Apparatus for measuring and controlling the wall thickness of plastic pipes

Apparatus for producing plastic pipes by an extrusion process in which the wall thickness of the plastic pipe is controlled by the takeaway speed at which a soft plastic pipe is removed from a metal sizing sleeve. To measure the wall thickness of the soft plastic pipe advancing through the sizing sleeve for regulating the takeaway speed, a transducer emits ultrasonic sound through a plastic transmission line mounted in or on the sizing sleeve and interposed between the transducer and the plastic pipe. The time interval between the emission of a sonic pulse by the transducer and the detection of a reflected sonic pulse from the inner wall of the advancing soft plastic pipe is a function of the wall thickness of the plastic pipe in a soft plastic state.

BACKGROUND OF THE INVENTION 
The present invention relates in general to apparatus for producing plastic 
pipes, and more particularly to an apparatus for producing plastic pipes 
by the extrusion process in which the thickness of the plastic pipe is 
controlled by the takeaway speed of soft plastic pipe. 
In the patent to Graves, et al., U.S. Pat. No. 4,137,025, issued on Jan. 
30, 1979, for Apparatus For Measuring And Controlling The Wall Thickness 
Of Plastic Pipes and the patent to Graves, et al., U.S. Pat. No. 4,152,380 
issued on May 1, 1979, for Method Of And Apparatus For Measuring And 
Controlling The Wall Thickness Of Plastic Pipes, there is disclosed an 
extrusion process for the manufacture of plastic pipes in which the inside 
diameter of the pipe and, therefore, the wall thickness of the pipe is 
controlled by the takeaway speed at which the molten plastic pipe is 
removed from the sizing sleeve. An ultrasonic transducer is disposed at 
the upstream end of the sizing sleeve to provide signals representative of 
the thickness of the molten pipe advancing in the sizing sleeve. These 
signals are sent to devices for comparing the measured thickness of molten 
plastic pipe with the desired thickness of the molten plastic pipe for 
producing a correction signal. The correction signal is employed for 
correcting the takeaway speed at which the molten plastic pipe is removed 
from the sizing sleeve. 
In the patent to Boggs, et al., U.S. Pat. No. 3,916,676, issued on Nov. 4, 
1975, for Method Of And Apparatus For Measuring Automatically Successive 
Sections of An Elongated Material, there is disclosed four crystal 
transducers spaced equal angular distances about the periphery of an 
advancing cable. The cable is immersed in water and the crystal 
transducers extend into the water. The crystal transducers are pulsed 
sequentially for measuring the thickness of the jacket of the cable in 
successive sections. In the patent to Boggs, et al., U.S. Pat. No. 
3,827,287, issued on Aug. 6, 1974, for Method Of And Apparatus For 
Measuring The Thickness Of Successive Sections Of A Cable Jacket, there is 
disclosed a pulsed crystal transducer for measuring ultrasonically 
successive sections of a cable having a jacket for measuring the thickness 
of the jacket. 
The patent to Saint-Amour, U.S. Pat. No. 4,520,672, issued June 4, 1985, 
for Thickness Measuring discloses a system for measuring the wall 
thickness of flexible tubing through ultrasonic techniques. Four 
ultrasonic probes are spaced about the tube in quadrature relation. The 
probes are disposed in water surrounding the flexible tube. 
The patent to Livingstone, U.S. Pat. No. 4,487,072, issued on Dec. 11, 1984 
for Ultrasonic Testing Of Tubular Goods discloses a test head with 
circumferential array of transducers oriented for flow inspection of 
non-rotating goods. The patent to Falgari, et al., U.S. Pat. No. 
4,089,227, issued on May 16, 1978, for Apparatus For Measuring The Radial 
Dimensions Of A Cylindrical Tube By Ultrasonics discloses an ultrasonic 
transducer whose emitting part is shaped like a ring for emitting pulses 
to measure the radial dimensions of a tube by ultrasonics. 
Krautkramer Branson Company of Lewistown, Pa., manufactured and sold an 
Ultrasonic Thickness Measurement System For Production Monitoring And 
Control (WDM-U) in which a probe transmitted a sound pulse at regular 
intervals through a delay medium into the piece to be tested. An interface 
echo and a backwall echo were returned to the probe. The transit time of 
the sound pulses was measured between the two pulses as the sound pulse 
travels through the test piece. 
