Louisiana State University
`LSU Digital Commons
`
`LSU Historical Dissertations and Theses
`
`Graduate School
`
`2001
`
`Molding Large Area Plastic Parts Covered With
`HARMs.
`Mircea Stefan Despa
`Louisiana State University and Agricultural & Mechanical College
`
`Follow this and additional works at: https://digitalcommons.lsu.edu/gradschool_disstheses
`
`Recommended Citation
`Despa, Mircea Stefan, "Molding Large Area Plastic Parts Covered With HARMs." (2001). LSU Historical Dissertations and Theses. 277.
`https://digitalcommons.lsu.edu/gradschool_disstheses/277
`
`This Dissertation is brought to you for free and open access by the Graduate School at LSU Digital Commons. It has been accepted for inclusion in
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`MOLDING LARGE AREA PLASTIC PARTS COVERED WITH HARMS
`
`A Dissertation
`
`Submitted to the Graduate Faculty of
`Louisiana State University and
`Agricultural and Mechanical College
`In partial fulfillment of the
`requirements for the degree of
`Doctor of Philosophy
`
`in
`
`The Interdepartmental Program in Engineering
`
`by
`MirceaS. Despa
`B.S. in Chemical Engineering, UPB Bucharest, Romania, 1994
`M.S. in Chemical Engineering, LSU, May 1998
`May 2001
`
`Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
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`UMI Number 3016540
`
`UMI*
`
`UMI Microform 3016540
`Copyright 2001 by Bell & Howell Information and Learning Company.
`All rights reserved. This microform edition is protected against
`unauthorized copying under Title 17, United States Code.
`
`Bell & Howell Information and Learning Company
`300 North Zeeb Road
`P.O. Box 1346
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`
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`To the memory of my mother
`
`ii
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`ACKNOWLEDGEMENTS
`m u
`
`I wish to express my gratitude to Dr. Kevin Kelly for his help and guidance. I
`
`thank him for his advice and for his continuous challenge; I became a better person
`
`through both. Also, I want to thank my wife, Simona, for her advice with statistical
`
`issues, and more important, for her immense support and patience. Paul Rodriguez and
`
`Ron Bouchard were instrumental with their input in the design, manufacture, and imaging
`
`of devices and parts.
`
`iii
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`TABLE OF CONTENTS
`
`ACKNOWLEDGEMENTS.......................................................................................... iii
`
`ABSTRACT.................................................................................................................... v
`
`CHAPTER 1. INTRODUCTION.................................................................................... 1
`
`CHAPTER 2. INJECTION MOLDING......................... ................................................ 6
`2.1 Injection Molding Techniques for Molding Microstructures................................... 6
`22 Injection Molding Equipment................................................................................ 10
`2.3 Injection Molding Experiments.............................................................................. 16
`2.4 Results and Discussion.......................................................................................... 22
`2.5 Examples of Injection Molding Results.................................................................27
`2.6 Repeatability Reproducibility Study for LIGA Micromolding...............................37
`2.7 Conclusions Injection Molding..............................................................................49
`
`CHAPTER 3. HOT EMBOSSING................................................................................52
`3.1 Survey of Embossing Techniques.......................................................................... 52
`3.2 Theoretical Considerations.....................................................................................53
`3.3 Hot Embossing Equipment.....................................................................................58
`3.4 Hot Embossing Experiments.................................................................................. 63
`3.5 Conclusions Hot Embossing..................................................................................75
`
`CHAPTER 4. COMPARATIVE STUDY HOT EMBOSSING VS. INJECTION
`MOLDING.................................................................................................................... 77
`4.1 Design of Experiments........................................................................................ 77
`42 Mold Insert Making................................................................................................80
`4.3 Molding Experiments.............................................................................................82
`4.4 Conclusions on Comparison between Injection Molding and Hot Embossing
`90
`
`CHAPTER 5. FINAL CONCLUSIONS AND RECOMMENDATIONS.....................92
`
`REFERENCES............................................................................................................. 97
`
`VITA............................................................................................................................100
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`ABSTRACT
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`Injection molding and hot embossing proved to be suitable for use to reproduce
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`layouts of micropatterns manufactured by LIGA. The research work was based on two
`
`commercially available machines. Preliminary experiments with both machines indicated
`
`that several modifications and additions were necessary to adapt the injection machine
`
`and the embossing press for microreplication. For the injection machine, a mold
`
`geometry was designed to ensure minimal pressure loss upstream from the entrance into
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`the mold cavity,.and optimal shape for molding and demolding. For the hot embossing
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`press, a vacuum chamber was designed containing the embossing sandwich, with the
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`temperatures of the mold insert and of the substrate controlled through the press’
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`heating/cooling system.
