Watching for Wear
Considering how coatings wear over time extends the options for mold surface finishes beyond hard chrome and electroless nickel.
More intricate molds, corrosive materials and resins, and advances in coatings science have led to the development of multiple choices for mold surface finishes beyond hard chrome and electroless nickel. Consulting with a knowledgeable supplier is your best bet for selecting the right surface treatment for a specific application, but it’s always wise to also have a basic understanding of the pros and cons of various coatings. For example, in terms of wear resistance, an electroless nickel coating with nano-size diamond particles can be a better alternative to more “classic” options, but how do you measure its abrasion resistance?
Measuring Hardness and Wear
The most common indicator of hardness is the Rockwell hardness test, as defined in ASTM E-18 (standard test method for Rockwell hardness of metallic materials). This method involves applying force to an indenter, then measuring the depth that the indenter reaches when it impacts the material. However, resistance to indentation is not the most appropriate measure of abrasion resistance. That title of distinction belongs to the Taber Wear Index, which shows the amount of material removed by a Taber Abraser (an instrument for accelerated wear testing) during an operation. This is a standard test a plater may conduct in house as part of a company’s quality control and product development processes.
The Taber test calculates the loss in weight of the material over a specified number of abrasion cycles—either the loss in weight per cycle or the loss in weight over the number of cycles required to remove a unit amount of coating thickness.
The chart below shows the results of a Taber test conducted in accordance with ASTM D4060 (standard test method for abrasion resistance of organic coatings by the Taber Abraser), which provides three types of calculations for wear resistance:
1. Wear Index: Weight loss per specified number of revolutions (usually 1,000) under a specified load (500 or 1,000 grams).
2. Wear cycles per mil: Number of cycles required to wear through a 0.001-inch thickness of coating.
3. Cycles to failure: Number of test cycles to wear away the coating or to expose the substrate.