Theory of Holding Pressure and Process Window

The injection of the plastic into the cavity can be divided into two main phases. The first phase is the injection phase. The mold cavity is completely filled with the molten plastic in the injection phase. The second phase is the holding phase. The holding phase follows the Injection phase. The holding pressure must pack into the cavity, the plastic equivalent to this volumetric shrinkage that occurs during the cooling down of the plastic as it hits the cold walls of the mold. There are various parameters that need to be controlled during this phase. These are packing pressure, holding pressure, packing time and holding time. In most cases packing and holding are not differentiated and are collectively called the holding phase. This type of molding where the injection phase is separated from the holding phase is called de-coupled molding, a term coined by RJG Technologies.

The packing phase consists of packing the cavity with plastic equal to the theoretical weight of the part. The theoretical weight is equal to the density of the plastic multiplied by the volume of the cavity. Any less plastic will result in an under-packed part and any more will result in an over-packed road. Under-packed parts results in defects such as sinks and voids in the parts. Such parts usually exhibit post molding shrinkage. Over-packed parts can have in-built stresses that usually get relieved after molding resulting in defects like warpage or premature failure. This section will deal with the pressures related to the holding phase. The next section will deal with the times related to the holding phase.

The ideal holding pressure is determined by evaluating the Process Window of the mold. The Process Window is also called the Molding Area Diagram. This is the area in which acceptable parts are molded. The bigger the window, the robust is your process. Refer to the picture below.

In the picture above, outside the process window, the parts can either have sink or flash above or below the holding pressure limits. Outside the melt temperature limits, you may have unmelted plastic below the lower limit or degraded plastic if you go over the limit. Both these are detrimental to the part quality. The process is set to the center of this window so that any variations within the window would still yield acceptable parts. The larger the window, the more robust is the process