Kutz M. Handbook of materials selection

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964 METAL FORMING, SHAPING, AND CASTING

It is common practice to coat newly sharpened cutting edges of tools by dipping

them in thermoplastic material to provide mechanical protection during handling

and storage.

Organic Coatings

Organic coatings are used to provide pleasing colors, to smooth surfaces, to

provide uniformity in both color and texture, and to act as a protective ﬁlm

for control of corrosion. Organic resin coatings do not ordinarily supply any

chemical-inhibiting qualities. Instead, they merely provide a separating ﬁlm be-

tween the surface to be protected and the corrosive environment. The important

properties, therefore, are continuity, permeability, and adhesion characteristics.

Paints, Varnishes, and Enamels

Paints. Painting is a generic term that has come to mean the application of

almost any kind of organic coating by any method. Because of this interpretation,

it is also used generally to describe a broad class of products. As originally

deﬁned and as used most at present, paint is a mixture of pigment in a drying

oil. The oil serves as a carrier for the pigment and in addition creates a tough

continuous ﬁlm as it dries. Drying oils, one of the common ones of which is

linseed oil, become solid when large surface areas are exposed to air. Drying

starts with a chemical reaction of oxidation. Nonreversible polymerization ac-

companies oxidation to complete the change from liquid to solid.

Varnish. Varnish is a combination of natural or synthetic resins and drying

oil, sometimes containing volatile solvents as well. The material dries by a chem-

ical reaction in the drying oil to a clear or slightly amber-colored ﬁlm.

Enamel. Enamel is a mixture of pigment in varnish. The resins in the var-

nish cause the material to dry to a smoother, harder, and glossier surface than

is produced by ordinary paints. Some enamels are made with thermosetting

resins that must be baked for complete dryness. These baking enamels provide

a toughness and durability not usually available with ordinary paints and en-

amels.

Lacquers

The term lacquer is used to refer to ﬁnishes consisting of thermoplastic materials

dissolved in fast-drying solvents. One common combination is cellulose nitrate

dissolved in butyl acetate. Present-day lacquers are strictly air-dried and form

ﬁlms very quickly after being applied, usually by spraying. No chemical change

occurs during the hardening of lacquers; consequently, the dry ﬁlm can be re-

dissolved in the thinner. Cellulose acetate is used in place of cellulose nitrate in

some lacquers because it is nonﬂammable. Vinyls, chlorinated hydrocarbons,

acrylics, and other synthetic thermoplastic resins are also used in the manufac-

ture of lacquers.

Vitreous Enamels

Vitreous, or porcelain, enamel is actually a thin layer of glass fused onto the

surface of a metal, usually steel or iron. Shattered glass, ball milled in a ﬁne

particle size, is called frit. Frit is mixed with clay, water, and metal oxides,

7 SURFACE TREATMENT 965

which produce the desired color, to form a thin slurry called slip. This is applied

to the prepared metal surface by dipping or spraying and, after drying, is ﬁred

at approximately 1470

⬚F (800⬚C) to fuse the material to the metal surface.

Metallizing

Metal spraying, or metallizing, is a process in which metal wire or powder is

fed into an oxyacetylene heating ﬂame and then, after melting, is carried by

high-velocity air to be impinged against the work surface. The small droplets

adhere to the surface and bond together to build up a coating.

Vacuum Metallizing

Some metals can be deposited in very thin ﬁlms, usually for reﬂective or dec-

orative purposes, as a vapor deposit. The metal is vaporized in a high-vacuum

chamber containing the parts to be coated. The metal vapor condenses on the

exposed surfaces in a thin ﬁlm that follows the surface pattern. The process is

cheap for coating small parts, considering the time element only, but the cost of

special equipment needed is relatively high.

Aluminum is the most used metal for deposit by this method and is used

frequently for decorating or producing a mirror surface on plastics. The thin

ﬁlms usually require mechanical protection by covering with lacquer or some

other coating material.

Hot-Dip Plating

Several metals, mainly zinc, tin, and lead, are applied to steel for corrosion

protection by a hot-dip process. Steel in sheet, rod, pipe, or fabricated form,

properly cleansed and ﬂuxed, is immersed in molten plating metal. As the work

is withdrawn, the molten metal that adheres solidiﬁes to form a protective coat.

In some of the large mills, the application is made continuously to coil stock

that is fed through the necessary baths and even ﬁnally inspected before being

recoiled or cut into sheets.

