Myth and reality in the most popular extrusion

2022-08-22
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Myth and reality in extrusion

the key to understanding extrusion is quantification. You have to ask: how many? What is the actual physical size? What is the actual duration

energy is expensive

it takes a certain amount of heat to melt the plastic and raise it to the point where the spraying worker no longer needs to stir the resin material or wait for the liquid to rise and extrude at the temperature. For example, it takes 0.16kwh/kg to raise LDPE from 20 ℃ to 220 ℃. Although it doesn't seem to cost much, one thing is often ignored - how much heat loss is in this process? What is the ineffective effect

extruders are effective machines because they are closed and most of the heat is generated inside the barrel by friction. Except for small extruders, kruder & Nunn's research shows that generally speaking, the effective coefficient of extruders is 50%. For example, in theory, we need 0.7 cents/lb, but in actual production, we need 1.4 dollars/lb - of course, this is for semi crystalline polymers, because its processing requires more heat than amorphous polymers. Generally speaking, the theoretical value is 0.03kw · h/LB for UPVC (20 ℃ ~ 180 ℃) and 0.04 kW · h/lb for hips (20 ℃ ~ 210 ℃), and the cost is 0.6 cents/lb and 0.8 cents/lb respectively

of course, on the other hand, half of the energy or cost is definitely needed to eliminate the impact of irregular samples on the sensor, so how can the rest be saved? 2. It is suggested to establish a petrochemical free trade Experimental Zone on the east island of Zhanjiang? Moreover, is it worth saving it - compared with the efforts to save materials by reusing waste products or better thickness control, or by selling more products? By the way, it is worth noting that PVC, a common goal of environmentalists, is the best of all plastic products in terms of energy conservation

the quality of waste products/regrinding agents is worse than that of raw materials

this is a subtle statement, because most waste products are indeed of poor quality. Processors have added many charges to waste products, and of course some should be. Even some processors have been prosecuted for using waste products in their products. But quality does need a more precise definition - waste products (trimming materials or short gauge materials) are usually slightly soiled and more polluted by the treatment process, but in many applications, these do not matter, especially from the perspective of cost savings

aggressive filtration can filter out most pollutants, and adding antioxidant or stabilizer concentrate can compensate for molecular decay. For some polymers, especially HDPE, if the chain cross-linking reaction speed is faster than the chain fracture and there is enough anti oxygen set left to protect the substance, the reprocessed resin may actually be stronger than the original resin

the quality of waste products largely depends on how to deal with waste products and whether they are separated, covered and clean

correct screw design is very important

there is no doubt that screw design is important, but it is only one of many solutions, and may be more expensive. Before spending a lot of money to replace the screw, make sure you understand the problem and confirm that you can't solve the problem by changing the conditions or processing aids

even if a new screw may be used, be sure that the new screw is designed to run the desired material at the speed you want. For example, it is not enough to buy a screw for polypropylene, which can almost perfectly realize the closed-loop control of experimental force, deformation, displacement and other parameters. Screw designers should know the level and processing aids, or have a group of data of display function for reference. In this computerized world, simulation technology is easy and should simulate all screws (except the smallest screw), unless relevant data of the same material applicable to the actual situation can be obtained

screw wear is related to screw design. A worn screw is not necessarily a bad screw. Sometimes wear is even beneficial to mixing, or when there is a pressure peak, it is beneficial to increase production. It is useful to determine the location and cause of wear. However, what is actually needed is to prove that wear is causing trouble, which may be accomplished by reducing the capacity of the pump, which may mean that a higher melt temperature is required for a given input. Tracking the output of a known material can show whether the pump displacement of a screw is really declining

don't pay too much attention to appearance

here, appearance means the relationship between can body temperature: flat appearance makes all set values equal; The rising appearance includes aftercooler and front heater; Reverse appearance has reverse side; The middle temperature of camel like appearance is the highest; The middle temperature of Valley appearance is the lowest. Appearance is acceptable, but paying too much attention to appearance will distract attention from specific numerical settings and hinder thinking about each area of itself

for zoning thinking, a good example is to control the adhesion and slippage of the barrel wall by managing the post barrel temperature, so as to control the transportation of solids. Too much adhesion may overload the metering area and cause high pressure here. The subsequent excess temperature and too small adhesion will reduce the feed volume, which requires a higher speed for a given amount of flux, and may lead to excessive melt temperature

here is an example of focusing on shape and hindering clear thinking: a film extruder runs LDPE with a 450 ° f flat forming tool. A high flow pigment concentrate adheres to the narrow part or the root of the screw (it is not clear which one is), and the pigment will fall off with irregular intervals to form a large number of pigment particles in the product. In this regard, it is difficult to convince the processors that only the rear area needs to be reduced. Generally speaking, for a modeling machine professional, if one temperature is reduced, then all temperatures should be reduced

the faster the better

who will object to increasing production? Generally, it is desirable to operate at a reasonable speed, but there are practical constraints - if the thickness variable is increased at a faster speed, its cost (too thick will waste material, and too thin will have the risk of fracture) may be greater than the benefit brought by speed. Sometimes an extruder runs faster, but it is restricted by the puller speed, printing, sealing, forming and even inspection and packaging conditions, and you must answer the question: can you sell the increased output? If it can, the efforts and costs of faster operation may be repaid in the increased total income; If not, then running at 500lb/h for 4H has no advantage over running at 400lb/h. (end)

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