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`P LASTI C
`BLOW MQLDING
`HANDBU~K
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`Edi ed b Norman Lee ~.
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`CopyrightO 1990 by Van Nostrand Reinhold
`
`Library of Congress Catalog Card Number 89-22674
`,~
`ISBN 0-442-20752-2
`
`All rights reserved. No part of this work covered by the copyright hereon
`may be reproduced or used in any form by any means—graphic, electronic,
`or mechanical, including photocopying, recording, taping, or information
`storage and retrieval systems—without written permission of the publisher.
`< H
`
`Printed in the United States of America ,~
`~ ,
`
`Van Nostrand Reinhold
`115 Fifth Avenue
`New York, New York 10003
`
`Van Nostrand Reinhold International Company Limited
`11 New Fetter Lane
`London EC4P 4EE, England
`
`Van Nostrand Reinhold
`480 La Trobe Street
`Melbourne, Victoria 3000, Australia
`
`Nelson Canada
`1120 Birchmount Road
`Scarborough, Ontario
`Canada MiK SG4
`
`1G 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1
`
`Library of Congress Cataloging-in-Publication Data
`
`t
`
`6
`
`Plastic blow molding handbook /edited by Norman C. Lee.
`p. cm.
`ISBN 0-442-20752-2
`1. Plastics—Molding—Handbooks, manuals, etc.
`C., 1934-
`TP1150.P54 1990
`668.4'1~dc20
`
`I. Lee, Norman
`
`89-22674
`CIP
`
`

`

`CHAPTER
`
`Extrusion Blow Molding
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`Blow molding is the forming of a hollow object by "blowing" a thermoplastic ;r
`molten cube called a parison in the shape of a mold cavity. Extrusion blow '~~
`molding is the most widely used of many blow molding methods.
`There are probably more differences in equipment fox blow molding
`than -for any other plastics fabrication technique. A blow molding machine
`maybe the size of an office desk ar may occupy a large zoom, making hollow
`objects as small as a pencil or as large as 5,000 gallons capacity or greater.
`There are also a great many operating variables in a blow molding process,
`which make it one of the more complex processes. This chapter covers five
`topics:
`T
`
`Blow molding markets, summarized briefly.
`• Processes and Equipment
`~ Controlling wall distribution, the heart of blow molding. Unlike in-
`jectian molding, in which the wall thickness is automatically set by
`the dimensions of the mold core and cavity, many variables in blow
`molding affect wall thickness.
`~ Special blow molding techniques using moving secticsns mold with
`which the Phillips Plastics Technical Center has done much work.
`• Blow molding applications and resins
`
`21
`
`

`

`44
`
`Processes and Equipment
`
`MOVING SECTION MOLDS
`
`The Phillips Plastics Technical Center has done extensive work with moving
`section molds. One technique is being used commercially to make a threaded
`neck off the parting line of a mold. This is done with a reciprocating plug.
`The sequence is:
`
`1. Blow the parison against the exterided,plug (Fig. 2-33~
`2. Retract the plug during the blow operation (Fig. 2-34)
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`Figare 2-34 Plug assist: plug retracted.
`
`

`

`Extrusion Blow Molding
`
`45
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`Figure 2-35 Plus assist wall distribd~ion.
`
`Figure 2-3G Scorpion snowmobile fuel tank.
`Wall distribution is significantly impxoved hom conventionally molded necks,
`as shown in Figure 2.35. Blow time and plug movement times must be
`precise. Thzs type of neck was made commercially on a snowmobile fuel
`tank (Pig. 2-36) by Crosby Manufacturing, using high molecular weight HDPE.
`It saves either a pastmolding operation of afusion-welded neck, or a molded-
`in encapsulated anchor zing for postmold nec~C attachments.
`Moving section molds are used to make water cooler lids having integral
`Handles. Many millions of cooler lids`have been made by both Igloo and
`Gott. An integral handle lid has been in production at Igloo for twenty years
`
`