Electroplating

Coatings of many metals can be deposited on other metals, and on nonmetals

when suitably prepared, by electroplating. The objectives of plating are to pro-

vide protection against corrosion, to improve appearance, to establish wear- and

abrasion-resistant surfaces, to add material for dimensional increase, and to serve

as an intermediate step of multiple coating. Some of the most common metals

deposited in this way are copper, nickel, cadmium, zinc, tin, silver, and gold.

The majority are used to provide some kind of corrosion protection but appear-

ance also plays a strong part in their use.

Temporary Corrosion Protection

It is not uncommon in industry for periods of time, sometimes quite long periods,

to elapse between manufacture, assembly, shipment, and use of parts. Unless a

new processing schedule can be worked out, about the only cure for the problem

is corrosion protection suitable for the storage time and exposure. The coatings

used are usually nondrying organic materials, called shushing compounds, that

can be removed easily. The two principal types of compounds used for this

966 METAL FORMING, SHAPING, AND CASTING

purpose are petroleum-based materials, varying from extremely light oils to sem-

isolids, and thermoplastics. The most common method of application of shushing

compounds for small parts is by dipping. Larger parts that cannot be handled

easily may be sprayed, brushed, or ﬂow coated with the compound.

7.3 Chemical Conversions

A relatively simple and often fully satisfactory method for protection from cor-

rosion is by conversion of some of the surface material to a chemical compo-

sition that resists from the environment. These converted metal surfaces consist

of relatively thin [seldom more than 0.001 in. (0.025 mm) thick] inorganic ﬁlms

that are formed by chemical reaction with the base material. One important

feature of the conversion process is that the coatings have little effect on the

product dimensions.

Anodizing

Aluminum, magnesium, and zinc can be treated electrically in a suitable elec-

trolyte to produce a corrosion-resistant oxide coating. The metal being treated

is connected to the anode in the circuit, which provides the name anodizing for

the process. Aluminum is commonly treated by anodizing that produces an oxide

ﬁlm thicker than, but similar to, that formed naturally with exposure to air.

Anodizing of zinc has very limited use. The coating produced on magnesium is

not as protective as that formed on aluminum, but does provide some protective

value and substantially increases protection when used in combination with paint

coatings.

Chromate Coatings

Zinc is usually considered to have relatively good corrosion resistance. This is

true when the exposure is to normal outdoor atmosphere where a relatively thin

corrosion ﬁlm forms. Contact with either highly aerated water ﬁlms or immer-

sion in stagnant water containing little oxygen causes uneven corrosion and

pitting. The corrosion products of zinc are less dense than the base material, so

that heavy corrosion not only destroys the product appearance, but also may

cause malfunction by binding moving parts. Corrosion of zinc can be substan-

tially slowed by the production of chromium salts on its surface. The corrosion

resistance of magnesium alloys can be increased by immersion of anodic treat-

ment in acid baths containing dichromates. Chromate treatment of both zinc and

magnesium improves corrosion resistance but is used also to improve adhesion

of paint.

Phosphate Coatings

Phosphate coatings, used mostly on steel, result from a chemical reaction of

phosphoric acid with the metal to form a nonmetallic coating that is essentially

phosphoric salts. The coating is produced by immersing small items or spraying

large items with the phosphating solution. Phosphate surfaces may be used alone

for corrosion resistance, but their most common application is as a base for paint

coatings. Two of the most common application methods are called parkerizing

and bonderizing.

BIBLIOGRAPHY 967

Chemical Oxide Coatings

A number of proprietary blacking processes, used mainly on steel, produce at-

tractive black oxide coatings. Most of the processes involve the immersing of

steel in a caustic soda solution, heated to about 300

⬚F (150⬚C) and made strongly

oxidizing by the addition of nitrites or nitrates. Corrosion resistance is rather

poor unless improved by application of oil, lacquer, or wax. As in the case of

most of the other chemical conversion procedures, this procedure also ﬁnds use

as a base for paint ﬁnishes.

BIBLIOGRAPHY

Alting, L., Manufacturing Engineering Processes, Marcel Dekker, New York, 1982.

Amstead, B. H., P. F. Ostwald, and M. L. Begeman, Manufacturing Processes, 8th ed., Wiley, New

York, 1988.

DeGarmo, E. P., J. T. Black, and R. A. Kohser, Material and Processes in Manufacturing, 7th ed.,

Macmillan, New York, 1988.

Doyle, L. E., G. F. Schrader, and M. B. Singer, Manufacturing Processes and Materials for Engineers,

Prentice-Hall, Englewood Cliffs, NJ, 1985.

LeGrand, R. (ed.), American Machinist’s Handbook, McGraw-Hill, New York, 1973.

Kalpakjian, S., Manufacturing Processes for Engineering Materials, Addison-Wesley, Reading, MA,

1994.

