13.11 What is the distinction between plastic sheet and film?
Answer. The distinction is based on thickness. Sheet stock has a thickness greater than 0.020 in (0.5
mm), while film stock is less than 0.020 in (0.5 mm) thick.
13.12 What is the blown-film process for producing film stock?
Answer. The blown-film process is a widely used process for making thin polyethylene film for
packaging. It combines extrusion and blowing to produce a tube of thin film. The process begins
with the extrusion of a tube that is immediately drawn upward while still molten and simultaneously
expanded in size by air inflated into it through the die mandrel.
13.13 Describe the calendering process.
Answer. 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 a
series of rolls to work the material and reduce its thickness to the desired gage.
13.14 Polymer fibers and filaments are used in several applications; what is the most important application
commercially?
Answer. Textiles.
13.15 Technically, what is the difference between a fiber and a filament?
Answer. A fiber is a long, thin strand of material whose length is at least 100 times its diameter; a
filament is a fiber of continuous length.
13.16 Among the synthetic fiber materials, which are the most important?
Answer. Polyester is the most important commercially, followed by nylon, acrylics, and rayon.
13.17 Briefly describe the injection molding process.
Answer. 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.
13.18 An injection-molding machine is divided into two principal components. Name them.
Answer. The components of an injection-molding machine are (1) the injection unit and (2) the
clamping unit.
13.19 What are the two basic types of clamping units?
Answer. The clamping units are: (1) mechanical toggle clamp and (2) hydraulic. In addition, there
are hydromechanical units which combine hydraulic and mechanical actuations.
13.20 What is the function of gates in injection molds?
Answer. 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.
13.21 What are the advantages of a three-plate mold over a two-plate mold in injection molding?
Answer. As the mold opens, the three-plate mold automatically separates the molded part(s) from
the runner system.
13.22 Discuss some of the defects that can occur in plastic injection molding.
Answer. 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