Patent Application
20130207317
A process for making a stretch-blow molded container having an integral handle.
Integral handles formed using a stretch blow molding process can be advantageous. An integrally molded handle can be generally less expensive than a separate handle, such as a clip-on handle. Approaches for providing an integral handle typically require the formation of a pair of opposing depressions or cavities in the body of the bottle that form the structural basis of the handle. These depressions can either then be welded together and the central section, encompassed by the weld, can be removed such as to form a completely open space through which the fingers and/or thumb can be inserted (a ‘through’ handle), or, alternatively, left to simply form a grip. If the grip is formed sufficiently wide and deep so that a hand can close on the grip without having the tip of the fingers touch the bottom of the recess, then the grip may be ergonomically acceptable to a degree comparable to a through handle.
One of the problems associated with available approaches for providing an integral handle is that the distribution of material in these depressions can be uneven. This can occur as a result of the differing degrees of stretch needed to deform the preform into different sections of the depressions of the handle of the container. The differing degrees of stretch can result in irregular wall thicknesses and irregular mechanical and aesthetic properties.
One approach to forming an integral handle requires using moving sections of the mold to compress the expanding preform and form the deep depressions during the blowing process. This process can result in two problems. First, this process requires significant levels of stretching of the material after the material contacts the mold. This can result in highly irregular wall thicknesses in the handle area and failure of the material under stress. Second, the complexity that is required to move mold sections against the high blowing pressure, for example more than 20 bars, required to blow mold a container requires mechanically complicated and expensive mold designs.
An alternative approach is to produce an intermediate container with a convex section which can be mechanically deformed inwardly about one or more articulation zones to form the concave grip section. This process allows for more even stretch ratios and hence more even wall thicknesses. However, the inversion of the convex section can result in significant deformation of the area around the handle which can create aesthetic defects. These defects can be difficult to control, as the exact nature of the deformation around the handle will be highly dependent on very small variations in wall thickness. Using multiple articulation zones to minimize the problem of providing for a clean inversion of the grip geometry from convex to concave can result in a restrictive design geometry and may not eliminate residual stresses in the plastic sheets forming the grip resulting in undesirable wrinkles and poor ergonomics.
It is an object of the present invention to provide a process to create a deep, concave grip which provides an ergonomic handle.
A process for making a container having an integral handle, comprising the steps of: a) providing a preform (6); b) stretch blow molding said preform (6) to form an intermediate container (8) which comprises a convex portion (9) extending outwardly from said intermediate container, said convex portion (9) having a glass transition temperature; c) at a pressure within said intermediate container (8) above about 1 bar and with the temperature of said convex portion (9) being below said glass transition temperature, inverting said convex portion (9) with an inwardly moving plug (5) having a drive direction to form a concave gripping region whereby locations on said convex portion translate in a substantially straight line path as said convex portion is inverted; and d) releasing excess pressure within the container; wherein said plug has a contact surface having a contact surface area, said contact surface comprised of an initial contact surface and a secondary contact surface proximal said initial contact surface, said secondary contact surface oblique to said drive direction; wherein said contact surface has a contact surface periphery having a projected plane area; wherein said contact surface area is greater than said projected plane area; wherein said contact surface is substantially convex relative to said contact surface periphery; and wherein said contact surface substantially corresponds with said concave gripping region.
By stretch-blow molding, what is meant herein is the process in which a preform is heated above its glass transition temperature, and then blown in a mold using high pressure air to form a hollow body, such as a container or bottle. The preform can be stretched with a core rod as part of the process.
By preform what is meant herein is a molded form which is produced prior to expansion to form the finished object. A preform is necessarily somewhat smaller than the finished object. A preform is generally produced by, for example injection molding, at an elevated temperature in excess of the melt temperature.
As used herein, oblique means not parallel.
Thermoforming is one of many manufacturing processes for converting plastic resin into usable products. The basic concept of thermoforming is as follows. A pre manufactured thermoplastic sheet is heated until it becomes soft and pliable. The sheet can be, for example, a flat structure or container perform. It is then forced against the contours of a mold until it cools to its original state. Once it has cooled it is removed from the mold while still maintaining the shape of the mold. Thermoforming is a broad term and there are many different types of thermoforming processes. For deep thermoforming, bubble plug-assist forming can be employed. An advantage to this forming technique is that it improves material distribution because of its pre-stretching procedure. By this process, it is possible to control the thickness of the formed article as the sheet is stretched to ensure an even thickness of walls. Once the sheet has been placed in the frame and heated, controlled air pressure creates a bubble. This bubble stretches the material to a predetermined level. The male plug assist is then lowered forcing the stretched stock down into the cavity. The male plug is normally heated to avoid chilling the plastics prematurely. The plug is made as large as possible so the plastic is stretched close to the final shape of the finished product. The female mold can be vented to allow trapped air to escape from between the plastics and the mold.
