Holding Pressure Phase

As soon as the plastics melt fills the mold, it begins to cool. This cooling process begins at the mold wall and proceeds toward the center. For a certain period, the melt remains fluid inside the molding. As it cools, the molding material decreases in volume in other words, it shrinks.  

If the pressure were completely switched off after the injection phase, the molding would detach itself fi-om the wall as it cools. There would no longer be any control over the molding s dimensions. It is easy to imagine the consequences in the example of the CD the surface would become uneven, the scanning by laser would become less reliable-in short, the CD would be utterly useless. 

In contrast to the injection phase, no flow resistance must be overcome in the holding pressure phase. Furthermore, practically no movement of material occurs in this phase. For these reasons, the holding pressure can generally be set to a lower level than the injection pressiue. 

For the injection molding machine setter, internal mold pressure curves of this type are an invaluable aid for finding the optimum times and pressures for the injection- and holding pressure-phases. This kind of graph enables one to find much data regarding the quality of production run and moldings alike. 

The following status applies to the machine s modular structural units during the holding pressure phase  

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During the holding pressure phase (phase C) the two cylinders compensate for the shrinkage of the plastic because the screw is hydraulically disconnected from the mold (Fig. 14.19) [17]. 

Calculation of the Holding Pressure Phase, Shrinkage, and Internal Stresses... 

It is important, to take a closer look at the position of the screw in the holding pressure phase. At the start of the holding pressure phase, the cavity has already been filled with most of the melt, that it is meant to receive. A small quantity is subsequently injected to compensate for shrinkage. But even at the end of the holding pressure phase, some residual material should still be in the injection chamber. This melt cushion enables pressure to be transmitted between the screw and the cavity or sprue. This melt cushion is absorbed in the next shot. 

The hydraulic pressure is at a lower level during the holding pressure phase change-over... 

The injection and holding pressure phases are followed by two other phases, which proceed concurrently ... 

For the material within the mold cavity, the holding pressure phase is concluded as soon as the sprue solidifies-in other words, as soon as it is sealed. From this point on, no more material can enter the cavity. During the injection and holding pressure phases, the material in the cavity has already begun to cool against the relatively cold mold wall. The outer layers have soldified very quickly. The cooling time therefore starts as early as injection. 

The status of mold and clamping unit during the holding pressure phase is identical with the one of the cooling phase. 

During the feed period - following the completion of the injection and holding pressure phases - material for a new shot must be prepared and provided in the proper quantity. The material is fed or metered. ... 

The preparation of new material begins immediately after the completion of the holding pressure phase. If a shut-oflF nozzle is used, it will close as soon as the holding pressure phase has timed-out In this case the plasticating unit will be retracted from the mold to avoid excessive heat-transfer from the hot nozzle to the cooler mold. 

Rheological calculation of the filling- and holding-pressure phase... 

The fllling characteristics of the cavities are determined in the rheological mold layout. The filling phase and holding pressure phase determine the properties of the molded part. The rheological mold layout can include several steps ... 

Rheologieal ealeulation of the filling- and holding-pressure phase (quantitative, e.g. pressures and temperatures) and... 

After the volumetric filling, the dynamical phase which is dominated by the injection pressure of the filling process moves on to the quasi-static phase. This holding pressure phase can on one hand get the melt to compress and on the other hand deliver additional melt during the initial cooling, where the pressure in the cavity remains often almost constant. Such a post-supply of melt is possible as long as the gate, the junction of the runner into the cavity is still permeable (not yet sealed ). 

In the shrinkage and warpage analysis, the process parameters, in addition to the material data, also play a crucial role. The data for both the filling phase and the holding pressure phase are important for the calculation results. Despite the fact... 

Modern controis of an injection molding machine switch from a speed-controlled injection phase to a pressure-controlled holding pressure phase. For this purpose, various methods are available, which differ mainly in terms of their accuracy. For all methods, however, the goal is to switch over the target when the cavity is volu-metrically filled. If the holding pressure switchover is done too early, a momentary drop in pressure usually occurs, and the cavity is filled under holding pressure in an undefined way. If the holding pressure switchover is done too late, the melt is still compressed in the injection phase, which results in pressure peaks, overloads, or so-called residual pressure. Both phenomena usually prevent the production of an optimum molded part. 

During the hold pressure phase the pressure gauges are monitored to detect any change in pressure. (Exposure of the piping to a hot sun can falsify the readings.)... 

At the end of the holding pressure phase the clamping force can be reduced. Generally this will be half... 

Injection pressures 800-1700 bar Holding pressure must be sufficiently high and of sufficient duration. No pressure jumps should occur during the holding pressure phase, so as not to interfere with the uniform crystallisation cycle. For precision components, gates of around 2/3 of maximum wall thickness are required (so that the holding pressure can operate as long as possible)... 

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