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`Experiments were performed to identify the important process parameters in
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`injection molding of HARMs. It was established that mold insert temperature is of
`
`primary importance for the process, followed by melt flow rate. In hot embossing, it was
`
`argued that temperature of the polymer has most evident effects on the process, followed
`
`by variations in embossing force and embossing time. It was shown that the embossing
`
`time should preferably be long, to allow for relaxation of the macromolecular chains
`
`resulting in stress-free moldings.
`
`Compared to HDPE, it was shown that PMMA is heat-sensitive, and that it
`
`requires special care including drying before dosage, and dosage under controlled
`
`temperature and shear conditions. When used in hot embossing, PMMA is advantageous
`
`compared with polycarbonate- PC, because it allows embossing at lower temperatures.
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`Three micro heat exchanger geometries were molded (and fabricated for testing),
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`one stacked pattern and two cross flow patterns. Also, several DNA sequencing chips
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`were microreplicated, using both injection molding and hot embossing. Also, a test
`
`pattern was designed for comparison between molding techniques. Molding was
`
`performed into two inserts of different heights (200 and 500 microns) using the same
`
`material (PMMA). An overall mean value of 1.25 % was reported for shrinkage of the
`
`plastic compared to the mold insert Qualitative interpretation of the molding results
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`showed differences in the demolding success rate between the two molding techniques
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`combined with the two mold insert heights
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`CHAPTER 1. INTRODUCTION
`
`The LIGA process involves three processing steps: X-ray Lithography,
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`Galvanoformung (electroplating) and Abformung (molding) [1]. The LIGA process, as
`
`used at LSU, is based on bonding a sheet of poly(methyl-methacrylate) (PMMA) resist
`
`to an electrically conductive metal substrate, as shown in Figure 1.1. The resulting
`
`PMMA/metal substrate laminate is positioned behind an X-ray mask containing a
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`pattern to be reproduced, and exposed to collimated radiation emitted from a
`
`synchrotron storage ring. During exposure, the molecular weight of the PMMA
`
`decreases in the irradiated areas due to bond breaking in the macromolecular chain.
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`After the PMMA sheet is exposed, it is immersed in a developer and the irradiated areas
`
`are completely removed.
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`PMMA
`
`Nickel Substrate
`
`L U
`
`HARMs
`
`figure 1.1 Schematic of LIGA
`
`The resulting PMMA template is used to electroplate microstructures onto the
`
`substrate. After the electroplating process is completed, the remaining PMMA is
`
`stripped, resulting in a metal substrate covered with microstructures. The
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`microstructure-covered substrate can represent the final product. Alternatively, it may
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`serve as a mold that can be inserted into a molding machine to repetitively produce
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`secondary polymer or ceramic structures having the same geometry as the primary
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`PMMA template. These molded structures represent an end-use object or can be used as
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`a template for electroforming of new metallic objects. The commercial implementation
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`of LIGA depends on the ability to rapidly generate inexpensive, mass-produced parts.
`
`Since the first two steps of this technique are inherently employing expensive and/or
`
`time consuming operations, the last step, molding, has to compensate for these expenses
`
`by mass producing low cost parts.
`
`In the microreplication area, aspect ratio is defined as [2,3]:
`
`.
