WO2011047226A1

WO2011047226A1 - Method and apparatus for closed loop blow molding

Info

Publication number: WO2011047226A1
Application number: PCT/US2010/052784
Authority: WO
Inventors: Sumit Mukherjee; Long Fei Chang; Frank E. Semersky
Original assignee: Plastic Technologies, Inc.
Priority date: 2009-10-15
Filing date: 2010-10-15
Publication date: 2011-04-21

Abstract

A method and apparatus for blow molding containers is disclosed, the method including the steps of; a) heating a preform to a desired temperature in an oven having a plurality of heating units in a stack; b) blow molding a container from the heated preform; c) measuring a wall thickness at a desired location along a longitudinal axis of the container; d) generating a signal representing the wall thickness; e) comparing the signal representing the wall thickness to a standard range of wall thickness measurements; and f) adjusting the energy output of the plurality of heating units of the oven to affect the production of subsequent containers if the signal is not within the standard range, wherein the energy output of at least one heating unit is increased and the energy output of at least another heating unit is decreased by an amount equal to the increase.

Description

TITLE

METHOD AND APPARATUS FOR CLOSED LOOP BLOW MOLDING

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit of priority of United States

Provisional Patent Application Serial No. 61/251 ,964 filed on October 15, 2009 hereby incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

[0002] The present invention relates to a closed loop blow molding method and apparatus. More particularly, the invention is directed to a method for automatically adjusting a blow molding process based on a sidewail thickness profile of a blow molded plastic container to minimize unacceptable variation in the sidewail thickness profile. Furthermore, the invention facilitates the use of preforms having a lighter weight than standard preforms by maintaining the thickness of the sidewalls of the blow molded container within a narrower range.

BACKGROUND OF THE INVENTION

[0003] Biaxially oriented containers may be manufactured from plastic materials such as polyethylene terephthalate (PET) using various methods, including blow molding. One method of blow molding containers involves preforms carried in molds having an inner cavity in the desired configuration or shape of a finished container to be formed. To prepare the plastic material of the preforms to be stretched and blow molded, the preforms are heated to a predetermined temperature, exceeding the glass transition temperature of the plastic material by from about 5°C and about 40°C. Upon reaching the desired temperature, high pressure fluid, such as compressed air, is sequentially introduced into the hollow interior of each of the preforms. The preforms are thereby caused to expand and assume the shape of the associated mold. The process can produce a container that has excellent properties for use in many packaging applications, especially for use as a carbonated soft drink container. [0004] Completed containers are subsequently inspected for defects such as holes, deformations, unacceptably thin sidewalis, or unacceptable thick sidewalis. Variations in the sidewall thickness of the containers may be attributed to improper heating of the preform in an oven prior to the introduction of the pressurized fluid. If the sidewalis of the container are unacceptably thin, the sidewalis may rupture during a filling operation or transportation of the container. If the sidewalis of the container are unacceptably thick, the thickness of adjacent portions of the sidewall may be unacceptably thin or the container may have an undesired appearance.

[0005] Defective containers are removed from a production run and discarded as waste or recycled for later processing. Minimizing the number of defective containers minimizes the time and costs associated with a particular production run of containers. Further, minimizing the number of defective containers minimizes the time and cost associated with an operator investigating defects and diagnosing any problems with the blow molding machinery to make the appropriate adjustments thereto.

[0006] It would be desirable to develop a method for automatically adjusting a blow molding process based on a sidewali thickness profile of a blow molded plastic container to minimize unacceptable variation in the sidewali thickness profile.

SUMMARY OF THE INVENTION

[0007] Concordant and congruous with the present invention, a method for automaticaliy adjusting a blow molding process based on a sidewali thickness profile of a blow molded plastic container to minimize unacceptable variation in the sidewall thickness profile has been surprisingly discovered.

[0008] In an embodiment of the invention, a method of producing of blow molded containers comprises the steps of a) heating a preform to a desired temperature in an oven having a plurality of heating units in a stack; b) biow molding a container from the heated preform; c) measuring a wall thickness at a desired location along a longitudinal axis of the container; d) generating a signal representing the wall thickness at the desired location; e) comparing the signal to a standard range of wall thickness measurements for the desired location; and f) adjusting the energy output of the plurality of heating units of the oven to affect the production of subsequent container if the signal is not within the standard range, wherein the energy output of at least one heating unit is increased and the energy output of at least another heating unit is decreased by an amount equal to the increase.

