ABSTRACT
Aluminum molds are frequently used in the plastics manufacturing industry. Relative to steels, aluminum alloys offer such benefits as lower weight, improved machining and polishing, and reduced cycle time due to aluminum’s significantly higher thermal conductivity. However, corrosion within cooling water channels in aluminum alloy molds is a common challenge. Exfoliation and intergranular corrosion are the main types of attack observed. Copper-bearing alloys, such as those in the 7XXX series, are commonly used. Those alloys are particularly susceptible to exfoliation corrosion, though such corrosion can occur in other alloys as well. Factors that promote corrosion of aluminum alloys will be discussed, including microstructural features, water chemistry, and operational practices. Case histories will be presented that illustrate the exfoliation corrosion mechanism and the problems that occur in plastics manufacturing systems. Methods for control of corrosion in these systems will be proposed.
INTRODUCTION
Aluminum offers numerous benefits compared to steel for the manufacture of molds in the plastics manufacturing industry. Fabrication of molds and tools is easier in many cases because of better machinability and polishing of some high-strength aluminum alloys. The thermal heat transfer can be 4-5 times better than steel. This results in improved cooling rates and reduced cycle time for producing plastic components. The significantly lower weight of aluminum reduces power consumption and wear and tear on equipment and personnel. However, the service life of a mold needs to be considered when using aluminum. High-strength aluminum alloys are typically chosen so that the molds are durable and cost-effective for an extended run time. The molds include cooling water channels for temperature control; however, the interaction of the cooling water with the aluminum alloy is often not considered as part of the design process. With good design, maintenance practices, and careful control of water chemistry and treatment, molds can last for 10-20 years or more; however, corrosion of the aluminum, especially exfoliation corrosion, can cause failures, sometimes within a few months. Corrosion also results in increased expenses for retooling and decreased heat transfer. This paper discusses material, design, operational, and chemical factors that can influence corrosion of aluminum alloys used for molds and provides detailed examples in case histories that illustrate the factors.
