模具设计外文翻译.doc

编号： 毕业设计(论文)外文翻译（译文）学 院： 机电工程学院 专 业：机械设计制造及其自动化学生姓名： 学 号： 指导教师单位： 艺术与设计学院 姓 名： 梁惠萍 职 称： 讲师 2012年 5 月 15 日The Effects of Mold Designon the Pore Morphology ofPolymers Produced withMuCell_ TechnologyABSTRACT: In this study two molds were designed and used in MuCell_technology to generate implants with a porous structure. To arrive the desiredpore structure many process parameters were investigated for indicating theeffects of process parameters on the pore morphology. This process parameterinvestigation was performed on each mold respectively, so that the influencesof the mold design on the pore morphology have been researched by the sameprocess parameter setting. It was found that the mold design also had effectson the pore structure in MuCell_ technology. A proper mold design couldimprove the generated pore structure, such as porosity, pore diameter, andinterconnectivity.KEY WORDS: mold design, cell morphology, MuCell_, injection molding,medical implant, porous polymer, polyurethane.INTRODUCTIONMuCell technology, as an effective microcellular injection moldingprocess, is widely used in automobile and furniture industries.In most cases, MuCell_ technology is used to save raw materials, but itis also used to produce implants with closed porous structure 1. It usesCO2 as blowing agent, which is injected in the plasticization section ofthe injection molding machine (Figure 1). The blowing agent is injectedinto the polymer melt through the gas supply line and injector, in itssuper critical state, by the plasticization phase of the injection moldingmachine. After the plasticization the mixture of polymer melt and gas isinjected through the nozzle into the mold, where the foam structure canbe generated due to the quick pressure drop in the mold. The mainproducts which are produced today with MuCell_ technology have closedcellular foam 24.The MuCell Microcellular Foam injection molding technology is a complete process and equipment technology which facilitates extremely high quality and greatly reduces production costs. The MuCell Process involves the controlled use of gas in its supercritical state to create a foamed part. The MuCell Technology is targeted at precision and engineered plastic components with maximum wall thicknesses of less than 3mm.The MuCell Process generally offers a 50-75% improvement in key quality measures, such as flatness, roundness, and warpage, also eliminating all sink marks. These improvements result from the fact that relatively uniform stress patterns are created in the molded part rather than non uniform stress characteristic of solid molding.As a direct result of the uniform stress and shrinkage associated with the MuCell Process (which occurs because the pack and hold phase of the molding cycle is eliminated), the parts that are produced tend to comply far more closely with the mold shape and, presumably, the dimensional specifications of the part itself. This means that when using the MuCell Process, fewer mold iterations are needed to produce a compliant part, saving time and cost.The quality advantages of the MuCell Process are complemented by certain direct economic advantages, including the ability to produce 20-33% more parts per hour on a given molded machine, and the ability to mold parts on lower tonnage machines as a result of the viscosity reduction and the elimination of the packing requirement that accompanies the use of supercritical gas. This 25 page processing handbook covers all aspects of the process from set-up to troubleshooting to optimizing results. It is primarily useful to companies who are manufacturing or are planning to manufacture parts using the MuCell Injection Molding Process. Please contact Trexel for a copy of this publication.The MuCell Injection Molding Process involves the highly controlled use of gas in its supercritical state (SCF) to created millions of micron-sized voids in thin wall molded parts (less than 3mm). With the correct equipment configuration, mold design, and processing conditions these microcellular voids are relatively uniform in size and distribution.The voids are created or nucleated as a result of homogeneous nucleation that occurs when a single-phase solution of polymer and gas (commonly nitrogen, but occasionally carbon dioxide) passes through the injection gate into the mold.The single-phase solution is created through the operation of a conventional injection molding machine which has been modified to allow the creation of a single-phase solution. The key modifications to the system involve the use of a precision SCF delivery system to deliver SCF to special injectors based on mass flow metering principles. The SCF is then injected into the barrel where it is mixed with the polymer via a specially designed screw. A shut off nozzle maintains the single phase solution while the injection molding screw maintains sufficient back pressure at all times to prevent premature foaming or the loss of pressure which would allow the single phase solution to return to the two phase solution. Trexel has recently published a comprehensive MuCell Process Guide in English, Chinese, Japanese, and German which explains in step by step detail how to apply the MuCell process in the manufacture of MuCell Injection Molded components. This 25 page processing handbook covers all aspects of the process from set-up to troubleshooting to optimizing results. It is primarily useful to companies who are manufacturing or are planning to manufacture parts using the MuCell Injection Molding Process. Please contact Trexel for a copy of this publication.The MuCell microcellular foam injection molding process for thermoplastics materials provides unique design flexibility and cost savings opportunities not found in conventional injection molding. The MuCell process allows for plastic part design with material wall thickness