When cavity is nearly full during injection molding, movement of screw switches from flow rate control to pressure control. This conversion point is called holding pressure switching control point, that is, V/P conversion point.
Correctly setting turning pressure point is a very important process setting in injection molding production. If pressure is changed too early, it is necessary to use holding pressure to push too much molten material into cavity, which is easy to cause material shortage. If holding pressure is too late, product will be compressed too much, causing many parts to have flash and internal stress is too high, demoulding is not stable enough, and weight of product will be increased.
General switching methods include pressure switching, time switching, position switching and so on.

 

①Pressure switching means that priority of machine is pressure. When machine detects that instantaneous pressure of injection reaches V/P switching pressure you set, injection action will switch to pressure-holding action until pressure-holding is completed. Since this method is based on a stable and reliable pressure absolute value signal, this switching is the most effective. Due to use of pressure monitoring to eliminate impact of screw travel and check valves, this method cannot compensate for changes in hydraulic oil, melt, and mold temperature, that is, changes in injection rate.
The faster pressure build-up in compression phase, the more effective this method is, since in this case precise and timely switching avoids pressure peaks. When pressure is switched, because minimum detection data of pressure can reach 1KGF, it is sensitive to pressure accuracy, and is suitable for small precision parts.
②Position switching (or stroke switching) is different. When screw injection action reaches V/P switching position you set, injection action will directly switch to pressure-holding action. This method is the most commonly used method, and it has been proved to be effective in practice. Switching signal is issued by limit switch at holding pressure position. Since injection stroke is basically constant, it can be considered that this switching occurs once when same volume is filled. With very short packing strokes, this method is very unreliable because small changes will cause switch to not actuate accurately every time, in which case it is best not to switch. Since position switch does not consider switching pressure, the highest injection pressure should be as loose as possible.
③Method of time switching requires that a switching signal is released after a predetermined time after injection starts, and pressure holding is switched when time is up. This method does not take into account changes in melt compression, melt viscosity, feed accuracy, or hydraulic pressure at the front of screw, nor does it take into account changes in screw advance rate, while changes in hydraulic pressure will cause changes in screw position (end of feeding stage, beginning of pressure holding stage) and corresponding stroke change (feeding stroke, injection stroke). As a result, quality of workpiece, especially weight and size of workpiece, fluctuates greatly. It is suitable for making some ordinary products with low requirements. Therefore, method of switching based on time is generally not recommended.
Usually, injection pressure on injection molding machine is switched from position to holding pressure, and when it reaches holding stage, it is transferred from position switching to time switching multi-stage holding pressure.
Turning point is generally set according to fluidity of material:
Such as PC, PP, PE, PA and other plastics with good flow properties, generally when product is not thick-walled, it should be turned to holding pressure when product is 85-90% full, and holding pressure is generally within 60-80%.
However, for products with average fluidity, pressure-holding is generally turned on at 90-95%, and pressure-holding setting is generally around 70-85%; while plastics with poor fluidity, such as PMMA, will switch to holding pressure when mold cavity is about 95% full, and holding pressure is generally higher; in some cases, when wall thickness of product is relatively thick, it can be squeezed with high pressure and low speed in a short time, and then turned to maintain pressure.