`_
`Maximum, Height
`Aspect Ratio = -----------------------------------
`Minimum Lateral Dimension
`
`High aspect ratio microstructures, HARMs, have a height (or depth) measured in
`
`hundreds of micrometers and an aspect ratio of at least five. The present commercial
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`molding processes, such as compact disk (CD) manufacture are limited to producing
`
`parts that have an aspect ratio on the order of 1 or less [4]. In addition, except for CDs,
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`the available parts consist of small areas covered with microstructures [5,6,7]. Molding
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`of large area plastic parts covered with HARMs is of particular interest for applications
`
`such as heat transfer, microfluidic elements, ceramics, etc. [8,9].
`
`A brief yet comprehensive review of the LIGA technique and its potential for
`
`microsystems was done by Bacher et al. in a paper dated in 1993 [2]. The authors of this
`
`paper are members of the German research team that first developed LIGA at KfK
`
`(Nuclear Research Center, Karlsruhe, Germany, and now Research Center Karlsruhe -
`
`FzK). Their earliest efforts date since the late 70’s [10], thus they have the most
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`significant experience in the field. In this paper, Bacher and his coworkers present all
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`the aspects of LIGA, including the modifications and extensions added to the technique.
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`Madou’s review of LIGA [10] is also a comprehensive study. However, he bases
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`his text on the experimental work and on the results of the same aforementioned
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`German team, especially when presenting the molding step (also referred to as
`
`micromolding). Madou also briefly mentions the early efforts initiated at LSU in the
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`field of molding, efforts that represent the beginning of the work hereby presented.
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`From his knowledge, these are the first attempts to systematically approach the field of
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`molding of LIGA microstructures in the United States. Harmening et al. [11] and
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`Ruprecht et al. [1,12] also part of the German team, give a condensed description of the
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`molding techniques used in LIGA, briefly mentioning the principles of operation.
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`Referring to the work of the German research team at KfK, the representation in
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`Figure 1.2 is an excerpt from Bacher’s et al. paper showing a schematic classification of
`
`the molding methods used in LIGA. Gassical molding methods were used, in principle,
`
`but several, sometimes severe, adaptations were made to fit the specific requirements
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`presented by micromolding. The classical methods employed were reaction injection
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`molding (RIM, figure 1.2 a), thermoplastic injection molding (figure 1.2 b) and
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`compression molding (figure 1.2 c) also known as hot embossing. Independent of which
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`method is used for molding, the final purpose is to produce a part that reproduces the
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`geometrical shape and dimensions of the micropattem on the metallic mold insert hi
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`order to achieve that the material for molding has to completely fill up the microvoids
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`on the insert it then has to solidify sufficiently for separation (demolding), and it has to
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`preserve its original geometrical definition over the period of usage.
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`(a)
`
`screw
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`I f1 J;Sh
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`s s s
`
`(b)
`
`(C)
`
`Mold Insert
`
`Mold Insert
`
`Thermoplastic
`Polymer
`
`Figure 1.2 Classification of LIGA Molding Techniques:
`a- RIM, b- Injection Molding, c- Hot Embossing [2]
`
`These requirements are also applicable in the case of larger molded parts, but they
`
`become much more significant in the case of micromolding, since micromolding is
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`performed on a small size scale on the order of micrometers. At this scale, the
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`penetration into the narrow gaps is more difficult and the flow and thermal issues
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`become more stringent For instance, a high viscosity of the plastic melt or solution will
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`make the filling step more difficult Also, demolding can become a problem if the
`
`forces required to separate the plastic part from the metallic insert are on the same order
`
`as the structural strength of the microstructures.
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`Ehrfeld et al. [6] compiled a list of materials used in the LIGA process. A wide
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`variety of thermoplastic polymers have been tried and applied for molding of LIGA
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`microstructures, including polyoxymethylene, polystyrene, PMMA, polycarbonates,
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`polyamides, and polyvinylidenefluoride. These polymers are suitable for specific
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`applications, and the list of materials will continue to expand as new applications
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`requiring new types of polymers are developed. The morphological structure of a
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`certain polymer, its degree of crystallinity, the intermediate changes the polymer
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`undergoes when changing phases, are all expected to affect the behavior of the polymer
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`throughout the molding process and during the period of usage of the final product.
