VVaalliidd ssiiggnnaall rreeggiioonn

TTiimmee

1: Start of injection

2: Melt reaches the sensor 2-3: Filling phase

3: Volumetric filling

3-4: Melt compression phase 4: Maximum cavity pressure 4-5: Holding phase

5: Gate sealing point

5-6: Additional cooling phase 6: Atmospheric pressure

profile; however, the molded product of the cavity will be faulty。 If the sensor is installed in an appropriate runner position to obtain a history profile approximating that of the cavity, it not only monitors the process, but will also increase productivity。 In addition, for small products, the pressure sensor cannot be installed in the cavity, but in the runner; however, the internal geometric appearance of the runner is apparently different from the cavity。 Therefore, this study attempts to determine the appropriate position in the runner for installing a pressure sensor, which can represent the posi- tion in the cavity in order to implement monitoring, control, and increase productivity。

2 Cavity pressure

Considering the increasing demand for high-quality injection molding, the cavity pressure and temperature control of an injection molding machine are crucial technology; however, only the pressure profile can continuously describe overall the filling, compression, packing, and cooling stages。 The cavity pressure is closely related to the features of a molded part, such as weight, crystallinity, flash, shrinkage, and warpage [38]。 Therefore, recording the cavity pressure not only guar- antees product quality but also monitors the tolerance limits。 Typical and constantly recurring characteristic points along the cavity pressure profile are as shown in Fig。 1 [5, 38]。 The

Fig。 1  Cavity pressure profile at the near gate

To achieve precision control, the hydraulic injection mold- ing machine used servo valves within which an electrohydrau- lic system offers the advantages of both electronic and hy- draulic systems。 The merits of these machines include superior control accuracy, larger power output, and higher response speed; the system has been widely used in the injection molding machine for precision position control。 Kamal et al。 [40] and Abu Fara [41] have developed cavity pressure models for the cavity pressure history of an injection molding process。 A deterministic (step) test was implemented in the injection molding machines with electrohydraulic systems, and pressure sensors were installed at different locations in the mold cavity of a rectangular plate。 Similar cavity pressure history was obtained, and a time delay in the response was observed as a result of the percent change in the servo valve opening during the injection molding process。

Curve fitting is conducted according to the cavity pressure profile obtained from the experiment; thus, the cavity pressure model of the near gate at the filling stage based on various step changes in the servo valve opening can be obtained。 In com- parison to the first-order model, the second-order model has better accuracy; however, the difference is slight。 Therefore, the simple first-order plus a time delay model superimposed on a constantly increasing (ramp) pressure component of the form is reasonably expressed, as follows [40, 41]:

。 。

figure indicates the pressure commutation point related to the

different stages, while the cavity pressure profile reflects the entire injection molding process。 The cavity pressure profile at

PcðtÞ ¼ K1t þ K2  1−e−ðt−DÞ=τ

ð1Þ

the near gate is pided into four stages: filling, melt compres- sion, packing, and additional cooling stages。 There are three commutation points: filling end (volumetric filling), maxi- mum cavity pressure, and gate sealing。 The volumetric- filling point represents the position where the pressure rises sharply。 When the melt is injected into the cavity end, the melt is compressed, and the cavity pressure reaches its maximum。 After packing and cooling time, as molded part’s surface is solidified, the pressure gradually decreases。 In summary, the cavity pressure profile is in close relation to the position of the sensor in the cavity [14–16, 23, 39]。