Meaning
Hardness scales such as Rockwell C define the resistance of the metallic components used in production to the mechanical stresses of the moulding cycle. Over time, the repetitive flow of resin and the clamping forces of the machine lead to the gradual loss of material from the cavity, a process known as tool steel wear. This degradation can take several forms, including abrasion, adhesion or erosion, and it directly affects the quality and dimensions of the moulded parts.
It is a significant factor in the total cost of ownership for an injection mould, as it determines how long the tool can stay in production before needing repair. The rate of wear depends on the material selection, the resin properties and the process conditions.
Abrasion Mechanism
The primary cause of material loss in many moulding applications is the mechanical scratching of the steel by hard particles in the resin. When glass filled or mineral reinforced polymers are processed, tool steel wear is accelerated by the constant scouring action of these fibres against the cavity wall. This is particularly intense at the gates and in areas of high turbulence where the melt velocity is highest.
The abrasive particles cut into the metal surface, removing small fragments and leaving behind a rougher finish. This not only changes the appearance of the parts but also increases the friction during ejection. Choosing a steel with a high carbide content or a higher hardness rating can help to slow down this process.
However, harder steels can be more brittle and difficult to machine, so a balance must be found.
Material Selection
Choosing the right grade of steel for a specific application is a critical decision in the tool design process. Different types of tool steel wear require different material properties for effective mitigation. For example, a stainless steel like 420 is often chosen for its corrosion resistance, while a high speed steel might be used for its resistance to heat and abrasion.
The heat treatment of the steel is also a key factor, as it determines the final hardness and toughness of the tool. Many moulders use P20 for general purpose applications, but high volume runs often require H13 or S7 to withstand the higher stresses. The cost of the material is balanced against the expected life of the tool and the value of the parts produced.
A well chosen steel will maintain its dimensions and surface finish for hundreds of thousands of cycles.
Operating Lifecycle
Managing the long term performance of a mould requires a proactive strategy for monitoring and repairing the tool surfaces. As tool steel wear progresses, the quality of the parts will eventually begin to decline, leading to increased scrap rates and production delays. Regular inspection and measurement of critical dimensions are necessary to identify the point at which the tool needs to be refurbished.
This might involve polishing out minor scratches, laser welding damaged areas or replacing entire inserts. A well documented maintenance history allows the moulder to predict the remaining life of the tool and plan for its eventual replacement. This planning is essential for maintaining a high level of efficiency and competitiveness in the manufacturing process.
The use of specialized coatings can also provide an additional layer of protection, extending the time between repairs. Consistent monitoring ensures that the tool continues to produce high quality parts.
