The goal of all quality conscious companies is zero defects. This is achieved through the use of capable processes and statistically based monitoring; it cannot be achieved by inspection.
Continual improvement is needed to increase the capability of processes. In today’s rubber industry, this capability is relatively low and many aspects are difficult to monitor. For example, rubber is pliable and dimensions such as cylindrical diameters often cannot be gauged quickly and accurately. In mixing, dispersion cannot be easily checked; in preparing blanks, actual volume cannot easily be measured; and in moulding, the injection process is often more sensitive to material variation than any existing rheometer.
Rubber materials crosslink and are time and temperature sensitive. This results in significant viscosity variations during flow. Friction heat through mould gates and channels creates differences between material and mould temperatures which cannot easily be measured.
Despite such problems, much progress has been made in recent years through the emphasis on consistent processing and with modern computer-based injection presses which are self-adjusting, within limits, to material variations.
At present, world class levels of internal rejects should not be assumed in rubber but they can be achieved through careful design and adherence to the procedures outlined above. Across the industry, internal reject rates run at a typical but unacceptable level of 2%, but rates of 500 ppm (0.05%) – well within world class levels (Anderson Report) – can be sustained across a range of parts, materials and machines.
Clearly, any single part with well designed processes can achieve lower ppm reject levels. As more and more parts are designed on the right basis, and continuous improvement is applied to processes, capabilities will rise and reject levels fall.
It is important to consider present capabilities. The following is intended to provide a general guide using typical results, but every material, machine and process has its own capability and these can vary considerably.