Kutz M. Handbook of materials selection

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6 SHEET THERMOFORMING 975

Fig. 8 Calendering process: R, R⬘, feed rolls; Rⴖ,R⬘ⴖ, gauge / ﬁnishing rolls; M, feed material;

B, material bank; N, feed nip; N⬘, intermediate nip; Nⴖ, gauge nip; P, product. (From Charrier,

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

Composites, Hanser Publishers, Munich, 1990.)

Fig. 9 Calendering rolls conﬁgurations: , vertically aligned; , inverted L; , Z-like. (FromILZ

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

Composites, Hanser Publishers, Munich, 1990.)

arranged in a variety of conﬁgurations (roll axis pattern or sheet path) as shown

in Fig. 9 The last roll, or pair of rolls, in the calendering stack determines the

surface ﬁnish of the product.

Materials and Products

Vinyl thermoplastics (PVC and copolymers) and vulcanizable rubbers are the

most commonly calendered materials. Film thickness is not normally below

about 0.05 mm. Sheets can be as thick as about 6 mm. Examples of applications

include vinyl ﬁlms made into products such as baby pants, inﬂatable toys,

shower curtains, tablecloths, pool liners, wall covering, and veneered panels as

well as vinyl sheets for credit cards, blister packaging, ﬂoor tiles, and ﬂoor

covering (linoleum).

6 SHEET THERMOFORMING

Principle

Sheet thermoforming, or simply thermoforming, involves the heating of a ﬂat

thermoplastic sheet to a softened state (above the glass transition temperature T

for noncrystallizing thermoplastics or near the melting temperature T

for crys-

tallizing ones), followed by the deformation (forming) of the softened sheet into

a desired shape by pneumatic or mechanical means, and ﬁnally its solidiﬁcation

into this shape by cooling. Thermoforming transforms a ﬂat sheet of uniform

thickness into a contoured product of variable thickness resulting from the non-

uniform biaxial stretching. Figure 10 illustrates this concept.

976 PLASTIC PARTS PROCESSING I

Fig. 10 Typical cuplike product: , blank; , product; E, edge; S, side; C, corner; A, apex;BP

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B, bottom; F, ﬂange; W, web. (From Charrier, J.-M., Polymeric Materials and Processing:

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

Fig. 11 Straight vacuum thermoforming: , heating; , positioning; , forming; , unload-HP F U

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ing; M, blank; O, oven; C, clamp; M⬘, softened blank; T, tool (mold); V, vacuum; H, vacuum

hole(s); W, web; P, product. (From Charrier, J.-M., Polymeric Materials and Processing: Plastics,

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

Thermoforming normally involves a one-surface (single-surface) tool, and it

is convenient to distinguish between negative (female, cavity) tools, which are

basically concave and into which the sheet is formed (drawn), and positive (male,

plunger) tools, which are basically convex and over which the sheet is formed

(draped). Pneumatic deformation is most commonly done by vacuum (suction).

Mechanical deformation tends to be used primarily for preforming or prestretch-

ing, before ﬁnal vacuum or air pressure forming. The simplest thermoforming

technique is perhaps that referred to as straight vacuum thermoforming, which

is illustrated in Fig. 11.

Equipment

The simplest commercial type is probably the single-station shuttle thermo-

former, shown in Fig. 12. The two-station (double-end) shuttle thermoformer is

a variation that uses a single oven with two workstations.

Tooling

Tools for thermoforming are not normally subjected to high stresses since the

pressure differential is often limited to 1 atm in vacuum forming or 3–4 atm in

pressure forming. They are often made from cast or machined aluminum for

small and medium sizes. Epoxy and polyester resins, with ﬁber reinforcement

and a heat-conductive ﬁller, are suitable for the relatively slow production of

large parts. Tools are generally water-cooled to temperature in the range 40–

⬚C, while mechanical assist devices (plugs) are usually maintained at a some-

7 BLOW MOLDING 977

Fig. 12 Single-station shuttle thermoformer: O, oven; M, blank; C, clamp; V, air cylinder;

R, rack; P, P⬘, platens; B, frame. (From Charrier, J.-M., Polymeric Materials and Processing:

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

what higher temperature (60–150⬚C). Vacuum is applied through ﬁne holes,

slots, or ports that do not leave signiﬁcant marks on products.

Auxiliaries

Heating of the sheet stock (blank) is most commonly carried out with radiant

electric heaters (nichrome wire, calrod, ceramic, quartz, etc.), which correspond

to electric power densities around 15–25 kW/m

. After forming, products must

be separated from the surrounding web by a trimming operation.

