylene is 100 % removed by a subsequent colorless PP after a cleaning time of only 5-6 times the melt blender volume (Fig. 2). Following this in-line cleaning, no traces of red color could be found, not even when disassembling the blender. This optimum self-cleaning behavior prevents building of deposits and enables the change of polymer or color within a very short period of time. However, only if the blender housing is exactly tuned to the melt blender and its interfaces an optimum cleaning behavior can be achieved. It is a known fact that the devil is in the details, which is why it is recommended to purchase melt blenders as system solutions including their specially designedhousings and suitable connection adapters.
Mechanical strength is a critical aspect of melt blenders. Under extreme circumstances, e. g. start-up after a shortened heating phase, incorrect lay-out or insufficient design of melt blenders may even lead to destruction of the mixer structure, as well as of downstream components such as feed blocks, dies and calibration units. This is why melt blenders must have a robust design and must be made of corresponding high-quality materials by suitable production methods. While, in industrial practice, one always starts from the worst-case scenario, it is recommended to use melt blenders made of highly rigid materials and withvery thick-walled structures only, as can be seen in the Title figure.
In particular, when retrofitting an extrusion line with a melt blender, the spaceavailable is often limited. Highly efficient mixers with short installed lengths and low pressure loss have the edge over larger units here. In most cases, such types of mixers can be integrated very easily into the existing plant, without having to displace the extruder or cooling line. If using pressure drop-optimized melt blenders, the extruder is usually able to make up for the additional pressure drop without problems. If required, an additional melt pump is very helpful here, but, in fact, is rarely necessary.
Retrofitting extrusion lines with melt blenders is relatively easy and inexpensive. This improves not only product qualities, in many cases it also steps up outputs and reduces start-up times. The key to success is to make sure the melt blender is designed to fit the respective case of application in an optimum way. Apart from mixer geometry and size, this comprises tailored mixer housing, the method of installation as well as the robustness of the overall solution.
Plastic Injection Molding: Improved Part Quality and Considerable Cost Savings Result With Mixing Nozzles