IN SITU MEASUREMENTS
3.2 Results and Discussion
3.2.4 Configuration 4: Plastic Syringe With an Air Gap
79 The larger strains measured with a plastic syringe do not necessarily indicate that the maximum principal stress σ1 is larger with a plastic syringe than with a glass syringe. Equations3.4and3.5are used to estimate the wall stresses. The maximum principal stressσ1≈ σθalong the barrel is estimated to be 14 MPa, a value which is less than the 22 MPa previously obtained with a glass syringe. The peak compressive axial stress in the plastic syringe is estimated to be -4.4 MPa, a magnitude which is less than the -8.3 MPa previously obtained with a glass syringe. This difference is explained usingEquation 2.19, which suggests the peak axial stress depends linearly on the density of the syringe material.
To summarize, the results for a glass and a plastic syringe without an air gap are qualitatively similar. In both cases, pressurization of the liquid content and acceleration of the syringe occur nearly simultaneously. As a result, cavitation is suppressed. The system formed by the plastic syringe and the syringe carrier is more compliant than the system formed by the glass syringe and the syringe carrier.
The increased compliance alters the behavior of the system during the final stage of the rebound. The increased compliance of the plastic syringe also results in larger strains, not to be mistakenly associated with larger stresses. Because the Young’s modulus of glass is approximately 26.5 times larger than the Young’s modulus of plastic (seeTable 3.1), the strains are expected to be approximately 26.5 times larger in the plastic syringe than in the glass syringe for the same stress σapplied on the syringe wall ( ∼σ/E).
(a) Position
(b) Velocity
Figure 3.20: Position and velocity of the moving components in a SureClick autoin- jector – plastic syringe with an air gap (test SC-201).
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Figure 3.21: Liquid pressure in a SureClick autoinjector – plastic syringe with an air gap (test SC-201).
Figure 3.22: Hoop and axial strains on the barrel of the syringe in a SureClick autoinjector – plastic syringe with an air gap (test SC-201).
Earlier results obtained with a glass syringe have shown that adding an air gap in the syringe below the plunger-stopper affects the results substantially by creating cavitation, and the same is expected when adding an air gap in a plastic syringe.
The fact that this is not the case indicates there are fundamental differences in the performance of the plastic and the glass syringes in the SureClick autoinjector.
The most significant difference between the glass and the plastic syringes is in the timing between pressurization of the liquid content and acceleration of the syringe.
With the plastic syringe the large acceleration of the syringe begins 0.5 msafterthe impact of the driving rod on the plunger-stopper (see Figure 3.20b). This leaves sufficient time for the air gap to become substantially compressed, and for the liquid pressure to increase before the syringe is rapidly accelerated. Pressurization of the syringe prior to acceleration suppresses cavitation. The situation is the opposite when a glass syringe is used: the acceleration of the plunger-stopper and the syringe occur almost simultaneously (seeFigure 3.12b), not leaving sufficient time for the air gap to be compressed and the liquid pressure to increase prior to syringe acceleration.
Several experiments were performed with non-instrumented Daikyo syringes with and without an air gap. The results confirm that cavitation is generally not observed.
In the few cases where cavitation is observed, the cavitation event is very mild; the cavities or bubbles remain very small, and they do not collapse as violently as in the case of a glass syringe.
An interesting question arises in light of those results: what could explain the substantially different relative timing between acceleration and pressurization of the syringes? Figure 3.23is used to answer this important question. It is a sequence of images showing the motion of the driving rod, the plunger-stopper, and the syringe.
The sequence at the top ofFigure 3.23is for a glass syringe, and the sequence at the bottom is for a plastic syringe. The syringes used in those experiments are empty of liquid, which means the motion of the syringes results almost entirely from the friction between the plunger-stopper and the syringe barrel. All frames are separated by 0.2 ms. The black horizontal lines visible in each frame are on the syringe outer surface. The motion of the syringe is indicated with the red, continuous curve, and the motion of the top surface of the plunger-stopper is indicated with the blue, dashed curve.
Figure 3.23indicates the motion of the syringe and the plunger-stopper are signifi- cantly different for a plastic and a glass syringe. The plunger-stopper appears to stick to the barrel of the glass syringe. The glass syringe and the plunger-stopper, soon
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(a) Glass syringe (test SC-215)
(b) Plastic syringe (test SC-214)
Figure 3.23: Sequence of images showing the motion of the driving rod, the plunger- stopper and the syringe (0.2 ms between successive frames). The red, continuous curves indicate the position of the syringe. The blue, dashed curves indicate the position of the plunger-stopper.
after the impact event, travel at a similar velocity; there is minimal relative motion of the plunger-stopper into the barrel. In the contrary, the plunger-stopper does not stick to the barrel of the plastic syringe. The friction is minimal, and the relative motion of the plunger-stopper into the syringe is large. Those results indicate the coefficient of friction between the plunger-stopper and the syringe is much lower for the Daikyo than the BD HyPack syringes in the highly dynamic regime under consideration.3
The reduced friction force between the plunger-stopper and the plastic syringe barrel accounts for the absence of cavitation. The reduced friction results in a substantial air
3Note that the results could be different in a quasi-static regime more representative of the extrusion phase.
gap compression before the syringe is accelerated. This is because the acceleration of the syringe primarily results from the liquid pressure applied on the bottom wall of the syringe rather than the friction between the plunger-stopper and the syringe, even when an air gap is present. Because the syringe is pressurized prior to being accelerated, the tension waves created upon acceleration are not sufficiently large to reduce the pressure to sub-atmospheric values, and cavitation is inhibited.
To summarize, the friction force between the plunger-stopper and the syringe barrel during the transient events is much smaller with the plastic syringe than with the glass syringe in a SureClick device. This results in the syringe pressurization occurring before the syringe acceleration, even when an air gap is present. This has the effect of suppressing cavitation. The results suggest the syringe material (i.e., glass vs plastic) is not the likely cause for the different friction forces. The different lubrication film between the plunger-stopper and the syringe barrel likely explain the differences, but this needs to be confirmed experimentally. One possibility would be to test plastic syringes lubricated with silicone oil and to compare the results with those discussed in this chapter. Note that the results could be different when using the same syringe models in combination with a different autoinjector device.