Panametrics Ultrasonic Thickness Gauging System (5215 Series) manufactured 
and sold by Panametrics, Inc. of Waltham, Massachusetts used an ultrasonic 
pulse-echo technique. Short duration electrical pulses were sent to a 
piezoelectric transducer and the transducer converted the pulses into 
short bursts of high frequency sound energy. The sound energy was 
transmitted into the test material through water and through the test 
material and was reflected from the inside surface of the test material. 
The reflected sound waves were returned to the transducer and were 
converted into electrical pulses. 
LFE Corporation of Waltham, Massachusetts manufactured and sold the LFE 
System 535 for measuring and controlling the wall thickness of extruded 
plastic pipe lines. The system used ultrasonic sensors. For small diameter 
vacuum-sized pipes, four fixed probes were mounted at the sizing sleeve 
and were either submerged in water or operated in a spray tank. For larger 
diameter pressure-sized pipes, a moving probe scanned the entire 
circumference of a pipe by reversing direction of travel along the 
circumference of the pipe. The scanning probe was mounted at the sizing 
sleeve in a cooling tank and was either used in a spray tank or was 
submerged in water. 
In the manufacture of plastic pipes by the extrusion process, the wall 
thickness of the pipe is controlled by the takeaway speed at which the 
plastic pipe is removed from the sizing sleeve. Sizing sleeves have been 
made of metal. A metal sizing sleeve presents an acoustical impedance 
mismatch between the plastic pipe wall and the metal sizing sleeve during 
the transmission of ultrasonic sound. A large impedance mismatch between 
the metal sizing sleeve and the plastic pipe results in substantial energy 
dissipation of the ultrasonic sound at the interface between the metal 
sizing sleeve and the plastic pipe. 
In the manufacture of plastic pipes by the extrusion process, the 
ultrasonic sound has been generated by a transducer and transmitted to the 
plastic pipe through a sizing sleeve and water. The transducer and the 
sizing sleeve may be submerged in cooling water under vacuum in a vacuum 
sizing/cooling tank. Heretofore a separate vacuum cooling water system was 
installed for the sizing sleeve. 
SUMMARY OF THE INVENTION 
Apparatus for producing a hollow article by an extrusion process in which 
the wall thickness of the hollow article in a soft state is controlled by 
the takeaway speed at which a soft hollow article is removed from a sizing 
sleeve. To measure the wall thickness of the soft hollow article advancing 
through the sizing sleeve for regulating the takeaway speed, a transducer 
emits ultrasonic sound through a transmission line mounted on the sizing 
sleeve and interposed between the transducer and the soft hollow article. 
The time interval between the emission of a sonic pulse by the transducer 
and the detection of a reflected sonic pulse from the inner wall of the 
advancing soft hollow article is a function of the wall thickness of the 
soft hollow article. 
A feature of the present invention is the use of a plastic transmission 
line mounted on or in a metallic sizing sleeve and disposed between a 
sound transducer and a hollow soft plastic article for acoustical 
impedance matching to reduce the dissipation of sound energy between the 
transducer and the hollow soft plastic article and to improve the accuracy 
of the measurement of thickness of the hollow soft plastic article 
advancing through the sizing sleeve and to improve the quality and power 
of echo signals. The transmission line may be in the form of a rod when 
the transducer is fixedly positioned and may be in the form of segmental 
sections within or on the sizing sleeve when the transducers travel about 
the circumference of the hollow soft plastic article. Another feature of 
the present invention is the use of a sizing sleeve that enables the 
forming of a plastic pipe with a generally constant outer diameter and 
with minimum defacing or marring of the finish of the plastic pipe. 
Another feature of the present invention is the use of a sizing sleeve that 
enables cooling water under vacuum to fully circulate into the sizing 
sleeve, thus obviating the need of a separate water-vacuum system for the 
sizing sleeve. 
Another feature of the present invention is that the transmission line 
through which the ultrasonic sound is transmitted is disposed in the 
sizing sleeve in the vicinity of the generation of the wall thickness of 
the soft hollow article being produced. Thus, the location of the 
measurement of the thickness of the soft hollow article being produced for 
correcting the takeaway speed at which the soft hollow article is removed 
from the sizing sleeve is at the vicinity of the generation of the wall 
thickness of the advancing soft hollow article. As a consequence thereof, 
the time lag between the generation of an error in the wall thickness of 
the soft hollow article and the detection of an error in the wall 
thickness of the soft hollow article for correction is reduced. 
