Blow molding method and apparatus

Abstract

In a method of forming a container from a thermoplastic material, a thermoplastic preform can be introduced into a blow cavity at a temperature equal to or greater than the glass transition temperature of the thermoplastic material and less than the melting temperature of the thermoplastic material. A fluid pressure can be introduced into the preform which can expand the preform radially outwardly to form the semi-finished container. An outer surface of the semi-finished container can be engaged with a wall of the blow cavity. The wall of the blow cavity can be rapidly heated. Heat can be transferred to the outer surface of the semi-finished container. A surface finish can be imparted onto the outer surface of the semi-finished container from the wall of the blow cavity.

Description

[Technical field]

[0001] The present invention relates to thermoplastic containers, and more particularly, the present invention relates to a method and apparatus for achieving a high quality surface finish on blow molded thermoplastic containers. [Background technology]

[0002] Blow molding is the process of forming a molten tube (hereafter "parison" or "preform") of thermoplastic material, including polymers and / or resins, and placing the parison or preform into a blow cavity, as shown in Figure 1. The molten tube is expanded with compressed air to take the shape of the cavity and is allowed to cool before being removed from the mold.

[0003] Current conventional methods of achieving a high, high-end surface finish on blow molded thermoplastic containers may require secondary decorative processes, such as spraying, heat transfer, silk screening, mechanical embossing, etc. These secondary processes are generally applied to the container after molding. In other words, achieving a high, high-end finish on a thermoplastic container generally requires subjecting the container to further processing after it is molded.

[0004] Heating and cooling techniques can be used in injection molding applications to obtain a transfer or surface finish from the mold to the part. In injection molding, the thermoplastic material has a melt temperature above its melting point to enter the mold or cavity with the desired surface finish. The melt temperature is the actual temperature of the thermoplastic material as it exits the nozzle. Thus, the thermoplastic material is in a completely molten state and can solidify in the mold during the process so that the molded surface finish can be transferred to the part as the thermoplastic material solidifies. This molten state of the thermoplastic material can facilitate the transfer of the surface finish to the part. Summary of the Invention [Problem to be solved by the invention]

[0005] The present invention is provided to solve the above-referenced problems and other problems, and to provide advantages and aspects not offered by previous thermoplastic containers and methods of making same. A complete discussion of the features and advantages of the present invention is set forth in the following detailed description, which proceeds with reference to the accompanying drawings.

[0006] One aspect of the present invention is directed to a method of forming a container from a thermoplastic material, the method comprising the steps of: i. introducing a thermoplastic preform into a blow cavity at a temperature equal to or greater than the glass transition temperature of the thermoplastic preform and less than the melting temperature of the thermoplastic preform; ii. introducing a fluid pressure into the preform to expand the preform radially outwardly to form a semi-finished container; iii. engaging an exterior surface of the semi-finished container with a wall of the blow cavity; iv. Heating the walls of the blow cavity; v. transferring heat to an outer surface of the semi-finished container; vi. Providing a surface finish on the walls of the blow cavity to the exterior surface of the semi-finished container.

[0007] This first aspect of the invention may include one or more of the following features, taken alone or in any reasonable combination: The method may further include softening an outer surface of the semi-finished container during the step of transferring heat. The heating step may be accomplished by induction heating. The method may further include applying an electric current to a conductor in the mould to induce a magnetic field in the mould. The electric current may be an alternating current between 10 kHz and 50 kHz. The temperature of the walls of the blow cavity may be heated to a range of 60°C to 130°C. The method may further include performing the heating step for less than 6 seconds. The method may further include cooling the walls of the blow cavity after the heating step. The cooling step may be performed after performing the heating step. The cooling step may include introducing a cooling fluid pressure into the mould via a passage embedded in the body of the mould. The heating and cooling steps may be collectively performed for less than 40 seconds or until the mould reaches a temperature of less than 60°C.

[0008] A second aspect of the invention is directed to a thermoplastic blow molding apparatus. The apparatus includes a mold having a mold body and a concave blow cavity formed therein. One or more electrical inductors are within the mold body. A source of alternating current is connected across the inductors, the alternating current having a frequency between 10 kHz and 50 kHz.