Thermoforming can take place in two dimensions, whereby a surface is deformed, or in three dimensions where, in addition to two dimensional deformation, a change of the thickness occurs.
The bubble plug-assist thermoforming technique can be used to address the problems of forming a deep grip on a stretch blow molded container.
The term deep grip is used herein to denote a blind handle which is a gripping feature which permits the user's thumb and fingers to wrap around a handle, but which does not allow the fingers to pass completely behind and through the handle. A through-type of handle can be achieved by cutting away part or all of the web of material which is formed between the handle and the body of the container.
Plastic resin materials that can be used to form an integral handle can include thermoplastic materials. Plastic resin materials that can be used to form an integral handle include common polyesters such as polyethylene terephthalate (PET). Other materials suitable for use in a container having an integral handle include polypropylene (PP), polyethylene (PE), polystyrene (PS), polyvinyl chloride (PVC) and polylactic acid (PLA).
The temperature history of the polymer employed can be an important factor in the deformation behavior. The glass transition temperature is defined as the temperature below which the polymer behaves like a brittle, glassy solid and above which the polymer behaves like a rubber and is easily deformable. The melt temperature is the temperature at which all crystallites are melted and the polymer is behaving as a fluid. The re-crystallization temperature for semi-crystalline polymers, Tc, is the temperature at which an un-oriented polymer when cooled down from the melt is showing significant crystal growth within a specific period of time, typically a few minutes. Glass transition temperature and melt temperature are measured following ASTM D3418.
Next, the perform 6 is stretch blow molded in a mold cavity 1 to form an intermediate container 8 which comprises a convex portion 9 extending outwardly from the intermediate container 8, as shown in
Next, as shown in
After the inverting step is complete, the excess pressure in the finished container 10 is released. The finished container is released from the mold in which the inverting step is performed, with the finished container 10 having a deep gripping region 13 shown in
A side view of a schematic of a plug 5 is shown in
The contact surface 20 can be comprised of an initial contact surface 25 and a secondary contact surface 30 proximal the initial contact surface 25 and oblique to the drive direction of the plug 5. The secondary contact surface 30 has a secondary contact surface area 32. The initial contact surface 25 can have an initial contact surface area 27. The secondary contact surface 30 can be abutting, or adjacent to, or proximal the initial contact surface 25. The secondary contact surface 30 can be separated from the initial contact surface 25 by one or more concave portions of the plug 5 and still be considered proximal the initial contact surface 25 if the surface area of the concave portion is less than the initial contact surface area 27.
The drive direction can be a straight line. The initial contact surface 25 of the plug 5 is the part of the plug 5 that that first comes into contact with the convex portion 9 as the plug 5 is moved in the drive direction. The secondary contact surface 30 of the plug 5 comes into contact with the convex portion 9 after the initial contact surface 25 has contacted the convex portion 9. By having the secondary contact surface 30 come into contact with the convex portion 9 after the initial contact surface 25 has contacted the convex portion 9, locations on the convex portion 9 are translated in a substantially straight line path as the convex portion 9 is inverted upon itself. Furthermore, it is thought that having the secondary contact surface 30 oblique to the drive direction can also be desirable because it can allow the final container 10 to be easily separated from the plug 5 when the final container 10 is ejected from the mold in which the inverting step is performed. Without being bound by theory, it is thought that the shear resistance along the sides of the plug 5 can resist the force applied to the final container 10 to eject the final container 10 and thereby prevent or hinder ejection.
The contact surface 20 can have a contact surface periphery 35. The contact surface periphery 35 is defined by the maximum extent of the plug 5 that comes into contact with the convex portion 9 as the convex portion 9 is inverted. If, by way of non-limiting example, the plug 5 has only an initial contact surface 25 and a secondary contact surface 30, the contact surface periphery 35 would bound the portion of the plug 5 containing the initial contact surface 25 and the secondary contact surface 30.
