Shot molding: loading by quantity, not weight

When processing via “Scientific Molding”, the injection of plastic into the mold or cavity is divided into two stages: filling (or “first stage”) and pack-and-hold (“second stage”). I’ve received a lot of emails over the past few weeks and had several discussions about how full the part should be at the end of the first phase. And since we are asking more than 90% portion at the end of the first stage, does this mean weight or volume? Good questions. Where do we find the answers?

First we define the terms, as there are a variety of different names, designations or expressions for the first and second level. Terms such as fill, pack, hold, high pressure, low pressure, V-to-P, etc. have been used. Do they all mean the same thing or are there differences? Having a lot of terms that aren’t clearly defined drives me crazy. This makes communication more difficult, leads to misunderstandings and confusion and makes training particularly difficult. To be clear what I mean by first and second stages, see Fig. 1This is a graph of injection pressure (plastic pressure) versus time that depicts the first and second stages. This diagram is typical of most, but not all, injection molding processes. (This does not apply to microforming, for example, where the parts are too small to separate the first stage from the second stage.)

In short, the first stage fills the part more than 90% and the second stage unpacks the part with the remaining amount of plastic needed to completely fill the part, remove the sinks, and provide the appropriate part dimensions.

Next, we’ll cover the procedure to create a part for the first stage only.

The goal is to take a short shot that is more than 90% full. Seems simple enough; Finally, the instruction to make a part for the first stage only is to take off the second stage. The problem is there are three different ways to start the second stage:

  1. Set the second stage timer to zero.
  2. Set the second stage pressure to a very low value. I suggest 5 to 50 psi (0.5 to 3.5 bar) and leave the second level timer on for at least 0.50 seconds or more.
  3. Reset the second stage timer to zero and reduce the pressure to a low value.

Try each one … I bet you won’t produce the same part just for the first stage. So which one is the best? I propose the second approach and let me explain why. Most of the molding machines have the problem of impulse or overflow. This means that the screw does not stop at the set cut-off position. It is important that the processor sees, understands, and deals with this crossing when developing the first stage.

It is also important that the processor ensure that this part of the first stage is only performed under conditions where the injection or the first stage is not pressure limited. Many operate with a set or available pressure 10% higher than the actual peak pressure during the injection. I find this general rule of thumb insufficient.

Now that we know the procedure for creating a part of the first stage, how full should it be? No easy answer here. Somewhere between 90 and 99.9% full – based on volume, not weight. Factors influencing this are the type of part, the number of voids, the balance of the infill, a weld at the end of the infill, a living hinge, etc. Conclusion for 99% of the parts to be molded: The part of the first stage (n )) should be visibly short, not full of sink. But why by volume and not by weight?

The emails and blogs I received and read covered a variety of reasons for volume or weight that looked at melt density versus solids density, sinking, rebounding, and other factors. For me, the answer – volume – is to examine the cavity pressure curves and observe the wear on the parting lines and the resulting development of the lightning on the parts.

Figure 2 provides the injection pressure curve in the first and second stages together with a pressure curve at the end of the cavity for a first stage developed by weight. Note the rapid, almost vertical, instantaneous increase in cavity pressure. This is a significant force on the mold parting line and will likely wear it down prematurely, which will eventually result in a flash.

Figure 3 provides the injection pressure curve for the first and second stages along with a pressure curve at the end of the cavity for a volume developed first stage. Note the rounded cavity pressure curve at the end of the infill. This suggests that less tension is being created on the parting line, so it can withstand significantly more blows before wearing and then developing the lightning. Please understand that there are a number of factors that influence the wear and tear of the parting line. This is just one of several. The shape of a cavity pressure curve at the end of the filling is also partly dependent.

In short, develop the first stage over time on the second stage timer and based on the volume. It should cause less wear and tear on the parting line.

ABOUT THE AUTHOR: John Bozzelli is the founder of Scientific Molding Solutions in Midland, Michigan, a provider of training and consulting services to injection moulders, including LIMS, and other specialties. Contact [email protected];

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