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72 Cards in this Set
- Front
- Back
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The total volume of polymers now ____________ that of metals. |
Exceeds |
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Why are plastic shaping processes important? |
- Almost unlimited variety of shape geometries - Plastic molding is a net shape process - Less energy is required than for metals due to much lower processing temperatures. |
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What is a polymer melt? |
A polymer in its liquid form. |
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What are two important properties of polymer melts? |
- Viscosity - Viscoelasticity |
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What is viscosity? |
A fluid property that relates shear stress to shear rate during flow. |
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Due to its _____________________, a polymer melt is a thick fluid with ____ viscosity. |
1) high molecular weight 2) high |
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Flow rates are often large, leading to high ____________________ and ________________, so significant pressures are required to accomplish the processes. |
shear rates and shear stresses |
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Viscosity ___________ with temperature. |
decreases |
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What is viscoelasticity? |
A combination of viscocity and elasticity. - Possessed by both polymer solids and polymer melts - Example: Die swell in extrusion, in which the hot plastic expands when exiting the die. |
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What is extrusion used for in thermoplastics and elastomers? |
To mass produce items such as tubing, pipes, hose, structural shapes, sheet and film, continuous filaments, and coated electrical wire. |
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What are the two main components of an extruder? |
- Barrel - Screw |
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What are the typical diameters of an extruder barrel? |
25 to 150mm |
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What are the L/D ratios for ext barrels? |
Usually between 10 and 30. Higher ratios for thermoplastics, lower ratios for elastomers. |
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How is feedstock melted? |
Through electric heaters, mixing, and mechanical working. |
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What are the sections of the extruder screw? |
- Feed section - Compression section - Metering section |
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What does the feed section do? |
Feedstock is moved from hopper and preheated. |
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What does the compression section do? |
The polymer is transformed into a liquid, the air is extracted, and the material is compressed. |
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What does the metering section do? |
The melt is homogenized and sufficient pressure is developed to pump it through the die opening. |
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What happens before the melt reaches the die? |
It passes through a screen pack, which is a series of wire meshes supported by a stiff plate containing small axial holes. |
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What is a screen pack for? |
- It filters out contaminants and hard lumps - It builds pressure in the metering section - It straightens the flow of the melt and removes its memory of circular motion from the screw |
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What is the principal transport mechanism in an extruder? |
Drag flow (Qsubd) |
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How does drag flow arise? |
It results from the friction between the viscous liquid and the rotating screw. |
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What is back pressure? |
The pressure that comes out of the compressing of the polymer melt through the die. It reduces flow transport. Qsubb |
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What is the resulting flow in an extruder? |
Qsubx=Qsubd-Qsubb |
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What do hollow profiles require? |
They require mandrel to form the shape. |
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How are mandrels held? |
Using a spider. |
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Mandrel often includes an air channel through which air is blown to maintain ____________________________________. |
Hollow form of extrudate during hardening. |
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What is film? |
When thickness is below 0.5 mm. |
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What is sheet? |
Thickness from 0.5 mm to 12.5 mm |
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Which plastics are used for polymer sheet and film/ |
All the thermoplastic polymers. |
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What are the most widely used processes for sheet and film production? |
Continuous and high production ops. - Slit-die extrusion of sheet and film - Blown-film extrusion process - Calendering |
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What is slit-die extrusion of sheet and fil,? |
The production of sheet and film by conventional extrusion, using a narrow slit as the die opening. |
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What are the characteristics of slit-die extrusion? |
- The sleet may be up to 3 m wide and as narrow as 0.4mm - A problem is the uniformity of the thickness, due to the drastic shape change of polymer melt - The edges must be trimmer because of thickening. |
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What is blown-film extrusion process? |
It combines extrusion and blowing to produce a tube of thin film. |
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What is the process of blown-film ext like? |
- Extrusion of tube - Tube is drawn upward while still molten and expanded by air inflated into it through die. The air is blown to maintain uniform film thickness and tube diam. |
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What is fiber? |
A long, thin strand whose length is at least 100 times its cross-section |
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What is a filament? |
A fiber of continuous length |