Materials

Noncrystallizing thermoplastics are normally easier to thermoform then crystal-

lizing ones because of their highly elastic behavior over a fairly wide range of

temperature above T

. They include styrenics such as high-impact (HI) or crys-

talline polystyrene (PS), and copolymers such as acrylonitrile-butadiene-styrene

(ABS).

Products

Products made by sheet thermoforming fall into two distinct categories. Small

products made in high-output machines, often of the roll-fed, in-line type, using

multicavity tools, are associated primarily with packaging and disposable items

such as those used in the food industry and for medical applications. Larger

products are generally made from cut sheets at much slower rates.

7 BLOW MOLDING

Principle

The basic principle of the blow-molding process is to inﬂate a softened ther-

moplastic hollow preform against the cooled surface of a closed mold, where

the material solidiﬁes into a hollow product. The extrusion blow-molding (EBM)

process, depicted in Fig. 13, involves an extruder ﬁtted with a die or die head

that produces the preform by extruding downward a tube of thermoplastic gen-

erally called preform or parison at stage . While the preform is still soft, it is

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then pinched (stage between the two halves of a mold; the bottom is pinchedP)

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shut and sealed, while the top is pinched around a metal tube or blow pin

protruding from the die core. At the following stage the soft preform is

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blown against the cooled mold surfaces by air injected through the blow pin.

978 PLASTIC PARTS PROCESSING I

Fig. 13 Extrusion blow molding: , extrusion; , pinching; , blowing; , demolding;EPBD

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E, extruder; D, die; F, preform; S, squeezed area; T, tool; B, blow pin; A, air pressure;

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

and Composites, Hanser Publishers, Munich, 1990.)

Fig. 14 Injection blow molding: , preform injection; , preform demolding; , tool replace-ID P

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ment; , product blowing; , product demolding; T, injection tool, C, core pin; R, neck ring;BD⬘

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F, preform; T⬘, blowing tool; V, air valve; A, air pressure; P, product. (From Charrier, J.-M.,

Polymeric Materials and Processing: Plastics, Elastomers and Composites,

Hanser Publishers, Munich, 1990.)

After the thermoplastic material has solidiﬁed, the mold is opened and the prod-

uct is pulled out (demolded) at stage .

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The injection blow-molding (IBM) process, illustrated in Fig. 14, ﬁrst involves

an injection stage , during which the thermoplastic material is injected into

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an injection mold featuring two important elements, a split neck ring and a core

pin. Upon demolding , the preform remains attached to the neck ring and core

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pin, but the injection mold is replaced by a blow mold (stage ). Air is sub-P

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sequently injected through a valve in the core pin, to blow the soft preform

against the cooled blow mold surface, to form the product. The core is retracted

and the neck ring and the mold simultaneously split open to demold the product.

Equipment

Extrusion blow-molding (EBM) machines basically consist of two major ele-

ments: (1) the parison former, which comprises an extruder, a die, and sometimes

a special accumulating device; and (2) the mold-handling device (mold carrier).

Injection blow-molding (IBM) machines often feature the rotary system with

stationary injection mold as well as blow mold and revolving hub, which trans-

fers the preform from the injection station to the blowing station, and the product

7 BLOW MOLDING 979

Fig. 15 Blow-molded products: , bottle; , jar; , canister; , drum; N, narrow neck;BJC D

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T, thread; S, shoulder; C, body; F, base; M, wide mouth; E, eye; H, handle; O, off-center neck;

A, annular rib; B, bungs; R, ring. (From Charrier, J.-M., Polymeric Materials and Processing:

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

to the demolding station. Blowing pressures are often limited to normal plant

air pressure (about 120 psi) and clamping forces are thus relatively small (typ-

ically about 1 ton for a bottle and about 50 tons for a shipping drum).

Tooling

Tooling for a speciﬁc extrusion blow-molded product involves the mold as well

as the die to produce the parison. In the case of injection blow molding, an

injection mold is needed for the preform, in addition to the blow mold. Blow

molds are subjected to moderate pressures and clamping forces, compared to

injection molds; they can thus be constructed more lightly, and aluminum is

often used, which has advantages in terms of weight and thermal conductivity.

Auxiliaries

Extrusion blow molding produces a signiﬁcant amount of process scrap, which

is usually recycled after regrinding. Process scrap is most often associated with

the ﬂash resulting from the pinching operation. Such scrap can amount to as

much as 25% for bottles and 50% for toys.

Materials

The process of blow molding requires a good stretchability of the softened ther-

moplastic (melt strength) over a reasonable temperature range. A relatively high

molecular weight material is normally required, particularly for crystallizing res-

ins. Polyethylene (PE) is, by far, the most widely used resin for large products.

