Evaluating extrudable material by forcing such material through an outlet of a chamber by means of a movable member then stopping and securing the movable member at a predetermined position and measuring the force on the material as it decays with time.

This invention relates to a method and apparatus for evaluating extrudable 
materials and more particularly for evaluating the processability of 
elastomers. Still more particularly, the invention provides an improved 
method and apparatus for determining viscosity and stress relaxation of 
elastomers. 
Stress relaxation is a useful indicator of the behavior of elastomer 
compositions when subjected to various processes such as extruding, 
molding and calendering, and is useful to characterize elastomer 
compositions in terms of dimensional stability, flow throughput, etc. 
Prior methods of measuring stress relaxation have been by indirect means 
such as compressing coupled with measurement of the recovery by transducer 
means or by photographing. Photographing a sample is a static, 
time-consuming method subject to operator error. Compression methods 
operate at low shear rates. A necessary requirement for a meaningful 
processability test is that the shear rates directly relate to those 
encountered in the actual process. Processing rates are typically in the 
range of 10 - 10.sup.5 sec.sup.-1. 
A quick and simple method of evaluating extrudable materials has been 
discovered which, in effect, comprises determining viscosity from stress 
on the material at high shear rate and at low shear rate and determining 
the stress relaxation. A supply of extrudable material is charged to a 
chamber having an outlet and a movable member for forcing the material 
through the outlet. The member is moved to extrude material from the 
outlet as in known procedures for determining viscosity with high shear 
rate instruments. The improvements comprise stopping the movable member at 
a predetermined position, securing it in such position and measuring the 
force as it decays with time. As a preliminary step, viscosity may be 
evaluated by measuring the force required to extrude the material at a 
constant rate of extrusion (constant shear rate) or by measuring the time 
to extrude a predetermined volume of material subjected to constant force 
(constant shear stress). 
The method may be adapted for automatically testing a number of elastomeric 
samples. For example, the samples may be cut from a sheeted elastomer by a 
pneumatic press and injected directly into a cassette. After the cassette 
is filled with samples, it may be placed in an oven until the samples 
reach equilibrium test temperature, removed from the oven and placed in 
the tester. The samples are automatically fed to the tester and subjected 
to the evaluation as described.

Referring to FIG. 1, the sample cutter (A) comprises a pneumatic cylinder 1 
which operates a plunger 2 at the bottom of which is a circular cutter 3. 
The assembly is arranged so that the cutter dies out the sample 5 from 
sheeted elastomer 4 and the plunger injects the sample into the cassette 
7. An advance mechanism 8 allows the cavities in the cassette to be 
accurately aligned with the plunger. The oven B which may be either 
microwave or electrical heating includes doors 9 and 10 which may be 
automatically controlled by timer 11 to allow the passage of the sample 
cassette in a predetermined time. The sample time in the oven should be 
sufficient to arrive at equilibrium test temperature without sample 
degredation. 
The tester C is preferably a Capillary Rheometer operating under constant 
shear rate conditions. The drive system 12 for the Capillary Rheometer 
preferably comprises a closed loop servo hydraulic cylinder, controlled by 
digital pulses supplied by the drive electronics 13. For example, the 
drive system may comprise an Olsen Linear Electro-hydraulic Pulse Drive 
Model No. LS300 manufactured by Olsen Control, Inc., Bristol, Conn., and 
described in U.S. Pat. No. 3,899,956. The Control 13 which includes the 
piston drive electronics as a programmer/controller may be a microcomputer 
such as the MCS40 microcomputer available from Intel Corporation. Guide 
rods 15 and 16 mounted between base support 17 and top support 18 guide 
the crosshead 19. The drive system drives the crosshead 19 at a constant 
rate independent of the loading effects of the material under test. 
Framing members 20 and 21 support the various components. The cassette 
"moving" member 8, comprises an air cylinder and indexing arm as better 
seen in FIG. 2, controls the movement of the cassette. The piston 22 is 
mounted to the crosshead via insulator 6 and is driven by the crosshead 19 
into the barrel assembly 23 to extrude strand 24 of test material. The 
stress electronics 25 measure the force required to extrude the tire 
material. 
