injection molding 1-9-08

injection molding 1-9-08 - Injection Molding Douglas J....

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1 Injection Molding Douglas J. Gardner Professor Wood-polymer Hybrid Composites Injection Molding Schematic • A hot, molten polymer is injected into a cold mold. A screw apparatus can be used to inject the polymer into the mold, as shown in the schematic. After the part cools and solidifies, the mold is opened and the part is ejected.
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2 Typical products • Automotive, appliance, computer, communication and industrial equipment • Both amorphous and crystalline thermoplastic resins are used in injection molding. • Short glass fibers are commonly used as reinforcements. Basic Process Factors in Injection Molding • Material Parameters – Amorphous, Semicrystalline, Blends and Filled Systems – Pressure-Volume-Temperature (PVT) Behavior – Viscosity • Geometry Parameters – Wall Thickness of Part – Number of Gates, Gate Location, Gate Thickness and Area – Type of Gates: Manually or Automatically Trimmed – Constraints from Ribs, Bosses and Inserts • Manufacturing Parameters – Fill Time – Packing Pressure Level – Mold Temperature, Melt Temperature
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3 Amorphous, Semicrystalline, Blends and Filled Systems The chains in an amorphous polymer are randomly distributed. Some examples of amorphous polymers are Polycarbonate, Polyphenylene oxide and Acrylonitrile Butadiene Styrene. In a semicrystalline polymer, chains are partially ordered to form crystallites. Some examples of such polymers are Polybutylene terephthalate and Nylon. Engineering polymers are often blends of two or more polymers. Polymers are frequently blended in a twin screw extruder Fillers such as short glass fibers, coloring agents, fire retardants etc. are frequently added to the polymers to enhance their mechanical properties or other performance characteristics. Semicrystalline polymers have the highest shrinkage followed by blends and amorphous polymers. Systems with fillers have the lowest shrinkage. Pressure-Volume-Temperature (PVT) Behavior Volumetric expansion data for polymeric materials are obtained under equilibrium. Such data represent fundamental thermodynamic properties of the material and reflect the transitions as the material moves from glassy to crystalline to melt state.
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4 PVT Amorphous vs. Semi-Crystalline
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This note was uploaded on 12/21/2010 for the course AEWC 430 taught by Professor Dg during the Fall '08 term at University of Maine Orono .

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injection molding 1-9-08 - Injection Molding Douglas J....

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