Factors in the Economics of Extruder Screw Design

For any technical problem, it is entirely wrong to ignore the economics of the design. In certain cases, a more comprehensive analysis is required—that is, the rationality of the screw design must also be evaluated from an economic perspective.

The economics of extruder screw design are related to the entire extruder and mainly involve the following aspects:

1. Energy Consumption
   If the specific energy consumption drops from 0.25 kWh/kg to 0.15 kWh/kg, then for an extruder with a throughput of 100 kg per hour, operating 300 days per year, the annual electrical energy saved per extruder is:

E = (0.25 – 0.15) × 100 × 24 × 300 = 72,000 kWh

Assuming an electricity cost of 0.088 yuan per kWh, the cost saved on this item alone amounts to as much as 5,976 yuan. Considering that the price of a medium-sized extruder is merely tens of thousands of yuan (at the then prevailing price), this is clearly a considerable saving.

Energy loss is not only related to the specific energy consumption figure; certain unreasonable designs often lead to even greater energy losses.

For example, if the screw speed is too high, the output may be very high, but the power consumption will inevitably be substantial. If the downstream cooling equipment cannot keep up, the potential of the screw cannot be fully utilized, and the screw speed must be reduced. As a result, the motor designed for high speed and high productivity will operate far below its rated load. Running the motor at low speed results in very low efficiency and extremely poor economics. From this point of view, such a screw design philosophy is clearly far from comprehensive.

2. Service Life of the Screw and Barrel System
   If the corrosive effects of the plastic are disregarded and the machining accuracy is assumed to meet specifications, the service life is obviously directly related to the screw speed and the melt pressure. Wear will inevitably be more severe in areas with higher melt pressure. In particular, in the feed section, where the plastic is still in a solid state, if high pressure develops there, rapid wear of both the screw and the barrel at that location will occur. This explains why, when glass-fiber-reinforced plastic is fed directly into the hopper, the glass fiber inlet port should be designed in the middle section of the barrel.

3. Manufacturability
   In view of international development trends, there is a gradual tendency toward increasing the screw length-to-diameter ratio (L/D ratio). With the advent of new screw designs, screw geometry has become increasingly complex. Therefore, from the standpoint of design rationality, when a large L/D ratio is not necessary, one should choose as small an L/D ratio as possible; when a complex screw configuration is not required, the screw structure should be simplified as much as possible. Arbitrarily selecting complex mixing elements without comparison or optimization is obviously unreasonable design practice, and indiscriminately increasing the L/D ratio is also clearly unscientific.

Given the importance of defining clear performance criteria for screws, we have systematically summarized this issue. Several key topics within it, such as output fluctuation, wear, and service life, will be discussed in future articles.