As better seen in FIG. 2, the cassette 7 is transported sequentially by 
mechanism 8 placing the samples under the piston 22. A thermal system 
comprising temperature controllers (not shown) and heated chamber 
maintains the barrel 23 and orifice 26 at a predetermined temperature 
(35.degree.-290.degree. C). A heated piston 22 mounted on a moving 
crosshead 19 (FIG. I) under control of the electronics 13 (FIG. 1) drives 
the sample 5 into the barrel 23. The cassette 7 is preloaded with 
elastomer samples and usually pre-heated to the required test temperature. 
Following the pre-heat, the cassette is loaded into the tester under the 
command of the control electronics 13. The cassette advance mechanism 8 
will move the sample in the cassette 7 to the test position, which is 
directly beneath the heated piston 22. FIG. 2 indicates the initial 
position with the piston 22 raised and the test sample 5 in place. 
FIGS. 3 and 4 illustrate the transposition of the sample. In FIG. 3a, it 
can be seen that the sample under the applied force of the piston starts 
to extrude through the orifice 26 held in place by retaining nut 33. The 
pressure at the barrel is sensed by a pressure transducer 27 and an 
electronic signal is produced which is processed by the stress electronics 
25 (FIG. 1) and can be displayed on a chart recorder 34 FIG. 6. An 
illustration of a typical recorder trace is shown in FIG. 3b, which shows 
the pressure (stress) plotted with respect to time. After a short period, 
the stress reaches a substantially constant value. The constant position 
of the curve represents the equilibrium value of stress E. Since the 
piston speed is constant, and corresponds to a pre-selected shear rate, 
the viscosity of the material is found for that shear rate. The piston 
continues to move down until a limit is imposed by the mechanical stops 28 
and 29 coming in contact. 
The stopped position is illustrated in FIG. 4a. At this point, the piston 
is locked in a fixed position by the pressure exerted upon it, and the 
action of the stops. Flow of the material continues under the constant 
force but the pressure decays at an approximate exponential rate. The 
decay is illustrated in FIG. 4b. Points S1 and S2 are preselected stress 
values which are set in the stress electronics. The stress relaxation 
curve F, G and H can be characterized by detecting the average time for 
the stress to fall from value S1 to S2. 
It has been found that the point at which the piston is stopped should 
correspond to a constant sample volume in the barrel, for example, the 
volume designated V2, FIG. 5, to enable reproducable results from 
different samples. V.sub.1 in FIG. 5 illustrates the initial volume 
charge, which is not critical, although it should be sufficient to enable 
a constant stress level to be obtained consistent with the selected speed 
of the piston. The values corresponding to S1 and S2 can be selected to 
represent any two points on the relaxation curve, and will be selected to 
accommodate the wide range of elastomer characteristics. 
The preceding description referred to a piston which was driven at a 
constant speed (shear rate). As an alternative the piston can be driven at 
a constant stress by an air cylinder, not shown. The speed of the piston 
will then vary and to obtain a measure of viscosity the average rate of 
travel (FIG. 5) for a known volume is measured. The measurement is 
accomplished by arranging preferably two optical electronic switches 30 
and 31 spaced apart to represent a fixed sample volume. Alternatively, 
mechanical switches or any suitable means to detect a known volume could 
be used. The two switches 30 and 31 can be switched on by a light source 
33 attached to the moving piston assembly. By coupling the switches to an 
electronic timer (part of control electronics), the time `t` is 
established. The relaxation method previously described remains unchanged 
for this mode. Apparent viscosity can be calculated from the equation: 
EQU n = Fr.sup.3 t/8R.sup.2 LV 
f = force on piston 
r = orifice radius 
R = barrel radius 
L = orifice length 
V = volume of material 
t = extrusion time 
Viscosity n = shear stress/shear rate 
It can be seen that the variables will be piston force F for a constant 
rate of travel and t for a constant stress mode. All other terms will be 
machine constants for given conditions. Hence, the electronic system can 
generate viscosity data for either drive alternative. 