`

`46
`
`Processes and Equipment
`
`and Gott Corporation" `has made an integral handle, internally threaded,
`doublE-wall cooler lid for seventeen years Fig. 2=37,x. Gott's screw-on lid
`carries the cooler. bs~ly. w'~he one-piece handle avoids having to install a
`handle~~n a subsequent operation. One mold half consis~s~ of a split cavity
`(Fig. 2-38) powered by air cylinders to form the integral handle. The other
`half has a threaded, rotatable .core to form the internal threads (Fig. 2-39~.
`The molding sequence of this part is as follows. The extruding parison is
`prepinched (sealed at the. open ends. Verb lo~v'pressure is introduced into
`the parison before mold closure (tl~e slight;posit~4~e pressure results in the
`parisbn covering the handle area between t;~e s~Tit cavities, and at the same
`time insures that the parison will be~wi~eenough to flash the core. The
`mold halves and split cavity mold sections close almost simultaneously,
`resulting in a two-plane pinch through the handle and around the lid skirt.
`Blow air is introduced, forming the lidT~e part is cooled. The,thread core
`rotates, unscrewing from the internally threaded lid. The split cavities and
`mold halves all open at about the same time (Fig. 2-40~, ejecting a part with
`flash as shown in Figure 2-41, which trims to the li~'shown previously (F~g.
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`2-37~.
`Mauser drums, made wbrjd wide (Fig. 2-47~~, use an allie~technique to
`make the L-ring that m~,es it possible to handle a plastic drum similarly
`to a metal drum. The P'f'L built a moving section mold that compression-
`molds ahandling ring on a 30-gallon drum (Fig. 2-43) in an effort to improve
`the handling characteristics and therefore the market for such a drum. By
`using this technique, a solid chime, or ring (Fig. 2-44), can be molded at the t~ ~►.
`
`Figure 2-37 One-gallon Gott cooler. `
`
`

`

`p
`
`Extrusion Blow Molding
`
`47
`
`Figure 2-38 Prototype mold; split cavity.
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`Figure 2-39 Prototype mold, core.
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`used.
`
`

`

`48
`
`Processes and Equipment
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`Figure 2-40 Prototype mold, mold opening.
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`Figure 2-41 One
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`gallon Gott lid with flash.
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`

`

`Extrusion Blow Molding
`
`49
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`Figure 2-42 Mauser drum.
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`Figure 2-43 Thirty-gallon drum.
`
`r>
`
`

`

`50
`
`Processes and Equipment
`
`''
`
`Figure 2-44 Section of a drum.
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`Sections of the chime are show in Figure 2-45. Although these drums can
`be molded with excellent control of wall thickness, and produced consis-
`tently, poor impact resistence can become a problem with this design. The
`reduced impact tolerance is due to three factors:
`~d
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`1. A thick, solid, relati~;e7y nonflexible ring is adjacent to a much thin-
`ner flexible side wal~
`2. Normally a notch is created by the moving mold section at the
`;
`juncture of the solid ring and side wall
`3. Built-in stresses result from the greatly different cooling rates of the ~~
`heavy solid ring and thinner adjacent side wall
`
`For these reasons, it was necessary to use high molecular weight (MW) resins
`(in the 2 HLMI [high loss melt index] range) to pass asix-foot, 0° F drop .
`impact test. The standard L-ring drum made by Mauser also requires a high
`MW resin, usually about 1.7 to 2.3 HLMI. Recent improvements in L-ring
`drum design now allow successful use of 5-10 HLMI resins in these drums.
`With this molding technique, ahollow-wall, semihollow, or completely
`solid chime can be molded by changing the timing when the mold sections
`move. The three chimes are shown in Fig. 2-45. The key to the success of
`this technique is precise control of the process so the part will be reproduced
`identically time after time. A solid chime can be molded with good consis-
`tency, but hollow and semihollow chimes are more difficult. Advances in
`resins and equipment technology have resulted in this being a practical
`molding method.
`
`~~'
`
`