The contact surface periphery 35 has a projected plane area that is the planar area of the contact surface 20 projected in a direction aligned with drive direction of the plug 5. The contact surface area 21 is greater than the projected plane area. One possible geometry for the contact surface 20 is that of a conical frustum, which is a frustum created by slicing off a cone with a cut made parallel to the base. If the contact surface 20 has the shape of a conical frustum and the drive direction of the plug 5 is aligned with the radial axis of the conical frustum, the projected plane area of the contact surface periphery is a circle having the radius of the base of the conical frustum. The initial contact surface 25 would be the area of the top of the conical frustum and the secondary contact surface 30 would be the slanted surface of the conical frustum. The initial contact surface area 27 would be the area of the top of the conical frustum. The secondary contact surface area 32 would be pi times the slant height times the sum of the radius of the top of the conical frustum and the radius of the bottom of the conical frustum. The combined initial contact surface area 27 and the secondary contact surface area 32 for a plug 5 that is a conical frustum is greater than the projected plane area of the conical frustum.
The contact surface 20 can be substantially convex relative to the contact surface periphery 25. That is, the plug 5 can be like a finger that pushes in the convex portion 9 to invert the convex portion 9. The contact surface 20 can substantially correspond with the concave gripping region 13 formed by inverting the convex portion 9. The contact surface 20 can completely correspond with the concave gripping region 13 formed by inverting the convex portion 9. Without being bound by theory, it is believed that by having the contact surface 20 be a mirror image of the shape of the grip in the finished container 10, undesirable wrinkling and folding does not occur as the convex portion 9 is inverted. Undesirable wrinkling and folding can have adverse impacts on the aesthetics and performance of the finished container 10.
A schematic of how the plug 5 can invert the convex portion 9 is shown in
The process of the present invention can be understood with reference to
The step shown in
The step shown in
In a variation, the convex portion 9 can be inverted in the mold cavity 1 if the mold cavity 1 is provided with a movable plug 5 or plugs 5 to invert the convex portion 9 in the same mold in which the intermediate container 8 is formed. Such an approach can have the advantage that the time between blow molding and inverting the convex portion(s) 9 is minimized and that the tolerances on the container specifically around the convex portions(s) 9 are tighter as there is no relative movement of the container versus the plug. The integration in one cavity complicates the construction of the blow mold as the blow mold needs to have moveable plug(s) 5, the blow molding machine needs to be adapted to control the thermoforming step, and the total cycle time increases as the movement of the plug adds to the blowing cycle.
In the step shown in
An acceptable integral handle can be provided by the above process by having the temperature of the convex portion (9) below the glass transition temperature Tg when deforming the convex portion 9 or each convex portion 9 by means of an inwardly moving plug 5 to form one or more concave gripping region(s) 13. Unexpectedly, the material in the convex portion (9) need not be above the glass transition temperature, Tg, to allow for the deformation from the intermediate container 8 in to the finished container 10. The temperature of the convex portion (9) can be below the glass transition temperature Tg and within about 35° C. of the glass transition temperature Tg. The temperature of the convex portion (9) can be below the glass transition temperature Tg and within about 25° C. of the glass transition temperature Tg. The temperature of the convex portion (9) can be below the glass transition temperature Tg and within about 15° C. of the glass transition temperature Tg. For PET, the material in the gripping region can have a temperature between about 50° C. and about 81° C. In the temperature range between about 50° C. and about 75° C. the material stiffness of PET is greater than the material stiffness above Tg (81° C.) thereby limiting large material deformations. As the plug(s) 5 forms the concave gripping region(s) the material strain and strain rate is moderate. The deformation is typically bending with no or limited extensional strain. If there is no extensional strain, no strain rate is applied. It is believed to be beneficial to have the material bending accompanied with application of a small amount of extensional strain. Extensional strain can be about 1% to about 100% or can be about 10% to about 50%. The strain rate can then be about 50 to 1000 mm/sec or can then be about 100 to 500 mm/sec.
Use of a temperature below Tg during the step of deforming the convex portion 9 or convex portions 9 can reduce the energy needed to produced the finished container, thereby lowering the cost of the container, and can increase conversion rate since time is not needed to reheat the convex portion 9 above the glass transition temperature Tg prior to deformation. Further, heating the convex portion 9 or convex portions 9 to a temperature above Tg can result in problems with material distribution in and proximal to the convex portion 9 and the deep grip region 13 resulting from inverting the convex portion 9 or convex portions 9.
In the step shown in
In one embodiment, the deep grip can be a mirror shape of the convex portion 9. In another embodiment, it can be of advantage if the surface area of the convex portion 9 is somewhat smaller than the surface area of the deep grip (about 1% to about 50% smaller). In the case that the surface area of the convex portion 9 is smaller than the surface area of the deep grip there is a three dimensional deformation of the convex portion 9 to form the deep grip. The resulting wall thickness reduction of the convex portion 9 to form the deep grip can be of advantage to give a good definition of the deep grip detail. Without being bound by theory, it is believed that the surface area of the convex portion 9 should not be larger than the area of the deep grip as the “excessive” surface can form folds and wrinkles that can have negative impact on aesthetics and performance. The convex portion 9 can have the shape of a bubble.
