Patent Application
20070252304
1. Field of the Invention
The present invention relates generally to an apparatus and method for making a multi-layer, plastic preform that can be utilized in the stretch-blow molding process. More particularly, the invention relates to such a method and apparatus that utilizes standard machine equipment to produce a compression quality multi- or mono-layer, plastic preform.
2. Related Art
In the manufacture of plastic containers, such as monolayer or multilayer PET containers, it is conventional to mold a container preform having a body and a finish with one or more external threads. The finish is typically molded to its final geometry, while the body of the preform is subsequently blow molded to the desired geometry of the container body. Although this manufacturing technique is satisfactory for fabrication of mono-layer plastic containers, the throughput quality of the process is greatly reduced when employed for fabricating multi-layer, plastic preforms and containers. For example, a preform formed by compression or injection molding involves expensive machine and mold costs due to the complex molds with moving parts that are required to form threads of the finish. In addition, multi-layer capability is limited with compression molding as the multi-layer plastic smears disrupting the orientation of the layers as the preform parison is compressed against the plastic. That is, in a compression formed multi-layer preform, the multi-layers of the parison are smeared, resulting in an inner layer being moved to an outer layer, as the pressure of the compression mold moves the layers of the multi-layer plastic.
The other conventional process for forming thermoplastic articles is blow molding, which employs air blowing plastic against a mold instead of a physical body compressing the plastic. However, performs formed by air-blowing tubular parisons are problematic as the distribution of the preform wall thickness is inconsistent. For example, in a multi-layer preform, the orientation of the layers are important after the air-blowing process. Since the outer layer of the multi-layer parison is pressed against a mold, the outer layer remains in its original orientation an outer layer of the resultant preform 2, as the pressure of the compression mold or preform core moves the layers of the multi-layer parison. The disturbed orientation of the layers is shown in
The other conventional process for forming thermoplastic articles is blow molding, which employs air blowing plastic against a mold instead of a physical body compressing the plastic parison. However, preforms formed by air-blowing tubular parisons are problematic as the distribution of the preform wall thickness is inconsistent. For example, in a multi-layer preform, the orientation of the layers are important after the air-blowing process. Since the outer layer of the multi-layer parison is pressed against a mold, the outer layer remains in its original orientation with respect to the other layers of the parison in the blow molding process. As
In addition, during a blow-molding operation, the inner layers of the multi-layer parison are blown against the mold and, as a result, some areas of the resultant preform are thicker than other areas. This is due to many factors, but mostly because of the flow head that makes the parison tube. When plastic is forced into the flow head, it flows around various passages to form the hollow tube. During this process, the plastic is subjected to various pressures and temperatures. These pressures and temperatures have a big affect on the plastic's flow characteristics (viscosity). As this plastic exits the flowhead, it is stretchable at different rates when subjected to air in the blow molding process thereby resulting in pools of thicker plastic which is adjacent to thinner plastic areas. Consequently, when a preform 2′ is blow molded into a container or other article 7, as illustrated in
What is needed then is a method of forming preforms that balances the above two processes to form a preform with a consistent thickness and without disrupting the orientation of the layers. In addition, an system that utilizes existing equipment and thus, is less expensive than the specialized molds associated with injection molding is needed.
In summary, the method and system according to the invention involves a two-step process where an extruded tubular parison is initially air-blown against the preform mold forming a preform with walls disposed against the mold, and thereafter, a preform core is inserted into the air-blown preform with walls to even out the thickness of the walls or smooth the walls thereby finish the resultant preform made from a multi-layer parison.
This invention succeeds where previous efforts have failed because it combines the cost-saving advantages obtained with blow molding with an inexpensive compression molding technique to improve the finishing of multi-layer preform.
This invention is in a crowded and mature art.
This invention differs from the prior art in modifications which were not previously known or suggested.
The invention is achieved with a method of making a multi-layer plastic preform that includes the steps of generating a multi-layer parison with at least an inner layer and an outer layer, capturing the multi-layer parison within a preform mold, blowing sufficient air within the parison so that the outer layer of the parison makes contact with the preform mold, while the orientation of the multi-layer parison is not disrupted, thereby forming a preform shape having walls and thereafter, inserting a core within the blow-molded preform shape and compressing the inner layer walls of blow-molded preform shape thereby finishing the inside of the preform.