`

`~
`
`~ :
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`
`CHAPTER
`
`:.}
`
`~. 23
`
`,;,_
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`Extrusion Bl~~v~1~Iolds
`;: .
`
`JAMES P. PARR
`
`rN
`The success of any blow molding operation is dependent: upon the molds `
`- which are used in this~p,~ocess. These molds must be accurate, efficient,
`durable and, most of ail, cost effective. The material and method of con-
`struction are chosen for their suitability for the desired end product pro-
`duction quantity, and for the resin type.
`The detail of. the mold is important because multiple molds must pro-
`duce interchangeable parts of the same volumetric and dimensional accu-
`racy. Without extremely. good duplication, processing can be a problem,
`especially with remote trimming in multiple-mold operations.
`Efficiency in processing results from a mold that cools the molten ex-
`trudate into a form as free from warpage as possible in as short a time as
`possible. An efficient mold also reduces the number of parts that are rejected
`due to design imperfections.
`Durability is needed for long mold life. The molds are closed under
`pressure, which can eventually wear down the mating surfaces. The pinch-
`off sections also can be abraded by the material it cuts and welds. Corrosion
`is sometimes a key factor in mold material selection when, for example,
`processing PVC or acrylonitrile.
`Of course, the selection of the mold material and construction must be
`based overall on the expected sales volume of the product. Alternative types
`can then be evaluated based on economic criteria, in addition, to the practical
`
`'s
`
`Cf:3+7
`
`

`

`510
`
`Mold Design and Engineering
`
`cause of the continual frictional contact, the plate can be rotated a quarter-
`or half-turn to put one of the~xemaining unworn surfaces in service. Main-
`~~"'
`tenance is there#ore a fast;' simpleroperation.
`
`Ejectors
`
`,;,,
`Depending upon the type of blowing method, varous:~options exist for
`'`~ '-
`removing the part from the mold.
`~,
`When a blow pin inserted into the top of;t'he.?c6ntainer is used, with-
`drawal mechanisms can be arranged to grasp the tail flash through special
`slots in the base of the mold. The bottles can then be moved onto a cooling
`bed or conveyor belt for subsequent handling.
`If a horizontally shuttling mold is used iri conjunction with a vertical
`blow pin, the part is retained on the pin as the mold indexes over to grasp
`the next parison. The part is dropped onto the collection bed ,when the blow
`pin is vertically retracted.
`After blowing, the article can be stripped off a bottiom .T~lo~ pin li j~
`means of a plate, sleeve, or gripper. This type of blowing'technique is usually
`utilized for large articles such as' g5=gallon drums.
`In needle blow methods the needle is withdrawn slightly before full
`mold open. The part is retained in one mold half by a special undercut.
`Knockout rods or pins force the container out of the cavity.
`
`Miscellaneous Features
`
`rf
`
`A hole should be drilled and tapped into the top of heavy molds to allow
`insertion of an eyebolt. This allows the use of a crane or hoist to position
`the mold in the machine. Some arrangements should be provided to lock
`the halves of large rriolds together to prevent accidental opening during
`installations. A slot in the parting line can help in prying the halves apart
`for modification or repair when not mounted in the machine. These features
`are shown in Figure 23-10.
`When molding large parts with extensive pinch-off lengths, landing pads
`can be used to reduce the loading on and increase the life of the pinch-off
`edges. These pads can be square and are preferably made. from steel. When
`new, the pads should stop the closing of the mold before the pinch-off edges
`meet. The pads can later be worked down until the edges are the proper
`distance apart.
`
`MOVING SECTION MOLDS
`
`If it is desired to produce undercut features in a molded part thatTcould not
`be released from a simple two-piece mold, moving sections must be used
`
`

`

`Extrusion Blow Molds
`
`511
`
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`Figure 23-13 Moving section molds are used to produce parts that could not oth-
`erwise be molded. A. Open quarter-mold sections and thread forming core. B.
`Quarter-mold sections closed on thread forming core. Courtesy of Phillips 66
`Company, Plastics Division.
`
`[ 10,15]. This is how the handle is produced in some types of screw-on lids.
`The sequence begins with the moving sections each fully extended, as shown
`in Figure 23-13~a~. The sections close together during the molding step to
`form the handle, as shown in Figure 23-13b. When molding, the slides move
`in sequence with mold opening, which allows part removal.
`This same mold utilizes a retractable core to produce the internal threads.
`
`