[0009] A second aspect of the invention may include one or more of the following features, alone or in any reasonable combination: The mold may be manufactured from a magnetic material. The magnetic material may be a tool steel. The thermoplastic blow molding apparatus may further comprise a cooling passage in the mold body. An electrical inductor of the one or more electrical inductors may be located within the cooling passage. An annular space may be formed within the cooling passage around the electrical inductor. The one or more electrical inductors may include equally spaced portions around a circumference of the concave blow cavity. The thermoplastic blow molding apparatus may further comprise a surface finish on a wall of the blow cavity configured to engage a semi-finished container in the blow cavity. The thermoplastic blow molding apparatus may further comprise a cooling fluid source fluidly connected to the cooling passage.

[0010] Other features and advantages of the invention will become apparent from the following specification taken in conjunction with the following drawings. [Brief description of the drawings]

[0011] For an understanding of the present invention, reference will now be made, by way of example, to the accompanying drawings in which: [Figure 1] FIG. 1 is a diagram of a prior art blow molding process. [Diagram 2] FIG. 2 is a diagram of the blow molding operation of the present invention. [Diagram 3] FIG. 3 is an elevational view of a mold for use in the blow molding operation of the present invention. [Figure 4] FIG. 4 is a cross-sectional view of a mold half for use in a blow molding operation of the present invention showing temperature variations within the mold. [Diagram 5] FIG. 5 is an alternative view of the type of FIG. [Figure 6] FIG. 6 is a schematic diagram of a blow molding apparatus of the present invention showing, inter alia, the expansion of the preform radially outward until the semi-finished container engages the walls of the blow cavity. [Figure 7]FIG. 7 is a partial cross-sectional view of a mold for use in the blow molding operation of the present invention showing an enlarged view of the inductor with annular cooling passages around the tubular conductor and the inductor and cooling passages through the tubular conductor. [Figure 8] FIG. 8 is a graph of a blow molding rapid heating and cooling cycle. [Figure 9] FIG. 9 is a tabular comparison of a first container (hereinafter "Container E") made according to the prior art method and a second container (hereinafter "Container R") made according to the method of the present invention and using the apparatus of the present invention. [Figure 10] FIG. 10 is a side-by-side comparison of the container on the right (hereinafter "R") made in accordance with the present invention and the container on the left (hereinafter "L") made in the same blow cavity without the application of rapid induction heating. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] While the invention is susceptible to embodiment in many different forms, preferred embodiments of the invention are shown in the drawings and described in detail herein, with the understanding that the present disclosure should be considered as an exemplification of the principles of the invention and is not intended to limit the broad aspects of the invention to the embodiments illustrated.

[0013] In the blow molding process, the parison or preform (hereinafter "preform") enters the blow cavity in a solid state above its glass transition temperature (Tg). This makes high replication and transfer of the etched surface finish from the blow cavity walls to the finished or semi-finished part difficult. However, the inventors have discovered that through rapid heating of the blow cavity walls and by controlling the blow pressure, a thermoplastic material above its glass transition temperature can be forced into the etched surface of the blow cavity walls, imparting a replication or transfer of the etched surface to the exterior surface of the finished or semi-finished part.

[0014] Process control of rapid heating, preform temperature, and blow pressure are key variables in obtaining the desired finish. Controlling these parameters achieves much higher consistency, repeatability, and part quality compared to traditional secondary methods of applying surface finishes. Furthermore, without rapidly heating the blow cavity, the fine surface finish on the surface of the blow cavity does not transfer to the part in the blow molding process, unlike injection molding, where most of the surface finish transfers without thermal cooling, although at a lower resolution.

[0015] Therefore, the primary factor affecting the surface finish of the finished blow molded part is the cavity wall temperature. However, the preform temperature is important to ensure that the part can be blown properly. The preform temperature varies depending on the material, but should be at least above the Vicat softening temperature of the material. It should be understood that the Vicat softening temperature can be utilized to compare the thermal properties of materials.

