The invention relates to blow molding and more particularly to an improved method and apparatus for blow molding an article.
In a typical blow molding process a plastic material is heated to a molten state and is extruded through a die head and formed into a parison or preform. The parison is a tube-like piece of molten plastic open at one end. The parison is introduced into the mold cavity and the two halves of the mold close around it. A pressurized gas, typically air, is delivered to the interior of the parison through the open end of the parison via a blow pin. The gas expands the hot parison of plastic and presses it against the walls of the mold cavity. The pressure is held until the plastic cools and solidifies inside the mold cavity. The mold is opened and the formed plastic article is removed from the mold. The exterior of the article assumes the shape of the mold cavity and the interior of the article is hollow.
A method of blow molding an article comprises providing a mold having a single mold cavity and a length and a width where the width may be less than the length. The mold may be oriented such that the length is oriented horizontally or transversely to the direction of parison extrusion. A first parison and a second parison are simultaneously extruded into the mold cavity. The first parison and the second parison are expanded such that the material of the first parison contacts the material of the second parison to form a structural rib inside of the article.
The method may further include blowing a gas into the first parison and the second parison. The gas may be blown into the first parison and the second parison through a first blow pin and a second blow pin. Heat causes the material of the first parison and the material of the second parison to permanently weld together to form the structural rib. A first compartment and a second compartment may be formed by the structural rib. Additional parisons may be simultaneously extruded into the mold cavity with the first and second parisons and expanded such that the material of the additional parisons contacts the material of the first and/or second parison to form additional ribs and compartments inside of the article.
A blow molded article is also provided comprising an outer wall formed of material of a first parison and material of a second parison. A structural rib is formed where the material of the first parison contacts the material of the second parison. The material of the first parison and the material of the second parison are permanently welded together at the structural rib. The article may comprise a first compartment and a second compartment formed by the structural rib. Material of additional parisons may contact the material of the first and/or second parison to form additional ribs and additional compartments inside of the article.
The blow molding method and apparatus allows larger articles to be blow molded without requiring larger blow molding machinery. In the blow molding technique of the invention the mold is modified to allow processing of the mold with the cavity geometry rotated from the standard orientation. Consider a mold that is a single cavity tool having a length L and a width W where the length L is greater than the width W. The mold has a single mold cavity that is rotated 90° as compared to existing systems such that the length dimension L is oriented in the mold press horizontally and the width dimension W is oriented vertically. The mold is processed with the short dimension W in-line with the direction of parison extrusion. The blow molding method and apparatus may also be used where the length dimension L and the width dimension W are the same.
In standard processing, a mold having a large horizontal direction requires a blow molding machine that has a die with a diameter size compatible with the large horizontal dimension which increases the size of the extrusion head and, in turn, greatly increases the cost of the equipment. In an existing panel mold, the mold has as single blow pin channel where the long dimension of the mold is vertically in-line with the direction of the parison extrusion. For example, consider an existing mold that is a single cavity tool with article dimensions of approximately 30″ wide by 60″ long. Using standard processing techniques this mold is processed with the length dimension (60″) in-line with the direction of parison extrusion.
With the mold redesigned for the rotated press orientation, a molding trial was conducted in a twin-head accumulator style blow molding machine with two 15 lb capacity die heads mounted 30″ apart on centerline with a mold having article dimensions of 30″×60″ where the 30″ dimension is in-line with the direction of parison extrusion. Each die head had 12″ dies installed (rather than a single die head having a 30″-35″ diameter that would otherwise be required with a 60″ wide mold) and by extruding and expanding two parisons simultaneously, a single article was produced. The blow molding technique allows plural smaller die heads to be used to make articles that would otherwise require a single larger die head resulting in cost savings.
Referring to
As the gas is introduced, the parisons 8 and 10 are expanded and the first plastic material of the first parison 8 and the second plastic material of the second parison 10 are pressed against the interior of the mold cavity 4 as shown in
Referring to
By using blow mold grade HDPE improved strength can be achieved versus using extrusion grade HDPE. The blow molding technique may also be applied to the profile extrusion process; however, this process requires additional components to close the ends of the article in order to make discreet articles. The blow molding technique provides an economic advantage because larger articles can be made with lower equipment capital and because of an improved standard cost of the finished product due to lower overhead rates from the use of a smaller blow molding machine.
Specific embodiments of an invention are disclosed herein. One of ordinary skill in the art will recognize that the invention has other applications in other environments. Many embodiments are possible. The following claims are in no way intended to limit the scope of the invention to the specific embodiments described above.
This application claims benefit of priority under 35 U.S.C. §119(e) to the filing date of to U.S. Provisional Application No. 61/122,543 as filed on Dec. 15, 2008, which is incorporated herein by reference in its entirety.
Number | Date | Country | |
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61122543 | Dec 2008 | US |