Kronenberg, M., Machining Science and Application, Pergamon, London, 1966.

Lindberg, R. A., Processes and Materials of Manufacture, 2nd ed., Allyn and Bacon, Boston, MA,

1977.

Machining Data Handbook, 3rd ed., Machinability Data Center, Cincinnati, OH, 1980.

Metals Handbook, 8th ed., Vol. 4, Forming, Vol. 5, Forging and Casting, American Society for Metals,

Metals Park, OH, 1985.

Moore, H. D., and D. R. Kibbey, Manufacturing Materials and Processes, 3rd ed., Wiley, New York,

1982.

Niebel, B. W., and A. B. Draper, Product Design and Process Engineering, McGraw-Hill, New York,

1974.

Schey, J. A., Introduction to Manufacturing Processes, McGraw-Hill, New York, 1977.

Shaw, M. C., Metal Cutting Principles, Oxford University Press, Oxford, 1984.

Society of Manufacturing Engineers, Tool and Manufacturing Engineers Handbook, Vol. 2, Forming;

Vol. 3, Materials, Finishing and Coating, McGraw-Hill, New York, 1985.

Zohdi, M. E., ‘‘Statistical Analysis, Estimation and Optimization of Surface Finish,’’ in Proceedings

of International Conference on Development of Production Systems, Copenhagen, Denmark,

1974.

969

CHAPTER 32

PLASTIC PARTS PROCESSING I

William E. Biles

Department of Industrial Engineering

University of Louisville

Louisville, Kentucky

1 INTRODUCTION 969

2 EXTRUSION 969

3 EXTRUSION COVERING 972

4 FILM BLOWING 973

5 CALENDERING 974

6 SHEET THERMOFORMING 975

7 BLOW MOLDING 977

8 COATING 980

9 ROTATIONAL MOLDING 980

10 CASTING 982

11 COMPRESSION MOLDING 983

12 TRANSFER MOLDING 984

13 INJECTION MOLDING 986

14 REACTON INJECTION

MOLDING 990

15 SUMMARY AND

CONCLUSIONS 991

BIBLIOGRAPHY 991

1 INTRODUCTION

The processing of plastics materials is characterized by a wide variety of tech-

niques, including (1) techniques involving the continuous manufacture of a prod-

uct with a uniform cross section, such as extrusion, extrusion covering, ﬁlm

blowing, and calendering; (2) techniques involving the shaping of a deformable

polymer preform against a mold surface, including sheet thermoforming and

blow molding; (3) techniques involving the gradual buildup of a polymer layer

against a mold surface, such as coating and rotational molding; and (4) tech-

niques involving the ﬁlling of a mold cavity, including casting, compression

molding, transfer molding, injection molding, and reaction injection molding.

This chapter discusses each of these techniques, including the basic principles,

equipment, tooling, and auxiliary apparatus involved in the application of each

technique.

2 EXTRUSION

Principle

The extrusion process consists of continuously shaping a ﬂuid polymer through

the oriﬁce of a die and subsequently solidifying it into a product. The feed

HandbookofMaterialsSelection.EditedbyMyerKutz

970 PLASTIC PARTS PROCESSING I

Fig. 1 Single-screw extruder: M, electric motor; R, speed reduction device; D, thrust bearing;

F, feed hopper; B, barrel; S, screw; H, heater; C, cooler; A, adapter; T, tool (die). (From Charrier,

J.-M., Polymeric Materials and Processing: Plastics, Elastomers and Composites,

Hanser Publishers, Munich, 1990.)

Fig. 2 Simple extruder screw: , solid-conveying zone; , transition zone; , melt-STM

䡺䡺䡺

conveying zone; H, channel depth; L, zone length. (From Charrier, J.-M., Polymeric Materials

and Processing: Plastics, Elastomers and Composites, Hanser Publishers, Munich, 1990.)

material, usually a thermoplastic in powder or pellet form, is heated to a ﬂuid

state and forced through the die using a screw extruder. The extruded product

is solidiﬁed by cooling after it exits the die.

Equipment

A conventional single-screw extruder consists of several important elements, as

illustrated in Fig. 1. The screw rotates within a cylindrical barrel. The material

enters the extruder through a feed hopper and exits through the die, which is

ﬁtted to the extruder through an adapter. The mechanical drive system consists

of a variable-speed electric motor and a speed reduction device. The barrel of a

conventional extruder has a circular cylindrical bore. Extruders are generally

rated by the barrel bore diameter. This diameter can be as low as about in.

–

(20 mm) for laboratory extruders and from 2 to 6 in. (65 to 150 mm) for com-

mercial extruders.

