What goes into rubber molding? The two important steps in this process are prototyping and production; you can’t have one without the other. In both stages, you need to consider tooling, tolerances, costs, and quality considerations to ensure the best possible result.
The Differences in Tooling
With rubber molding, tooling is the actual hardware of inserts and cavities used to shape the raw rubber into parts through heat and pressure. The mold dictates the physical dimensions and sealing capabilities of the rubber. During the prototyping process, steel is used for quick machining and fast adjustments to design as required. At this stage in the process, tooling is meant to validate early designs and perform low-volume sample rubs for material evaluation. Instead of automation, it uses manual extraction.
In production, tooling is engineered to manufacture high-volumes for thousands of cycles or more via thermal management and multi-cavity plates. These help with ensuring high repeatability. Tool steel or mold steels are used because of their abilities to withstand elevated temperatures and pressure; stainless steel is also an option if there’s a risk of corrosion from some rubber compounds.
What are the Tolerances Between Prototype and Production?
Tolerances are the acceptable range of a part’s properties, like hardness and dimensions, that it can experience. For molding, tolerances ensure that the part works and fits correctly even with rubber’s natural tendency to shrink in size. In prototyping, tolerances help make sure a part fits in an assembly and seals before production occurs. During the initial stages of prototyping, tolerance limits can use preliminary shrinkage factors based on the chosen material to determine its operating ability and fit. At this stage in the process, tolerances focus on validating the design and determining the functionality of the material.
In production, tolerances strive for consistency with future repeatability, larger volumes, and compliance to industry standards. Rubber needs to endure the thousands or more of molding cycles, along with variables like mold wear and temperature fluctuations. Regulatory requirements need to be abided by as well; for example, the Association for Rubber Products Manufacturers (ARPM) has four designations for dimensional tolerances: A1 (“High Precision”). A2 (“Precision”), A3 (“Commercial”), and A4 (“Basic”).
The Importance of Quality Control
You can think of quality control in the prototyping stage as the first step toward getting the rubber ready for full-scale production. Here, proper quality control can help you spot tooling or design issues that can be problematic and expensive to alleviate during the production process. Prototyping helps validate cure time and temperature stability within actual molding conditions. Performing quality control in a small-volume environment that prototyping allows helps to prevent potential high rejection rates in production.
Quality control is especially important in the production process. Any sort of variation like temperature fluctuations or pressure drops can have an immensely negative impact on the rubber. Quality needs to be upheld because these parts need to withstand intense usage in real-world applications. Cured rubber cannot be readjusted or fixed; if it doesn’t meet proper hardness or compression set, it is unfit for use and is considered as waste.
What’s the Cost?
Prototyping uses cost-effective materials for molds as tools used for prototyping last for a smaller amount of cycles than production. Regarding lead times, molding at the prototype stage offers quick turnarounds, usually between days to a few weeks because there’s no heat-treatment cycles and the mold designs are usually simpler, helping design verification.
Production uses more expensive tools like hardened steel, precise multi-cavity plates, thermal management monitoring, cold runner systems, and automated ejection pins, all making for a high and costly investment. The key word here is “investment,” though; these tools are built to endure and handle a high number of parts. With automation, multiple cavities, and the rubber part being ready for production, the actual per-part-cost is low. The catch is that lead times are long (usually at least a few weeks) and post-tooling processes, like post-curing and deflashing, can be expensive.
The Next Step
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