`

`512
`
`Mold Design and Engineering
`
`The core unscre~zs during`mold opening to allow part removal. This feature
`is commonly encountered in many,injection molds.
`:.
`L-ring drums<.~.re pro~ucet~lri~'z?i`bving section molds. A compx~sion-
`molded lifting ring is produced on the top circular edge of the drum. This
`ring provides a site for attachment of various lifting devices. They mold is
`sequenced to close the vertically opened top section a predetermined time
`after the parison is inflated; producing the solid ring. _' ~ .
`.
`~r,
`
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`
`ACKNOWLEDGMENTS
`
`"` 'r ~ I f''~
`~'< <
`This project would not have been possible without the Hoechst blow molding
`handbook, which provided the foundation upon which this chapter was written,
`and the men who were responsible for it: Eckard Raddatz and Christian Gondro,
`Hoechst AG, Frankfurt, West Germany.
`i ""
`
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`
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`13.
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`9
`
`The authors extend special thanks to:
`H.R. Glover, Chevron Chemicals, Orange, TX
`Dieter Wunderlich, FGH Systems, Denville, l~lJ
`Scott Hartung, Seajay Inc., Nepxune, 1VJ
`Christopher Phillips, Johnson Controls!' Manchester, MI
`Don Peters, Phillips Chemical Co., Bartlesville, OK
`Brush Wellman Inc., Warren, MI
`Ampco Metals, New Berlin, WI
`Uddeholm Corp., Totowa, NJ
`'~
`NGK Metals Corp., Reading, PA. ,~
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`
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`P
`
`REFERENCES
`
`1. Suit, M., "Blow Molds." Modern Plastics Encyclopedia. New York: McGraw-
`Hill, 1986-1987. Pp. 331-333.
`2. Cantillon, W. FI. Beryllium-Copper Molds for Blow Molding, Brush Wellman
`Corp.
`3. "Cast Beryllium Copper: Some Tips on Using It as a Mold Material." Industrial
`Models and Patterns (March—April 1962.
`4. A Short Course in B1owMolding Righ-Density Polyethylene. Chevron Chemical
`Co., 1970.
`5. Menges, G., and P. Mohren. How to Make Injection Molds. New York: Hanser,
`1986.
`G. Branscum, T. E., and R. Doyle. "Blow Mold Design for Large Irregular Shapes."
`Modern Plastics 407) pp. 144-148 (March 1963).
`7. Gondro, C. Modern Applications and Equipment for Blow Molding of Polyole-
`fins. Ausplas, 1982.
`8. Segura, J. S. "Cryogenic Inner Cooling for Extrusion Blowmolding—The Ex-
`trublas Process." Proc., 2n Ann. Blowmold Technical Conference. Soc. Plastics
`Eng., November 1985.
`9. 5chubbach, R. "Optimierung des Quetschkantenbereic~s and des Schliessvor-
`gangs fuer Extrusionsblaswerkzeuge.",Plastverabeiter 10 pp. 608-614 (1~73~.
`
`

`

`Extrusion Blow Molds
`
`513
`
`4 ~
`
`10. Peters, D. L., and J. R. Rathman. "Blow Molding Highly Irregular Shaped Parts
`with Multiple Parting. Lines." Proceedings, ANTEC. Soc. Plastics Eng., 1985.
`1 1. Irwin, C. Extrusion Blow Molding Tools:..~enter for Professional Advanceme~'
`(February 1985.
`12. "Blow Molding Marlex." Phillips Chemical Co. V.2~2, 3, 5, 6~; v.3(2, 3, 4~; v.4~ 1,
`4, 6~. (March 1961—December 1963.
`13. Hasl, H. V. M. "In-mold Labeling: The New Competitive Edge." Proceedings,
`ANTEC. Soc. Plastics Eng., 1986.
`14. Rathgeber, J. "Blow Molding: The Evolution of an Imaginative Techn~logy.~"
`Proceedings, ANTEC. Soc. Plastics Eng., 1987.
`15. -Peters, D. L. 'Blow Molding Highly Irregular Shaped P ts
`Sections." Proceedings, ANTEC. Soc. Plastics Eng., 19~
`
`ith Moving Mold
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