The control and signal processing circuitry are shown in greater detail in 
FIG. 6. In the embodiment illustrated, the drive system of the capillary 
rheometer is controlled by pulses to provide a constant rate of extrusion. 
The heart of the system is the programmer control 13 which in the 
preferred embodiment would be a micro-processor. The programmer controller 
13 and piston drive electronics 13a are shown separately in FIG. 6; also, 
the function of blocks 35, 41, 42, 43 and 44 would be part of the 
processor function, but are shown separately to give a better 
understanding of the operation. The operation of a micro-processor is well 
known, a suitable type being the aforementioned Intel Model #MCS40. The 
functions of the various blocks can be described as follows: 
At the start of the test sequence, a pulse from the programmer controller 
13 will cause the Cassette Indexing Mechanism 8 to advance the cassette to 
the test station. At this point, the programmer will cause the piston 22 
to move at a pre-selected rate which is determined and selected in the 
controller by selector means (not shown). The piston travel will continue 
downward and push the sample into the barrel of the test instrument. The 
pressure inside the barrel is monitored by the pressure transducer 27. The 
signals from the pressure transducer are fed to the stress electronics 25 
and can be fed to a chart recorder 34 on which the resultant stress signal 
is plotted. The signal from the stress electronics is converted to a 
digital form by the analogue to digital converter 35 and at a defined 
point during the test corresponding to the equilibrium stress value shown 
in FIG. 3b the gate 44 will be enabled by the programmer controller to 
record the stress value for display in digital display 36. When the piston 
travel is finally limited by the mechanical stops 28, 29 (FIG. 2), then a 
second Digital Display 37 will begin to record the time for the stress to 
decay from S1 to S2 (FIG. 4b). The points corresponding to S1 and S2 are 
selected on the stress switches shown as in Block 38. The points which are 
selected by stress selection switches 38 are compared with the actual 
decaying stress signal by a comparitor 41 in conjunction with the 
programmer/controller 13. The gate 42 is enabled when the stress level 
corresponds to S1 and pulses from the pulse generator 43 will be fed to 
the Display 37 for a period determined by S2 minus S1. This value 
represents the stress relaxation time constant. 
If desired, time constant limits or stress limits can be assigned in the 
micro-processor and the information displayed in 36 and 37 compared with 
such preselected limits. The limits and the actual test values may then be 
recorded on the printer tape punch 45. Furthermore, the values which are 
outside the preselected limits can be indicated. The points S1 and S2 
provide a measure of the average slope of the stress decay curve. If more 
precise measure of the slope (average rate of stress decay) is required 
means may be included to differentiate the stress values over the selected 
period of stress decay. This can be accomplished by arranging a circuit 
such as that described in Operational Amplifiers, Design and Application, 
Burr Brown, page 220, FIG. 6.18, published by McGraw Hill. The output of 
the differentiating circuit can be displayed and will represent the 
instantaneous slope of the relaxation curve at a specific time interval. 
This would be controlled by the programmer 13 and displayed on 37. It will 
also be apparent that the tests may be conducted at a plurality of 
different shear rates resulting in different peak or equilibrium values of 
stress (and corresponding values for relaxation). Testing at a plurality 
of different shear rates may be programmed into the apparatus and can be 
performed automatically. 
In the alternative embodiment described earlier and shown in FIG. 5, the 
viscosity is determined by the time for the piston to extrude a known 
volume of material. This time is controlled by the piston rate switches 
39. In this case, the apparent viscosity will be the result of gating 
through gate 44 the required number of pulses corresponding to "t" (FIG. 
5) the time to extrude a predetermined volume and will be displayed on 36. 
In either embodiment the addition of machine limit switches 40 is a 
desirable safety feature. 
Although the invention has been illustrated by typical examples, it is not 
limited thereto. Changes and modifications of the examples of the 
invention herein chosen for purposes of disclosure can be made which do 
not constitute departure from the spirit and scope of the invention.