[0009] In another embodiment of the invention, a method of producing of blow molded containers comprises the steps of: a) heating a preform to a desired temperature in an oven having a plurality of heating units in a stack; b) blow molding a container from the heated preform; c) measuring a wall thickness at a desired location along a longitudinal axis of the container; d) generating a signal representing the wall thickness at the desired location; e) comparing the signal to a standard range of wall thickness measurements for the desired location; and f) adjusting the energy output of the plurality of heating units of the oven to affect the production of subsequent container if the signal is not within the standard range, wherein the energy output of at least one heating unit is increased and the energy output of at least another heating unit is decreased by an amount equal to the increase; and g) repeating the comparing and adjusting steps for subsequent containers until a signal of a container is within the standard range.

[0010] In another embodiment of the invention, a blow molding apparatus comprises a) a preform feed and alignment device for transporting preforms through the apparatus; b) an oven including having a plurality of heating units for receiving and heating the preforms; c) a blow molding machine adapted to blow mold the heated preforms to form containers; d) an inspection device adapted to measure a plurality of wall thicknesses along a longitudinal axis of each container; and e) a controller in electrical communication with said oven and said inspection device, wherein the energy output of at least one heating unit is increased and the energy output of at least another heating unit is decreased by an amount equal to the increase. BRIEF DESCRIPTION of THE DRAWINGS

[0011] Other objects and advantages of the invention will become readily apparent to those skilled in the art from reading the following detailed description of a preferred embodiment of the invention when considered in the light of the accompanying drawings in which:

[0012] Fig. 1 is a schematic diagram of a blow molding system as described herein; and

[00 3] Fig. 2 is a schematic diagram of a preform and the heating units of Fig. 1.

[0014] It is to be understood that the present invention is not limited in its application to the details of construction and arrangement of parts illustrated in the accompanying drawings, since the invention is capable of other

embodiments, and of being practiced or carried out in various ways within the scope of the appended claims. Also, it is to be understood that the phraseology and terminology employed herein is for the purpose of description, and not of limitation.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[00 5] The following detailed description and appended drawings describe and illustrate various exemplary embodiments of the invention. The description and drawings serve to enable one skilled in the art to make and use the invention, and are not intended to limit the scope of the invention in any manner. In respect of the methods disclosed, the steps presented are exemplary in nature, and thus, the order of the steps is not necessary or critical.

[0016] Fig. 1 is a schematic illustration of a blow molding apparatus 0 for practicing the invention. The blow molding apparatus 10 includes a preform feed and alignment device 12, an oven 14 adapted to provide thermal energy to the preforms 24, a preform transfer device 16, a blow molding machine 18 where the preforms 24 are inflated and shaped into a container, a container transfer device 20, a container output 22, and an inspection device 23. The blow molding apparatus 10 further includes operating controls (not shown), drives (not shown), actuators (not shown), valves (not shown), switches (not shown), relays (not shown), wiring (not shown), plumbing (not shown), blowers (not shown), ducting (not shown), utility connections (not shown) and other related components common to blow molding machines.

[0017] The preform feed and alignment device 12 is a conveyor system adapted to receive a plurality of preforms 24 for transport through the blow molding apparatus 10. The preform feed and alignment device 12 includes spindles 26 adapted to receive the preforms 24. As best shown in Fig. 2, the preforms 24 may include a threaded finish 28 for receiving an associated closure cap and a shoulder 30 for handling and transporting the preform 24 or the container formed from the preform 24. The spindles 26 may orient the preforms 24 as shown in Fig. 2 or in an orientation where the threaded finish 28 is at a bottom of the stacks of heating units 34, as desired.The preforms 24 may be formed by an injection molding process as known in the blow molding industry. The preforms 24 may be at ambient temperature, or the preforms 24 may be preheated prior to introduction to the blow molding apparatus 10, as desired.

