Handbook of Plastics, Elastomers and Composites Part 3 docx

Kroschwitz, J.I., Concise Encyclopedia of Polymer Science and Engineering, John Wiley and Sons, New York, 1990, p... Kroschwitz, J.I., Concise Encyclopedia of Polymer Science and Enginee

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EV A PA6

P A6,6 PC HDPE LDPE PMMA POM PP PPO mod.

PS–GP PS–HI PBTP TPU PVC (soft) SAN TPR PETP

PV A C

PSU PC–PBTP PC–ABS

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■ Free blowing

Drape forming, as shown in Fig 1.69, involves the either lowering the heated sheetonto a male mold or raising the mold into the sheet Usually, either vacuum or pressure isused to force the sheet against the mold In vacuum forming (Fig 1.70), the sheet isclamped to the edges of a female mold, then vacuum is applied to force the sheet againstthe mold Pressure forming is similar to vacuum forming, except that air pressure is used

to form the part (Fig 1.71) In free blowing, the heated sheet is stretched by air pressureinto shape, and the height of the bubble is controlled using air pressure As the sheet ex-pands outward, it cools into a free-form shape as shown in Fig 1.72 This method wasoriginally developed for aircraft gun enclosures Matched die molding (Fig 1.73) uses

Figure 1.69 Drape forming process.469

Figure 1.70 Vacuum forming process.469

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two mold halves to form the heated sheet This method is often used to form relativelystiff sheets.

Multi-step forming is used in applications for thicker sheets or complex geometrieswith deep draw In this type of thermoforming, the ﬁrst step involves prestretching thesheet by techniques such as billowing or plug assist After prestretching, the sheet is

Billow drape forming consists of a male mold pressed into a sheet prestretched by thebillowing process (Fig 1.74) A similar process is billow vacuum forming, wherein a fe-male mold is used (Fig 1.75) In vacuum snap-back forming, vacuum is used to prestretchthe sheet, then a male mold is pressed into the sheet, and, ﬁnally, pressure is used to force

Figure 1.71 Pressure forming.470

Figure 1.72 Free-blowing process.470

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the sheet against the mold as seen in Fig 1.76 In plug assist, a plug of material is used toprestretch the sheet Either vacuum or pressure is then used to force the sheet against thewalls of the mold as shown in Figs 1.77 and 1.78 Plug assist drape forming is used toforce a sheet into undercuts or corners (Fig 1.79) The advantage of prestretching thesheet is more uniform wall thickness.

Materials suitable for thermoforming must be compliant enough to allow for forming

materials generally exhibit a wider process window than semicrystalline materials

are thermoformed include PS, ABS, PVC, PMMA, PETP, and PC Semicrystalline als that can be successfully thermoformed include PE and nucleated PETP Nylons typi-cally do not have sufﬁcient melt strength to be thermoformed Table 1.16 showsprocessing temperatures for thermoforming a number of thermoplastics

materi-1.6.4 Blow Molding

Blow molding is a technique for forming nearly hollow articles and is very commonlypracticed in the formation of PET soft-drink bottles It is also used to make air ducts, surf-

parison (a tubular profile) and expanding it against the walls of a mold by inserting surized air into it The mold is machined to have the negative contour of the final desiredfinished part The mold, typically a mold split into two halves, then opens after the part hascooled to the extent that the dimensions are stable, and the bottle is ejected Molds are

pres-Figure 1.73 Matched die thermoforming.471

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Figure 1.74 Billow drape forming.472

Figure 1.75 Billow vacuum process.473

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commonly made out of aluminum, as molding pressures are relatively low, and aluminumhas high thermal conductivity to promote rapid cooling of the part The parison can either

be made continuously with an extruder or it can be injection molded; the method of

pari-son production governs whether the process is called extrusion blow molding or injection

blow molding Figure 1.80 shows both the extrusion and injection blow molding

extruded into a two-plate open mold; the mold closes as the table rotates another mold der the extruder’s die The closing of the mold cuts off the parison and leaves the charac-teristic weld-line on the bottom of many bottles as evidence of the pinch-off Air is thenblown into the parison to expand it to ﬁt the mold conﬁguration, and the part is then cooledand ejected before the position rotates back under the die to begin the process again The

un-Figure 1.76 Vacuum snap-back process.473

Figure 1.77 Plug assist vacuum forming 474

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blowing operation imparts radial and longitudinal orientation to the plastic melt, ening it through biaxial orientation A container featuring this biaxial orientation is moreoptically clear, has increased mechanical properties, and has reduced permeability, which

strength-is important in maintaining carbonation in soft drinks

Injection blow molding has very similar treatment of the parison, but the parison itself

is injection molded rather than extruded continuously There is evidence of the gate on thebottom of the bottles rather than having a weld line where the parison was cut off The par-ison can be blown directly after molding while it is still hot, or it can be stored and re-heated for the secondary blowing operation An advantage of injection blow molding isthat the parison can be molded to have ﬁnished threads Cooling time is the largest part ofthis cycle and is the rate-limiting step HDPE, LDPE, PP, PVC, and PET are commonlyused in blow molding operations

1.6.5 Rotational Molding

Rotational molding, also known as rotomolding or centrifugal casting, involves ﬁlling a

mold cavity, generally with powder, and rotating the entire heated mold along two axes touniformly distribute the plastic along the mold walls This method is commonly used for