A system for making a multi-layer plastic preform according to the invention may be achieved with an extruder (or extruders), which generates (generate) a multi-layer plastic parison, a preform mold that has at least one section that captures a multi-layer parison, and a blow stem assembly that has a device which introduces air into a multi-layer parison captured by the preform mold in a blow-mold operation, a compression rod movably attached adjacent to the device, and a compression preform core attached to an end of the compression rod wherein when the compression preform core is retracted into the device, air can be blown into the preform mold and when, the compression rod extends in a stroke through the device, the compression preform core extends out of the device into the preform mold.
Another advantage of the present invention compared to known blow molding processes is that the preform can be made with decreased cooling times. This is because the compression preform core is cooled via water being circulated there through. Thus, the plastic of the formed preform can be cooled both on the inside (core-side) and the outside (mold-side) due to the insertion of the preform core. This results in faster cycle times thereby increasing production.
Further objectives and advantages, as well as the structure and function of preferred embodiments will become apparent from a consideration of the description, drawings, and examples.
The foregoing and other features and advantages of the invention will be apparent from the following, more particular description of a preferred embodiment of the invention, as illustrated in the accompanying drawings wherein like reference numbers generally indicate identical, functionally similar, and/or structurally similar elements.
Embodiments of the invention are discussed in detail below. In describing embodiments, specific terminology is employed for the sake of clarity. However, the invention is not intended to be limited to the specific terminology so selected. While specific exemplary embodiments are discussed, it should be understood that this is done for illustration purposes only. A person skilled in the relevant art will recognize that other components and configurations can be used without parting from the spirit and scope of the invention.
Looking at
In an exemplary embodiment, the six barrier layers forming the parison may include an outer layer, which may be a polypropylene layer, a regrind layer, an outer adhesive layer, an EVOH layer, an inner adhesive layer, and an inner layer for contacting food products, which may be a polypropylene layer. To form this layer structure, four extruders would be used: one extruder to supply the inner and outer layers; a second extruder to supply the inner and outer adhesive layers; a third extruder to supply the regrind layer; and a fourth layer to supply the EVOH layer. That is, four extruders could be used to form a six-layer parison. The exemplary embodiment of
Preform mold 14 may have an opening (not shown in
An alternative shuttle machine mold 22 is illustrated in
After the capturing step 20, air may be pre-blown in the parison 12 to activate inflation of the parison within the mold (step 30) before the actual blowing step that forms an unfinished preform. A hot knife 32 cuts the parison as shown in step 30. Then, either the extruded parison 12 or the preform mold 14 with the cut parison 34 is moved so that a blow stem assembly 36 can be positioned above the preform mold 14 with the cut parison 34, as shown in step 40. That is, the preform mold 14 can be indexed to a location underneath the blow stem assembly 36, or the blow stem assembly can be moved to a location above the preform mold 14. In a preferred embodiment, the preform mold 14 would be indexed from one location in its open position, move to surround extruded parison 12, and then close to capture the parison within the preform mold. Then, the preform mold 14 would be indexed back to its original location underneath a blow stem assembly. The upper diameter of the cut parison 34 may be pre-blown and cut so that its diameter has a one to one correspondence with the inner diameter of mold alignment plate 42 of the blow stem assembly 36. The blow stem assembly 36 is lowered to the perform mold 14 so that mold alignment plate 42 comes into contact with the cut parison 34 and pinches the parison to the preform mold 14. Then, air is activated though the blow stem assembly to blow into cut parison 34.
In a preferred embodiment, the plastic of the multi-layer preform is polypropylene (PP). PP material is less expensive when compared with polyethylene terephthalate (PET) material. An acceptable PP container with the desired barrier properties (achieved with PET blown containers) has not been produced with known technology. If the polypropylene preform formed by the inventive method is used in a stretch-blow molding process to form a container or an article, a soft package feel is achieved as compared to the hard plastic of a polyethylene terephthalate (PET) preform. In addition, the orientation of the polypropylene preform has clear, optical quality when properly processed. In order to generate a clear container (obtain molecular orientation of the plastic), the preform is heated to a critical temperature and then stretched. The combination of stretching and heating will cause molecules of the plastic to stretch and align resulting in a clear container. Thus, with the present invention, PP containers with the desired barrier technology can be produced and, as a result, a low cost PP container can be produced that can replace the more expensive PET containers. While a multi-layer preform may be formed of two or more layers, the preferred amount of layers is six; however, the number of layers depends upon the preform being formed and its use. Thus, a multi-layer container may be made from a preform with less than six (6) layers or more than six (6) layers.