Additionally, the measurement of the wall thickness of the soft hollow 
article is taken while the outer skin of the soft hollow article is more 
rigid, but before the remainder of the soft hollow article reaches a more 
rigid state.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Illustrated in FIG. 1 is an apparatus 10 embodying the present invention 
employed in the extrusion process for the manufacture of a hollow article, 
such as a plastic pipe. The apparatus 10 comprises a hopper 11 in which a 
discharged well-known dry raw material for the production of plastic pipe. 
The hopper 11 discharges the dry raw material into a well-known multiple 
auger extruder 15. In a well-known manner, the extruder 15 uses one or 
more well-known auger-type devices, not shown, to knead and compress the 
raw material. External heat is applied to the extruder 15 and the 
combination of heat and pressure converts the dry raw material to a soft 
plastic material in a well-known manner. 
At the discharge end of the extruder 15, the soft plastic material is 
forced through a well-known die, not shown, which forms a hollow tube T in 
a soft plastic state. The hollow tube T has the approximate dimensions of 
the desired end product. 
After the hollow tube T is discharged from the die of the extruder 15, it 
is advanced into a metallic sizing sleeve 20 (FIG. 2) that is disposed on 
or in a conventional vacuum sizing/cooling tank 25 at the entrance or 
forward end thereof. The inside diameter (FIG. 2) of the sizing sleeve 20 
is dimensioned to form the outside diameter desired for the finished 
plastic pipe P (FIG. 1) plus an allowance for shrinkage. The length of the 
sizing sleeve (FIG. 2) is dimensioned so that a soft plastic pipe M 
advancing therethrough is in a generally soft state, except for the 
outside skin thereof which is rigid so that the soft plastic pipe M 
discharged by the sizing sleeve 20 will be of a predetermined, present 
outside diameter. 
The soft plastic pipe M advancing through the sizing sleeve 20 is urged 
against the inner cylindrical wall 20a of the sizing sleeve 20 by 
pressurizing the inside of the pipe M or by evacuating the vacuum 
sizing/cooling tank 25 and allowing atmospheric pressure to force the pipe 
M to be urged against the inner cylindrical wall of the sizing sleeve. A 
suitable vacuum pump 26 (FIG. 1) provides the vacuum for urging the pipe M 
against the inner cylindrical wall 20a of the sizing sleeve 20 by virtue 
of the pressure differential between the forces on the outside wall of the 
pipe M and the inside wall of the pipe M. 
In the vacuum sizing/cooling tank 25 is chilled water. In the alternative, 
the vacuum sizing/cooling tank may include a well-known water spray system 
of conventional spray heads, not shown, discharging chilled water. The 
chilled water extracts heat from the hollow tube T advancing into the 
vacuum sizing/cooling tank 25 and the pipe M advancing through the sizing 
sleeve 20. Fresh cooling water is continuously introduced into the vacuum 
sizing/cooling tank 25, while water heated by the cooling process is 
removed by means of a suitable pump 27. When the pipe M exits from the 
vacuum sizing/cooling tank 25 it is in the form of a rigid plastic pipe P 
(FIG. 1). #For purposes of clarity, the hollow tube T advances into the 
vacuum sizing/cooling tank 25 to be formed in the soft plastic pipe M and 
when the soft plastic pipe exits from the vacuum sizing/cooling tank 25, 
it is in the form of a rigid pipe P. For advancing the plastic pipe in its 
formation sequence through the sizing sleeve 20 and through the cooling 
tank 25, the rigid plastic pipe P is gripped and advanced by suitable 
takeaway means (FIG. 1). In the exemplary embodiment, the takeaway means 
30 comprises rollers 31 and 32, which grip the rigid plastic pipe P and 
advance the rigid plastic pipe P in the direction of an arrow (FIG. 1). A 
suitable drive motor 34, such as a d.c. drive motor, imparts rotation to a 
drive belt 35. The drive belt 35, in turn, rotates a sheave 36. The sheave 
36 is fixed to a common shaft with the roller 32 so that the sheave 36 
imparts rotation to the roller 32. The sheave 36 also drives a drive belt 
37, which, in turn, imparts rotation to a sheave 38. Fixed to the shaft of 
the sheave for rotation therewith is a gear 39, which meshes with a gear 
40 to impart rotation to the roller 31. The rollers 31 and 32 advance the 
rigid pipe P in the direction of the arrow 33. 
