Steel hardness and rubber molding
In the complex world of rubber molding, the hardness of steel plays an essential role. Yet, such a role is too often underestimated. From the moment the rubber first meets the mold, flowing between the runners of the plates, the steel hardness not only affects the overall quality of the final product, but also influences the entire manufacturing process. In this article, we are going to delve into the fascinating realm of mold making to give you an idea of how we carry out our daily work: a world where the hardness of steel meets the resilience of rubber and where every single cavity tells a story of durability, precision, and cost-effectiveness optimization.
Steel hardness in rubber moulding: why it matters
The hardness of steel plays a crucial role in the molding of rubber items. Hardened steel molds, which usually measure 38 to 45 on the Rockwell hardness scale, are commonly used in rubber molding due to their durability and longevity.
As you may already know, the Hardness Rockwell C Scale (see HRC) is an ordinal qualitative scale based on the measurement of the depth to which an indenter is driven by a heavy (major) load. This test follows the sequence of application of minor and major loads (indentation tests) and it is widely employed to assess the hardness of materials.
In rubber molding, hardened steel molds are preferred as they can meet pressure and heat requirements, thus ensuring uniform and accurate shaping of the rubber material.
For instance, in rubber compression molding, many variables come into play. Choosing the right type of mold, understanding the compression force requirements and the temperature properties of the rubber compound in use are all essential parameters one has to take into account to obatain high-quality outputs. While molding, the overall weight of the steel mold affects the amount of energy required. Therefore, trying to reduce the weight as much as possible while maintaining structural integrity is critical to successful operations. Which, by the way, can prove to be a real challenge, as each project has a unique design, whose geometric characteristics are often one of a kind, not to mention specific manufacturing needs to be met in order to comply with the customer’s manufacturing process requirements.
Consequently, mold design should consider factors such as flexibility, vibration absorption, mechanical properties, pressure requirements, and thermal decoupling to optimize the process and, obviously, minimize costs. Nothing is left to chance when precision is at stake.
Therefore, the hardness of the steel mold used in rubber molding directly influences the efficiency, quality, precision of the molding process, and durability of the mold itself, which must withstand several production cycles, while always guaranteeing high-quality products.
Why not use >50HRC steel then?
Well, because, as with everything, there is a price to pay: the hardness of steel, in fact, significantly affects costs in various ways. Harder steels may require more expensive materials and manufacturing processes, as they often involve higher carbon content or additional alloying elements. This leads to higher costs per gram of raw material. +60 HRC (like 64 or 68), for example, are rather rare and extremely expensive because they involve particular powder metallurgy processing techniques. Also, they are more difficult to maintain and fine-tune (as in the case of mechanical grinding). As a consequence, they are best suited for products where less steel is needed, such as kitchen knives.
But it doesn’t end here, because, apart from powder metallurgy, there are other techniques that can be employed to harden a steel plate. Take, for instance, heat treatment processes such as quenching and tempering. They are often used to achieve higher hardness levels but, on the other hand, can be energy-intensive and require precise control of temperature and cooling rate. And we’re just talking about the metallurgical process for die casting and hardening of steel plates; we are not considering the costs of CNC machining, which, in our case, is the next step in the supply chain for the manufacturing of custom molds.
Applying surface treatments to steel plates, such as chrome plating, is another common practice used to improve their hardness, wear resistance, and corrosion resistance, but it further increases manufacturing costs. However, no matter how often it can be employed, chrome plating is a story in itself, a complex process that involves many variables. Therefore, we are going to delve into the pros and cons of chrome plating and give you a full insight into it in our next article. Stay tuned!