Optical Profilometry and ISO 25178 Parameters for Cavity Wear
ISO 25178 areal parameters like Ssk, Sku, and Vvc measured via optical profilometry quantify cavity wear, predicting ejection failures before part dimensions drift.
Corrosive Wear
Meaning
Material safety data sheets for halogenated polymers often highlight the potential for chemical reactions to degrade the surface of the injection mould. This type of degradation occurs when the polymer melt releases acidic gases or reactive species during processing. Corrosive wear specifically describes the loss of metal from the tool surface due to these chemical attacks, which often manifest as pitting or discoloration.
It is a significant concern when moulding materials like polyvinyl chloride or flame retardant polyamides that can release hydrochloric or hydrobromic acid. The process continues as long as the corrosive environment and the metal surface are in contact at high temperatures.
Chemical Attack
Molten plastic can undergo thermal degradation if it is held at high temperatures for too long in the barrel or the manifold. This degradation releases volatile compounds that react with the iron, chromium or other alloying elements in the tool steel. Corrosive wear often starts at the microscopic level, where the acid eats away at the grain boundaries of the metal.
This creates a rough, porous surface that is more susceptible to further mechanical wear. The presence of moisture in the resin or the ambient air can accelerate these reactions by forming liquid acids on the tool surface. Over time, the polished finish of the cavity becomes dull and pitted, which directly affects the surface quality of the moulded parts.
Using corrosion resistant steels or specialized coatings can provide a barrier against this type of damage.
Degradation Rate
The speed at which the tool surface is lost depends on several factors, including the melt temperature, the residence time and the chemical stability of the resin. Higher temperatures generally increase the rate of chemical reactions, making corrosive wear more aggressive during fast cycle times or high heat applications. If the venting system of the mould is inadequate, the corrosive gases can become trapped in the cavity, leading to localized areas of intense attack.
These areas often appear near the ends of the flow paths or in deep ribs where air is easily compressed. Regular cleaning of the mould and the use of neutralising agents can help to slow down the process. However, the most effective prevention is the selection of materials that are chemically compatible with the intended polymer and its additives.
Material Protection
Protecting the investment in high quality tooling requires a strategy that combines material selection with proper process control. Chrome plating or nickel based coatings are frequently used to provide a sacrificial or inert layer that resists acid attack. When corrosive wear is expected, the mould design should include enhanced venting to allow gases to escape before they can react with the steel.
Technicians must also be trained to recognize the early signs of corrosion, such as a slight yellowing of the steel or the appearance of fine rust. Removing the tool from the press and applying a protective oil during downtime is a standard practice to prevent corrosion from moisture. These steps are necessary to ensure that the mould remains functional for its intended service life and produces high quality parts.