Extruder machine degassing is one of several operations required to blend polymers with additives and fillers to achieve desired physical properties. The resulting granular compound must be free of voids and residual volatiles that would cause defects in extruded or injection molded parts.
Extruder machine compounders must be aware that the customer's thing, the single-screw extruder, usually does not have any venting capability, and any volatiles in the pellets can only be released in the final part - causing surface defects, holes, etc. For hygroscopic polymers, effective outgassing during modification is critical to maintaining physical properties.
According to process requirements, the vent hole in the extruder barrel can be operated under atmospheric pressure or vacuum, and the most typical mixing process includes two types of venting. Extruders can be equipped with multiple vacuum vents for degassing bulk solvents or monomers while producing low levels of residual volatiles in the finished product.
A variety of vent designs have been developed for co-rotating twin screw extruders to suit a wide range of applications and materials; these include vent plugs, extended vent barrels and side vent holes. Each of these designs has been developed considerably to overcome some of the problems associated with the standard vent design located at the top of the extruder barrel.
Whether atmospheric or vacuum, the requirements for stable operation of the extruder machine vent are a low filling degree in the screw in the area of the vent and a "melt seal" (filling screw) upstream of the vent.
1. exhaust problem
The extruder machine operation problem, polymer (and/or other raw material) coming out of the extruder vent. This process disturbance often requires twin-screw operator intervention to clear the vent or requires shutting down the line.
Residual volatiles/gases left in the compound pellets due to insufficient venting in the extruder barrel. In this case, increased exhaust efficiency is required to produce acceptable product quality.
Each of these problems has unique solutions, as described below.
2. Why is "something" coming out of the extruder vent?
This type of problem is common in all twin-screw compounding extruders. Extruder atmospheric vents are designed to release air, moisture and other volatile gases after the initial melting of the polymer. The velocity of air and/or steam exiting these extruders vents is a function of volumetric flow and vent area.
When the extruders steam velocity is too high (due to too much volume or too small an open area), the exhausted gas tends to entrain solids, causing the melt to flow out of the extruders vent. The solution here is to provide a larger exhaust area, and additional extruders exhaust ports may be required to achieve this.
Particulate fillers (talc, mineral fillers, CaCO3, etc.) are fed into the molten polymer from the downstream side via a vent port. These extruders vents are designed to release air entering the extruder machine through the extruder side feeder with powder. The lower the bulk density of the filler, the more air enters the extruder barrel and must escape. If the extruder vent opening area is too small (relative to the volume of air that must be expelled), the resulting high exit velocity tends to carry fines and powders out of the vent. Solving this problem requires enlarging the exhaust area, as previously mentioned, and possibly additional exhaust ports.
The presence of unmelted polymer at the first extruder side feeder also causes filler to exit the atmospheric extruder vent associated with that side feeder. To diagnose if this is related to the vent, a physical inspection of the melt quality at the side feed must be done to verify that there is no unmelted resin present. If this is the case, the solution requires modification of the screw design in the upstream part of the extruder screw (where the polymer is melted).
Optional extruder vent designs are available to meet specific degassing requirements. The heated side vent is designed to prevent contamination (eg black spots) that can occur when polymer builds up and degrades within the vent.
Extruder vacuum vents are usually located near the prilling die and are designed to release any residual vapors under extruder vacuum. Melt will flow out of the extruder vacuum vent and block the vent, which seems to be a very common problem. When this happens, the gas is not removed from the melt and the resulting particles become porous with voids etc. Clogged vents require operators to manually clear the vents, which in some cases may require shutting down the line.
There are several possible causes and solutions for this problem: The melt may reach the extruder vacuum vent directly due to excess spare screw length (the length of the filling screw required to generate the required pressure).
As extruder screen or die pressure increases (for example, when the screen becomes clogged with contaminants), the reserve length increases accordingly until it reaches the extruder vent in the extruder barrel. Melt flows continuously from the vent even without vacuum. The solution to this problem is to reduce the pressure (e.g. increase the screen area or the number/size of die holes), increase by moving the extruder vacuum vent one barrel upstream (or add a barrel after the extruder vacuum vent) The pumping length of the extruder, or install a melt pump to pressurize the downstream equipment. Note that most of these solutions are capital intensive (expensive).
The same happens when the pumping screw elements at the end of the twin-screw extruder machine wear out (to the spare length of the vacuum vent). In the case of a worn screw, melt will flow out of the extruder vent more frequently over time, eventually becoming a chronic condition. The solution to worn threaded elements is simple - replace worn elements.
If the supportable pressure of the melt sealing element is less than the extruder vacuum pressure, the melt will also flow out from the extruder vacuum vent. This condition causes the melt to be "pulled" out of the extruder as the vacuum pump draws air through the extruder barrel. Since there is no pressure sensor installed on the extruder barrel, the only clue that this is the cause of the problem is to look at the vacuum gauge. If the pressure gauge stabilizes over time, the extruder vacuum system is "tight tight". If you can see the gauge drop, air is being drawn through the system (the exhaust will be full of molten plastic at this point). If melt flows out of the vacuum port only when extruder vacuum is applied, this indicates that the vacuum seal is not generating enough pressure.
Some specialty materials exhibit unique properties when exposed to high temperature and vacuum. These materials tend to expand and foam when they reach the extruder barrel opening and do not easily flow back into the extruder screw. When using conventional open vents, these materials are exhausted from the machine under all conditions. These compounds can only be processed under vacuum using vented fillers: the mechanical twin-screw system prevents the melt from expanding beyond the extruder screw channels, but allows the gas to move axially through the screw with vented fillers installed in the extruder vents.
Side-fed low bulk density fillers at high fill (up to 80%) and high production rates require multiple atmospheric vents to facilitate the removal of large volumes of air from the extruder barrel.
3. Why can't the things that should be discharged come out?
These solutions often require modification of the extruder screw design and/or barrel configuration if the current machine configuration, vent design, and operating conditions cannot remove sufficient quantities of volatiles.
The removal of volatile gases from the melt is accomplished by vacuum, which provides an additional driving force for mass transfer. Most compounding lines operate at vacuum pressures in the range of 20 to 27 inches of mercury (approximately 100-300 mPa). There are many solutions available to further improve devolatilization:
• If the residence time of the melt under vacuum is the limiting factor (diffusion limited), one option is to reduce throughput to increase the average residence time. This is not a popular option, but the easiest to implement.
• Another solution to increasing residence time requires repositioning the melt seal element further upstream in the extruder barrel. The vacuum section therefore extends further upstream.
• Increase extruder vacuum to maximum–this usually requires replacing the existing extruder vacuum pumping system.
• Reduce extruder vacuum section filling by increasing conveying element spacing. A lower packing in the extruder screw channel creates a thinner melt layer through which the gas must diffuse.
• Reconfigure the machine with additional extruder vacuum vents. This may require extending the current screw length to add an additional extruder vacuum exhaust port (if the L/D of the existing screw is insufficient to accommodate a second vacuum exhaust cartridge).
• Inject a stripping agent (usually water) into the barrel upstream of the extruder vacuum vent to reduce the partial pressure of volatile species.
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