Synchronization of parison profile in a plastic container molding system

In a blow-molding system in which a parison programmer is coupled to orifice tooling in an extrusion die head for controlling wall thickness of an extruded parison fed to a blow-molding apparatus, a method of synchronizing operation of the parison programmer contemplates placement of observable indicia on the parison after the parison exits the die head. This indicia placement preferably is carried out under control of the parison programmer to place the indicia at a predetermined position along the parison profile, such that mis-positioning of the indicia indicates absence of phase synchronization of the parison programmer. The indicia is then observed on a molded article to determine phasing of the parison programmer relative to the molding apparatus, and operation of the parison programmer is adjusted to correct any mis-positioning. In the preferred implementation of the invention, the readable indicia comprises an optically readable spot or dot, and position of this spot or dot on the molded article is determined by an optical sensor that provides an electrical signal for automatic closed-loop correction of parison programmer phasing without operator intervention.

The present invention is directed to systems for blow-molding articles such 
as hollow plastic containers, and more specifically to a method and 
apparatus for synchronizing parison wall thickness profile to the molding 
apparatus. 
BACKGROUND AND SUMMARY OF THE INVENTION 
In conventional systems for blow-molding articles such as hollow plastic 
containers, an extrusion die head contains orifice tooling that is 
controlled by a parison programmer to vary the wall thickness according to 
a predetermined thickness profile lengthwise of the parison. The hollow 
extruded parisons are fed in sequence to blow molding apparatus, typically 
comprising a plurality of mold-half pairs that pass in sequence adjacent 
to the extrusion head for receiving the parisons. A blow molding system of 
this character is disclosed, for example, in U.S. Pat. No. 4,523,904. The 
purpose of the parison programmer is to control the orifice tooling to 
provide a parison wall thickness that changes along the length of the 
parison in correspondence with changes in material requirements along the 
length of the molded container. Since any change in wall thickness also 
changes the length of the parison in a free-extrusion system, or the 
stretch of the parison in a captured-extrusion system, it is difficult to 
coordinate programmer position along the parison relative to the desired 
position in the molded container (i.e., programmer "phase" 
synchronizations). 
It has heretofore been proposed to attempt to synchronize operation of the 
parison programmer by making a radical profile change at a specific point 
along the profile, and then checking the position of this radical change 
in the molded container. However, the act of making such a radical change 
itself tends to change synchronization or phase of the parison profile. 
Furthermore, all containers made during this process are not usable and 
must be scrapped. It is a general object of the present invention to 
provide a method and apparatus for synchronizing operation of the parison 
programmer in a blow-molding system of the described character that may be 
readily implemented in existing systems without substantial modification 
or expense, and that may be readily employed by relatively unskilled 
personnel. Another and more specific object of the present invention is to 
provide a method and apparatus of the described character that 
automatically synchronizes operation of the parison programmer to the 
molding apparatus in a closed loop system without operator intervention. 
In a blow-molding system in which a parison programmer is coupled to 
orifice tooling in an extrusion die head for controlling wall thickness of 
an extruded parison fed to a blow-molding apparatus, a method of 
synchronizing operation of the parison programmer in accordance with the 
present invention contemplates placement of observable indicia on the 
parison after the parison exits the die head. This indicia placement 
preferably is carried out under control of the parison programmer to place 
the indicia at a predetermined position along the parison profile, such 
that mis-positioning of the indicia indicates absence of phase 
synchronization of the parison programmer. The indicia is then observed on 
a molded article to determine phasing of the parison programmer relative 
to the molding apparatus, and operation of the parison programmer is 
adjusted to correct any mis-positioning. In the preferred implementation 
of the invention, the readable indicia comprises an optically readable 
spot or dot, and position of this spot or dot on the molded article is 
determined by an optical sensor that provides an electrical signal for 
automatic closed-loop correction of parison programmer phasing without 
operator intervention. 