The inside diameter of the pipe P, and, hence, the wall thickness of the 
pipe P, is controlled by the speed at which the pipe M is removed from the 
sizing sleeve 20. Since the extruder 15 is discharging the hollow tube T 
at a constant rate and since the outside diameter of the pipe M, which is 
advancing through the sizing sleeve 20, is determined by the inside 
diameter of the sizing sleeve 20, the inside diameter of the pipe M is 
determined by the rate at which the pipe M is removed from the sizing 
sleeve by the takeaway means 30. The faster the rate of removal of the 
pipe M from the sizing sleeve 20 by the takeaway means 30, the thinner 
will be the wall thickness of the rigid plastic pipe P. Conversely, the 
slower the rate of removal of the pipe M from the sizing sleeve 20 by the 
takeaway means 30, the greater will be the wall thickness of the pipe P. 
In the exemplary embodiment, the rate at which the takeaway means removes 
the pipe M from the sizing sleeve 20 is controlled by the speed of 
operation of the d.c. motor 34. In the preferred embodiment, the thickness 
of the pipe M is measured while the pipe M is advancing through the sizing 
sleeve 20 for the forming of the inner wall of the pipe M. Hence, the 
correction of the takeaway speed for controlling the thickness of the 
plastic pipe P is made substantially while the soft plastic pipe M is 
being measured for the wall thickness thereof or in a very short time 
interval thereafter. 
In one embodiment of the present invention, the metallic sizing sleeve 20 
(FIG. 2) comprises an annular flange 20b, which is secured to the forward 
wall 25a of the vacuum sizing/cooling tank 25 by suitable means, such as 
nuts and bolts. The nuts and bolts are spaced equal angular distances 
apart and positioned equal radial distances from a common axis. Integrally 
formed with the annular flange 20b at the inner end thereof is an outer 
hollow or cylindrical wall 20c. Spaced radially inward from the outer 
cylindrical wall 20c is the inner hollow or cylindrical wall 20a. An 
annular disc 20d is integrally formed with the forward ends of the 
cylindrical walls 20c and 20a. The inner cylindrical wall 20a is 
concentric with the outer cylindrical wall 20c. The annular flange 20b is 
radially disposed relative to the cylindrical wall 20c and the inner 
cylindrical wall 20a. 
Formed along the inner surface of the inner wall 20a are grooves 45. 
Radially disposed holes 46 extend through the inner cylindrical wall 20a. 
The grooves 45 and the holes 46 serve to provide paths through which 
cooling water travels to contact the outer diameter of the pipe M to 
provide a cooling action and lubricating action. As a consequence thereof, 
the pipe M slides along the inner surface of the inner cylindrical wall 
20a and reduces the tendency of marking the outer wall of the soft plastic 
pipe M as it cools. The holes 46 also function to assure the forces 
applied by the atmospheric pressure on the inner wall of the soft plastic 
pipe M are greater than the forces applied to the outer wall of the soft 
plastic pipe M to cause the pipe M to be continuously urged against the 
inner surface of the inner wall 20a of the sizing sleeve 20. 
The above arrangement enables a plastic pipe P to be formed with a 
generally constant outer diameter and with minimum defacing or marring of 
the finish of the plastic pipe P. Cooling water under vacuum freely 
circulates between the inner cylindrical wall 20a and the outer 
cylindrical wall 20c of the sizing sleeve 20. 
Mounted within the sizing sleeve 20 and extending between the inner surface 
of the inner wall 20a and in the vicinity of the outer surface of the 
outer wall 20b are spaced apart, sound transmission lines 50 (FIG. 2). In 
the preferred embodiment, the sound transmission lines 50 are made of a 
non-metallic material, such as plastic material. In the preferred 
embodiment, the plastic material is VESPEL manufactured by DuPont 
Corporation or TORLON manufactured by AMOCO. VESPEL is a polyimide resin 
and TORLON is a poly(amide-imide) resin. 
The sound transmission lines made of VESPEL and TORLON material have been 
found to have a low acoustic impedance. The velocity of sound through the 
plastic transmission lines 50 is generally equivalent to the velocity of 
sound through the soft plastic pipe M for impedance matching. Thus, the 
transmission ines 50 have the capacity to withstand the high temperatures 
of the soft plastic pipe M without breaking down and also have good wear 
resistance for avoiding the defacement of the plastic pipe P. 
While reference is made herein to transmission lines 50, it is common for 
one skilled in the art to refer to the elements 50 as delay lines. 