The overall deformation of the convex portion 9 to the grip in the finished container 10 can be large. The grip recess (dimension z in
The deep grip can be ergonomically shaped to help the consumer to hold the container and pour product from the container. Anthropometric studies indicate that the minimum grip diameter of the circle formed by the thumb and forefinger when enclosing a cone (in accordance with DIN33402) for women aged 20-59 is 34 mm, which corresponds to an inside perimeter of 107 mm. Accordingly the deep grip can provide at least this amount of grippable developed length to ensure ergonomic functionality equal to that of a through handle.
The deep grip can be as deep as possible, the principle limitation being the footprint of the bottle. The deep grip can have a depth no less than that required to generate a developed length of about 107 mm. In the case of a symmetrical deep grip design as shown in
The palm rest of the deep grip (x) can be sufficiently large so that bottle can rest intuitively against the palm of the hand and opens the hand sufficiently wide that fingers do not touch in the bottom of the deep grip recess. It has been found by ergonomic studies that a palm rest width of at least about 30 mm (x>about 30 mm) can be required to provide comfortable handling at parity with that of a through handle.
The depth of each deep grip recess (z) can be no less than about 10 mm (z>about 10 mm).
Opposing deep grip halves do not need to be symmetrical in terms of depth and shape as the asymmetry can improve the ergonomic performance of the container
It can be advantageous to include venting holes in the plug(s) 5 that are designed similar to venting holes conventionally used in blow molds. When the pre-stretched bubble gets thermoformed to form the concave gripping region, the surface of the material will conform closely to the outer profile of the plug(s) 5, and the air between the convex portion (9) and the plug 5 can escape through the venting holes. Furthermore, when the plug 5 is withdrawn just prior to the container ejection, the venting holes prevent a vacuum from building up between the concave gripping region and the plug 5 that could lead to a distortion of the deep grip.
It can also be advantageous to provide means for interlocking opposing concave gripping regions against each other such as to eliminate any relative movement when gripped. An example of one such means is a peg and pin where both concave gripping regions meet. The peg and pin can be aligned and interlocked after the convex portions 9 are inverted. In one embodiment the peg and pin can be formed concavely in the convex portions 9 and then formed into their final convex shape during the step of inverting of the deep grip.
The advantages of this invention can be that formation of the final deep grip area is produced with material that is already substantially stretched to its correct ratios. This prevents the requirement of significant stretching against a cool mold wall. The plugs also only need to move against a relatively low pressure (typically less than 5 bars), greatly simplifying the mold construction. In addition as deep grip formation is not occurring in an intermediate container that has had significant intimate contact with cool mold walls but instead by low strain bending during a step subsequent from blowing the intermediate container, the material may be much less stressed on deep grip forming, resulting in lower internal stresses in the deep grip area.
Equipolymer® C93 PET is used in stretch-blow molding at 90-95° C. a container having a convex portion. The container is transferred from blow cavity to the conforming mold 3 while the convex portion 9 is at temperature between 30 and 81° C. The conditions for the transfer are selected so that the PET material remains essentially amorphous before and during the deep grip forming step. Further, the temperature/time profile for transfer are selected to limit crystal growth beyond that which can occur during stretch blow molding. The container is pressurized in the thermoforming cavity to 1-5 bar and then the deep grip is thermoformed by use of pneumatic cylinders. The deep grip is formed at a temperature below Tg. A plug having a contact surface matching the final deep grip shape is used to invert the convex portion. The container is vented and then ejected at a temperature of below 61° C.
For PET it can be of advantage to heat the blow cavity up to 60° C. to achieve the desired temperature of the blown container (between 30 and 81° C.) at the thermoforming step.
The dimensions and values disclosed herein are not to be understood as being strictly limited to the exact numerical values recited. Instead, unless otherwise specified, each such dimension is intended to mean both the recited value and a functionally equivalent range surrounding that value. For example, a dimension disclosed as “40 mm” is intended to mean “about 40 mm”.
All documents cited in the Detailed Description of the Invention are, in relevant part, incorporated herein by reference; the citation of any document is not to be construed as an admission that it is prior art with respect to the present invention. To the extent that any meaning or definition of a term in this document conflicts with any meaning or definition of the same term in a document incorporated by reference, the meaning or definition assigned to that term in this document shall govern.
While particular embodiments of the present invention have been illustrated and described, it would be obvious to those skilled in the art that various other changes and modifications can be made without departing from the spirit and scope of the invention. It is therefore intended to cover in the appended claims all such changes and modifications that are within the scope of this invention.