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What are the applications of fibers and filaments? |
- Textiles - Reinforcing materials in polymer composites |
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What is spinning? |
The extrusion of polymer melt or solution through a spinneret then drawing and winding onto a bobbin. |
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What is a spinneret? |
A die with multiple small wholes. |
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What are the three variations of spinning? |
- Melt spinning - Dry spinning - Wet spinning |
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What is melt spinning? What are its steps? |
The starting polymer is heated to its molten state and pumped through the spinneret. The filaments are then drawn and air cooled before being spooled onto bobbin. Significant extension and thinning of the filaments occur while the polymer is still molten, so final diameter wound onto bobbin may be 1/10 of extruded size. |
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What is the typical spinneret like? |
It is 6 mm thick and contains approx 50 holes of 0.25mm |
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What is dry spinning? |
It is similar to melt spinning, but the starting oplymer is in solution and solvent can be separated by evaporation. |
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What are the steps of dry spinning? |
- The first step is extrusion through the spinneret - The extrudate is then pulled through a heated chamber which removes the solvent, leaving the polymer |
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What is dry spinning used for? |
For filaments of cellulose acetate and acrylics. |
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What is wet spinning? |
It is similar to melt spinning, but the polymer is also in solution, but the solvent is not volatile. |
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How is the polymer separated in wet spinning? |
The extrudate is passed through a liquid chemical that coagulates or precipitates the polymer into coherent strands, which are then collected into bobbins. |
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What is wet spinning used for? |
To produce filaments of rayon (regenerated cellulose). |
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What is injection molding? |
A process in which the polymer is heated to a highly plastic state and forced to flow under high pressure into a mold cavity where it solidifies and the molding is then removed from cavity. |
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What does injection molding produce? |
Discrete components almost always to net shape. |
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What is the typical cycle time for injection molding? |
10 to 30 sec, but cycles of 1 minute or more are not uncommon. |
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What is injection used for? |
To produce complex and intricate shapes. |
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What are the limitations of injection molded parts |
The capability to fabricate a mold whose cavity is the same geometry as the part. The shape must allow for part removal from mold |
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What is injection molding economical? |
When there's large production, due to the high cost of the mold. |
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What is the most widely used molding process for TP? |
Injection molding, |
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What considerations must be taken into account when using injection molding for thermosets and elastomers? |
There must be modifications in equipment and operating parameters to avoid premature cross-linking of the materials.. |
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What are the two principal components of the injection molding machine? |
1) The injection unit 2) The clamping unit |
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What does the injection unit do? |
It melts and delivers the polymer melt. |
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What does the clamping unit do? |
It opens and closes mold each injection cycle |
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What does an injection unit of molding machine consist of? |
A barrel fed from one end by a hopper containing a supply of plastic pellets. |
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What does the barrel do? |
It rotates for mixing and heating the polymer, and acts as a ram to inject the molten plastic into the mold. There is a non-return valve near the tip of the screw that prevents melt flowing backward |
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What are the functions of the clamping unit of the molding machine? |
1) Holds two haves of mold in proper alignment with ea/o 2) Keeps mold closed during injection by applying a clamping force sufficient to resist injection force 3) Opens and closes mold at appropriate times in the cycle |
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Describe shrinkage in polymers. |
Polymers have high thermal expansion coeffs, so significant shrinkage occurs during solidification and cooling in the mold. |
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How are the dimensions of the mold calculated? |
Dsubc=Dsubp+Dsubp*S+Dsubp*S^2
Dsubc is the dimension of the cavity, Dsubp is the molded part dimension, and S is the shrinkage value. |
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Fillers in plastic tend to ___________ shrinkage. |
Reduce |
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Higher injection pressures __________ shrinkage. |
Reduce |
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Longer compaction time ____________ shrinkage. |
Reduces |
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Higher temperatures _______ polymer melt viscosity, ___________ shrinkage. |
lower/reduce |
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Many plastics are subject to degradation from ___________ and other forms or radiation. |
Sunlight |
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Variations in wall thickness can result in: |
Non-uniform plastic flow and uneven cooling, which can warp the extrudate. |
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What are reinforcing ribs good for? |
They achieve increased stiffness without excessive wall thickness. They are made thinner than the walls they reinforce to minimize sink marks on outside wall. |
