A processors is always ready to grasp new knowledge to reduce cost of production.
It is important to minimize barrel cooling to save energy and avoid resin degradation. Barrel cooling occurs when the extruder screw generates more heat than necessary; this is sometimes referred to as a hyperactive screw. In most cases, it indicates that the screw is not properly designed for the plastic it is running. Either the screw is too shallow or it has restrictive elements that choke the forward flow of plastic.
Poor screw design is a common cause of overheating the plastic and is usually the reason barrel cooling is needed. Operations where multiple extruder zones are cooled at 100% are inherently inefficient. The most energy-efficient extrusion operations are those where most of the heat is supplied by the extruder screw, with only a small amount of heating by the barrel and die heaters and without any need for cooling of the extruder. Also, the extruder should run at or close to full capacity.
The best solution to overheating by poor screw design is to change the screw geometry. If an extruder with poor screw design runs at 0.25 kWh/kg and with a good screw design at 0.20 kWh/kg, the energy savings will be $36,000/yr at $0.10/kWh and a throughput of 1000 kg/hr (2200 lb/hr) running 24 hr/day and 300 days/yr.
This example indicates that a well-designed extruder screw can pay for itself relatively quickly, just based on reduced energy cost. In addition, there will be benefits such as lower melt temperatures, less degradation of the plastic, better physical properties in the extruded product, better product appearance and consistency, etc.
In many extrusion operations the die is fully exposed to the ambient air. Obviously, this will cause heat losses. In addition, it will cause non-uniform temperatures in the die. These, in turn, will cause non-uniform flow of the molten plastic, which will affect product dimensions and appearance.
For these reasons it is good practice to wrap a heat-insulating blanket around the die or to mount heat insulation against the die. The cost of insulating the die is quite low and the benefits are substantial. It should be done in every extrusion operation. The same is true for transfer lines.
Energy use in extrusion is related directly to the melt discharge temperature. Energy use increases with melt temperature. It is important, therefore, to run at the lowest possible melt temperature. Low melt temperatures will also reduce the need for cooling. In processes that are limited by cooling, such as thick-walled pipe, reduced need for cooling will translate directly into increased line speed and production rate.
In many extrusion operations the plastic is dried before processing to remove moisture. This is particularly important in plastics that undergo hydrolysis, like PET. If the plastic is dried at elevated temperature, the plastic should be fed to the extruder at elevated temperature. The energy use of the extruder is directly linked to the temperature of the plastic entering the extruder. Increased temperature of the feedstock will reduce energy consumption by the extruder.
If the plastic is predried and allowed to cool back down to room temperature, the energy required to heat the plastic is wasted. For instance, if a plastic is dried at 80 C and extruded at 200 C, the specific energy requirement for the extruder can be reduced from 0.20 kWh down to 0.15 kWh, a 25% reduction. Therefore, if the plastic is dried at elevated temperature it is important to maintain this temperature and feed the plastic to the extruder at this elevated temperature.
Many extrusion operations are rate-limited by the melting capacity of the extruder. Preheating the plastic will increase that melting capacity. This means that in many cases higher throughput rates can be achieved. Also, with higher melting capacity it is less likely that melting-related instabilities occur in the extrusion process.
The screw is the heart of the extruder and a key component of any film extrusion system. The screw melts the polymer, prepares the homogeneous melt and pumps the melt through the die. Special attention needs to be taken into account while designing screws for the film extrusion process, particularly for multilayer barrier film extrusion, due to both the complexity of the process and the complex rheology of the materials being processed. The extruder is a key component of any extrusion process.
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