As more and more molders begin to understand the benefits of cavity pressure transducers, more questions will arise regarding how many transducers are actually required in a mold. This question has more than one answer and depends on what you want to accomplish with the technology.
Optimal instrumentation could be defined as two transducers in each cavity, one post-gate and one near the end of the fill. This allows for V-P transfer and quality monitoring. The reality is that this configuration can greatly increase the cost of of a mold, and one transducer per cavity properly placed can monitor the part quality and sometimes control V-P transfer. Using a standard decoupled two process, one transducer near the end of fill should be enough to monitor the part quality effectively as long as the process is robust. In most cases, one transducer per cavity is really all you need. Whenever a molder is forced to use less than one transducer per cavity it can get tricky and, in some cases, completely ineffective.
I will start off stating that if you have a full hot runner mold directly gated into the cavity you must have at least one transducer per cavity to ensure part quality. Anything less than this is very risky, even if you control the drop temperatures. A hot runner going through a normal heat cycle is more than enough to cause process conditions to change, and with less than one transducer per cavity, these condition changes can go undetected. This is definitely something to avoid.