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Cavity Pressure Profile - Fast acquisition measurement
In the plastic injection molding process, molten plastic at high temperatures is injected under pressure into a mold
cavity, where it fills the mold and solidifies to create the final product. This is a cyclic process that consists of four
phases: filling, melt compressing (or packing), holding, and cooling, each with a series of adjustable parameters that
determine the efficiency of the process and the quality of the final product.
A given configuration that produces a high quality final part will most likely not have the same result when producing
a different part with different characteristics. The optimal parameters for achieving a high quality product with high
efficiency depend directly on the mold used, the type of plastic, the machine, and many other aspects. Discovering
all these ideal parameters often involves a series of time-consuming trial-and-error iterations that do not guarantee to
find the perfect combination.
However, this can be considerably accelerated if the pressure profile inside the cavity of the mold during an injection
cycle is known: the cavity pressure profile is strongly linked to the final quality of the produced part and the production
efficiency.
A correctly shaped pressure transition between phases implies a high quality part produced. Moreover, a correct se-
tup will allow the machine run with lower energy consumption and faster cycle times. On the other side, a bad setup
can cause serious damages on the involved machine parts.
When adjusting the injection parameters, the goal is to achieve a Cavity Pressure Profile curve with a shape similar
to that of Figure A. Figures B to E show different scenarios where the pressure profile is not ideal, which will most
probably induce a production of a part with low quality.
A
A peak in the pressure profile, like the one shown in Figure B, denotes a late switch to the holding phase and/or a
high injection speed. Figure C implies an early switch to the holding phase. A short duration of the holding phase
reflects on the cavity pressure profile like shown in Figure D. Lastly, in Figure E a residual pressure is shown.
The high sampling rate and the high resolution of the QE2008-W sensors makes it possible to extract the shape of
the cavity pressure curve. In a nutshell, the high pressure inside the cavity of the mold generates a back-force on the
clamping unit, which is transfered as an elongation of the tie bars. This elongation during the injection cycle can be
measured via the sensors, obtaining the shape of the cavity pressure curve.
This method of analysis of the cavity pressure profile is often preferred among others because, as the sensors are
mounted non-invasively (magnetically), there is no need to modify the mold. The great advantage of the QE2008-W
sensors is that hey can be reused on several machines and many different molds.
Compression
phase
Filling
phase
B
Subject to technical modifications
Holding
phase
C
D
Residual
cavity
pressure
E
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