Radially disposed relative to the axis of the sizing sleeve 20 and spaced 
from the axis of the sizing sleeve 20 for engaging the transmission lines 
50 respectively are transducers 55, such as ultrasonic piezoelectric 
transducers. In one embodiment of the present invention, the transducers 
55 (FIG. 2) are stationary and are secured to the transmission lines 50 in 
a suitable manner, such as threaded engagement. The transducers 55, in the 
exemplary embodiment, are manufactured and sold by Krautkramer Branson of 
Lewiston, Pa., Aerotech Fingertip with removable delay line, style DFR. 
In another embodiment of the present invention, there are stationary, 
segmental, spaced apart sound transmission lines 50' (FIGS. 3, 4 and 8). 
The sound transmission line segments 50' are mounted within a sizing 
sleeve 20' and are disposed in two planes and are generally quadrantly 
spaced from one another with a relatively small overlap to allow the water 
under vacuum to freely circulate within the sizing sleeve 20'. In the 
exemplary embodiment, there are four radially disposed arcuate 
transmission line segments 50' with two segments in a first plane and two 
segments in a second plane with the first and second planes located 
generally adjacent one another while leaving sufficient space therebetween 
for the free circulation of water. The transmission line segments 50' are 
made of material similar to the material of which the transmission lines 
50 are made and function in a similar manner. 
The central axis about which the transmission line segments 50' are 
disposed is coextensive with the axes of the inner cylindrical wall 20a' 
and the outer cylindrical wall 20c'. Radially disposed relative to the 
axis of the sizing sleeve 20' and axially spaced relative to the sizing 
sleeve 20' for movement in close proximity to the transmission line 
segments 50' in respective planes are a set of transducers 55', such as 
ultrasonic piezoelectric transducers. In the exemplary embodiment, the 
transducers 55' are manufactured and sold by Krautkramer Branson of 
Lewiston, Pa., AEROTECH style IPS of the Gamma Series. In the preferred 
embodiment, there are two transducers 55'. 
For mounting the transducers 55' for movement in the proximity to their 
associated transmission line segments 50' is a suitable annular shoe 56. 
(FIG. 3). The annular shoe 56 is disposed about the cylindrical wall 20c' 
of the sizing sleeve 20'. The shoe 56 is rotatable about the cylindrical 
wall 20c'. Suitable retaining means, not shown, retain the shoe 56 against 
excessive axial displacement relative to the axis of the sizing sleeve 
20'. Formed in the shoe 56 are radially disposed openings 57 in which are 
disposed the transducers 55', respectively. 
The transducers 55' through a cycle of rotation successively and 
sequentially move in close proximity to the stationary transmission line 
segments 50'. Fixed to the annular shoe 56 for imparting rotation thereto 
is a ring gear 60 (FIGS. 3 and 4). For rotating the ring gear 60 and the 
transducers 55' about the axis of the sizing sleeve 20' in alternate 
clockwise and counter-clockwise directions is a suitable drive gear 61. 
The drive gear 61 is arranged to mesh with the ring gear 60 to impart 
rotation to the ring gear 60 and the transducers 55'. The drive gear 61 
imparts rotation to the ring gear 60 so that the transducers 55' scan over 
the entire cylindrical wall of the pipe M for 360.degree. during the 
clockwise and counterclockwise cycle. A drive belt 62 (FIG. 4) 
interconnects a drive motor 64 with the drive gear 61 for imparting 
rotation thereto. 
In another embodiment of the present invention which is similar to the 
embodiment shown in FIG. 2, sound transmission lines 65 (FIGS. 5, 6 and 
9), such as non-metallic or plastic sound transmission lines, are mounted 
within the sizing sleeve 20 and extend between the inner surface of the 
inner wall 20a and the outer surface of the outer wall 20b of the sizing 
sleeve 20. In the exemplary embodiment, there are four transmission lines 
65 disposed in quadrature. The sound transmission lines 65, in the 
preferred embodiment, have cylindrical configurations. The central axis 
about which the transmission lines 65 are disposed is coextensive with the 
axes of the inner cylindrical wall 20a and the outer cylindrical wall 20c 
of the sizing sleeve 20. The axis of each cylindrical transmission line 65 
is radially disposed relative to the axes of the inner cylindrical wall 
20a and the outer cylindrical wall 20c. In the preferred embodiment, the 
sound transmission lines 65 are made of non-metallic material or plastic, 
such as VESPEL, manufactured by DuPont Corporation or TORLON, manufactured 
by AMOCO. 