[0018] As shown in Figs. 1 and 2, the oven 14 has a substantially linear arrangement consisting of two pair of spaced apart rows 32 of heating units 34. Each row 32 contains a plurality of adjacent stacks of heating units 34. The oven 14 may have any number of adjacent stacks of heating units 34 and any number of heating units 34 in each stack, as desired. Favorable results have been obtained with the embodiment shown in Fig. 2 wherein each row 32 includes at least five adjacent stacks of heating units 34 and each stack of heating units 34 includes at least five heating units 34a, 34b, 34c, 34d, and 34e. Positive results have been obtained using stacks of heating units 34 having nine heating units 34. Furthermore, while the linear arrangement of heating units 34 in the oven 14 is a common arrangement, an array of heating units positioned aiong a circular path with an inner circular array of heating units may be used, as desired. The oven 14 also includes a reflective surface 36 adapted to reflect thermal energy from the heating units 34 to the preforms 24 passing through the oven 14. [0019] The preform transfer device 16 is adapted to transfer the heated preforms 24 from the oven 14 to an ingress of the blow molding machine 18. The blow molding machine 18 includes a rotating platform adapted to transport the preforms 24 therethrough during a blowing operation. A suitable rotating platform is manufactured by Sidel, a corporation of France and has a product number of SBO-1068. The container transfer device 20 is adapted to transfer blow molded containers from an egress of the blow molding machine 18 to the container output 22. The preform transfer device 16 and the container transfer device 20 may also be a rotating platform similar to that of product number SBO-1068. The inspection device 23 may be a wail thickness measuring apparatus as described in commonly owned United States Patent Number 6,985,221 issued on January 10, 2006, which application is hereby incorporated herein by reference in its entirety.

[0020] In use, the preforms 24 are disposed on spindles 26 of the preform feed and alignment device 12. As shown in Fig. 1 , a path of movement of the preforms 24 through the apparatus 10 is shown by a series of arrows A. Each of the preforms 24 is caused to travel through the oven 14 and pass between a pair of the rows 32 of the heating units 34. The preforms 24 are then caused to pass between a second pair of the rows 32 of the heating units 34. Thermal energy from the heating units 34 reflects off of the reflective surfaces 36 in the oven 14 and is redirected toward the preforms 24. The preforms 24 are heated to a desired temperature that does not exceed the glass transition temperature of the material forming the preforms 24. The preform transfer device 16 transfers the preforms 24 from an exit of the oven 14 to the blow molding machine 18. In the blow molding machine 18, the preforms 24 are disposed in molds (not shown) having an inner cavity in the desired configuration or shape of the finished container to be formed. High pressure fluid, such as compressed air, is sequentially introduced into the hollow interior of each of the preforms 24. The pressure fluid introduced into the hollow interior of the heated preforms 24 causes the preforms 24 to expand and assume the shape of the interior cavity of the mold as a completed container. [0021] As the pressure fluid is introduced into the molds, the platform of the blow molding machine 18 rotates the molds and preforms 24 through the blow molding machine 18. Completed containers are then transferred by the container transfer device 20 to the container output 22. Next, each container is inspected by the inspection device 23. The inspection device 23 measures the wall thickness of the container at desired locations along a longitudinal axis of the container. Each of the desired locations may correspond with the height of each heating unit 34a, 34b, 34c, 34d, 34e of the vertical stacks of heating units 34. In a controller 38, the wall thickness measurements are stored and compared to a standard range. The standard range is a stored range of acceptable wall thicknesses comprising an acceptably thin wall thickness and an acceptably thick wail thickness. A standard range is stored for each measurement location, for example adjacent the shoulder 30, at a base of the container, and at a plurality of locations along the body of the container between the shoulder 30 and the base. The wall thickness measurements of each container are also compared to the wall thickness measurements of preceding and subsequent containers.

[0022] The inspection device 23 generates an electric signal in response to a container having a sidewall thickness outside of the standard range at any of the measurement locations. The electric signal may be sent to an amplifier (not shown) and then to a logic circuit (not shown) in the controller 38. The logic circuit is operative to coordinate and keep track of the subsequent path of the container having the defective wall thickness and will send an appropriately timed rejection signal to an air blow-off station (not shown) to remove the container from the production line prior to filling or storage, if necessary. The air blow-off station may contain solenoid-operated valves controlling the flow of pressurized air capable of completing the rejection operation. The pressurized air is sufficient to remove the container having a wall thickness at a measurement location outside of the standard range for that measurement location from the production line. Containers having acceptable wall thicknesses at each measurement location are transferred from the inspection device 23 to a conveyor which transports the containers to a filling station (not shown). Finally, the filled containers are removed from the conveyor to be stored for later delivery or are immediately loaded on appropriate vehicles for delivery to the ultimate customer. Unfilled containers may also be off-loaded in a similar fashion.

