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The Blow Molding Process: |
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Extrusion-blow molding: For normal runs, extrusion-blow molding is used.
Here, a tubular "parison" is extruded downward, a "blow-pin" injected into it, and it is inflated against the mold wall. The mold opens, and the cooled part is ejected, and is ready for trimming. Aluminum
molds are used, and these parts are made with a relatively low mold cost and relatively high labor (trimming) costs. |
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Injection-blow molding: Designed for high-production molding, such as beverage
bottles, this is a more automated process. A "Test-tube shape" is injection molded, whose design is optimized for the blown finished part. While hot, it is moved to the blow mold, where it is inflated
against the cavity walls. In this case, the finished part typically does NOT need trimming, avoiding a high-labor operation. This labor reduction lowers the piece price, but the necessity for two molds
substantially increases the mold cost. For this reason, this is used only with high production items. |
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Process Variations:
Besides injection and extrusion blow molding, other process variations include multiple head machines, dropping multiple parisons between multiple mold cavities, and use
of barrier resins. With barrier resins, the head drops a parison with two or more layers. This allows a bottle to have a UV resistant layer over an impact resistent layer, or to allow an inside layer
of impermeable material to keep the carbonation high. Also, parison programming
can create a parison of varying thickness, so that it is thickest where the most thinning from stretching is likely to occur.
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Design considerations: |
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Part Geometry:
Material melts as it does in an extruder, but is extruded down, between the 2 mold halves. The blow pin inflates the tube to the cavity walls, meaning that this tube
stretches as it forms the part. The farther it stretches, the thinner the wall. Good design requires that corners have generous radii, to avoid excessive thinning in areas where impacts are the most
likely.
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Materials:
Most thermoplastics can be blow molded, but the most common is polyethylene, which is used for food or chemical or detergent bottles. PET or polyester is used for clear
beverage bottles such as water bottles or the familiar 2-liter beverage bottles. EVA is a rubber-like material used for blow molded elastomer parts. Generally the list includes: HDPE, PET, Polypropylene,
LDPE, PVC, Polycarbonate, ABS, EVOH, LLDPE, TPO, PBT,Nylon, TPE, ABS/PC Blend, Polystyrene, K-Resin®, MDPE, PUR, PETG and PPO,
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Blow Molding links:
Resin source: http://www.mrcpolymers.com/product_summer00.htm
Book: http://www.mastio.com/press/bmpress.html
News: http://www.blowmouldingmatters.co.uk/bmm_test2.htm
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