Term
| What are some reasons why plastic shaping processes are important? |
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Definition
| The reasons include: (1) many of the processes are net shape processes; (2) in general, less energy is employed than in metalworking processes; (3) lower temperatures are required to process plastics than metals or ceramics; (4) there is great flexibility in geometry; and (5) painting and other finishing processes are generally required. |
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Term
| Identify the main categories of plastics shaping processes, as classified by the resulting product geometry. |
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Definition
| The categories are: (1) extrusion; (2) molding; (3) forming of continuous sheets and films; (4) fibers; (5) foamed products; and (6) discrete formed sheets and films |
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Term
| Viscosity is an important property of a polymer melt in plastics shaping processes. Upon what parameters does viscosity depend? |
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Definition
| Viscosity of a polymer melt depends on (1) temperature and (2) shear rate. Also (3) the molecular weight of the polymer affects viscosity. |
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Term
| How does the viscosity of a polymer melt differ from most fluids that are Newtonian. |
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Definition
| A polymer melt exhibits pseudoplasiticity, which means that its value descreases with increasing shear rate |
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Term
| What does viscoelasticity mean; when applied to a polymer melt? |
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Definition
| Viscoelasticity is a combination of viscous and elastic properties which cause the melt to exhibit memory - the tendency to return to its previous shape, as exhibited by die swell in extrusion |
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Term
| Define die swell in extrusion |
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Definition
| Die swell is the tendency of the extrudate to expand in cross-sectional dimensions immediately on exiting the die orifice. It results from the viscoelastic properties of the polymer melt. |
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Term
| Briefly describe the plastic extrusion process |
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Definition
| In plastic extrusion, a polymer melt is compressed to flow through a die orrifice and thus the continuous length of the plastic assumes a cross-sectional shape that is approcimately the same as that of the orifice |
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Term
| The barrel and screw of an extruder are generally divided into three sections; identify the sections. |
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Definition
| The sections are: (1) the feed section; in which the feed stock is fed from the hopper and heated; (2) the compression section, in which the polymer changes to a viscous fluid; and (3) the metering section, in which pressure is developed to pump the plastic through the die orifice |
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Term
| What are the functions of the screen pack and breaker plate at the die end of the extruder barrel? |
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Definition
| The functions are to: (1) filter dirt and lumps (2) build pressure (3) straighten the flow and remove memory of the polymer melt. |
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Term
| What are various forms of extruded shapes and corresponding dies? |
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Definition
| The shapes are: (1) solid profiles, such as rounds and L-shapes (2) hollow profiles, such as tubes; (3) wire and cable coating; (4) sheet and film; (5) filaments (continuous fibers) |
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Term
| What is the distinction between plastic sheet and film? |
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Definition
| The distinction is based on thickness. Sheet stock has a thickness greater than 0.020 inches (0.5 mm) while film stock is less than 0.020 inches (0.5 mm) thick. |
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Term
| Describe calendering process |
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Definition
| Calendering is a process for producing sheet and film stock out of rubber or rubbery thermoplastics such as plasticized PVC. In the process, the initial feedstock is passed through series of rolls to work the material and reduce its thickness to the desired gage. |
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Term
| Briefly describe the injection molding process |
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Definition
| Injection molding is a process in which a polymer is heated to a highly plastic state and forced to flow under high pressure into a mold cavity, where it solidifies. The molding is then removed from the cavity. |
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Term
| An injection-molding machine is divided into two principle components. Name them. |
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Definition
| The components of an injection-molding machine are: (1) the injection unit and (2) the clamping unit. |
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Term
| What are the two basic types of clamping units? |
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Definition
| The clamping units are: (1) mechanical toggle clamp and (2) hydraulic. In addition, there are hydromechanical units which combine hydraulic and mechanical actuations. |
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Term
| What is the function of gates in injection molds? |
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Definition
| The function of gates in an injection mold is to constrict the flow of molten plastic into the cavity, which increases the shear rate and reduces the viscosity of the polymer melt. |
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Term
| Discuss some of the defects that can occur in plastic injection molding. |
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Definition
| The defects include: (1) short shots, in which the polymer melt solidifies before filling the cavity, (2) flashing, in which the polymer melt is squeezed into the parting surfaces between the mold halves and around ejection pins; (3) sink marks, in which the surface is drawn into the molding by contraction of internal material; and (4) weld lines where the melt has flowed around a core or other convex detail in the mold cavity and met from opposite directions, thus resulting in mechanical properties that are inferior to those in the rest of the part. |
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Term
| What are the significant differences in the equipment and operating procedures between injection molding of thermoplastics and injection molding of thermosets? |
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Definition
| The differences in injection molding of thermosets are: (1) shorter barrel length; (2) lower temperatuers in the barrel, these first two reasons to prevent premature curing; and (3) use of a heated mold to cause cross-linking of the TS polymer. |
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Term
| What is reaction injection molding? |
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Definition
| Reaction injection molding involves the mixing of two highly reactive liquid ingredients and immediately injecting the mixture into a mold cavity where chemical reactions leading to solidification occur. The two ingredients form the components used in catayst-activated or mixing-activated thermoset systems. |
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Term
| What kinds of products are produced by blow molding? |
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Definition
| Blow molding is used to produce hollow, seamless containers, such as bottles. |
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