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Understanding and applying the ‘Pack and Hold’ concept.

There are three phases during the filling of the mold. They are as follows:

  1. The Injection Phase: In theory, during this phase the cavity is fully filled with molten plastic. However since plastic melt is compressible, in practice one does not know if the mold is 100% filled with molten plastic or is filled more than a 100%. For this reason the mold is typically filled to about 95 – 98% in the injection phase.
  2. The Pack Phase: During this phase additional plastic is further injected into the cavity to compensate for the shrinkage that is occurring in the plastic that was injected in the Injection Phase. This is a pressure and velocity controlled phase.
  3. The Hold Phase: Once the required amount of plastic is injected into the cavity, it must be held in there till the gate freezes off.  This is a time controlled phase.

In most cases molders do not differentiate between the pack phase and the hold phase and will typically have only one pressure setting and one time setting for both phases. They call this collective phase as the hold phase. The optimization of this phase is done by the gate seal study where the part weight is plotted against time. Once the gate freezes off the part weight stays constant. The processor adds a second or so to the time where the part weight stabilizes and sets this as the gate seal time. See Figure 1 where the part weight stabilizes at 6 seconds and therefore the hold time on the machine is set to 7 seconds.

In some cases, such as in case of softer materials or larger gate sizes the part weight does not stabilize within a practical time limit of molding cycle times. This is also true in cases of hot runners and valve gated systems. Adding more time than required does nothing else other than pack the gate area when the rest of the part is already below the no flow temperature of the plastic. In such cases the same study mentioned above will yield a graph as shown in Figure 2 where the part weight does not stabilize.

In such cases where gate is not seen the phases of pack and hold must be differentiated from each other. In the pack phase the required amount of plastic must be injected and in the hold phase this plastic must be held in there till the gate freezes off. If the hold phase is terminated before the gate is frozen then the pressurized plastic the cavity will flow back out of the cavity often causing sink and/or dimensional variations and issues. (This is the reason that a molder will notice sink on parts with high pack and hold pressures. When the molder lowers the pressure the sink disappears often baffling the molder since it is opposite to what he expects.)

Following is the procedure that has been used for optimizing the pack and hold times. It is best to illustrate this with an example. Consider the graph shown in Fig 3. This is the same graph in Figure 2. It can be observed that at about 5 seconds there seems to be a change in the slope of the graph. In other words, the % increase of part weight with incremental hold time seems to be lower compared to that before 5 seconds. We therefore can consider that the part has reached the required part weight or in other words the pack phase has been completed. It is similar to visualizing when one is packing his travel bag where initially clothes can be placed in till the bag seems physically full but the remaining clothes can only be put inside after compressing the clothes that were first put in. As the bags are filled more and more, lesser and lesser amounts of clothes can be placed in there. So after the initial quick fill further additions slow down. The pressure used during this initial phase can now be considered as the pack pressure and the time that this pressure is applied for as the pack time.

Going back to the travel bag example, once we have packed the required amount of clothes we must now zip it up in order to hold the clothes in there. If not, the bag top or cover will not be able to keep the clothes in there. Similarly, once the required amount of plastic is now present inside the cavity it must be held in there. This is done by applying another pressure setting that will be lower in value than the pack pressure for a time until the part weight stabilizes or in other words the gate freezes off. The target part weight here will be the same part weight that was obtained at the end of the pack time. The following procedure will better illustrate the steps.

Procedure for determining pressures and times for pack and hold phases.

Note: Optimization of this phase is Step No. 5 in the 6-Step Study for Process Optimization. It is therefore assumed that the previous 4 steps have been completed. Click this link for more info: http://fimmtech.com/index.php?id=6&subid=38

Procedure: We will refer to the same info in the graph in Fig 2 starting from the steps to generate the graph.

  1. Set only one pressure collectively for the pack pressure and hold pressure. We will call this as the compensation pressure. Therefore Compensation Pressure = Pack Pressure + Hold Pressure.  As an arbitrary value, let us consider this pressure to be 8000 psi plastic pressure.
  2. Set only one time collectively for the pack time and hold time. We will call this as the compensation time. Therefore Compensation Time = Pack Time + Hold Time.  As an arbitrary value, let us consider this time to be 15 seconds.
  3. Set the compensation time to zero and generate a graph of Part Weight versus Compensation Time up to 15 seconds. (Fig 2).
  4. Observe the graph and the part weight table to estimate where the change in the part weight begins to slow down. A change in the slope of the graph can be seen. In Fig 3 this time can be considered as 5 seconds.  Based on this Pack Pressure = 8000 psi and Pack Time = 5 seconds
    Record the part weight and this will be the ‘Pack Only Part Weight’ = 11.10 gms.
  5. Initially we only had set one value for pressure and time and called it compensation pressure and time. We split the compensation pressure and time into two and identify them as pack pressure, pack time, hold pressure and hold time. Therefore on the molding machine we will now add another pressure and time profile. The first will be the pack and the second will be the hold. Set the first pressures to 8000 psi and the first times to 5 seconds.
  6. The second set of pressures are the hold pressures and hold time. Since the compensation time was set to 15 seconds, the pack time was set to 5 seconds set the hold time to 10 seconds (15 – 5 = 10).
  7. Set the hold pressures equal to the value of pack pressures = 8000 psi and mold parts.
  8. Record the part weight. This should be the same as the 15 second value above and should therefore be equal to 11.23 grams. It will be higher than the Pack Only Part Weight of 11.10 gms.
  9. Drop the hold pressures in convenient steps of about 250 psi and keep checking the part weight at every reduction. The pressure at which the part weight equals the pack only part weigh of 11.10 grams will be the holding pressure. In this example, the value was 5250 psi.
  10. Next reduce the hold times in steps of 1 second and note the time where the part weight drops below 11.10 grams. In this example, this time setting was 3 seconds since at 3 seconds of hold time the part weight was 11.08 grams. This means that at 3 seconds the plastic is coming back out from the cavity. Add a second to 3 seconds making this time 4 seconds which will bring the part weight back up to 11.10 grams. This time will be the set holding time.

The results are summarized in Figure 3.

Final Settings:

Happy Molding! Please send your questions and comments to suhas@fimmtech.com

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