Plastic is injected into the mold under gas pressure

2017-05-15 13:59:21 | 日記
Thanks to Carl Zimmer, Radiolab regular and author of the wonderful blog, The Loom, whose musings about evolutionary traps and the work of Bruce Robertson of Bard College, Jennifer Rehage of Florida International University and Andrew Sih of the University of California, Davis, got me thinking about all this. Also, thanks to two wonderful songwriters out of Britain, Flanders and Swann, who years ago wrote about the impossible love of an armadillo for an Army tank — one of the most poignant evolutionary traps ever.

This method differs from the above in that the plastic is injected into the mold under gas pressure. This process is more controlled and repeatable, producing bottles and other hollow containers with uniform, thick and clear walls. The surface quality is excellent, although it is slower and is not ideal for making thinner wall sections.Because this method is so often employed for disposable plastic drinking bottles the raw material is inexpensive and easy to recycle.

PET (polyethelene terephthalate) or PEEK (polyether ether ketone) are the typical choices here, for their clarity, structural strength, and because such material is rated as safe for consumables. It’s also easily recycled.Two chemically reactive polymers are introduced separately into the die cavity, where they are heated and allowed to undergo a thermal expansion that fills the die cavity. Remember, these are thermosetting plastics which undergo a chemical change, so waste material cannot be recycled.

The entire molding process takes more time than injection molding and the chemicals used tend to be more expensive, but the resulting parts are lightweight and strong by volume.They are big — much bigger than the males. But most important, they are covered, as you see here, with dimples, little Hollow Bottles.Within the oven the mold is rotated on two axes, but it’s done slowly. Centrifugal force is not used to displace the material; rather, it’s allowed to coalesce onto the mold walls under gravitational pressure.

Tooling costs for prototypes are relatively low, while production tooling is moderately expensive. The main cost is in the material, bearing in mind that the resulting part must always be finished, usually with a urethane-based gel coat or by painting, so the process is more labor intensive which increases the piece price.After the two halves are dried the mold can be opened up and the master removed.
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