Polyvinyl chloride (PVC) is widely used for bottles. Thermoplastic polyester

(PET) is primarily used in injection blow molding.

Products

Blow-molded products have certain inherent design limitations, concerning

shapes and wall thickness in particular. While the blow-molding process nor-

mally yields one-piece, closed, hollow products, pairs of open, hollow products

have been made by splitting closed moldings. Packaging is the major area of

application of small to medium-size disposable blow-molded products. Figure

15 shows several typical blow-molded products.

980 PLASTIC PARTS PROCESSING I

Fig. 16 Planar coating: , roller technique; , knife technique; W, web; C, coating material;RK

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R, pressure rolls; B, doctor blade; P, product. (From Charrier, J.-M., Polymeric Materials and

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

Fig. 17 Liquid dip coating: , dipping; , conditioning; S, substrate; L, coating liquid; T, tank;DC

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C, conditioning chamber; P, product. (From Charrier, J.-M., Polymeric Materials and Processing:

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

8 COATING

Coating may be deﬁned as the formation (application) of a layer (coat) of

polymer on a substrate; ‘‘planar coating,’’ is used for ﬂat ﬁlm and sheet substrates

and ‘‘contour coating,’’ for general three-dimensional objects.

Planar Coating

A variety of ﬂexible ﬂat substrates (webs) are coated with polymers for a variety

of reasons, including appearance, waterproofness, electrical insulation, and seal-

ability. The adhesion between substrate and coat is always important, and various

methods are used to favor a good bond.

Two basic techniques, as illustrated in Fig. 16, are used to form the coating

and control its thickness. The roller (roll) technique squeezes the ﬂuid coating

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material against the substrate web in the wedge (nip), between counterrotating

pressure (nip) rolls. In the knife technique , a blade (doctor blade) holds an

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excess of coating material, allowing the entrainment of a suitable amount

through a small gap.

Contour Coating

Objects of complex shapes can be coated by dipping them in suitable ﬂuids as

shown in Fig. 17 or by ﬂuidized-bed dip coating as illustrated in Fig. 18.

9 ROTATIONAL MOLDING

Principle

The process of rotational molding is depicted in Fig. 19. At the loading stage

, a suitable powdery thermoplastic material charge is introduced in the open

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9 ROTATIONAL MOLDING 981

Fig. 18 Fluidized-bed dip coating: , unﬂuidized bed; , ﬂuidized bed; , substrate heating;UFH

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, dipping; , conditioning; M, powdered coating material; F, ﬂuidized particles; D, porousDC

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diaphragm; G, gas inlet; S, substrate; C, heating chamber; C⬘, conditioning chamber;

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

and Composites, Hanser Publishers, Munich, 1990.)

Fig. 19 Rotational molding process: , loading; , heating; , cooling; , unloading; T, T⬘,LHCU

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mold halves; M, material charge; R, R⬘, axes of rotation; C, conditioning chamber; P, product.

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

and Composites, Hanser Publishers, Munich, 1990.)

mold (mold halves T and T⬘). The mold is then closed and mounted on a holding

device, which permits its double, biaxial, rotation around two perpendicular axes

(R and R

⬘) to produce a tumbling action. A heating stage follows in whichH

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the exterior of the mold is subjected to a high temperature in a suitable chamber,

while being roasted. Heat transfer through the mold wall eventually raises the

temperature of the inner mold surface to a level sufﬁcient to cause superﬁcial

melting of the thermoplastic particles and their sticking to the mold surface.

Equipment

The mold-handling device, which is capable of imparting double rotation, is the

central element of rotational molding equipment. There are two major types of

equipment: (1) the ‘‘shuttle cart system’’ and (2) the ‘‘swing/rotary arm system.’’

The shuttle cart system, shown in Fig. 20, features a cart (wagon), incorporating

the mold rotation device, which can be rolled in and out of the heating or cooling

chamber (bay). The swing/rotary arm system, illustrated in Fig. 21, features

arms or spindles that rotate in a horizontal plane around a hub.

Tooling

Molds for rotational molding are normally split (two-half, two-piece) negative

molds, with built-in mechanical locking. The need for rapid heat transfer, and

minimum weight to facilitate rotation, calls for relatively thin walls, which are

acceptable in view of the low pressures involved. Large molds are generally

982 PLASTIC PARTS PROCESSING I

Fig. 20 Shuttle cart rotational molding: C, conditioning chamber; R⬘, rotation axis; R, tilting

axis; W, cart; T, mold. (From Charrier, J.-M., Polymeric Materials and Processing: Plastics,

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

Fig. 21 Swing/ rotary arm rotational molding: , one-arm (swing); , three-arm (rotary);OT

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O, oven; H, hub; C, cooling chamber; T, mold; A, arm. (From Charrier, J.-M., Polymeric Materials

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

fabricated from steel or sometimes aluminum in sheet form. Smaller molds are

generally made from aluminum, which is light and heat conductive.

