Quiz 5 - Chapter 10-11

[Ch. 10 – Q1] Polymers can be divided into two general chemical classes. What are they, and which is most commonly used in packaging? What is the difference in molecular structure that characterizes the two general polymer classes?

Thermoplastics and thermosets. Thermoplastics are most common in packaging; their chains can move past one another when heated. Thermosets have cross-linked chains that cannot move freely without destroying the material.

[Ch. 10 – Q2] Complete the following sentences: (a) As molecular weight increases, melting point _____. (b) As branching increases, density _____. (c) A plastic with two monomers is called a _____. (d) Highly ordered polymer chains are _____. (e) Randomly arranged polymer chains are _____. (f) The stiffer HDPE has a _____ molecular weight.

a) Increases. b) Decreases. c) Dimer. d) Crystalline. e) Amorphous. f) Higher.

[Ch. 10 – Q3] What is the molecular structural difference between HDPE, LDPE and LLDPE?

HDPE has linear chains, LDPE has highly branched chains, and LLDPE has short-branched chains.

[Ch. 10 – Q4] Properties of plastic film can be improved by stretching the film in one or two directions. What is this process called, and what does it actually do?

Orientation. Stretching aligns and straightens polymer molecules in the direction of the applied stress.

[Ch. 10 – Q5] Identify these abbreviations: LLDPE, PP, PS, PVC, EVOH, PA, PET, PVDC, OPP, PVAC.

LLDPE = linear low-density polyethylene; PP = polypropylene; PS = polystyrene; PVC = polyvinyl chloride; EVOH = ethylene-vinyl alcohol; PA = polyamide; PET = polyethylene terephthalate; PVDC = polyvinylidene chloride; OPP = oriented polypropylene; PVAC = polyvinyl acetate.

[Ch. 10 – Q6] You are given an amorphous and a crystalline version of the same polymer. How could you probably tell the difference?

The amorphous version generally has better clarity (transparency), while the crystalline version is usually less transparent.

[Ch. 10 – Q7] How do the general properties of thermoplastics and thermosets differ?

Thermosets generally have higher usable temperatures and greater solvent resistance than thermoplastics.

[Ch. 10 – Q8] What properties of plastic are affected by the molecule's degree of polarity?

Melting point, solubility, barrier properties, and coefficient of friction.

[Ch. 10 – Q9] How are heat-shrink properties imparted to a polymer?

Through a combination of thermal and mechanical processing history, typically involving stretching and cooling to retain orientation.

[Ch. 10 – Q10] What is cold flow? Where are cold-flow considerations most important?

Cold flow is deformation of plastic under a sustained load (creep). It is especially important for plastic parts that must support significant loads over time.

[Ch. 10 – Q11] All plastics are clear in the melt state. Why?

Rapid molecular movement in the molten state prevents polymer molecules from forming an ordered crystalline structure.

[Ch. 10 – Q12] Which two common packaging plastics are predominantly nonpolar?

Polyethylene (PE) and polypropylene (PP).

[Ch. 10 – Q13] From what raw material source are most packaging plastics made?

Petrochemicals.

[Ch. 10 – Q14] What is the purpose of flame or corona treatment?

To increase surface polarity of low-polarity plastics, improving adhesion for printing, coatings, or bonding.

[Ch. 10 – Q15] Which would have the higher melting point: high-molecular-weight polysulfone or low-molecular-weight polysulfone?

High-molecular-weight polysulfone.

[Ch. 10 – Q16] What are the purpose and advantage of producing copolymers?

Copolymers combine favorable properties of different polymers into one material.

[Ch. 10 – Q17] What is the difference between addition polymers and condensation polymers?

Addition polymerization joins monomers directly into a chain. Condensation polymerization joins reactive monomers while releasing a small molecule, such as water.

[Ch. 10 – Q18] What would influence whether a particular PET would be amorphous or crystalline?

The filling or application temperature determines whether PET remains amorphous or becomes crystalline.

[Ch. 10 – Q19] Why can't the same mold be used to shape all plastic materials?

Different plastics have different shrinkage rates and thermal expansion properties, so molds require material-specific shrinkage allowances.

[Ch. 10 – Q20] Name four plastics that would sink in water.

Polyamide (PA), polyacrylonitrile (PAN), polycarbonate (PC), and polyvinyl chloride (PVC). PET, PVDC, and fluorocarbon plastics also sink.

[Ch. 10 – Q21] Plastic films can be made to have uniaxial or biaxial shrink properties. Name two applications for uniaxial shrink film. Why are uniaxial shrink films used for these applications?

Tamper-evident neck bands and shrinkable labels. Uniaxial films shrink in only one direction, allowing controlled tightening around containers.

[Ch. 10 – Q22] What are Tg and Tm? Describe the difference in transition temperature patterns (Tg and Tm) for amorphous and crystalline polymers.

Tg = glass transition temperature, where plastic changes from rigid to soft/rubbery. Tm = melting temperature, where plastic enters the melt phase. Amorphous polymers have a more prominent Tg; crystalline polymers have a more prominent Tm.

[Ch. 11 – Q1] Name four basic ways of forming plastics into semirigid shapes.

Extrusion blow molding, injection molding, profile extrusion, and thermoforming.

[Ch. 11 – Q2] Molds for plastic manufacture are generally designed for a particular polymer. What problem are you likely to encounter if you switch materials?

