Accumulator Head Machinery is used for the extrusion blow molding of large industrial hollow parts. Examples of parts produced on this machinery include drums, trash cans, automotive panels, playground equipment, and large containers, such as Jerry Cans, for liquid storage. Most parts produced on accumulator head machinery are single layer; however, specialized machinery is capable of producing parts with up to seven unique layers of plastic - these machines are used primarily to manufacture automotive gasoline tanks with barrier layers.
Accumulator Head Machinery is characterized by the accumulation of melted plastic resin in one or more extrusion heads. As extruders melt the plastic, it accumulates in the heads until the resin is ready to be extruded into parisons. An internal plunger is then activated, using hydraulic pressure, to extrude the parison through an extrusion die, between two open mold halves.
Unlike shuttle machinery or rotary wheel machinery, which are characterized by continuous extrusion, accumulator head machinery utilizes an intermittent extrusion process. This allows large, heavy parisons to be dropped in a few seconds, followed by the rapid closing of the molds. Due to the large, heavy weight of the parisons, it is not practical to slowly extrude the plastic while the prior parison is blown and cooled in the molds. Cycle times of 30 to 120 seconds or more are common in thick-walled parts, and the parisons would cool and sag if extruded slowly over this time period. The intermittent process also allows the machinery to function without shuttling the molds, which is not economical with large, heavy molds and clamping structures.
In some applications, the parison is extruded over one or more blow pins, which are used to form precise openings in the part, as well as provide an entry point for the blow air. In other applications, the blow air may enter the part through the center of the extrusion heads, or through needles, which puncture the parison.
Due to the size of parts produced, requiring large clamps, the extruders and flowheads are typically positioned on an upper, "mezzanine" level. The clamp, electrical cabinets, operator station, and hydraulic system are typically positioned on the lower "ground" level.
Sequence of Operation
Although there are numerous variations, based upon equipment customization and part molding requirements, in general accumulator head machinery is characterized by the following sequence of operations:
Plastic resin is melted by an extruder and fills an accumulation chamber in the extrusion heads. As the melted plastic fills the head, a hydraulic plunger is pushed back by the pressure in the extrudate.
When the accumulation chamber is full, hydraulic pressure is activated to the plunger, forcing the resin through a die and forming a parison.
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.
A part extractor moves into place. In most cases, the part is gripped between the mold and flowhead, where the unformed parison forms part of the scrap that will be removed later from the formed part.
The molds open, releasing the part, which is now held into place by the part extractor. If necessary, blow pins are retracted or "unspun" (if forming threads in the part).
The part extractor moves the parts from the molds. The parts are then ready for external trimming of "flash", or scrap plastic.
In some cases, parts are dropped from the molds, and are removed manually from the machine without an extractor. In some cases, the parts drop onto angled trays, which then slide the parts away from the clamps, for manual removal. This approach may require longer cycle times, to allow the operators to remove the parts. The use of drop slides may also require the clamp of the machine to be elevated, increasing cost and required factory ceiling height.
In some cases, the parts are moved from the molds into secondary cooling stations. This approach allows the operator to reduce the overall cycle time required to manufacture the part. Most Jerrycans are manufactured using secondary cooling stations.
Many industrial parts are manufactured using additional steps. For example, slides or pins may be hydraulically activated in the molds, allowing the production of parts with complex geometry or undercuts. In some cases, components are inserted into the molds, to be attached to or encapsulated by the parisons in the blow molding process. For example, some fuel tanks are manufactured with pumps encased in the plastic, and automotive panels may incorporate carpet backing or encapsulated fasteners.
In 1949, Reinhold Hagen of Kautex, Siegburg Germany, develops the first blow molding machine for processing polyethylene.
In the 1960's, reciprocating screw blow molders were developed, with single or double heads with up to 10 lb (4.5 kg) plastic shot capacity. These were precursors of modern accumulator head machines.
In 1964, the first prototype plastic fuel tank was produced by Kautex. Uniloy introduced the first "unitized block" construction machine.
In 1972, Barr Polymer produced the first American accumulator machine. This technology was later sold to Uniloy.
In 1973, the first commercial polyethylene fuel tanks were produced by Kautex and installed in the production series Volkswagen Passat.
The first Sterling single 10 lb (4.5 kg) accumulator machine, with a 36 x 30 in (914 by 762 mm) press and MACO IV controller was demonstrated at NPE 1979.
In the 1980's the advent of modern "engineering plastics" such as Noryl® Modified Polyphenylene Oxide, spurred a generational leap in accumulator head machinery, as early generations of these materials had reduced melt strength. To be able to process these resins, machinery was developed including modern features such as:
High press closing speeds, > 1200 in. per minute.
Proportional valve hydraulics used with variable displacement pumps
Hydraulic prefill valves for fast clamp closing and lock-up.
Entire hydraulic system filtration to 3-10 micrometres, eliminating the need for a separate tank for parison programming.
In 1994, the first coextruded (multilayer) fuel tank was utilized in series production. Milacron also produced their first commercial machines that year.
In 1995, Graham Engineering produced their first commercial accumulator head machine - a dual 10-lb. with 60x40 in. press and MACO 6500 controller. Krupp introduced the first 7-layer accumulator machine.
In 2002, Graham Engineering introduced the first accumulator machine with PC controls.
In 2005, Graham Engineering began delivery of 5-head machines, utilizing a shooting pot to accumulate the melt, for high volume production of mid-sized industrial parts.
There are many suppliers of accumulator head blow molding machinery. Some of the major global suppliers include: