Often times when we hear that there is a random filling imbalance related to a random part quality issue, we are skeptical that the problem is truly “random”. A majority of molds that we are asked to look at often times have a pattern to the data, and therefore, a common root cause behind the associated problems. These patterns may be a result of either non-uniform shear heating variations or steel related issues that affect a certain set of cavities. In order to first determine if there is a pattern, we need to break down a mold into flow groups and regions. By determining flow groups in a mold, not only can a person more easily find the true imbalance but it is also easier to determine the source(s) of the imbalances.
A flow group is a collection of cavities that has identical flow paths/shear history and therefore receives identical melt conditions (temperature and filler distribution, viscosity, shear rates, etc…).
Below is the first step to help determine how to break a mold down into flow groups:
Step 1 – Determine which cavities make up flow group 1
The first step is to determine which cavities belong to flow group 1. Flow group 1 typically represents the cavities that are being filled with the material that experiences the longest shear history. These cavities commonly will be heavier, possibly flash, or are over packed, but not always due to a variety of different reasons. To determine which cavities make up flow group 1, begin by tracing the outside edges of the primary runner, starting at the sprue and ending at a cavity. See the example below.
Example 1 – 8 cavity mold
To make sure that all of the cavities that belong to flow group 1 are accounted for you must trace the flow on BOTH sides of the primary runner and on ALL sides of the sprue. In the example above there are four trace lines. If a mold has four primary runners coming from a sprue there would be eight trace lines (one on each side of each runner). Each cavity should be marked with the number 1 representing that it is part of flow group 1.
Determining which cavities make up flow group 1 is sometimes confusing as most people think it will always be the inner most cavities (those closest to the sprue). But this is not always true. It depends on the runner layout itself, not the cavity position in the mold. Let’s look at a few additional examples to help illustrate how to find flow group 1:
Example 2 – 4 cavity mold
Here in a four cavity example where there is only one primary runner coming from the sprue so we only have two flow traces, with the two inside cavities making up flow group 1.
Example 3 – 16 cavity mold
In this 16 cavity example there are four primary runners coming off the sprue, so we have eight flow traces. Also note that the cavities that make up flow group 1 are on the outsides of each grouping.
Example 4 – 32 cavity mold
Here in the 32 cavity example the runner trace makes four turns. No matter how many turns the runner system makes the flow trace will always on the outsides of the runner channels. Again in this mold configuration, flow group 1 is not the inner-most cavities.
Note that in more complex molds such as three-plate molds or hot manifolds with level changes finding which cavities make up flow group 1 becomes a little more difficult as you need to think in three dimensions. Still the same concepts apply. The picture below shows an example of a flow trace in a multiple level runner system:
Determining which cavities make up flow group 1 is a critical step in properly determining the molds filling balance. Look at a few of your molds and see if you can determine which cavities make up this critical flow group 1. Check back for the next tech tip when we continue on to Step 2: How to determine the Regions of a Mold.