The simple screw shown in Fig. 2 consists of three zones corresponding to

the three stages in the process. Zone is primarily involved with the conveying

䡺

of the solid material and features a relatively deep channel (depth H

). Zone

is associated primarily with the metering and pressurizing (1000–3000 psi

䡺

or 70–200 atm) of the molten material and features a shallower channel (depth

). Most melting takes place over the transition zone . The ratio of the totalT

䡺

screw length to the barrel diameter (L /D) is usually referred to as the ‘‘screw

aspect ratio’’ and is usually in the range 20–30. The ratio of channel depths H

is called the ‘‘compression ratio’’ and is typically in the range from 2 to 6.

2 EXTRUSION 971

Fig. 3 Plate die conﬁgurations: , expanding transition; , contracting transition; E, extruderEC

䡺䡺

exit; I, transition piece (preland); T, die plate (land). (From Charrier, J.-M., Polymeric Materials

and Processing: Plastics, Elastomers and Composites, Hanser Publishers, Munich, 1990.)

Fig. 4 Tubing die: F, feed; S, spider; A, air channel; M, mandrel; B, bushing; P, product. (From

Charrier, J.-M., Polymeric Materials and Processing: Plastics, Elastomers and Composites,

Hanser Publishers, Munich, 1990.)

Tooling

The shaping tool associated with extrusion is usually called the die. Widely

different die designs are associated with ﬁlaments, proﬁle products, tubes or

pipes, and ﬁlms or sheets. Filaments are often made from plate dies featuring

multiple oriﬁces with streamlined (tapered) entrances. Proﬁles without hollow

sections are also made from plate dies, as illustrated in Fig. 3. The formation of

a hollow extrudate, such as a tube or a pipe, requires the use of a mandrel to

form the inner surface, while a bushing forms the outer surface, as shown in

Fig. 4.

Auxiliaries

Although the die is the major element in the formation of the extruded product,

downstream auxiliaries are often essential to meeting product speciﬁcations

while maximizing output. Auxiliaries include cooling, sizing, post forming, and

haul-off, as well as cutting and/or winding devices. Cooling is usually achieved

through air convection in the case of thin extrudates or through water convection

(immersion) in a tank trough. Evaporation spray, a more efﬁcient method, is

often used for pipes and proﬁles. Sizing depends on the shape and dimensional

speciﬁcations of the products, with the thickness of extruded sheets, ﬁlms, pipes,

and other conﬁgurations typically monitored with a variety of gauges.

Materials

Practically all thermoplastics can be processed by extrusion, but in order for the

extrudates to maintain their shape until they solidify, extrusion grades tend to

972 PLASTIC PARTS PROCESSING I

Fig. 5 Extrusion covering line: S, substrate; H, preheater; T, tool (die); E, extruder; C, cooler;

P, product. (From Charrier, J.-M., Polymeric Materials and Processing: Plastics, Elastomers and

Composites, Hanser Publishers, Munich, 1990.)

have relatively high molecular weights associated with high viscosity and melt

strength. Conventional rubbers and, to a much smaller extent, thermosets, are

extruded to form proﬁles, for example, but they must be subsequently cured.

Products

●

Fibers and monoﬁlaments are extruded from a number of thermoplastics.

●

Proﬁles are extruded in a wide variety of shapes, from ﬂat strips or simple

channels to very complex shapes, including hollow sections.

●

Flat products are usually subdivided into ﬁlms, when thinner than 0.010

in., and sheets above that.

3 EXTRUSION COVERING

Principle

The expression ‘‘extrusion covering’’ is used here to describe a process that

completely surrounds a continuous substrate with a cover of polymeric material.

Its most common use is for the formation of an insulating layer or a protective

jacket on power or communication electrical wires and cables.

Equipment

An extrusion covering line, shown in Fig. 5, comprises a substrate pay-off (input)

system, a substrate preheater [typically to around 100

⬚C(⬇200⬚F)], an extruder

ﬁtted with an appropriate extrusion covering tool, a cooling device, and a take-

up (output) system for the product.

Tooling

In its simplest form for the case of wire coating, an extrusion covering die (Fig.

6), mounted with its extrusion axis perpendicular to the extruder axis (crosshead

die), comprises a tapered guider or core tube for the substrate, and a suitable

channel to direct the covering ﬂuid polymer, from the extruder exit to an annular

gap between a bushing and the continuously moving substrate.

Materials

Plasticized polyvinyl chloride (PPVC) and polyethylene (PE) are the most

widely used thermoplastic covering materials, but other plastics such as poly-

amides, ﬂuoropolymers, and cellulosics are used for wire insulation.