[0023] If the wall thickness measurement performed by the inspection device

23 of at least one of the measured locations of the container is not with the standard range of wall thickness, the controller 38 sends an electrical signal to the oven 14 to adjust the power to the heating units 34 to control an amount of thermal energy produced thereby. To ensure that the energy generated by the change in power output of the heating units 34 does not cause a continuous increase or decrease of the overall temperature of the oven 14 and the preforms

24 traveling therethrough, an adjustment of the power output of a heating unit 34 is balanced by an equal and opposite adjustment of another heating unit 34. It is understood that the energy output adjustments may be balanced by adjustment of a plurality of heating units 34. The adjustment of the power output is made to heating units 34 in each stack of heating units 34. The adjustment may be made to each stack of heating units 34 or to selected stacks, such as heating units 34 in every other stack or in every third stack, for example.

[0024] The wall thickness measurements of a container are also compared to the wall thickness measurements of preceding and subsequent containers, thereby optimizing a power profile to be implemented in a subsequent adjustment of the operation of the apparatus 10. If the changes to the power output are deemed not to provide any improvement in minimizing a square error with respect to a measured wall thickness at a particular measurement location, the magnitude of the change of the power output is refined with a change thereto. The square error is defined as a weighted computed error square value between a target thickness and a desired, optimal thickness. Ideally, as the power output is iteratively adjusted and the sidewall thickness approaches the desired thickness, very minimal process changes are required, in contrast, if the sudden introduction of an adjustment in response to a high square error occurs, an algorithm for the closed loop process adapts to the change with a higher percent change to the power output of the heating unit(s) 34. [0025] Adjustments in the power to the heating units 34 may work in unison with a plurality of heating units 34 in the vicinity of the area of the preform being blow molded, and whose thickness is being tracked to be acceptable.

Accordingly, the algorithm may back calculate the power from particular heating units 34 that contribute a majority of the thermal energy to a particular

measurement location of the preform 24 and changes may be made to a single heating unit 34, or the power adjustment may be made to a plurality of heating units 34. If the power change is made to a piurality of heating units 34, the power change may be shared equally between the heating units 34 in a stack. The power change may aiso be prorated among the heating units 34 in the stack based on the linear distance of a particular heating unit 34 from a preform 24. For example, a tapered prefrom 30 has a wide portion closer to a heating unit 34 than a narrow portion thereof. The iterative calculation of power profiles also automatically takes into account an equilibration time for the power levels to stabilize and generate a steady temperature of the preforms 30. For example, the equilibration time may be between about 5 and about 50 preforms 24.

[0026] As a non-limiting and simple example, the power to the heating unit 34a may be adjusted by the controller 38 to provide an increase in energy produced thereby by five percent (5%). The five percent (5%) power adjustment may be made to one of the stacks of the heating units 34 or to a plurality of the adjacent stacks of heating units 34, as desired. Within the stack(s), the power adjustment may be made to only one of the heating units 34a with the power to the remaining heating units 34b, 34c, 34d, 34e each reduced by one-and-one- quarter 1.25% to provide an equal and opposite adjustment to power adjustment increase to the heating unit 34a. Alternatively, the power to heating unit 34e may be reduced by five percent (5%) to provide the equal and opposite change.

[0027] After the power adjustment, the wall thickness of containers produced are measured by the inspection device 23. The wall thickness measurements are then compared to the standard range of acceptable wall thicknesses at each measurement location of the preform 24. If the actual wall thickness

measurement is closer to the acceptable range but still outside the acceptable range, the power to the heating units 34 may be further adjusted by the controller 38 to provide a change of energy produced thereby by an additional three percent (3%). The power to the heating units 34 will be adjusted by the controller 38 until the actual wall thickness measurements are within the standard range of wall thickness measurements.

[0028] The feedback control according to the present invention will generate the required recommended process control functions. These controls will allow for better process monitoring, less need for labor to intervene in the process, less random variation in the product, and minimize the number of incidents of poor quality product being produced.

[0029] In accordance with the provisions of the patent statutes, the present invention has been described in what is considered to represent its preferred embodiment. However, it should be understood that the invention can be practiced otherwise than as specifically illustrated and described without departing from its spirit or scope.