Figure 1.78 Plug assist pressure forming.475

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making hollow parts, like blow molding, but is used either when the parts are very large(as in the case of kayaks, outdoor portable toilets, phone booths, and large chemical stor-age drums) or when the part requires very low residual stresses Also, rotomolding is wellsuited, as compared with blow molding, if the desired part design is complex or requiresuniform wall thicknesses Part walls produced by this method are very uniform as long asneither of the rotational axes corresponds to the centroid of the part design The rotomold-ing operation imparts no shear stresses to the plastic, and the resultant molded article istherefore less prone to stress cracking, environmental attack, or premature failures alongstress lines Molded parts also are free of seams Figure 1.81 shows a diagram of a typical

This is a relatively low-cost method, as molds are inexpensive, and energy costs arelow, thus making it suitable for short-run products The drawback is that the required heat-ing and cooling times are long, and therefore the cycle time is correspondingly long Highmelt ﬂow index PEs are often used in this process

The act of foaming a plastic material results in products with a wide range of densities

These materials are often termed cellular plastics Cellular plastics can exist in two basic

structures: closed-cell or open-cell Closed-cell materials have individual voids or cellsthat are completely enclosed by plastics, and gas transport takes place by diffusionthrough the cell walls In contrast, open-cell foams have cells that are interconnected, andﬂuids may pass easily between the cells The two structures may exist together in a mate-rial so that it may be a combination of open and closed cells

Blowing agents are used to produce foams, and they can be classiﬁed as either physical

or chemical Physical blowing agents include

pressure, which expands when the pressure is reduced

pressure is released

Figure 1.79 Plug assist drape forming.475

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Chemical blowing agents include

evo-lution of gas

The major types of chemical blowing agents include the azo compounds, hydrazine

common blowing agents, their decomposition temperature, and primary uses

A wide range of thermoplastics can be converted into foams Some of the most mon materials include polyurethanes, polystyrene, and polyethylene Polyurethanes are apopular and versatile material for the production of foams and may be foamed by eitherphysical or chemical methods In the physical reaction, an inert low-boiling chemical is

com-Figure 1.80 Extrusion and injection blow molding processes.442

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added to the mixture, which volatilizes as a result of the heat produced from the mic chemical reaction to produce the polyurethane (reaction of isocyanate and diol).Chemical foaming can be done through the reaction of the isocyanate groups with water to

polyure-thane is poured into a cavity and allowed to expand in the pour process In the spraymethod, heated two-component spray guns are used to apply the foam This method issuitable for application in the ﬁeld The froth technique is similar to the pour technique,except that the polyurethane is partially expanded before molding A two-step expansion isused for this method using a low-boiling agent for preparation of the froth and a secondhigher-boiling agent for expansion once the mold is ﬁlled

This process combines low-molecular-weight isocyanate and polyol, which are accuratelymetered into the mixing chamber and then injected into the mold The resulting structureconsists of a solid skin and a foamed core

Ex-truded polystyrene foam is produced by extrusion of polystyrene containing a blowingagent and allowing the material to expand into a closed cell foam This product is used ex-tensively as thermal insulation Molded expanded polystyrene is produced by exposing

loose-ﬁll packaging, then no further processing steps are needed If a part is to be made,the beads are then fused in a heated mold to shape the part Bead polystyrene foam is used

in thermal insulation applications, ﬂotation devices, and insulated hot and cold drink cups Polyethylene foams are produced using chemical blowing agents and are typically

terms of reduced weight and lower dielectric constant As a result, these materials ﬁnd plication in electrical insulation markets Polyethylene foams are also used in cushioningapplications to protect products during shipping and handling

ap-Figure 1.81 The rotational molding process.443

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°C

Upper temperature limit, °C

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9 A.W Bosshard and H.P Schlumpf, Fillers and Reinforcements, in Plastics Additives 2/e, R Gachter and H Muller, Eds., Hanser Publishers, New York, 1987, p 407.

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525

TABLE 1.17 Common Chemical Blowing Agents 460

Blowing agent

Decomposition temp., °C

Gas yield, ml/g Polymer applications Azodicarbonamide 205–215 220 PVC, PE, PP, PS, ABS, PA Modiﬁed azodicarbonamide 155–220 150–220 PVC, PE, PP, EVA, PS, ABS 4,4´–oxybis (benzenesulfohydrazide) 150–160 125 PE, PVC, EVA

Diphenylsulfone–3,3´–disulfo-hydrazide

155 110 PVC, PE, EVA

Trihydrazinotriazine 275 225 ABS, PE PP, PA

p-toliuylenesulfonyl semicarbazide 228–235 140 ABS, PE, PP, PA, PS

5-phenyltetrazole 240–250 190 ABS, PPE, PC, PA, PBT, LCP Isatoic anhydride 210–225 115 PS, ABS, PA, PPE, PBT, PC

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Tiêu đề	Bonding for Thermoplastic Combinations in Multicomponent Injection Molding
Trường học	McGraw-Hill Companies
Chuyên ngành	Materials Science and Engineering
Thể loại	thesis
Năm xuất bản	2004
Thành phố	New York

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