CA1068061A

CA1068061A - Plastics extrusion die head improvement

Info

Publication number: CA1068061A
Application number: CA254,433A
Authority: CA
Inventors: Thomas J. Bond; Richard C. Adams
Original assignee: Standard Oil Co
Current assignee: Standard Oil Co
Priority date: 1975-07-28
Filing date: 1976-06-09
Publication date: 1979-12-18
Also published as: NL7607010A; JPS5215557A; CH603339A5; BE843069A; IT1061457B; ZA763432B; DK321076A; FR2319480B1; DE2631838A1; SE7608533L; AU1512176A; GB1523809A; FR2319480A1

Abstract

PLASTICS EXTRUSION DIE HEAD IMPROVEMENT

Abstract of the Disclosure An improved plastic extrusion blow molding die head is described which permits the formation of extruded precision parisons having controlled wall thickness by partial restric-tion of the entering flow of the molten plastic material into the die head, and the improvement of including individual temperature control zones running parallel to the direction of plastic flow in the die.

Description

-` 1068061 This inverltion relates to an improved plastic extrusion die head and more particularly pertains to an improved plastic extrusion blow molding die head which permits the formation of extruded parisons of uniformly controlled wall thickness by pre-ferentially restricting the entering melt flow of the plastic material into the die head and carefully controllir.g the tempera-ture of zones in the die wall to insure even flow.
There has previously been described a plastics extrusion - -die head in which the rate of flow of thermoplastic material lO (melt) around the annular space of said die head is controlled !'.
and made uniform by preferentially restricting the entering melt flow of the plastic material into the die head. The present invention is an improvement over the apparatus described in U. S.
Patent No. 3,895,900.
In the present apparatus, the melt-flow control in pari-son formation includes elimination of parison hooking and insures ~
very uniform parison wall thickness throughout the entire circum- -ference of the parison at any given point along the parison, and makes possible the formation of preforms which are useful for the -~
2Q production of biaxially oriented plastic bottles and containers. ;
The present invention contemplates the use of heating zones around the circumference of the extru~ion die head which extend in the direction of melt flow and are individually con-trolled to adjust and fine tune plastic-flow control within the die head so as to enable one to produce routinely non-hooking parisons which can be used in the molding of biaxially oriented bottles of various shapes having uniform wall thickness.
It is an object of this invention to provide an appara-tus which enables one to extrude from an annular gap die a free-hanging plastic parison having uniform wall thickness around itscircumference without parison hooking. A parison of uniform wall thickness around its circumference is required to make biaxially

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oriented bottles having uniform walls from extrustion blow-molded preforms. Uniform parison wall thickness is also desirable but not as critical in normal extrusion blow molding of bottles and containers from thermoplastic materials.
In conventional blow molding, ninety-degree bends in the melt-flow path and die-head geometry cause uneven melt flow around the die end which often causes parison hooking or non-uniform parison formation. Parison hooking causes a high level of rejects in the formation of blow-molded containers therefrom. The melt flow-parison hooking problem has been remedied to a great extent by preferentially restricting the melt flow by means of a ring insert as in U.S. Patent No.

3,895,900. Without proper die-zone temperature control as provided herein, it is still possible to get some lack of uniformity in parison wall thickness around the circumference ; of the parison.
Thus, in accordance with the present teachings, there is provided an improvement in an extrusion blow molding die head comprising a die body which is equipped with a removable die bushing with the die body having an internally, somewhat generally conical hollow area fitted with a stationary tubular cone through which extends a movable internal shaft which has a core pin extending into and near the end of the opening in the die bushing. The die body also has a fixed `
gap and a channel for plastic flow near an insert ring which is located at the top of the tubular cone with an obstruction located at the bottom of the insert ring and extending about half the circumference of the tubular cone in the area where the plastic enters the channel for plastic flow. The improvement comprises adding a plurality of individual controlled and monitored temperature zones extending parallel to the direction of melt flow in the die with means being ~l ,",~ , ~ -3-~06~ )61 for monitoring and controlling individually the temperature of the zones.
The invention is more readily illustrated by the particular embodiment shown in the accompanying drawings.
Figure 1 is a side elevation of the die head of ' the present invention.
Figure 2 is a side-sectioned elevation of the die head of this invention.
Figure 3 is a view taken along line 3-3 in Figure 1.
Figure 4 is a diagrammatic view of an electrical heating arrangement for one embodiment of this invention.
The apparatus of this invention comprises a die body 1 having an internal, somewhat generally conical hollow area fitted with a stationary tubular cone 2. A
vertically movable internal shaft 3 extends through the cone ~-2, and protruding at the bottom end thereof is a core pin 4 ~ , which extends into and near the end of the opening in the ;~
die bushing 5. The die bushing 5 is equipped with an adjusting screw 6 for horizontal adjustment of the die ~-bushing 5 and is also equipped with a die bushing retainer 7.

