Raw material is fed to the extruder and is almost always the primary cost of the operation, sometimes as much as all other manufacturing costs put together. So we shouldn’t just take the suppliers’ word for everything, but just as we must pay attention to the nutritional value of what we eat, we must also pay close attention to the material/feed -- its condition, form and properties.
This presentation covers the following topics:
Some definitions: plastic, polymer, compound, resin, olefin, elastomer, vinyl, styrenic, etc.
The "vitamins" (additives): stabilizers, fillers, tougheners, colorants, viscosity aids, etc.
Q: When do we use PVC pellets or powder material?
A: Powder blends are the most economical and also offer the user the option of changing formulations. Pellets are used where quantities are too small to justify stocking and blending ingredients, and where a compound supplier has a proven recipe that works for the intended use.
Q: What factors change output of a screw as it speeds up, or over time while running?
A: Output of a single-screw machine for a given material depends on inpush (feed rate), screw dimensions (especially pumping capacity of last flights), and the resistance of everything after the screw.
This is a two-part question –
(a) If you speed up a screw (raise rpm) the output will rise as the screw is pumping out more, but the head resistance also increases as more is pushed through it. Further, the increased rpm and power draw is likely to raise melt temperature, which reduces viscosity. In general, deeper channels in the last flights make the system more pressure sensitive (more loss at higher head resistance).
(b) If the rpm stays the same, the output will slowly decline as the screens catch more and more contamination, and the head resistance rises accordingly.
Q: Is moisture cured PE (such as used in copper wire insulation) exposed to steam to speed up curing process but it will eventually "cure" without steam, or is it required to gain a full cure?
A: Water is required to cure the insulation, as a chemical cross-linking reaction is happening. Some systems will eventually cure (cross-link) from the moisture in the air, but it takes much longer; steam or hot water is faster. Here is a review cross-linking methods (pdf).
Q: If you are insulating with pvc and blended (high and low density) moisture cured PE, but are experiencing a rougher finish on the PE, yet smooth on PVC products, would the difference mostlikely be in temperatures used, the screw profile, or something else?
A: Could be the two PEs aren’t fully blended (mixing), or temperature is too low for the PE to melt and mix and react properly. By “screw profile,” you may mean screw dimensions (which will affect mixing and temperature) or you may mean the set temperatures on the barrel, which may also affect melt temperature. It is helpful to have a reliable melt thermocouple that projects into the flow (not a reading from a dual device set at the screw tip).
Q: Is curing for wire insulation simply for structural integrity of the coating?
A: It also adds heat resistance, dimensional stability and sometimes chemical resistance.
Q: We have variation and die drool problems running Estane Polyurethane.
A:
Variation (of dimensions) can come from many sources, too many to list here. Die drool depends on formulation (low-melting additives), die materials (some believe chrome-plating helps but I’m not convinced), lip edge angle, and temperature (too high and too low both may cause trouble). Remedies include additives that plate out on the inside die surfaces, also air blown at the emerging line, also management of melt temperature to find optimum.
Q: I'm guessing that round hoppers are, in general, better; how do you deal with the square feed throats on extruders?
A:
Round hoppers clean more easily, drying can be more uniform, but square ones hold more and need filling less often. Round is more common. If material always the same and there is no hopper-drying, cleaning is irrelevant so square is preferred.
Most throats (the vertical channel in the barrel wall) are round, but square or rectangular may be preferred for light/fluffy feeds.
Q: What temperatures would you use to extrude a mixture of 95% HDPE and 5% PP? A similar question is what temperature profile would you use to extrude 95% LLDPE and 5%LDPE?
A: Depends on viscosities of materials at melt/mixing conditions. If the minor component is used at 5%, its viscosity should ideally be around 5% of that for the major component at average melt temperature in the metering/mixing zone. If you have varying material or don’t know viscosities, try a few melt temps between 200 and 250 C and see what works best.
Note that I said melt temperature, not profile, as it’s melt temperature that counts, and you would use whatever profile works to gain the desired melt temperature.
Q: How can dual durometer scrap be salvaged?
A: Grind it up and reuse it by mixing with other compound to produce a salable product, adding more stabilizer if needed. If you don’t have such applications, you can probably get some money for it from a broker, especially if it’s kept clean.
Q: You said nylon in a big machine needs to be very well dried. I always experienced nylon as brittle if over dried on IM parts. Is there residual level of moisture to run flexible parts or is moisture all added during cooling?
A: The moisture has to be there at the beginning for it to do its “dirty work” of degrading (hydrolyzing) the polymer chains. I’ve heard figures of 0.1% as optimum, but wouldn’t use such a general guide. The reason I said large machines are more trouble is that the nylon is in there as a hot melt for a longer period of time. I think each operation must find the optimum drying time and temperature for the system and resin in use. (Some nylons absorb more than others, some react faster than others.)
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