There are three phases during the filling of the mold. They are as follows:
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.
Procedure: We will refer to the same info in the graph in Fig 2 starting from the steps to generate the graph.
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