Apparatus for synchronizing operation of the parison programmer in a system 
for blow-molding hollow plastic articles such as containers includes an 
ink jet device coupled to the parison programmer for placing optically 
readable indicia on parisons as they exit the extrusion die head at a 
preselected position lengthwise of the parisons coordinated with wall 
thickness profile programming prestored in the parison programmer. An 
optical sensor observes position of this optically readable indicia on 
containers molded by the molding apparatus, and operation of the parison 
programmer and orifice tooling relative to the die head and the molding 
apparatus is altered, preferably automatically, so as to place the indicia 
at a predetermined location relative to the molded container. In the 
preferred embodiment of the invention, the optically readable indicia is 
placed on a portion of the molded container that will be subsequently 
removed and discarded (or recycled), together with a scale molded into the 
container to facilitate operator observation of parison programmer phasing 
.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
The drawing illustrates a system 10 for blow-molding hollow plastic 
containers as comprising an extruder 12 that receives molten plastic from 
a source not shown. The plastic is fed by extruder 12 to a die head 14 
that includes orifice tooling 16 coupled to suitable positioning means 
such as an hydraulic servo actuator 18. Actuator 18 is connected to an 
hydraulic power pack 20 that is controlled by a parison programmer 22. 
Programmer 22 also receives a feedback signal from servo actuator 18. 
Programmer 22, which may comprise a PLC or microprocessor-based 
controller, contains prestored programming for controlling the position of 
orifice tooling 16 relative to extrusion head 14, and thereby controlling 
wall thickness of the parisons 24 formed by extrusion head 14. Such 
prestored programming relates parison wall thickness profile to lengthwise 
position along parison 24, and may be displayed on an operator screen 26 
as an incremental function of parison axial length. The successive 
parisons 24 are fed to molding apparatus 28, which may include a series of 
mold-half pairs 30, 32 on a moveable turntable. The sequential parisons 24 
are captured between mold halves 30, 32 and blow-molded to form blown 
containers generally indicated at 34. 
To the extent thus far described, system 10 is of generally conventional 
construction. Above-noted U.S. Pat. No. 4,523,904, assigned to the 
assignee hereof, shows a system of this character in greater detail, and 
is incorporated herein by reference for purposes of background. 
In accordance with the present invention, an ink jet device 36 is placed 
adjacent to the outlet of die head 14, such as by mounting on the die 
head, and is connected to a source 38 of suitable ink and other associated 
hardware. Ink jet device 36 is electronically controlled by a signal from 
parison programmer 22. Ink jet device 36 is controlled by parison 
programmer 22 to place a dot or spot of ink on parison 24 as the parison 
exits die head 14 and at a position lengthwise of the parison coordinated 
with the wall thickness profile stored within programmer 22. That is, ink 
jet device 36 is operated by programmer 22 to place optically readable 
indicia in the form of an ink spot or dot at a predetermined position 
along parison 24 coordinated with a predetermined position along the 
prestored parison profile. 
An optical sensor 40 is positioned downstream of molding apparatus 28 for 
detecting the position of the optically readable indicia on molded 
container 34, and for providing an electrical feedback signal to 
controller 22 as a function of such position. For example, in the 
illustration of the drawing, the optically readable indicia is placed on 
parison 24 so as to be located in the moil 42 of container 34. The 
position of the ink dot relative to the axial length of moil 42 and 
container 34 is fed back to controller 22, at which such actual position 
is compared with the desired position along the parison wall thickness 
profile. If actual position of the ink dot is misplaced relative to the 
desired position, such mis-positioning indicates absence of phase 
synchronization of the parison programmer. The variable profile output of 
programmer 22 to power pack 20 is then advanced or retarded so as to 
correct such mis-synchronization, and consequently to correct misplacement 
of the ink dot relative to the molded container. 
In the preferred embodiment of the invention illustrated in the drawings, a 
scale 44 is also molded into container moil 42, which will assist operator 
determination of misplacement of the ink dot. Operator observation of ink 
dot misplacement may lead to manual phase synchronization of the parison 
programmer, although fully automatic and continuous closed loop 
synchronization is preferred. Operation of ink jet device 36 and sensor 40 
may be continuously enabled, or periodically enabled at suitable control 
intervals. Placement of the ink dot and scale 44 in moil 42 means that the 
scale and ink dot will eventually be removed from container 34, and 
discarded or recycled. The ink dot is preferably, although not 
necessarily, of ultraviolet-responsive ink to enhance observability under 
suitable lighting.