People like hot runner molds if they behave. Behave means that the process doesn’t change over time, as it does with multi-process disease. There are two easy tests to detect multi-process disease:

(1) A dynamic test where short shots (known as fill only shots) are intentionally made over a period of time, perhaps once every hour, while a mold is running. This can be done in the early stages of qualifying the tool (the earlier the better actually) so that effects can be anticipated. If the short shots are not the same every time, the fill pattern is changing.

(2) Monitor the individual cavity weights periodically—once per hour. This shows the distribution of plastic in each of the cavities.

What we would like to see is that the average of all cavities trends the same as each individual cavity, What this means is that all cavities’ weights go up or go down in unison, indicating that there is really only one process to control here. Often, however, , where part weights for each cavity vary independent of one another. In this case, we have multiple processes. This four-cavity mold has four distinct processes or cavities that vary independently. Many times, as you can see, some of these cavities trend completely counter to other ones. As part weights go up in one area of the mold they go down in other areas of the mold.

A mold that does this can be highly problematic. Any mold that has multi-process disease will almost assuredly make defective parts and allow them to escape downstream to the customer. Currently, the only way to eliminate these escapes is to monitor pressure in each cavity with a cavity pressure sensor. This will detect when the pressure distribution has changed so that parts can be contained while corrective action is initiated.

Often the mold builder is the scapegoat for a multi-process disease situation. However, studies have found that it is a complex interaction between the mold, the manifold, the material, the machine, and the mold temperature controller that all contribute to the situation.