[0016] Here, "semi-finished" refers to the container when the preform is expanded into and contains a part during removal from the blow molding process. The term contemplates that the part may undergo further processes following the blow molding process, such as labeling, adding a container lid, etc. It is understood that the part may be a container, a bottle, a package, and other forms of parts.

[0017] Thus, an aspect of the blow molding method and apparatus disclosed herein is that fine, high quality surface finishes (macro, micro, nano) that are visually and physically similar to surface finishes created by secondary methods can be achieved within the blow cavity of the mold by transferring such fine surface finishes etched into the blow cavity onto the part blown into the blow cavity, thereby eliminating secondary processes.

[0018] Compared to conventional processes, this novel method and apparatus can provide improved consistency and repeatability of up to about 97%, improved decorative adhesion of silk screening, hot stamping, and heat transfer. Improved compatibility of the container can be achieved because there is no sprayed layer that affects potential adhesion. Surface treatments, including but not limited to spraying and secondary decoration, do not impart incompatibility and stress to the container. Thus, when using rapid cooling and induction heating, cycle time is improved by at least 25%. This can allow for a wider range of target surface finishes without the need for tooling or spray modifications. Furthermore, a wider range of materials can be utilized compared to injection molding because surface replication does not rely on high melting temperature characteristic materials. A "sprayed layer" as understood by those skilled in the art is a layer of coating formed on an article via a coating process in which material is sprayed onto the surface of the article.

[0019] Referring to FIG. 2, the novel method of the present invention includes the steps of introducing a thermally conditioned thermoplastic preform 10 (FIG. 6) into a blow cavity 14 (FIG. 6) (step 100), introducing a blowing fluid pressure 18 (FIG. 6) into the preform 10 (step 200) to expand the preform 10 radially outward against the walls 22 (FIG. 6) of the blow cavity 14, heating the walls 22 of the blow cavity 14 (step 300), and removing the finished or semi-finished container 26 (FIG. 6) from the mold (step 400). These steps are conventional as understood by those skilled in the art of blow molding. As shown in FIG. 6, the expansion of the preform 10 into the semi-finished container 26 provides an intermediate stage of the semi-finished container 26, shown in dashed lines.

[0020] The method further includes the step of rapidly heating the walls 22 of the blow cavity 14. This step enables heat transfer between the semi-finished container 26 such that the surface finish of the walls 22 of the blow cavity 14 is transferred to the outer surface 30 of the semi-finished container 26. This step requires a properly designed apparatus that includes a mold that includes a means for rapidly heating the walls 22 of the blow cavity 14. Such a mold is shown in Figures 3-7. The mold is heated via induction heating.

[0021] Induction heating is the process of heating a conductive mold (usually metal) by electromagnetic induction. Eddy currents generate heat within the mold.

[0022] An alternating current 34 from a source 36 is applied to the inductor 42 at a frequency between 10 kHz and 300 kHz. This induces a current in the opposite direction in a passage 50, and the electromagnetic field 38 propagates heat diffusion to the walls of the blow cavity 14 (see FIG. 6). This continues until the temperature of the walls 22 of the blow cavity 14 is heated to a range of 60° C. to 130° C. This temperature is suitable to soften the outer surface 30 of the semi-finished container 26 upon engagement with the walls 22 of the blow cavity 14. In combination with the blow fluid pressure 18 delivered to the interior 58 of the semi-finished container 26, the softening allows for the transfer of a desired surface finish to the outer surface 30 of the semi-finished container 26.

[0023] The mold is therefore manufactured from an electrically conductive material, such as a metallic material. Iron alloys are a common metallic material. 1.2343 (H11) magnetic tool steel has an electrical resistivity of about 40 Ω·m, a relative permeability of about 55, and a thermal conductivity of about 27 W / m·°K.

