Reciprocating screw machinery
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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.
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[edit] Description
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:
[edit] 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.
A large number of machine configuration possibilities exist, with variations in extruder size (3.5, 4.0 and 4.5 in. diameters are typical), head size (typically 100 mm or 140 mm), clamp sizes, number of heads (2, 3, 4, 5, 6, 8, 10, 12 and 16 head options), and downstream handling and trimming options.
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.
Handleware bottles may be trimmed in horizontal impact trimmers, as shown, or rotary trimmers. In these trimmers, the tails, top flash, and handle slug are removed. The trimmed bottle is then ready for filling and labeling.
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.
[edit] Variations
- Side Shift Clamps can be utilized to allow the production of bottles with offset necks.
- Neck-to-Neck molding can be used to make two bottles per parison simultaneously. This requires needle air blow, and eliminates the possibility of pre-finished necks. Although possible for small, single-serve spin trimmed bottles, this practice has not yet achieved commercial acceptability due to the added complexity of capturing two containers with a free-falling lightweight parison.
- Shot Pot Extrusion allows the use of a continuous extrusion system, and optional screen changer. In this approach, the resin is extruded into a shot pot accumulation chamber, then forced through the flowheads by activating a hydraulic ram cylinder. This can lead to an improvement in cycle time through faster parison drop speeds, and in some cases, lower melt temperatures.
- Parison Programming is utilized in some industrial applications, but is not normally required due to the fast drop speeds and relative uniform wall thickness in properly designed lightweight HDPE containers.
[edit] 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.
In the mid to late 1970’s the 5-gallon polycarbonate water bottle market began to develop and Improved Blow Molding, working with GE Plastics, designed and supplied a number of special "B30" model blow molding machines to meet the need. That “niche” market enjoyed significant domestic growth in the ‘80’s and significant growth worldwide in the ‘90’s.
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."
[edit] 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:
- Plastic resin is melted by an extruder. As the screw rotates to melt the plastic, it also retracts, allowing molten resin to form in the front of the barrel.
- When enough plastic is melted, hydraulic pressure is activated to push the screw forward, forcing the resin through flowheads and dies to form parisons.
- When the parison is fully extruded, the mold halves are closed under hydraulic pressure.
- Air enters the hollow parison, forcing it outward against the chilled mold halves. Air pressure is maintained until the plastic has cooled sufficiently to eject the part from the mold.
- Air pressure is relieved before opening the mold halves.
- The parts are removed from the mold. Parts may be released by gravity (with the aid of a part stripper) or removed using a part extractor. The parts are then ready for external trimming of "flash", or scrap plastic.
[edit] Advantages and Disadvantages of Reciprocating Screw Machinery
[edit] Advantages
- Very efficient container production, with fast cycle times
- Machines are mechanically simple and easy to operate - the clamps do not shuttle, and the flowheads do not require bobbing
- Moderate initial capital investment
[edit] Disadvantages
- Limited to monolayer production- Uniloy remedied this with a 3 layer, White/black/white machine in 2006-
- Integration of parison programming is expensive, as each head utilizes a separate parison control system
- Inability to utilize a screen changer (This limitation is overcome in the Shot Pot machinery variation)
[edit] History
- Lightweight Bottle Machinery
- The Rocheleau Tool & Die Co., Inc. is established in 1938 by Leopold A. Rocheleau. They later develop a niche in reciprocating screw machinery for small containers.
- In the 1950's, the Uniloy Division of Hoover Ball & Bearing becomes the first American supplier of blow molding machinery. They go on to develop machinery and patents for the production of "hollow-handle" plastic bottles for the dairy packaging market.
- In 1985, Uniloy was purchased by Johnson Controls, and continues market dominance in light weight dairy bottle machinery.
- APS Plastics Systems is formed by former Uniloy employees in 1997.
- Uniloy became part of Milacron Inc. in 1998.
- In 1998, Liberty Blow Molding also emerges, competing with Milacron in large bottle reciprocating screw blow molding.
- Liberty is purchased by Uniloy in 1992, after achieving 40% market share in four years, and shipping some 100 machines.
- Graham Engineering purchases the technology of APS Plastics Systems in 2003. GMG capitalizes on refinements such as shot pot technology as a viable alternative to reciprocating screw extruders.
- Water Bottle Machinery
- This equipment product line is relatively old, originating in 1960 and produced by IMPCO, a NH based injection molding machine manufacturer.
- IMPCO (a.k.a. Improved) subsequently became part of Ingersoll-Rand Co. in 1964.
Improved Blow Molding B30 Machine – Courtesy Graham Engineering, York, PA - In the mid to late 1970's the 5-gallon polycarbonate water market develops.
- IMPCO was subsequently purchased by Goodman Equipment Corporation of Chicago in 1979.
- The Improved Blow Molding B15-R35, B30-R180, and B37-R250 product line was purchased from Goodman Equipment Corp. by Graham Engineering in 2000. At that time, around 800 Improved machines has been sold.
- GMG redesigned the product line, introducing the C30 machine in 2001. This machine was noteworthy in that it was designed for optimum production of PC water bottles, at up to 150 bottles per hour, and utilized PC controls.
[edit] Major Suppliers
There are many suppliers of reciprocating screw blow molding machinery. Some of the major global suppliers include:
