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September 28, 2021
Type of blow molding
Extrusion blow molding
In Extrusion Blow Molding (EBM), plastic is melted and extruded into a hollow tube (a parison). This parison is then captured by closing it into a cooled metal mold. Air is then blown into the parison, inflating it into the shape of the hollow bottle, container or part. After the plastic has cooled sufficiently, the mold is opened and the part is ejected.
EBM processes may be either continuous (constant extrusion of the parison) or intermittent. Types of EBM equipment may be categorized as follows:
1. Continuous Extrusion Equipment
2. Intermittent Extrusion Machinery
Examples of parts made by the EBM process include dairy containers, shampoo bottles, and hollow industrial parts such as drums.
Basic polymers, such as PP, HDPE, PVC and PET are increasingly being co-extruded with high barrier resins, such as EVOH or Nylon, to provide permeation resistance to water, oxygen, CO2 or other substances. In dairy applications, it is possible to extrude a black light-blocking layer in the center layer of containers, with opaque white resin used in the inner and outer layers.
Compared to injection molding, blow molding is a low pressure process, with typical blow air pressures of 25 to 150 psi. This low pressure process allows the production of economical low-force clamping stations, while parts can still be produced with surface finishes ranging from high gloss to textured. The resulting low stresses in the molded parts also help make the containers resistant to strain and environmental stress cracking.
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 in between two open mold halves.
Unlike shuttle machinery or rotary wheel machinery, which is 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 flow-heads 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.