[0024] One or more inductors 42 are incorporated into the mold body 46 near the walls 22 of the blow cavity 14. The inductors 42 are uniformly distributed in the magnetic material around the walls 22 of the blow cavity 14. In other words, the inductors 42 are equally spaced around the periphery of the blow cavity 14 within the mold body 46. However, while it would be ideal to have the inductors evenly spaced, design constraints often prohibit such spacing and desired results are achieved when the inductors 42 are not evenly spaced.

[0025] The inductor 42 comprises electrical conductors 44, such as similar braided copper wire. These conductors 44 are located within passages 50, or a network of passages 50, within the mold. The passages 50 may take the form of tubular channels, grooves, conduits, etc. The conductors 44 are backed by one or more insulating layers 54.

[0026] An alternating current 34 at a frequency between 10 kHz and 300 kHz can be delivered from an alternating current source 36, which can be electrically connected to an inductor 42. This induces a current in the opposite direction in a passageway 50, and the electromagnetic field propagates the heat spread to the walls 22 of the blow cavity 14 (FIG. 6).

[0027] The method further comprises a step of rapidly cooling the mould after the heating step. The apparatus therefore comprises means for cooling the mould. This may take the form of a further passage 50 passing through the mould in the longitudinal direction and / or across the height of the mould, which may be fluidly connected to a cooling fluid pressure source 62. Cooling fluid may be injected into the passage 50 to rapidly cool the mould and / or to control heat diffusion. Alternatively or additionally, the cooling means may comprise a tubular conductor 70 which may allow a cooling fluid pressure 66 to flow therethrough.

[0028] Alternatively or additionally, the cooling means comprises a space 74 between the circumferential wall of the conductor passage 50 and the insulating layer 54 of the conductor 44. A cooling fluid can then be introduced into the space 74 to cool the mould. Here, the space 74 is created by increasing the cross-sectional area of ​​the passage 50 so that it is substantially larger than the cross-sectional area of ​​the conductor 44 and the insulating layer 54. When used in this context, the term "substantially" is intended to encompass a configuration in which the space 74 formed by the distance between the radially outermost portion of the conductor / insulating layer and the passage wall is large enough to allow the flow of a fluid therethrough. The space 74 is preferably annular such that the space 74 forms a circumferential gap around the circumference of the conductor / insulating layer. In each case, the cooling fluid is made to flow through the mould between the two heating cycles.

[0029] The apparatus and blow molding process are controlled by a suitable controller 78, such as a computer, microcontroller, wired control panel, data processor, etc. The controller 78 may have a non-transitory memory having one or more software routines. The software routines may control the operation of the blow fluid pressure source 82, the alternating current source 36, and the cooling fluid pressure source 62 in terms of heating / cooling cycles or duration, duration pressure, volume, etc.

[0030] In a particular embodiment of the present invention, the thermoplastic preform 10 is introduced into the blow cavity 14 at a temperature that is equal to or greater than the glass transition temperature of the thermoplastic preform 10 and less than the melting temperature of the thermoplastic preform 10. A blow fluid pressure 18 is introduced into the preform 10 to expand the preform 10 radially outward to form a semi-finished container 26. The blow fluid pressure 18 continues such that the outer surface 30 of the semi-finished container 26 engages the wall 22 of the blow cavity 14. The wall 22 of the blow cavity 14 is heated by induction heating. An electric current is applied to a conductor 44 in the mold body 46, inducing a magnetic field within the mold body 46. The electric current is an alternating current 34 between 10 kHz and 300 kHz. The heat is transferred to the outer surface 30 of the semi-finished container 26. The temperature of the wall 22 of the blow cavity 14 is heated to a range of 60° C. to 130° C. within a duration of less than 6 seconds. The heat from the heat transfer softens the outer surface 30 of the semi-finished container 26. The surface finish from the walls 22 of the blow cavity 14 is imparted to the exterior surface 30 of the semi-finished container 26. The walls 22 of the blow cavity 14 are cooled after the heating step. Cooling of the walls 22 involves cooling fluid pressure 66 to the mold via passages 50 embedded in the body 46 of the mold. The heating and cooling steps are collectively carried out for a total of less than 40 seconds or until the mold reaches a temperature of less than 60° C.