Different plastics shrink at different rates, causing dimensional inaccuracies because molds are designed with specific shrinkage allowances.

[Ch. 11 – Q3] What is coextrusion, and what advantages does coextrusion offer?

Coextrusion uses multiple extruders to combine different materials into layers in one sheet or product, allowing the favorable properties of different plastics to be combined.

[Ch. 11 – Q4] To mold plastic into a useful shape, it must first be heated and softened. What is the name of the machine that does this?

A plasticating extruder, consisting of a heated barrel with a rotating screw.

[Ch. 11 – Q5] What is the principal use for a thin-walled extruded tube?

Personal care products.

[Ch. 11 – Q6] Define the following terms: sprue, runner, flash, gate, parison.

Sprue: main channel where molten plastic enters the mold. Runner: channels distributing plastic to mold cavities. Flash: excess plastic between mold halves. Gate: small opening into the mold cavity. Parison: hollow plastic tube used in bottle making.

[Ch. 11 – Q7] There are two ways of manufacturing plastic film. What are they?

Cast film and blown film.

[Ch. 11 – Q8] How are plastic bags usually made?

Blown film is produced, cut to size, and sealed at one end.

[Ch. 11 – Q9] What are combinations of vacuum, pressure and mechanical assists used in thermoforming?

Plug-assist vacuum forming: a plug pushes heated plastic into the cavity, then vacuum completes forming. Blow forming: air pushes the film upward, a plug shapes it, and vacuum pulls it against the mold cavity.

[Ch. 11 – Q10] Discuss the importance of a programmed parison.

A programmed parison controls wall thickness in narrow-waisted containers. A moving extruder mandrel adjusts thickness along the bottle to maintain proper material distribution.

[Ch. 11 – Q11] What is the makeup of a high-oxygen barrier, multilayer bottle of the type typically used for ketchup?

PP/EVOH/PP layers, with bonding layers on both sides of the EVOH barrier layer.

[Ch. 11 – Q12] What molding process would be used to make a 1,200-liter bulk bin?

Rotational molding.

[Ch. 11 – Q13] What is the significance of the number that can be found on most blow-molded bottles and injection-molded parts?

It identifies the mold cavity that produced the item, allowing manufacturers to troubleshoot defects and investigate complaints.

[Ch. 11 – Q14] Why are carbonated beverage bottles injection-stretch blow molded?

The process produces bottles that withstand internal pressure and can be hot-filled. Bottles made this way are round to resist internal pressure.

[Ch. 11 – Q15] What designs can be made by extrusion blow molding that would be more difficult to make by injection blow molding?

Oil, detergent, and household chemical bottles with handles, pumps, and other hardware.

[Ch. 11 – Q16] Why isn't injection molding used to make narrow-mouthed bottles?

Injection molds are expensive, and narrow threaded openings require special provisions to remove the molded container from the mold.

[Ch. 11 – Q17] How does an extruder for injection molding differ from an extrusion profile extruder?

Injection molding extruders deliver a measured amount of molten plastic into a mold, while profile extruders produce a continuous flow of molten material.

[Ch. 11 – Q18] Parts made by injection molding have tighter tolerance ranges than parts made by extrusion blow molding or thermoforming. Why?

Injection molds use precisely controlled metal mold surfaces, allowing complex, dimensionally accurate parts. Extrusion blow molding and thermoforming provide less precise dimensional control.

[Ch. 11 – Q19] How is a cast film biaxially oriented?

The film is gripped along its sides and stretched sideways while also being stretched in the machine direction.

[Ch. 11 – Q20] What is an “undercut”?

An undercut is a feature that prevents a molded part from being removed normally without first moving or removing part of the mold, increasing mold cost.

[Ch. 11 – Q21] What are the advantages of injection blow molding, and for what specific applications and industries are these advantages particularly beneficial?

Advantages: no secondary trimming, high precision neck and finish, four mold cavities for small bottles, better material distribution, and wide-mouth jars are easy to make. Commonly used in bottle manufacturing.

[Ch. 11 – Q22] Molded open tubs can be made by injection molding or by thermoforming. What are the implications of choosing one method over the other? What feature can be incorporated easily into a thermoformed tub that would be difficult with injection molding? How can you tell which process was used to make the tub?

Injection molding has higher tooling costs but allows undercuts and precise details. Thermoforming has lower tooling costs, can use multilayer sheets, and easily incorporates lids, but undercuts are difficult. Injection-molded tubs have a small gate point and mold parting lines; thermoformed tubs have no obvious parting lines and may show vacuum holes.

[Ch. 11 – Q23] How can you recognize the process by which a bottle was made?

Look for manufacturing marks such as mold parting lines, gate marks, pinch-off seams, or other distinctive features that indicate the molding process.

[Ch. 11 – Q24] In thermoforming, cavity molds are preferred over plug molds. Why?

Cavity molds distribute material more evenly, while plug molds have limited draw and poorer material distribution.

[Ch. 11 – Q25] What is a sink mark, and what might cause it?

A sink mark is a localized depression caused by a substantial change in wall thickness and plastic shrinkage during cooling. It occurs in injection molding.

[Ch. 11 – Q26] What does melt flow rate measure, and what is the significance of this value?

Melt flow rate measures how much molten polymer passes through a specified orifice under a set load and temperature in 10 minutes. It helps determine suitable processing temperatures and methods.