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`CHAPTER 2. INJECTION MOLDING
`
`2.1 Iqjection Molding Techniques for Molding M icrostructures
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`Injection molding for LIGA is currently employed as a primary technique for
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`replicating micropattems produced by X-ray exposure/electrodeposition processes [12].
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`The injection molding cycle starts by first melting and homogenizing the polymer inside
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`the barrel of the injection machine, as shown in Figure 2.1. The polymer, in the form of
`
`pellets or flakes is loaded into a hopper. A screw, housed in a barrel, is used to shear,
`
`melt, and finally, pump polymer into the cylindrical zone located between the tip of the
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`screw and the forward end of the barrel ("dosing"). When the desired amount of melt
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`accumulates in front of the screw tip, the screw plunges forward and pushes the melt
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`through the nozzle and the sprue and into the mold cavity where the mold insert for
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`HARM replication is mounted. After the mold is cooled and the melt solidified, the two
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`platens that form the mold are separated and the plastic part is ejected. Bacher et al. [2],
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`Ruprecht et al. [12], and Ehrfeld et al. [6,7] all mention the use of thermoplastic
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`Hopper
`
`Barrel
`
`-Mold Cavity
`
`Mold In s e r t
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`Figure 2.1 Schematic of an Injection Molding Machine
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`injection molding as one of the techniques for replicating LIGA microstructures. They
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`mention the injection molding of compact disks (CD) as a method for molding
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`microstructures. However, they also mention that the pits imprinted on the CD’s surface
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`have an aspect ratio of about one, which makes the technique unsuitable for direct
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`application for molding HARMs. Ruprecht indicates, without going into any details,
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`that a conventional injection molding machine as those used for CD fabrication can be
`
`used for molding of microstructures. The machine needs to be "expanded with special
`
`devices allowing the evacuation and temperature control of the tool for the complete
`
`filling of the mold"[12]. When describing this technique, the authors limit themselves to
`
`listing the main factors influencing the process, without showing actual values for any
`
`experimental parameters. The list of important parameters includes the tool temperature
`
`and the temperature of the melt, both of which have to be higher than in the
`
`conventional injection molding to prevent incomplete filling [7,12]. They also indicate
`
`that "too high injection pressures or injection speeds lead to damages of the mold insert
`
`in an extreme case" [7]. Evacuation of the mold cavity is also mentioned as a critical
`
`factor for a successful injection [12].
`
`2.1.1 Related ejection molding applications
`
`The small nature of the LIGA mold inserts makes the molding step more
`
`challenging than the classical injection molding processes. However, some industrial
`
`applications of injection molding of macroscopic objects can be compared to injection
`
`molding of HARMs, because similar fundamental flow and heat transfer principles
`
`govern the process both at macro and at micro scale. For instance, molding of thin wall
`
`parts also involves filling of mold patterns having high aspect ratios. As the thickness of
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`the wall is decreased, incomplete filling (short shot) can become a problem due to
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`premature solidification of the melt. Typical applications of thin wall parts include
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`cellular phone components, laptop and notebook computer components, disposable
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`containers and cups, medical equipment components, etc [4]. Fasset [4] indicates that
`
`increasing the injection speed coupled with using a smaller injection machine leads to a
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`more efficient and safe production of such thin wall molded parts, hi order to achieve
`
`complete filling of a high aspect ratio mold, the polymer melt has to be as fluid as
`
`possible, therefore its viscosity must be kept minimal. To decrease the viscosity, the
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`polymer melt is heated to higher temperatures, thus increasing the potential for thermal
`
`degradation. Using increased injection speeds rather than elevated temperatures results
`
`in a decrease of the degree of degradation of the polymer. This is due to the shear
`
`thinning character of the polymer that leads to a lower viscosity without thermal
`
`degradation of the polymer. Fasset reports that an increase of the injection speed from
`
`10 cm/sec to 36 cm/sec led to a decrease by a factor of about 3.3 in polymer degradation
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`as measured by percent decrease in melt viscosity. Also beneficial was the selection of
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`an injection machine with a smaller barrel, because a shorter residence time was
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`imposed to the polymer melt, thus less time for the thermal degradation to occur. The
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`melt viscosity of the degraded polymer decreased 300% as compared to that of a
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`polymer that was used in a barrel four times smaller.