Formed in each transmission line 65 (FIGS. 5 and 6) is a threaded, axially 
disposed, cylindrical opening. Disposed in the threaded openings, 
respectively, in threaded engagement with the transmission lines 65 are 
transducers 70, such as ultrasonic piezoelectric transducers. The 
transducers 70 are fixedly secured by threaded engagement with the axes of 
the transmission lines 65, respectively. In the exemplary embodiment, 
there are four piezoelectric transducers 70 radially disposed in 
quadrature relation. Each transducer 70 is radially aligned and in contact 
with a respective transmission line 65. It is within the contemplation of 
the present invention, the transducers 70 may be slipped in type as well 
as being threaded. 
In the exemplary embodiment, the ultrasonic, piezoelectric transducers 70 
are of the type manufactured and sold by Krautkramer Branson of Lewiston, 
Pa., Aerotech Fingertip with removable delay line, style DFR. Sound pulses 
produced by the previously disclosed transducers 50 are transmitted and 
reflected back through the associated transmission lines 50. Similarly, 
sound pulses produced by the transducers 70 are transmitted and reflected 
back through the associated transmission lines 65. 
The transmission lines 50 and 65, which are preferably cylindrically 
shaped, are disposed in intimate contact with the outer surface of the 
soft plastic pipe M advancing through the sizing sleeve 20 to enable 
cooling water under vacuum in the vacuum sizing/cooling tank 25 to freely 
circulate into the sizing sleeve 20. As a result thereof, the need of a 
separate vacuum cooling water system for the sizing sleeve 20 has been 
obviated. 
A fluid couplant, such as EXOSEN couplant manufactured by Krautkramer 
Branson of Lewiston, Pa., is placed between transducers, such as the 
transducers 55 and 70, before the transducers 55 and 70 are screwed in or 
placed into the transmission lines 50 and 65 to assure that sound pulses 
do not travel through air. In the case of the rotating transducers, the 
couplant is a steady stream of water, i.e. tap water, running through the 
hose 57' and through the suitable hole 58 to a cavity formed in the shoe 
56. A film of water is always present between the transducers 55' and the 
transmission line segments 50'. 
In the preferred embodiment, the transmission lines 65 and 50 have grooves 
or 60 degree V-grooves 65', 0.006 to 0.008 inches deep and spaced 
approximately 40 grooves per inch on their outer diameters (FIG. 5). The 
grooves 65' reduce the tendency for sonic waves to be reflected by the 
outer diameter of the transmission lines, thereby causing noise at the 
signal processing unit 75. 
In practice, the stationary or fixedly positioned transducer arrangement is 
employed when plastic pipes of relatively small outer diameter are 
produced, i.e. 1/2 inch outer diameter to 4 inches outer diameter. On the 
other hand, the rotatable transducer arrangement is employed when plastic 
pipes of relatively large diameters are produced, i.e. outer diameters in 
excess of 4 inches. 
In operation, the transducer 55 (FIG. 2) transmits a sonic or a supersonic 
pulse A (FIG. 7) at a time T.sub.1 through the transmission line 50 and 
through the soft plastic pipe M. The sonic energy is returned or echos 
back to the transducer 55 to be detected as a pulse B at a time T.sub.2 
(FIG. 7). The pulse B is sonic energy reflected from the near end of the 
transmission line 50. A second pulse of sonic energy will be reflected and 
returned to the transducer 55 for detection as echo C at a time T.sub.3. 
The echo C is generated at the outer wall of the soft plastic pipe M. A 
third echo of sonic energy will be reflected and returned to the 
transducer 55 for detection as echo D at a time T.sub.4. The echo D is 
reflected from the inner wall of the soft plastic pipe M. 
The time interval between the emission of the source pulse by the 
transducer 55 and the detection of the reflected echo pulse from the inner 
diameter of the soft plastic pipe M by the transducer 55 is a function of 
the wall thickness of the soft plastic pipe M and the sonic velocity of 
the material through which the sonic pulse travels. Measurement is taken 
of the time it takes for sonic energy to travel between the near and far 
wall of the pipe M. 
Each transducer 55 (FIG. 2) transmits a sonic or supersonic pulse A (FIG. 