Products

Wire and cable coating, sheathing, and jacketing is the most common application

for extrusion covering.

4 FILM BLOWING 973

Fig. 6 Extrusion covering die: S, substrate; C, covering material; G, substrate guider; B,

bushing; P, product. (From Charrier, J.-M., Polymeric Materials and Processing: Plastics,

Elastomers and Composites, Hanser Publishers, Munich, 1990.)

Fig. 7 Film-blowing line: E, extruder; I, molten tube; B, bubble; P, product; R⬘, pinch / take-off

rolls; C, collapsing device; F, freeze line; R, cooling air ring; T, tool (die); A, air inlet.

(From Charrier, J.-M., Polymeric Materials and Processing: Plastics, Elastomers and

Composites, Hanser Publishers, Munich, 1990.)

4 FILM BLOWING

Principle

The ﬁlm-blowing process basically consists of extruding a tube of molten ther-

moplastic and continuously inﬂating it to several times its initial diameter, to

form a thin tubular product that can be used directly or slit to form a ﬂat ﬁlm.

Equipment

As illustrated in Fig. 7, the ﬁrst element of a conventional ﬁlm-blowing line is

the extruder, which supplies one, and sometimes two or more, tubular dies,

described subsequently in the section on tooling. The molten tube, as it emerges

from the die, is subjected to both a moderate internal air pressure by way of an

air inlet running through the die mandrel, and a longitudinal force exerted by

take-off rolls. The tube is simultaneously cooled by air from an external air ring.

The molten tube gradually deforms into a stable solid cylindrical bubble beyond

the frost (freeze) line. The bubble is gradually ﬂattened in the collapsing device

(tent frame), which consists of a pair of converging, ladderlike sets of idler rolls

or wood slats, and beyond the pinch (nip) rolls it is handled as a thin ﬂat product.

974 PLASTIC PARTS PROCESSING I

Startup of the ﬁlm-blowing operation normally involves the pulling of the

uninﬂated extruded tube, with the help of a cable, until it becomes pinched

between the nip rolls. Internal air pressure is subsequently applied to form the

bubble. The blow-up ratio (BUR), deﬁned as the ratio of bubble to die diameters

(or circumferences), is normally in the range 1.5–4, with larger values requiring

high melt strengths. The air pressure in the bubble, which is responsible for the

blowing, is normally in the range 0.05–0.2 atm.

Tooling

The extrusion die that produces the tube of molten polymer is the key element

of the process. Crosshead dies of the bottom-fed/spider type tend to cause prob-

lems with weld lines, while it is difﬁcult to obtain ﬂowrate uniformity with

side-fed/spiderless types. Modern ﬁlm-blowing dies are increasingly of the bot-

tom-fed/spiral mandrel type. Dies for coextruded, multilayer blown ﬁlms feature

several concentric spiral groove systems with associated clearances, leading to

individual prelands and the common die land.

Auxiliaries

While the normal output of a ﬁlm-blowing line is a tubular, lay-ﬂat product, the

introduction of slitters, cutters, punches, gusseting and sealing devices, and other

instruments can allow in-line production of slit-open or lay-ﬂat products, and

gusseted products, as well as a variety of bags.

Materials

Polyethylenes of various types [low-density polyethylene (LDPE), linear low-

density polyethylene (LLDPE), high molecular-weight high-density polyethylene

(HMWHDPE), etc.] are by far the most common ﬁlm-blowing resins.

Products

Films produced by the ﬁlm-blowing process are widely used in many areas.

Thick-gauge, heavy-duty ﬁlms are often in the range 0.004–0.008 in. Films are

also widely used for a variety of packaging applications. Blown ﬁlms often ﬁnd

applications as shrink-wrap ﬁlms. Multilayer, coextruded blown ﬁlms are in-

creasingly used for barrier packaging.

5 CALENDERING

Principle

The calendering process, depicted in Fig. 8, is used for the fast production of

ﬂat ﬁlms or sheets of plastics or rubbers. A suitable amount of plasticated ma-

terial is regularly fed between two counterrotating heated cylinders (rolls) (R

and R

⬘). The material is entrained at the surface of the rolls and squeezed in the

wedge, and a layer of a thickness corresponding approximately to the shortest

distance between roll surfaces (gap) is formed through the nip and entrained on

the surface of one of the rolls (R

⬘).

Equipment, Tooling, and Auxiliaries

The diameter of calendaring rolls is generally in the range 40–90 cm and their

length is generally in the range 1–2.5 m; this corresponds to length: diameter

ratios between 2 and 3. Calendering units generally feature 3–5 rolls (2–4 nips),