`
`Breuer takes the comparison between CD manufacture and microinjection
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`molding one step further and shows that producing free standing miniature parts is more
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`challenging than molding CD storage media because the overall size of a CD is much
`
`larger than that of a LIGA part [13]. hi the case of micromolding individual parts, the
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`runner system dwarfs the actual microparts, leading to a complicated design and an
`
`uneconomical production. He proposes a few modifications of the classical plasticizing
`
`unit such as eliminating or reducing the runner system, or the introduction of open hot-
`
`runner systems such that the molded part can be reached without a runner. Moreover,
`
`he suggests that given the very small shot weight (on the order of a few grams or less) a
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`reciprocating plasticizing screw is not suitable anymore. A reduction of the plasticizing
`
`unit cannot be done past a certain limit dimension of the screw diameter because of the
`
`increased probability of a break and because the positive plasticizing characteristics are
`
`reduced with a low screw stroke. Breuer presents an alternative to using a machine with
`
`reciprocating screw plastication by using an injection unit where the melt is prepared
`
`into a separate plastication unit, and then sent into an injection unit used to fill the mold.
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`Such a machine is now commercially available, being produced by Battenfeld
`
`Kunstatoffmachinen [14]. The machine offers the advantage of a more accurate control
`
`of the shot size and a more efficient usage of the material to be injected (a ratio of 60/40
`
`part/sprue+runners as opposed to 34/66 for older systems). Cycle times are also reduced
`
`by minimizing cooling time and handling time.
`
`Summarizing the information with respect to thermoplastic injection molding, it
`
`can be concluded that the most important process parameters are melt and mold
`
`temperatures, coupled with the injection pressure/ injection speed. The experience
`
`gathered from related molding processes indicates that selection of the injection
`
`machine, combined with a proper design of the mold (runner-gate system and cavity)
`
`and evacuation of the mold prior to injection are essential for a successful injection
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`molding of HARMs, as those produced by LIGA.
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`23 Injection Molding Equipment
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`The |iSET group at LSU is currently developing injection molding methods for
`
`producing large area plastic parts covered with HARMs. The work has been centered on
`
`using a standard injection molding machine for a nonstandard application such as
`
`micromolding. Immediately, one should notice that the equipment costs are much lower
`
`if a standard, commercially available molding machine is used for micromolding
`
`instead of a complicated, non-standard application-specific machine. The injection
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`machine, type Arburg Allrounder® 170 CMD 150-45 [15], is designed with a
`
`reciprocating screw that both melts and injects the polymer into the mold cavity. The
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`reciprocating screw is capable of two types of motion: it can rotate around the
`
`longitudinal axis of the barrel and it can translate along the same axis. Inside the barrel,
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`the material is melted by thermal input from the electrical heating jackets (bands) and
`
`by the shearing action of the rotating screw. As stated above, the work done throughout
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`this study was based on using a commercially available, general-purpose injection
`
`molding machine. Thus, given the differences between a classical injection molding
`
`process and the more special process of micromolding, a few adaptations had to be
`
`made to the equipment, none of which was cost-intensive.