Materials

The nature of the process normally calls for the use of a relatively thin powder

of a thermoplastic polymer. Particles featuring sharp edges and corners process

better than purely spherical ones, probably because of the rapid softening of

those areas. Polyethylene is widely used, particularly in its high-density form

high-density polyethylene (HDPE). Polypropylene (PP) and polymethyleneoxide

(PMO) are among other crystallizing thermoplastics used in this process.

Products

The part thickness, which is heat-transfer-controlled, can be quite uniform and

is usually in the range of 3–6 mm. Corners and edges are often thicker as a

result of concentrated heat transfer, and, conversely, insulated areas do not be-

come covered with polymer, thus forming openings.

10 CASTING

Principle, Materials, and Products

Casting corresponds to the pouring or casting of the liquid resin system into a

mold (gravity or atmospheric pressure casting), as illustrated in Fig. 22. In some

cases, the chemical reaction that is taking place during a casting process converts

a low-molecular-weight monomer into a high-molecular-weight thermoplastic;

the most common examples are acrylics and polyamides. Problems often asso-

ciated with the chemical reactions involved in casting include cure shrinkage,

11 COMPRESSION MOLDING 983

Fig. 22 Casting molds: , open mold; , two-piece mold; F, feed runner; C, cavity; V, vent.OT

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

and Composites, Hanser Publishers, Munich, 1990.)

Fig. 23 Compression-molding presses: , downstroking; , upstroking; P, stationary platen;DU

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P⬘, moving platen; F, frame; C, clamping cylinder; L, bolster plate; B, tie bar. (From Charrier,

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

Composites, Hanser Publishers, Munich, 1990.)

which can be as high as about 20% for acrylics, and heat evolution (exothermic

reactions), which can lead to runaway situations through autoacceleration.

11 COMPRESSION MOLDING

Principle

The compression-molding process is used almost exclusively for temperature-

activated thermosetting polymers. Compression molding basically involves the

pressing of a deformable material between the two halves of a heated mold, as

shown in Fig. 23, and its transformation into a solid product under the effect of

the elevated mold temperature. Compression molding temperatures are often in

the range of 140–200

⬚C; mold pressures can vary from about 35 atm (⬃500 psi)

to 700 atm (

⬃10,000 psi); and setting (curing, vulcanization) times can vary

widely from about 1 min for relatively thin phenolic parts to over 1 h for very

thick rubber components. Material charges are often preheated or preplasticated

to speed up the initial softening stage.

Equipment

Compression-molding molds are used on compression presses that may be of

the downstroking or upstroking types, and are generally slow-acting and

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hydraulically operated (clamping ram or cylinder).

Tooling

Compression molds for thermosetting resins and vulcanizable rubbers are heated

to temperatures typically in the range of 140–200

⬚C(⬃280–400⬚F) and are

984 PLASTIC PARTS PROCESSING I

Fig. 24 Flash-compression mold: C, cavity; J, parting line; T, T⬘, mold halve; L, ﬂash land;

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

and Composites, Hanser Publishers, Munich, 1990.)

Fig. 25 Flash molding: , loading; , unloading; M, material charge; P, product.LU

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

Composites, Hanser Publishers, Munich, 1990.)

subjected to high forces and pressures. In compression molding, it is the closing

of the two-half mold that shapes the material charge and establishes the pressure

required to obtain a good molded part. Flash molds are illustrated in Figs. 24

and 25.

Auxiliaries

The starting material for compression molding is often in powder form for ther-

mosetting systems such as phenolic resins and amino resins. The commercial

molding of small parts calls for the use of multiple cavity molds. Material

charges are generally preheated to temperatures somewhat below those initiating

rapid curing.

Materials

Temperature-activated thermosetting plastics most commonly processed by com-

pression molding include phenolic, amino and alkyd resins, and diallyl phthalate

(DAP), as well as unsaturated polyesters (UP) and epoxy resins (EP). They are

often associated with particulate ﬁllers and ﬁber reinforcement.

12 TRANSFER MOLDING

Principle

In transfer molding, a softened temperature-activated thermoset, or a vulcaniz-

able rubber, is transferred through a narrow gate into the closed cavity of a

heated mold, where it cures to a solid state.

Equipment

Hydraulic presses similar to compression-molding presses are used for transfer

molding.