Mixing Nozzles. It is a well-known fact that problems with color streaks in the injection molding process can be rectified through the correct use of static mixers - so-called mixing nozzles. What is less well known is that it is possible to solve a large number of other quality problems in this way, in addition to boosting reliability and reducing production costs.
Over the past few years, the screw geometry and electronic controls of injection molding machines have been increasingly refined so as to en- sure that plasticization and the incorporation of masterbatch and other additives can be performed both efficiently and rapidly. Since injection molding is a dis- continuous process, however, the melt remains in the cylinder for different periods of time. To keep the melting process as short as possible, it is also necessary to heat the cylinder from the outside, employing high temperatures.
These two effects mean that the melt displays different temperatures over both its cross-section and the injection stroke. This, in turn, leads to different viscosities and flow rates. It is precisely here that mixing nozzles come into their own. The static mixer integrated in the injection nozzle homogenizes the temperature, viscosity and flow rate over the entire injection cycle.
Many of the quality problems encountered in injection molding are directly associated with the different temperature, viscosity and flow rate. By achieving a better distribution of the temperature and flow rate, it is possible to considerably improve the dimensional tolerances and avoid surface effects such as matter glossy areas. Weak points in the part that are caused by flow lines can be greatly reduced, as is shown by the example of the outlet zone of a cartridge (Fig. 3). It proved possible to eliminate this shortcoming through the use of a mixing nozzle (manufacturer: Promix Solutions AG, Winterthur, Switzerland) and increase the strength of the part at the trouble spot.
Savings on Masterbatch Costs and Increased Energy Efficiency
It is well known that static mixers can provide an effective remedy for problems with color streaks on account of the better mixing action they permit. Employing a mixing nozzle in the production of closure caps, it proved possible to considerably reduce the amount of color streaking that started at the injection point. Static mixers are not just useful when it comes to solving problems, though. Due to the bigger process window that results, they can further optimize the injection molding process and, in a large number of cases, contribute towards considerably reducing costs.
A growing number of processors are now switching to direct coloring with masterbatch or liquid coloring agents in order to save on raw material costs and make for more flexible production. It is not, however, always possible to achieve constant color shades with direct coloring, since process conditions such as the temperature, back pressure or screw speed have an influence on the color shades, and uniform metering of the coloring agent has to be guaranteed during the process. Precise metering of the masterbatch in combination with a mixing nozzle is the solution required here. The mixing nozzle ensures that identical color shades are obtained - irrespective of the type of injection molding machine and its mode of operation. Depending on the particular color additive and the requirements, this process can additionally reduce the amount of masterbatch and liquid coloring agent required by 15 to 25 %, since the color pigment agglomerates are efficiently broken up and distributed in a homogeneous manner.
Quite often the melt temperature can also be reduced typically by 10 °C as a function of the part and the material, through the more balanced temperature control. This considerably boosts the energy efficiency and additionally shortens the cooling time, which dominates the cycle in many cases. Hence a leading company was able to reduce the cycle time for the production of a 2.5 kg PP container from 120 to 110 s by reducing the melt temperature.
Mixing Screw versus Static Mixer
Problems with color streaks can, in some cases, also be alleviated by increasing the back pressure and the quantity of master- batch. It goes without saying, however, that this approach is not an economical one, since it leads to higher masterbatch costs and, depending on the polymer type, to a reduced service life for the plas- ticizing screw. Is it not possible to solve color-mixing problems by employing a mixing screw? Basically, yes - but this also has considerable drawbacks compared with the use of a correctly configured static mixer.
The mixing quality is then conditioned by the screw speed and the back pressure. Color agglomerates can only be broken up to a limited extent in a mixing screw, since, although the shear input can be very high at certain points, it is very un- even by comparison to a static mixer anddo the size of the injection molding ma- chine and the clamping force play a role but also the molded part involved and the plastic used. An incorrectly configured static mixer will either only partially re- solve the problem or not resolve it at all. An error of this type can also cause a sig- nificant increase in the pressure loss or lead to decomposition of the polymer. A correctly configured mixing nozzle should not normally cause an additional pressure loss of more than 10 to 15 % of the injection pressure.
The geometry of the mixer also affects the processing result, and this will only be satisfactory if the mixer operates highlyefficiently over the entire cross-section for its very short installation length. In many cases, an inappropriate mixer geometry or incorrect configuration is the reason for a disappointing result, which is then wrongly interpreted as being a fundamental problem of static mixer technology. Key criteria for practical use are a minimal pressure loss and efficient self- cleaning behavior.
Pay Attention to Cleaning Behavior and Material Quality
The widely held opinion that static mixers tend to accumulate material deposits and are essentially difficult to clean has long since been refuted. In fact, quite the opposite holds true, even though big differences are seen here too, these being due to the mixer geometry, on the one hand, and to the housing tolerance and the way in which the housing is installed on the other. A static mixer does not fundamentally need to be dismantled to be cleaned. Ideally, it should be cleaned inline by purging it with additional polymer. Cleaning trials with a Promix mixing nozzle have been documented which show that a cleaning volume of just three times the mixer volume is sufficient to fully clean the mixer. This is much less than is needed to clean an empty pipe or a plasticizing screw, for instance.
The mechanical load acting on a static mixer in the injection molding process is enormous. During the injection phase, the mixer has to withstand constantly alternating loads of up to several hundred bars. This is only possible if the material, geometry and layout are all coordinated. Particular importance must therefore be attached to careful, continuous quality assurance. No spare parts are required with high-quality mixers, the right layout and correct operation. Even when abrasive fillers are used, evidence has been acquired of service lives of several years.
Static mixer solutions improve process dependability in injection molding, reduce the amount of masterbatch used and optimize the injection molding process in overall terms. They permit many quality problems to be solved simply, cheaply and rapidly. This does, however, require the correct layout and a mixer geometry and mixing nozzle configuration that is suit- ably tailored to the application in question.
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