optimized for functionality and not for the injection molding process. The combination of density reduction and design for functionality often results in material and weight savings of more than 20%. By replacing the pack & hold phase with cell growth, lower stress parts are produced which have enhanced dimensional stability and substantially reduce warpage. Cell growth also results in the elimination of sink marks.Unlike chemical foaming agents, the physical MuCell process has no temperature limitation and does not leave any chemical residue in the polymer; making consumer products perfectly suitable for recycling within the original polymer classification and allowing re-grind material to reenter the process flow. The numerous cost and processing advantages have led to rapid global deployment of the MuCell process primarily in automotive, consumer electronics, medical device, packaging and consumer goods applications.Microcellular foams refer to thermoplastic foams with cells of the order of 10 m in size. Typically these foams are rigid, closed-cell structures; although recently there is much interest in creating open-cell, porous structures that have cells in this size range. The microcellular process that sparked the growth in this field over the past two decades was invented at Massachusetts Institute of Technology, USA, in early eighties 1, in response to a challenge by food and film packaging companies to reduce the amount of polymer used in their industries. As most of these applications used solid, thin-walled plastics, reducing their densities by traditional foaming processes that produced bubbles larger than 0.25 mm was not feasible due to excessive loss of strength. Thus was born the idea to create microcellular foam, where we could have, for example, 100 bubbles across one mm thickness, and expect to have a reasonable strength for the intended applications. It would be reasonable to say that the potential of microcellular foams has yet to be realized. These materials have not yet appeared in mass produced plastic items, and the promised savings in materials and associated costs have yet to materialize. This is largely due to manufacturing difficulties encountered in scaling up for large scale production. However, enthusiasm for these materials remains high, and today researchers and commercial enterprises on every continent are in a global race to harness the potential benefits.Much has been learned about the processing and properties of microcellular foams since the first patent was granted in 1984 2. An early review of the subject appeared in 1993 3. In this chapter the state-of-the art of processing will be reviewed in the next section, followed by a discussion of structure and properties. This chapter will conclude with a look at some of the current research directions involving microcellular technology. Although innovations in processing have developed at a rapid pace, the property data on microcellular foams has been slow in coming. The early publications on microcellular foams conjectured that the microcellular structure, believed to be on a scale that was smaller than the critical flaw size for polymers, would enable these foams to retain their mechanical properties even as the density was reduced. No quantitative information on the critical flaw size was ever presented, nor was any property data presented in support of the hypothesis. This is likely to be due to the emphasis placed on process development, as opposed to property characterization, in the early years of evolution of this field. Over time, however, this conjecture has become a myth that microcellular materials are as strong as the solid polymers but have a lower density, thus providing an opportunity to lower costs with no penalty in performance.The tensile property data 4 shows that the tensile strength of microcellular foams decreases in proportion to the foam density, and can be approximated quite well by the rule of mixtures. Thus a 50% relative density foam can be expected to have 50% of the strength of the solid polymer. Figure 11.5 shows relative tensile strength as a function of relative foam density for a number of microcellular polymers. In this figure the relative tensile strength, is obtained by dividing the tensile strength of the foam by the tensile strength of the solid polymer. Similarly, the relative density is foam density divided by the solid polymer density. In Figure 11.6 we have plotted the strength data on a specific basis. Thus the specific relative tensile strength for the foam of a given relative density is obtained by dividing the relative tensile strength by the relative density. Figure 11.6 shows that on a specific basis, the tensile strength of microcellular foams is essentially constant over the entire range of foam densities. Unfortunately, similar data on conventional foams is not readily available for a direct comparison with microcellular foams.A unique aspect of data in Figure 11.5 is that in the relative density range of 0.1 to 0.5, the microcellular foams represent novel materials for the engineer with properties not previously available. Most conventional foams fall either in the low-density region (relative density less than 0.1) or belong in the structural foams category (relative density greater than 0.5). The modulus of microcellular foams can be reasonably estimated by the Gibson-Ashby cubic cell model 5, which predicts that the relative tensile modulus equals the square of the relative density. The gas composition in the cell may affect the long term thermal conductivity of the foams 6. Microstructures, tensile strength, and thermal expansion properties for a number of low density foams have been reviewed by Williams and Wrobleski 7.Fatigue and creep behaviours of microcellular polycarbonate foams have been investigated 8-10. An interesting result from fatigue