Claims

WHAT IS CLAIMED IS:

1. A method of producing of blow molded containers comprising the steps of:

a) heating a preform to a desired temperature in an oven having a plurality of heating units in a stack;

b) blow molding a container from the heated preform; c) measuring a wall thickness at a desired location along a

longitudinal axis of the container;

d) generating a signal representing the wall thickness at the desired location;

e) comparing the signal to a standard range of wall thickness

measurements for the desired location; and

f) adjusting the energy output of the plurality of heating units of the oven to affect the production of a subsequent container if the signal is not within the standard range, wherein the energy output of at least one heating unit is increased and the energy output of at least another heating unit is decreased by an amount equal to the increase in energy output.

2. The method of producing of blow molded containers according to Claim 1 , further comprising a step of heating the preform prior to heating the preform in the oven.

3. The method of producing of blow molded containers according to Claim 1, further comprising a step of repeating the comparing and adjusting steps for subsequent containers until a signal of a container is within the standard range.

4. The method of producing of blow molded containers according to Claim 3, wherein an equilibration time elapses between subsequent adjusting steps to facilitate a thermal equilibration within the oven.

5. The method of producing of blow molded containers according to Claim 1, wherein the oven includes a plurality of stacks of heating units disposed in a row.

6. The method of producing of blow molded containers according to Claim 5, wherein the oven includes a pair of spaced apart rows of stacked heating units.

7. The method of producing of blow molded containers according to Claim 6, wherein the oven includes a plurality of pairs of spaced apart rows of stacked heating units.

8. The method of producing of blow molded containers according to Claim , wherein the preform is heated to a temperature below a glass transition temperature of a materia! forming the preform.

9. The method of producing of blow molded containers according to Claim 1 , further comprising the step of generating a rejection signal in response to a container having a wall thickness measurement outside of the standard range. 0. The method of producing of blow molded containers according to Claim 1, wherein the comparing step is performed by a controller in electrical communication with a measurement device for measuring the wall thickness and the oven.

11. The method of producing of blow molded containers according to Claim 10, wherein the controller performs the adjusting step by sending an electrical signal to the heating units.

12. A method of producing of blow molded containers comprising the steps of:

longitudinal axis of the container;

d) generating a signal representing the wall thickness at the desired location;

e) comparing the signal to a standard range of wall thickness

measurements for the desired location; and

f) adjusting the energy output of the plurality of heating units of the oven to affect the production of subsequent container if the signal is not within the standard range, wherein the energy output of at least one heating unit is increased and the energy output of at least another heating unit is decreased by an amount equal to the increase to the energy output; and

g) repeating the comparing and adjusting steps for subsequent containers until a signal of a container is within the standard range.

13. The method of producing of blow molded containers according to Claim 12, further comprising a step of heating the preform prior to heating the preform in the oven. 4. The method of producing of blow molded containers according to Claim 12, wherein the oven inciudes a plurality of spaced apart rows of heating units, each row having a plurality of stacks of heating units.

15. The method of producing of blow molded containers according to Claim 12, further comprising the step of generating a rejection signal in response to a container having a wall thickness measurement outside of the standard range.

16. The method of producing of blow molded containers according to Claim 2, wherein the comparing step is performed by a controller in electrical communication with a measurement device for measuring the wall thickness and the oven.

17. The method of producing of blow molded containers according to Claim 16, wherein the controller performs the adjusting step by sending an electrical signal to the heating units.

18. A blow molding apparatus comprising:

a) a preform feed and alignment device for transporting preforms through the apparatus;

b) an oven including having a plurality of heating units for receiving and heating the preforms;

c) a blow molding machine adapted to blow mold the heated

preforms to form containers;

d) an inspection device adapted to measure a plurality of wall thicknesses along a longitudinal axis of each container; and

e) a controller in electrical communication with said oven and said inspection device, wherein the energy output of at least one heating unit is increased and the energy output of at least another heating unit is decreased by an amount equal to the increase in the energy output.

19. The blow molding apparatus of Claim 18, wherein said oven includes a plurality of spaced apart rows of heating units, each row having a plurality of stacks of heating units.

20. The blow molding apparatus of Claim 18, wherein at least one row of heating units includes a reflective surface.