A manufacturer was producing black masterbatch, 40% carbon black, using EVA wax as a dispersant. The material is pre-mixed by a high-speed mixer and then added to the hopper. The temperature is 160°C, 165°C, 170°C, 180°C, and the machine head is 175°C. As a result, some of the strips were very brittle, and some were very strong. It was judged that the carbon black was unevenly dispersed.
Problems:
① The material in the hopper does not go down, and the strips often break. The workers think there is no material in the hopper, but when feeding, they find that there is still a lot in the hopper, and they need to manually push it. This problem does not occur when the carbon black is 20% or 30%.
② The speed of the outlet is uneven, and the middle two outlets are faster.
③ There is no detection method on-site, and it is unclear how to judge the masterbatch quality easily.
Solutions:
① Increase the amount of EVA wax
② Lower the temperature by 10°C
③ Reduce or increase rotating speed
The hopper is a part for storing plastic raw materials. The shape of the hopper is generally conical at the bottom and cylindrical at the top. The cone slope of the cone has different optimal values for plastic parts with different pellet sizes, different pellet shapes, and different friction coefficients and bonding coefficients between particles. Otherwise, either the storage capacity of the feeding hopper is wasted, or the phenomenon of “bridging” or “funnel forming a tube” occurs when the feeding is not smooth or the feeding is not done at all.
Definition of Fluidize phenomenon:
Too rapid a change in the diameter of the hopper near the feeder of the extruder can cause the material to form a compacted solid that prevents the material from entering the extruder.
The fluidized phenomenon is a nightmare for many production process designers and production managers.
Causes of fluidize phenomenon:
Plastic pellets form an open bridge at the small mouth of the cone that can support the material above it, which is easier for recycled materials with larger pellets and irregular shapes.
“Funneling into a tube” is because the pellets flowing down are not strong enough to pull their adjacent pellets to flow together, which often happens when the plastic pellet size is small.
Especially when the filler itself is very light or the proportion of filler is very large, to eliminate or minimize the possibility of bridging phenomenon, it is necessary to separate the mixed air before the powder enters the twin-screw extruder. Reduce the vertical height between the feeding port and the screw as much as possible, and the powder should enter the bottom of the screw instead of being added directly to the top of the screw. In addition, the screw structure should ensure that the air moves along the screw groove and is discharged in the exhaust section, the gas should not flow back to the feeding port for discharge.
The general solution is to install a vibration device on the hopper or reduce the cone slope. If the heat from the barrel is transferred to the hopper and the temperature of the hopper is too high, the surface of the plastic pellets will soften or stick together into blocks, which is more likely to form bridges or blockages.
Adding too much calcium powder will also cause cutting difficulties. There are two main reasons. One is that the calcium powder itself is difficult to flow, and the other is that the external lubricant added for processing will also hinder the flow of dry powder. The solution is to pay attention to the mixed use of light calcium and heavy calcium. Generally, heavy calcium is easier to feed than light calcium; at the same time, the method of adding a vibrator to the lower hopper can also be used; in terms of formula, it is necessary to add more internal lubricants. Among these methods, adding a vibrator is the most effective.
Since most of the conveying of the extruder is conveyed by friction and dragging, the temperature in the feeding area is generally controlled between 120-160°C. If the temperature in this area is too low, the resin will not be able to adhere to the inner wall of the barrel, but will just idle with the screw rotation, resulting in a lack of material in the melting area; if the temperature in this area is too high, the material will be melted and plasticized in advance, A ring-shaped molten film is formed on the inner wall of the barrel to lubricate and also cause a material shortage.
There is a large internal friction force and relatively high viscosity between thermoplastic elastomer particles, so it is easy to be short of material during processing, which will cause fluctuations in production output and degradation of materials. Generally, it is solved by increasing the aspect ratio of the feeding section, forced cooling, and vibrating feeding.
There are also some elastomers that have been processed once or more times, and there are more or fewer ingredients such as filler oil, filler, and resin in these raw materials. Although the internal friction and viscosity are not as large as the raw materials, the presence of oil makes the feeding of materials difficult, and it is generally not recommended to crush such materials too small. At the same time, it is also necessary to increase the friction coefficient of the flight of the screw in the feeding section.
Melt granulation of toner mixed with resin requires attention to the pre-dispersion of the toner (“rubbing”). Whether a toner is dispersed well in post-processing depends largely on the dispersant and pre-dispersion in the process of configuring the toner. If a toner is not pre-dispersed, the base toners that make up the toner will not mix evenly during the melt-mixing process, resulting in different areas of the material (“colors”). Therefore, if the blending manufacturer finds that the toner you use has a pattern in the production process, it means that the toner supply steel frame may not be very professional.
The color masterbatch has been pre-dispersed and mixed with the carrier, theoretically there is no poor dispersion. But one thing to note is that the same carrier with different colors will have different performances during processing. If the MFR of the masterbatch is too different from the MFR of the blended resin, it is entirely possible to cause color mixing. At the same time, if the natural color of the masterbatch is too flexible and cannot be crushed into fine masterbatch chips as soon as possible during the mixing process, even if the MFR match between the masterbatch and the resin is good, it may lead to the failure of the final product.
Problems:
① Make the material dry, if there is wet material, it will melt at the material mouth first, but the melting is not complete, so it will be squeezed there, and the dried material is easier to enter than the wet material.
② Reduce the temperature of the material inlet, even very low, such as 100 ℃, so that the material is not melted first, but is directly squeezed in, relying on the power of the screw instead of heat. The disadvantage of this method is that the material is made for a long time, and the damage to the screw is relatively large.
③ Enlarge the material inlet. If you have been making this kind of material for a long time, you can cut the material inlet bigger. Of course, a large optical cut does not mean that more materials can be fed, a mixing device can be installed, and you can refer to the production equipment of PS foam material.
④ Replace the disc of the motor, and increase the diameter of the rotating disc, so that the screw speed will be accelerated, and the material will be stirred in before it is dissolved. The disadvantage is that the screw is easy to stir, so you need to be more cautious. After the speed becomes faster, you need to increase the temperature in the heating zone behind, otherwise the screw will bear too much force.
Content source: Internet; Liansu.com; organized by Xinda
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