As shown in
Compression preform core 47, in the exemplary embodiment, is attached to the front end 46f of compression rod via screw threads. The rear end 46r of compression rod is screwed into mounting plate adapter 51 and compression spring 48 surrounds compression rod 46 between alignment plate 54 and mounting plate adapter 51. As shown in step 50 of
By using the air blowing process to form the preform shape, the multi-layer walls of the preform are moved against the preform mold 14. In particular, the outer wall is moved against the preform mold without disrupting the orientation of the multi-layers. This is achieved in that air introduced from air blowing device 44 provides multi-axial pressure that acts normal to the multi-layer parison. Consequently, the cut parison is caused to move against the preform mold in a manner that does not disrupt the orientation of the multi-layer parison. Thus, when a sufficient amount of air is blown within the parison, the parison is expanded to contact the walls of the preform mold 14 thereby forming a preform shape having walls. Since the amount of air is intended to open up parison 34 so that compression preform core 47 can be inserted in to the parison, air needs to activated for less than approximately one (1) second compared to approximately 20 plus seconds for a typical blow molding operation. As discussed above, this preform shape formed by introducing air for approximately less than one second is not complete because blow molding may cause areas of thicker plastic adjacent areas with thinner plastic. That is, preform achieved by the shortened blow-molding process needs the thickness of the inner layer to be smoothed out over the inner portion of the preform so that the preform has a consistent thickness throughout.
Known blow molding operations introduce blow air for the entire duration of the cooling process, in addition to the time for blowing the parison to the sides of the mold. The cooling times are generally greater than approximately 20 seconds. The air pressure during the cooling stage forces the plastic against the outside of the mold to promote cooling of the plastic (via heat transfer from the plastic to the cooled mold) and to maintain the desired package shape. The present invention inserts a compression preform core 47 that forces the plastic of the preform against the outside mold thereby maintaining the desired preform shape. The preform core 47 has means to circulate coolant there through. Thus, the present invention eliminates the need for air to be blown throughout the cooling process thereby resulting in cycle times that are decreased substantially. This is because heat transfer via conduction (cooled core or cooled mold absorbing heat from plastic) is more effective then heat transfer via convection between air and the outside of the plastic.
After blowing sufficient air to form a preform shape having walls, the air is deactivated. Then, as shown in steps 60, 70, 80 of
As a result of the blow molding process followed by the finishing, compression step, the alignment of the multi-layer parison is maintained and the thickness of preform is consistently formed. As a result, a container formed in a subsequent stretch-blow molding process of the formed preform does not rupture and does not have areas where barrier layers are missing. Step 50 shows the bottom portion of the blown parison with a rounded dotted line. The area beneath the dotted line represents excess parison that can be trimmed off at a later stage.
After the finishing, compression step is achieved, the mold parts 23, 24 may open to release the formed preform from the mold. The finished, preform would remain attached to the compression preform core until the blow stem assembly is retracted thereby removing the finished, resultant preform from compression preform core 47. Alternatively, a robotic arm or other mechanical device may grab the preform.
One point of novelty of the above-described invention is that the blow stem assembly can be easily attached to conventional or standard shuttle machine equipment. For example, the mounting plate adapter 51 can be inexpensively bolted to standard equipment.
The embodiments illustrated and discussed in this specification are intended only to teach those skilled in the art the best way known to the inventors to make and use the invention. Nothing in this specification should be considered as limiting the scope of the present invention. All examples presented are representative and non-limiting. The above-described embodiments of the invention may be modified or varied, without departing from the invention, as appreciated by those skilled in the art in light of the above teachings. For example, instead of shuttle machine blow molding platform, this invention can be used with continuous blow molding machines, shuttle machines with multiple flow heads that produce multiple parisons, and a tangential extrusion platform. It is therefore to be understood that, within the scope of the claims and their equivalents, the invention may be practiced otherwise than as specifically described.
