CAM 120 Chapter 13

Thermoplastics that have not been previously molded are called?

Pure

Common gases used in polymer foams are three of the following.

air, carbon dioxide, and nitrogen.

two of the following are reasons why injection molding of thermosets is more difficult than injection molding of thermoplastics.

allowances must be made for cross-linking of thermosets, and cycle times are longer because of curing.

A three-plate mold offers two of the following advantages when compared to a two-plate mold.

automatic separation of parts from runners and gating is usually at the base of the part to reduce weld lines.

Stretch blow molding is a variation of the following.

injection blow molding

The coating of a flat plastic or rubber film onto materials such as fabrics, paper, and cardboard is called.

planar coating

The two most common polymer foams are which of the following.

polystyrene and polyurethane

A thermoforming mold with a convex form is called?

positive mold

Blow molding can only be used on one of the following polymer.

thermoplastics

The length-to-diameter ratio of the extruder barrel is the highest of the following polymer type.

thermoplastics

Thermoforming can only be used on one of the following polymer types.

thermoplastics

The length-to-diameter ratio of the extruder barrel is highest in the following polymer types.

viscoelasticity

The parting line in injection molding is the following.

where the mold halves open to remove the part or parts

Extrusion is used for thermoplastics and elastomers but rarely for thermosets.

yes it is True

The distribution channel in an injection mold consists of three of the following components.

gates, runners, and sprue

Two of the following properties do plastic parts often compare favorably with metals.

impact resistance and strength of weight ratio.

For relatively large parts the molds in reaction injection molding are relative to the corresponding molds in conventional injection molding.

less expensive

One of the following thermoforming processes requires the most expensive mold.

mechanical thermoforming

how is one of the following not a function of the breaker plate and screen pack through which the polymer melt flows just before entering the die?

reducing the viscosity of the melt.

Structural foam molding and sandwich molding both produce plastic parts that have an inner core that is foam surrounded by a dense outer skin, but only one produces a smooth part surface. which is called.

sandwich molding

The coefficient of shear viscosity is defined as one of the following.

shear stress divided by shear rate

As the term is used in the production of synthetic fibers and filaments, spinning involves the extrusion of a polymer melt through the following.

spinneret

The backpressure flow in the extruder barrel increases as which two of the following parameters is increased.

the diameter of the barrel and head pressure

The drag flow of polymer melt in the extruder barrel increases as two of the following parameters is increased.

the diameter of the barrel and rotational speed of screw.

Polyethylene is widely used in blow molding for product containers. More low-density PE is used than high-density PE because LDPE is less expensive.

False, Even though HDPE is more expensive than LDPE, wall thicknesses of HDPE containers can be made thinner so less material is required and cost per container is less.

When the plastic begins to solidify in the mold cavity during the injection-molding cycle, the following actions occurs next.

The ram pressure is maintained to pack additional melt into the cavity,

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 and is then removed from the cavity.

True, The most common application of injection molding is thermoplastics

The screw-preplasticizer machine is also known by.

Two-stage machine

A polymer melt exhibits which four of the following properties and characteristics:

becomes less resistant to flow as temperature increases, less resistant to flow at high shear rates, high viscosity, and viscoelasticity

Three of the following processes would be applicable to produce hulls for small boats.

blow molding, rotational molding, and vacuum thermoforming.

Three of the following processes are used to produce plastic sheet and film?

blown film extrusion process, calendering, and chill roll extrusion

Toggle-clamp units on injection-molding machines are most suited for one of the following tonnages.

Low

Relative to conventional injection molding, the clamping forces in structural foam molding are

Lower

Relative to conventional injection molding, the injection pressures in structural foam molding is.

Lower

Compression molding differs from injection molding in which two of the following respects:

molds are simpler, and presses are oriented vertically rather than horizontally

Bambooing is a severe case of the following defects in plastic extrusion.

Sharkskin

The extruder screw rotates at a speed that is closest to.

60 rev/min

Use of a parison is associated with the following plastic shaping processes

Blow molding

The amount of molding compound loaded into the heated mold during each compression molding cycle is called?

