Thermoplastic material for compression molding

Topics: Plastics industry, Viscosity, Plastic Pages: 7 (1923 words) Published: April 14, 2014

1. How do you prep a Thermoplastic material for compression molding? a. Thermoplastic resins may be loaded into the mold either in the form of pellets or sheet, or the mold may be loaded from a plasticating extruder. Thermoplastic materials are heated above their melting points, formed and cooled. For both thermosets and thermoplastics, the better the feed material is distributed over the mold surface, the less flow orientation occurs during

2. What are the advantages, Strengths of Rotomolding? List at least 4. a. Ability to select from a variety of surface textures.
b. Ability to mold in inserts for exceptional pull out strength. c. Strong outside corners in virtually stress free parts.
d. Process allows for reasonably quick mold changes.
e. Ability to produce double wall parts for additional rigidity. f. Cheap startup tooling costs compared to injection or blow molding.

3. Describe dry flow and bulk density in Rotomodling
a. Dry flow in Rotomodling is the time it takes 100 grams of resin powder to pass through a funnel of given dimensions. Powder dry flow properties are important throughout rotational molding because they determine how the polymer distributes itself within the mold and how polymer melts into complex shapes. Dry flow depends mainly on particle shape and size.

Bulk density in Rotomodling is the quantity of undisturbed powder than can fit inside a beaker of specified dimensions. It is also a measure of efficiency with which the powder particles are packed together.

Bulk density and dry flow are dependent on particle shape, particle size, and particle size distribution of powder

4. Define a Newtonian Fluid
a. Newtonian fluid is a fluid whose stress versus strain rate curve is linear and passes through the origin. The constant of proportionality is known as the viscosity. It is also any fluid exhibiting a linear relation between the applied shear stress and the rate of deformation.

5. Define a non Newtonian Fluid
a. Non-Newtonian fluid is a fluid that its viscosity depends on the force that is applied to it. It is a fluid whose rate of shear is not proportional to the corresponding stress

6. Why are melt flow indexers used in Polymer processing
a. The melt flow indexer is used in polymer processing because of quality control.

7. Describe the process of Compression Molding
a. The compression molding starts, with a selected amount of plastic or gelatin placed over or inserted into a mold. Later the material is heated to a flexible state by the mold. Soon after that the hydraulic compresses the flexible plastic against the mold, which result in a perfectly molded piece by retaining the shape of the inside surface of the mold. After the hydraulic press releases, an ejector pin is injected in the end of the mold rapidly and ejects the finish piece out of the mold.

8. What are the machine selection consideration for injection molding, list at least 4 a.
i. Screw and Barrel Selection
The screw performs the following function in the injection molding process. 1. Conveys the material through the barrel. 2. Mixes the material to the proper molten state. 3. Compresses the material to maximum density. 4. Forces the material into the mold. Adjustments to the injection process often involve the screw to some degree. The screw, when perform its function provides important contribution to the overall process. The screw speed for the polypropylene material ranges 50-125 rpm. Therefore it is important to be aware of the type of screw that is being used for each applied specific Barrel and screw designs are recommended for maximum machine wear resistance and proper material plasticization. While molding Polypropylene products, should be aware of two critical parameters are L/D ratio and compression Ratio. (L/D)= (Flight Length of screw)/ (Outside diameter of screw) Compression Ratio= (Depth of feed section)/ (Depth of metering...
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