Automotive air conditioning compressors utilizing aluminum scrolls offer improvements in engine performance and fuel economy by lowering energy consumption, reducing weight, and allowing smaller packaging. This alternative compressor design was first commercially produced in 1981. Since this time, scroll compressors have continually increased their share of the original equipment compressor market for Japanese, European, and US automotive manufacturers. Two essential components in the compressor assembly are the aluminum scroll castings (fixed and orbiting).
The first production aluminum scrolls were machined from sand castings. This process was then replaced by squeeze casting, which has now been in use for over 12 years. Forging has recently emerged as an alternative process. The design and structural requirements of the aluminum scroll component challenge both squeeze casting and forging processes. Scrolls are a unique combination of thin and thick sections with deep low draft walls requiring minimum machining stock for optimum machinability. Over 90% of the scroll surfaces are machined. In some areas, the remaining stock is less than 2 millimeters wide and over 30 millimeters tall. Operating conditions for the scroll compressor require the aluminum scroll component to withstand the very high temperatures and pressures of the discharge gases. These requirements have established new standards for dimensional capability, internal integrity, and material properties (selection and control) in the high volume automotive aluminum casting market.
This paper will first describe the basic design and operational benefits of scroll compressors over traditional piston types. The discussion will include operational differences between the compressors and the effect on manufacturing/assembly tolerances. Secondly, the paper will identify the key design, material, and quality requirements of the aluminum scroll. The discussion will focus on how machining and operational requirements drive design requirements such as machining stock, draft, dimensional control, and internal integrity. Finally, a comparative summary will explain the difference in processes and materials used in the manufacture of aluminum scrolls. Benefits and potential issues for squeeze castings and forgings will be discussed relative to manufacturing, processing, and compressor performance/reliability testing.