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1068~61 A fixed gap 8 is provided in the die body 1 through which the plastic melt passes on its way out of the die bushing 5.
Near the top of the die body 1 is a channel for plastic flow 9 created near an insert ring 10 which is located at the top of the tubular cone 2. An obstruction which can be in the form of a welded bead 11 is located at the bottom of the insert ring 10 and extends for about half the circumference of the tubular cone in the area where the plastic melt enters the channel for plastic flow 9. The plastic melt is supplied to the die body by -:
an extruder (not shown) which is usually equipped with at least one screw. The end of the extruder is equipped with a screw tip 12 which is located at the end of the extruder barrel 13. Adapter ~ :
14 having an orifice 15 connects the screw tip 12 of the extruder to the die body 1 and allows passage of the plastic melt from the ~ -end of the extruder into channel 9 and through the die bushing 5. .
The plastic parison 16 (in phantom) emerges from the . .
die body 1 through an adjustable gap 17, the width of which can be -~
varied in one dimension by vertical adjustment of the internal shaft 3 and correspondingly core pin 4.
Heater zones I8 are provided with heat~monitoring and heat-control means 19 which are connected to a temperature monitor so that the individual heat monitoring and control means 19 can control and monitor independently each of the heating zones 18 for careful plastic~flow control to provide a parison wall with -extremeIy uniform thickness.
The novel extrusion blow molding die head of the present invention allows to a substantial degree balanced flow of the plastic out of the orifice in the production of parisons and more careful flow control through temperature control of the die :~
3a portions~ The parisons are subsequently used in the blow molding of bottles and the like, and are particularly useful for the for- :
mation of blow-molded preforms used to fabricate biaxially orien-.~.. . .

ted bottles having very uniform wall thickness. This is especially important in the use of "programming heads" where any unbalanced -flow of plastic causes the extrudate to "hook" and have poor tem-perature uniformity.
Uniform melt flow of the parison 16 around the annular space of the die 8 is obtained by employing four or more equally spaced heating zones 18 parallel to the direction of melt flow.
- sy individually controlling the temperature (watt density) in each zone, the melt flow around the circumference of the extrusion die can be bàlanced with the die adjusted for uniform wall thick-ness around the circumference of the parison. Temperature dif-ferences between zones (0-50F) cause changes in melt viscosity of the thermoplastic which in turn controls its flow. ~-Electrical strip heaters 18 can be employed to obtain the heater zone's required temperature for flow control. Shorter control response may be obtained by either using air cooling holes -in the extrusion head parallel to the melt flow or a liquid tem-perature-controlled zone parallel to melt flow, where both heating and cooling are available.
When using a conventional single-band heater in this apparatus, the wall thickness of a parison will be 0.125 inch plus or minus about 0.010 to 0.030, whereas with the zone-heated apparatus of this invention, the wall thickness will vary only from plus or minus 0.0005 to 0.005 inch from 0.125 inch thickness around the circumference of the parison. The more uniform wall th;ckness provided by the apparatus of this invention gives very much superior products in the biaxially oriented containers pre-pared therefrom. Closer tolerance in circumferential wall thick-ness of the parison also allows for weight reduction for the parison which results in reduction in costs of the finished container.
The apparatus of the present invention is particularly ~068~
useful in the extrusion blow molding of viscous resins such as the high-nitrile barrier resins, some of which are described in ~ .
U. S. Patent Nos. 3,426,102, 3,586,737 and 3,763,278 as well as rigid polyvinyl chloride and others.

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Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In an extrusion blow molding die head comprising a die body equipped with a removable die bushing, said die body having an internally, somewhat generally conical hollow area fitted with a stationary tubular cone through which extends a movable internal shaft having a core pin which extends into and near the end of the opening in the die bushing, said die body also having a fixed gap and a channel for plastic flow near an insert ring located at the top of said tubular cone and an obstruction located at the bottom of the insert ring and extending about half the circumfer-ence of the tubular cone in the area where the plastic enters the channel for plastic flow, the improvement comprising a plurality of individually controlled and monitored temperature zones extending parallel to the direction of melt flow in the die and means for monitoring and controlling individually the temperature of said zones.

2. The apparatus of claim 1 wherein the zones are elec-trically heated.