studies is that introduction of very small bubbles in PC, with less that 1% reduction of density, led to a thirty-fold increase in fatigue life compared to the solid PC. This might suggest a process similar to heat treatment of metals, where a PC part may be saturated with carbon dioxide at 5 MPa and then heated to say 60 C to introduce the microcellular structure without an appreciable density change, to increase the fatigue life of a part. Due to the low processing temperatures, very little dimensional change was observed in the experiments.The tensile data for all gas-polymer systems investigated falls on one reduced plot where relative tensile strength can be plotted against the relative density, as is shown in Figure 11.5. However, energy absorption measures, such as an impact test, are more sensitive to variations from polymer to polymer, and the results cannot be generalized. Gardner Impact Strength for PVC foams 11 with relative densities of 0.5 and higher. It is seen that the impact strength decreases linearly with foam density. This result is contrary to the popular belief, long held without proof, that the microcellular structure will always improve the energy absorption behavior due to the increased resistance to crack propagation offered by the micro voids 12.Some studies have investigated the relations between the key processparameters in MuCell_ technology and produced cellular foam structure1,5,6. It was found that the pore morphology in MuCell_ process couldbe adjusted through varying the process parameters. However, there iscurrently no literature regarding the effects of mold design on the poremorphology by MuCell_ technology.In this study two molds were designed and used in MuCell_ process togenerate implants with a porous structure for medical use. The researchof process parameters was independently performed on these two molds.By comparing the pore structure of implants made from two molds atthe same process parameter setting, the influences of the mold design onthe porous structure were investigated.Figure 1. Draft of the MuCell_ technologyMATERIALS AND METHODSPolymer ProcessingMedical grade thermoplastic polyurethane TPU (Texin_ 985, Bayer,Pa, USA) was chosen as raw material for the implant. An injectionmolding machine (KM 125-520C2, KraussMaffei Technologies GmbH,Munich, Germany) with a temperature control unit for cooling the mold(90S/6/TS22/1K/RT45, Regloplas, St. Gallen, Switzerland) was used forthe production of the samples. The injection molding machine wasequipped with a MuCell_ package by the Trexel Inc., Woburn, MA, USA.The MuCell_ package is schematically shown in Figure 1. The blowingagent is injected into the polymer melt through the gas supply line andinjector, in its super critical state, by the plasticization phase of theinjection molding machine. After the plasticization the mixture ofpolymer melt and gas is injected through the nozzle into the mold, wherethe foam structure can be generated due to the quick pressure drop inthe mold.CO2 was used as blowing agent (CO2 protective gas DIN-32525-C1,Westfalen AG, Munster, Germany).In order to produce the implant, two particular molds were designedand used. The technical drawings of molded parts from mold A and moldB are shown in Figure 2. The mold A had six ring shaped implantsand was just used for the preliminary test of the feasibility of thefoaming process and parameter research. The mold B was designed withsix solid disk shaped implant based on the results of in vivo test ofimplants from mold A, for a higher biological requirement andprospective production.Figure 2. Different mold designs.Two molds have similar gate, runner, and sprues. The mold B has ashorter polymer melt flow of mold cavity and the L/D (length/thickness)of 2.8, whereas this L/D for mold A is 4.7. This means the molded partfrom mold B is relatively thicker but shorter. The advantage of mold B isthat the energy loss of melt flow, which dominates the cell nucleationand growth, is reduced due to the shorter flow path (low L/D). As aresult better pore morphology, such as bigger mean pore size, higherporosity, and so on, could be expected. On the other hand the mold B hasa bigger capacity which means more possibilities of parameter variation.The disadvantage of mold B is that relative thicker molded part will leadto an incomplete filling of the cavity of mold B, a long cooling time, andsignificant shrinkage of molded part, in normal injection moldingprocess. These problems could be partially or wholly resolved if thefoaming process is applied due to the expansion of foamed polymer.Experimental StrategyThe choice of the changeable parameters was made based on theknowledge given by nucleation theory and literature search 5,7. Theranges of variable parameters and the values of fixed parameters arepresented in Table 1. The experiments were done by varying one of variable parameters while keeping the others constant. The wholeprocess parameters investigation was performed on two molds respectively.The implants from two molds, which were used to be compared,were produced at exactly same process parameters, so that the effects of different molds were shown.Characterization of Macro- and MicrostructuresScanning electron microscopy (SEM; Jeol JSM-6060LV, JEOL Ltd.,Tokyo, Japan) was used for the observation of the pore morphology ofthe cross section of implant. The samples were sliced with a scalpel andthen coated with a thin gold layer by using a sputter-coater (SCD 005,BAL-TEC AG, Balzers, Lichtenstein) under high vacuum with a voltagerange between 5 and 15 kV. Characteristics of porous structure such aspore size and porosity can be calculated by counting the average cellnumber and size of several SEM-images from one sample.One cut area with certain size was chosen and all pores were measuredmanually with the help of software of digital microscope