`
`First, a new mold, composed of two halves, was designed to hold the micro­
`
`mold insert The two mold halves were designed such that when closed together they
`
`form a cavity inside which the insert is placed and where the plastic melt is injected to
`
`fill the micro-pattem onto the insert. The mold design was based on what was expected
`
`to be specific requirements related to pushing the plastic melt into the high aspect ratio
`
`microstructures. Because a high pressure drop was expected to occur within the
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`microgaps on the mold insert, a runner/gate system was completely eliminated from the
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`mold design. This allowed for a direct access of the melt with minimal pressure loss
`
`upstream from the mold cavity. The design resulted in a single cavity mold; given the
`
`experimental nature of this work, together with the small capacity of the injection
`
`machine, the purpose was not to design a multi-nested mold but one that could hold a
`
`single mold insert. Knowing that complete filling of the microvoids will be influenced
`
`by the temperature of the insert, the design included a heating/cooling system. The
`
`system was mounted onto the mold platen holding the mold insert The heating system
`
`consists of four heating cartridges (rods) mounted in channels running vertically into the
`
`mold platen. Heating the mold is controlled from a temperature controller mounted on
`
`the control panel of the injection machine and coupled with a thermocouple that senses
`
`the mold platen temperature. The cooling system consists of three channels interplaced
`
`between the heating elements. The heating/cooling channels were uniformly distributed
`
`to cover the mold insert area of the mold cavity. The channels were drilled such that a
`
`better uniformity of the temperature can be achieved.
`
`Aluminum was used as the material for the mold platens because it provides
`
`several advantages over the more classical solution using stainless steel. First,
`
`aluminum is strong enough to resist the relatively small loads imposed by the clamping
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`system to the mold, given the relative small shot size. Also, aluminum has the
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`advantage of being easier to machine. Second, aluminum offers the advantage of a
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`lower thermal inertia than stainless steel. This is advantageous because the mold has to
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`be heated at the beginning of the cycle in order to prevent premature freezing of the
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`melt; it then has to be cooled down to the demolding temperature for the part to be
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`Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
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`ejected, and the low thermal inertia of aluminum shortens the portion of cycle time
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`dedicated to heating/cooling the mold.
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`Another requirement for micromolding, in addition to controlling the barrel
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`temperature, the injection rate, and the mold temperature, is to vent the micromold prior
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`to injection [12]. At macroscale, air entrapment inside molds is usually avoided by
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`venting the air through very fine slits. These slits are machined on the interior surfaces
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`of the two mold platens, in the parting line, and the air is expelled as the melt fills the
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`mold. In other cases, the clearance gaps around the ejector pins provide sufficient
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`volume to vent the mold, without the need for venting slits. In micromolding, a
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`complicated fluid front may lead to air entrapment if air is present inside the mold prior
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`to injection, i.e. if the mold cavity is not under vacuum before injection begins. Thus,
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`the injection machine has to be fitted with a vacuum system for venting the mold prior
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`to injection. A rotary vane vacuum pump was connected to the mold through a vacuum
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`port machined into the stationary mold plate. To prevent plastic melt from being sucked
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`into the vacuum line upon injection, the vacuum port is interfaced with the mold cavity
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`through a pill of sintered metal (stainless steel). The pill is held in place with a hex cap
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`screw whose core was drilled out along its longitudinal axis. A schematic drawing of
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`the setup is shown in figure 2.2.
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`Additional equipment was necessary for the heating/cooling system and for the
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`vacuum system. Thus, a general-purpose water pump was connected with the cooling
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`channels and a 25-gallon water tank was built to economically fit inside the space
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`available in the injection machine’s chassis. The water flow could be controlled with a
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`ball valve, and the temperature could be lowered by adding ice to the water.
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`Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
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`figure 2.2 Schematic Drawing of Vacuum Pott
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`The vacuum was achieved using a laboratory size pump capable of evacuating
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`the mold cavity down to SO mmHg in less than 1 second. A photograph of the mold
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`halves is shown in the next sub-chapter, together with mold design solutions chosen to
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`fit the specific requirements of the injection molding material.
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`2^.1 Characterization of Molding Materials
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`• Differential Scanning Calorimetry (DSC)
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`DSC was used to determine the softening region for the thermoplastic polymer used in
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`our molding experiments, namely high density polyethylene, HDPE. This calorimetric
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`technique, described in detail elsewhere [16] is based on the fact that a transition such
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`as crystallization or vitrification is associated with a change in thermal properties of the
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`sample unde