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.
A mold with a full cold runner or a hot-to-cold runner can be monitored with strategically placed cavity pressure transducers, but I would not consider this to be fail-safe. The key to transducer placement in the cavity is to ensure they are in the last cavity to fill in each runner group. If this rule is not followed, the parts that fill after the cavity with the transducer are at risk and are not quality monitored. It is actually very difficult to get optimal balance on a multi-cavity mold and greater imbalance reduces the ability to effectively monitor. Optimal balance, in my opinion, is 97% or above but often times 95% is about as good you can get. To get these types of fill balances you must seek out technologies such as Beaumont's melt flippers.
Also note: Never try to balance the fill on an injection mold using differences in runner and/or gate diameters!
The reason you can't effectively monitor with a single transducer placed in a cavity filling first is due to the viscosity of the material in each cavity. Cavities that fill ahead of the others will actually see less pressure than the cavities to fill last. A full cavity has a tremendously high viscosity rate, because the cavity is full and plastic cannot flow. Considering one of the basic rules of processing—plastic flows in the direction of least resistance—you can understand the physics behind this. That is, plastic has stopped flowing in those cavities that were first to fill and now all the pressure is transferred to the cavities that have yet to fill.
This can cause nightmares when developing a robust process let alone trying to monitor it from another cavity. You can't guarantee the quality level of cavities that fill after the cavity that has been instrumented with a transducer. This is also why this method can't be considered fail-safe. Even if a mold is instrumented with a transducer in the cavity that is last to fill, process conditions can change that could cause the balance to shift.
So the question is:
- Will the transducer detect these condition changes when not placed directly into the effected cavity?
It is extremely difficult to guarantee the quality level of the parts because of the thousands of different scenarios that could potentially take place. "Possibly" is sometimes not good enough depending on the product's tolerances or intended use. You must decide whether the risk is worth the reward of a lower-cost mold.
When no other options are available and you are forced to use this method of instrumentation, there is one critical rule that must be followed when building the mold:
Each cavity must be built with provisions to install a cavity-pressure transducer.
Basically, the mold must be designed so that transducers will fit into each and every cavity, even if you intend to only use only a single transducer. Why? Well, how do you know which cavity is going to fill last? Put simply, you don't! After your initial process development you must complete a fill-balance study to determine where to place the transducer.
Remember, fill balance will change depending on fill speed so you also must wait until you have chosen the optimal fill time before you can determine transducer location. Cavity pressure transducers can be the ultimate means of monitoring part quality but understanding how to effectively utilize them is key to a robust, repeatable injection molding process.
About the Author: The Tech Group's Robert P. Gattshall has worked 17 years in the automotive and medical injection molding industries, including 12 years in process engineering and process development. Certified in John Bozzelli's Scientific Injection Molding for more than 10 years, Gattshall has developed more than 600 processes using scientific injection molding principles. Certified in Lean 5S and SMED, Gattshall has also trained more than 50 process technicians and engineers on the principles of decoupled molding.