[0031] Referring to Figure 8, an exemplary heating / cooling cycle is shown according to an embodiment of the present disclosure. The heating cycle duration is 5 seconds and can increase the mold temperature from less than 60°C to about 90°C to 108°C. Diffusion of heat continues to heat the mold above 110°C. A cooling step continues to commence until the mold temperature reaches about 50°C for a duration of about 35 seconds.

[0032] With reference to FIG. 9, an illustrative example of container R manufactured according to the above embodiment is compared to container E manufactured according to the prior art. Container E was manufactured using a conventional blow molding method with a plain (i.e., no surface finish) cavity. The surface finish of container E was achieved by a prior art secondary process of spray coating - a conventional spray method. Using the blown cavity with the surface finish, container R was manufactured. The surface finish was transferred to container R with over 97% replication. As shown in FIG. 9, container R has a surface finish of 100%. a is the average surface roughness, R z is the difference between the highest "peak" and the deepest "valley" in the surface, R Sm is the average peak width. Vessel E has an R of approximately 0.4012 micrometers (μm). a , R of about 2.7845 μm z , and an R of about 41.6655 μm Sm The container R can be provided with an R of about 1.5294 μm. a , R of about 8.5857 μm z , and an R of about 109.5578 μm Sm can be provided.

[0033] Referring to FIG. 10, the container R on the right was again produced according to the above embodiment and compared to the container L on the left, which was produced according to the prior art. Containers E and L were produced using blow cavities having substantially identical cavity wall surface finishes. The transfer of the blow cavity wall surface finish to container R is greater than the transfer of the blow cavity wall surface finish to container L. In other words, container L was produced without induction heating but within a similar surface finish on the blow cavity surfaces, whereas container R was produced with induction heating. Container L, blown into a cavity containing a fine surface finish, was not effectively transferred because induction rapid heating and cooling was not applied. Container R was blown into the same cavity, but with induction turned on, which allowed the fine surface finish to be successfully transferred.

[0034] Advantages of the present invention include the ability to blow mold high quality surface finishes on packaging within the mold itself by applying heating / cooling techniques to provide a fine finish that is transferred from the walls 22 of the blow cavity 14. This technique can be used in conjunction with, for example, extrusion blow molding, injection blow molding, injection stretch blow molding, etc. This process achieves high quality surface finishes at the macro, micro, or nano level. It allows for different types and levels of surface finishes to be blow molded without the need for different trimming or spray modifications. Importantly, secondary finish decoration of the container can be eliminated while achieving a fine and high quality surface finish. The new process achieves uniformity, consistency, and repeatability of the surface finish.

[0035] While particular embodiments have been illustrated and described, numerous modifications come to mind without significantly departing from the spirit of the invention, the scope of protection being limited only by the appended claims.

Claims

1. A thermoplastic blow molding apparatus comprising: a mold comprising a mold body and a concave blow cavity formed therein; one or more electrical inductors within the mold body; an alternating current source connected across the electrical inductors, the alternating current having a frequency between 10 kHz and 300 kHz; and cooling passages within the mold body; A thermoplastic blow molding apparatus, wherein an electrical inductor of the one or more electrical inductors is located within the cooling passage.

2. The thermoplastic blow molding apparatus of claim 1 , wherein the mold is made from a magnetic material.

3. 3. The thermoplastic blow molding apparatus of claim 2, wherein the magnetic material is a tool steel.

4. The thermoplastic blow molding apparatus of any one of claims 1 to 3, wherein an annular space is formed within the cooling passage around the electrical inductor.

5. The thermoplastic blow molding apparatus of claim 4 , wherein the one or more electrical inductors include portions equally spaced around a circumference of the concave blow cavity.

6. 6. The thermoplastic blow molding apparatus of claim 5, further comprising a surface finish on a wall of the mold body configured to engage a blank container in the blow cavity.

7. The thermoplastic blow molding apparatus of claim 6 further comprising a source of cooling fluid fluidly connected to the cooling passages.

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