7) at a time T.sub.1 through its associated transmission line 50 and 
through the soft plastic pipe M. The sonic energy is returned to the 
transducer 55 to be detected as a pulse at a time T.sub.2. The pulse B is 
sound energy reflected from the adjacent end of the associated 
transmission line 50. A second pulse of sonic energy will be reflected and 
returned to the transducer 55 for detection as a pulse C at a time 
T.sub.3. The pulse C is reflected from the outer wall of the soft plastic 
pipe M through the associated transmission line 55. A third pulse of sonic 
energy will be reflected and returned to the transducer 55 for detection 
as pulse D at a time T.sub.4 through the associated transmission line 55. 
The pulse D is reflected from the inner wall of the soft plastic pipe M. 
The time interval between T.sub.3 and T.sub.4 (FIG. 7) represents the wall 
thickness of the soft plastic pipe M substantially at the time of the 
formation of the inner wall of the soft plastic pipe M. 
The transducer 55 transmits electrical signals over conductors 72 
representative of time intervals between emitted pulses and detected 
pulses to a well-known signal processing unit 75 (FIG. 1). The wall 
thickness of the pipe M is determined by the time interval between echos 
T.sub.3 and T.sub.4 times the known sound velocity of plastic pipes. 
The signal processing unit 75 may be of the type manufactured and sold by 
LFE Corporation of Waltham, Mass. as the LFE System 535 console. The 
transducer generated electrical pulses are applied to the signal 
processing unit 75 successively and sequentially, and are representative 
of the time intervals between reflected sound pulses detected by the 
transducer. The electrical pulses are multiplexed and interrogated one at 
a time. The results of the readings are stored and averaged. The output 
signal of the signal processing unit 75 is applied to a suitable 
controller 80 such as the ACP-105 controller manufactured and sold by LFE 
Corporation for adjusting a conventional motor control to regulate the 
takeaway speed of the motor 34. 
The signal processing unit 75 may be the Model 5215 Ultrasonic Gauge 
manufactured and sold by Panametrics, Inc. of Waltham, Mass. An electrical 
pulse is applied to a piezoelectric transducer by the signal processing 
unit 75 and the transducer applied electrical pulses derived from 
reflected sound pulses to the signal processing unit. The procedure is 
multiplexed for each transducer step at a selectable rate. The Panametrics 
5215 Ultrasonic Gauge operates on the pulse-echo principle. It excites a 
piezoelectric transducer for a short time duration by an electrical pulse. 
The transducer converts the electrical energy into sound energy. The sound 
energy is applied to a soft plastic pipe. The sound energy travels through 
the wall of the soft plastic pipe and is reflected back to the transducer 
emitted from the sound energy. The same transducer converts the reflected 
sound energy into electrical pulses. The electrical pulses representing 
the reflected sound waves are applied to the Panametrics 5215 Ultrasonic 
Gauge. 
The 5215 Ultrasonic Gauge selects from the returned pulses the appropriate 
echo signals that will be used in the time-interval measurement. The 
time-interval is measured and the time-interval is electronically 
multiplied by the material velocity factor. The signal representing the 
thickness correction factor is applied to the controller 80, which, in 
turn, regulates the motor control 81. The motor control 81, in turn, 
adjusts the takeaway speed of the motor 34 to compensate for the error in 
wall thickness of the soft plastic pipe M. 
The present invention is concerned with the method of transmitting 
ultrasonic signals through a sizing sleeve. The specific form or 
arrangement of circuitry in the controller 80 is not a part of the 
invention. The controller may involve a microprocessor which would compare 
signals received from the signal processing unit 75 with prescribed limits 
set into the controller with a keyboard. When the measured thickness 
exceeds the preset limits the controller would cause the puller motor 
control 81 to either speed up or slow down the puller motor 34. A 
microprocessor based controller 80 can manipulate the signals received 
from the signal processing unit in any number of ways to control the wall 
thickness of the extruded plastic pipe by controlling the speed of the 
rollers 31 and 30. A simple on-off system would either speed up or slow 
down the rollers a very small preset amount and then interject a time 
delay before allowing another correction to be made in order to allow the 
extrusion system to "settle down". This would ensure that any thickness 
measurements made represent a stable extrusion line condition rather than 
a transient line condition. 
The controller might also average signals received from the signal 
processing unit 75 and compare them with preset standards at preset time 
intervals. A system of this general type may be utilized with the rotating 
transducer arrangement shown in FIGS. 3 and 4.