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Reciprocating screw machinery
Reciprocating screw machinery is used for the extrusion blow molding of hollow containers. Examples of parts manufactured from these machines include lightweight HDPE bottles for dairy and water, as well as large 3-5 gallon polycarbonate bottles for water coolers.
Additional recommended knowledge
Reciprocating Screw blow molding machines are characterized by the use of a reciprocating screw extruder, as is used in injection molding. As the screw melts the resin, the screw moves backward, allowing the melted plastic to accumulate in the end of the barrel. When the screw pushes forward under hydraulic pressure, the plastic is pushed out of the barrel, extruded through a flowhead and die, to form a plastic parison.
Unlike shuttle machinery or rotary wheel machinery, which are characterized by continuous extrusion, reciprocating screw machinery utilizes an intermittent extrusion process. This allows the parisons to be dropped quickly (in some cases less than one second), followed by the rapid closing of the molds.
In most cases, the molds are stationary, and do not shuttle sideways. Rather, the parts are extracted from the molds, and then the parison is dropped between the open mold halves.
Reciprocating extrusion blow molding machinery is used to manufacture monolayer parts, as intermittent processes are now well suited for coextrusion. These machines can be broken down into at least two major families:
Lightweight Bottle Machinery
Although typically referred to as “Lightweight Dairy” machines, this family of machines may also be used for the manufacture of bottles for water, as well as juice, household chemical containers, and some industrial parts. The introduction of this family of machines caused a massive conversion in the diary industry, with HDPE bottles replacing glass and paperboard.
Bottles may be manufactured with either a “pull-up” or “ram down” neck finish. In the United States, the pull-up finish is most common. A pull-up finish forms a pre-cut inner ring in a round, horizontal ledge at the top of the neck of the bottle - requiring a plug seal. A ram-down prefinish is capable of forming a "vertical" tube section at the top of the neck - without a horizontal ledge. This is analogous to a blowpin neck finish on shuttle machinery.
Most one-gallon dairy containers are manufactured on 4-head or 6-head machines, although recently more producers have gone to 8-head machines to drive down bottle costs. It is estimated that over 2500 reciprocating screw blow molding machines for production of HDPE containers have been delivered in the United States - and over 3200 worldwide.
Typically, bottles are removed from the molds by pulling downward via a takeout gripper device. Handleware bottles are often laid on "cooling beds" - which allow the plastic to cool while properly spacing the bottles and feeding them into an impact trimmer.
In some applications, particularly non-handled and small containers - the bottles may drop down through metal chutes, into a horizontal takeout device. This takeout device my utilize belt conveyors or screws to move the bottles out of the machine, for detabbing (removing the tails) and trimming.
Horizontal impact trimmers are the most common approach. Bottles may be fed into the trimmers by conveying off the cooling bed over a "live roller", which feeds the bottles into conveyor buckets. Alternatively, a pick and place unit may lift the bottles from the cooling bed using vacuum cups and place them into the conveyor buckets.
Water Bottle Machinery
This equipment was originally engineered to be simple, versatile, and capable of producing basic bottles and containers. However, without integral bottle trimming capability, most applications were centered on the production of various “niche” industrial, recreational, automotive, and novelty type products.
This family of machines utilizes a reciprocating screw extruder with a direct feed “flow through” die head design for forming the parison. Cantilevered clamp systems are non-shuttling for simplicity and provide open access for convenient part extraction.
Both Single or Dual Head configurations are common in the industry. Hydraulic extruders are commonly used. The use of hydraulic compaction blow pin assemblies to allow the production of necks with unblemished sealing surfaces has improved the performance of the containers by greatly reducing the number of "leakers."
Sequence of Operation
Although there are variations, based upon equipment customization and part molding requirements, in general reciprocating screw blow molding machinery is characterized by the following sequence of operations:
Advantages and Disadvantages of Reciprocating Screw Machinery
There are many suppliers of reciprocating screw blow molding machinery. Some of the major global suppliers include:
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Reciprocating_screw_machinery". A list of authors is available in Wikipedia.|