Charge

The turbulent flow of polymer melt through the die orifice is likely to cause which one of the following defects in plastic extrusion.

Melt fracture

Stretch blow molding is widely used to produce containers for carbonated beverages out of the following thermoplastics.

Polystyrene

The amount of polymer melt injected into the mold during each injection molding cycle is called?

Shot

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 and is then removed from the cavity.

True

Reaction injection molding is used for the following plastic types.

Thermosets

A fiber is defined as a long, thin strand of material whose length is at least 100 times its cross-sectional dimension, whereas a filament is a fiber of continuous length.

True

Because the polymers used in transfer molding are thermosetting, the scrap produced each cycle, called the cull, cannot be recovered

True

Blow molding is a molding process in which air pressure is used to inflate a soft plastic parison inside a mold cavity to produce hollow parts such as bottles.

True

Contour coating involves the application of a polymer coating onto a solid three-dimensional object.

True

Extrusion is a compression process in which material is forced to flow through a die orifice to provide a long continuous product whose cross-sectional shape is determined by the shape of the orifice

True

The term encapsulation refers to the following plastics shaping processes.

casting

The principal components of an injection molding machine are?

clamping unit and part ejection unit

these three of the following are sections of a conventional extruder barrel for thermoplastics

compression section, feed section, and metering section.

Spinning processes is the starting polymer in solution and the solvent is separated by evaporation by pulling the filament through a heated chamber.

dry spinning

The following factors will reduce the amount of shrinkage during solidification and cooling in thermoplastic molding.

higher injection pressure, higher molding temperature, and a higher proportion of filler in the plastic.

The extruder characteristic indicates that flow rate in the extruder barrel increases as head pressure.

it decreases

The die characteristic indicates that flow rate through the die orifice increases as head pressure.

it increases

The die swell ratio for a polymer melt has a value that is never less than one.

it is True

Relative to blow molding, rotational molding tends to be advantageous in the following situations

larger parts, lower production quantities, and more complex external part geometrie.

The function of the ejection system in injection molding is.

separate the part from the mold cavity after molding.

Polymers with low melt viscosities are the best candidates for extrusion because they hold shape better during cooling.

False Polymers with high melt viscosities hold their shape better during cooling

Shrinkage of high-density polyethylene is lower than for low-density polyethylene because of HDPE's greater crystallinity.

False the Shrinkage of HDPE is greater because of its greater crystallinity

Casting can only be used for thermosetting plastics, not thermoplastic polymers.

False, Casting can be used for both TP and TS polymers

In a reciprocating-screw injection molding machine, the injection of polymer melt into the mold cavity is accomplished by the rotation of the screw in the barrel.

False, Injection of polymer melt into the mold cavity is accomplished by reciprocating screw acting as a ram that rapidly moves forward in the barrel.

In rotational molding, centrifugal force is used to force the polymer melt against the surfaces of the mold cavity where solidification occurs.

False, It is the force of gravity in the doubly rotating mold that forces the polymer against the mold surfaces

Plastic sheet refers to stock thicknesses thinner than 0.5 mm (0.020 in), whereas plastic film refers to thicknesses greater than 0.5 mm (0.020 in)

False, it's the reverse.

The forward movement of polymer melt in an extruder barrel is resisted by drag flow, which is caused by the resistance to flow through the die orifice

False, the Drag flow is the forward motion of the melt caused by the Archimedian screw principle in the barrel. The resistance to forward flow is called backpressure flow

The three of the following defects are associated with injection molding.

Flash, short shots, and sink marks

The operating point in an extrusion operation consists of the values of flow rate Qx and pressure p determined by the intersection of the extruder characteristic and the die characteristic.

True

The product of an extrusion process is called the extaterud.

True

To compensate for die swell, the extrudate is often stretched to offset expansion from die swell.

True

Transfer molding is capable of molding part shapes that are more intricate than compression molding but not as intricate as injection molding.

True

When a three-plate mold opens, it automatically separates the sprue and runner from the molded parts

True