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ABSTRACT: A heated mold with temperature above the polymer-softeningtemperature is highly desired in precision injection molding. The elevated moldtemperature reduces unwanted freezing during the injection stage, thusimproving moldability and enhancing part quality. The resulting advantagesinclude, but are not limited to, longer flow path, improved feature replication and surface transcription, reduced molecular orientations and residual stresses,smoother surfaces of composites, better control of crystallization, stronger weldlines, etc. 29173
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However, the heated mold needs to be rapidly cooled during thecooling stage to maintain a short cycle time. Despite the large body of availableliterature, mold rapid heating and cooling does not represent a well-developedarea of practice. Development of capable techniques for rapidly heating andcooling a mold with a relatively large mass is technically challenging because ofthe constraints set by the heat transfer process and the endurance limits set by thematerial properties. This article attempts to offer a constructive overview on thestate of the art in mold rapid heating and cooling, with the goal of explaining theworking mechanisms and providing unbiased accounts of the pros and cons ofexisting processes and techniques. Successful applications of existing processesare highlighted and potential improvements of these processes are suggested. Thearticle further intends to provide a fundamental understanding of the constituentelements and corresponding building blocks needed in a workable mold with arapid heating and cooling capability. An increased understanding of theseelements will facilitate the development of more capable new processes. C 2009Wiley Periodicals, Inc. Adv Polym Techn 27: 233–255, 2008; Published online inWiley InterScience (www.interscience.wiley.com). DOI 10.1002/adv.20136KEY WORDS: Heat transfer, Injection molding, Precision molding, Rapidcooling, Rapid heating IntroductionIn injectionmolding, polymermelt is injected intoa closed cavity, held under pressure to compen-sate for thermal shrinkage until the gate freezes, andthen ejected out of the cavity after the part has suf-ficiently cooled. Since the polymer melt needs to becooled and solidified in a mold, the mold tempera-ture during the cooling stage has to be lower thanthe polymer phase transition temperatures (e.g.,the glass transition temperature for an amorphouspolymer and the crystallization temperature for asemicrystalline polymer). Themost economical wayfor mold temperature control is single set point con-trol, which is widely adopted in standard injectionmolding practice; that is, a constant mold tempera-ture is used (although the actual mold surface tem-perature may vary during the cycles due to the heatreleased fromthe incoming polymermelt).Auxiliaryheating and chilling equipment is usually utilized tomaintain this constantmold temperature. Thisman-ufacturing process is technically adequate in manyinjectionmolding applications, particularly for partswith thick walls and low requirements for dimen-sional accuracy and/or optical quality. The processis also economically attractive because of a very short cycle time, typically from several seconds to aminute.The standard injectionmolding processwith con-stant mold temperature control, however, suffersfrom problems caused by the large temperature dif-ference between the mold and the incoming poly-mer. Upon contact with the mold surface, the poly-mer melt starts to solidify, almost instantaneously,at the mold surface. The resulting frozen layer has anumber of adverse effects on molding performance.Because of this frozen layer, it is difficult to fill apartwith a large length/thickness (L/T) ratio; to ob-tain glossy surface finishes; to achieve a low level ofresidual stresses, etc. The premature freezing prob-lemduring the filling stage also results inweakweldlines because of the lack of molecular diffusion be-tween the joining melt fronts. More important, thefrozen outer layer deteriorates the optical and me-chanical properties of the molded part. The differ-entiated cooling process from the skin to the coreresults in undesired structural and morphologicalheterogeneity. In particular, the frozen-in molecularorientations in the skin result in distributed birefrin-gence and residual stresses, of which the latter is theprimary driver for part warpage and dimensionalinstability. The causes for the anisotropy in prop-erties are understandable considering the complexthermomechanical history the polymer experiences during themolding cycle, where a large thermal dif-ference is imposed on the polymer and a high in-jection pressure is applied to counterbalance prema-ture freezing and achieve the necessary moldability.Often, the injection pressure in thin-wallmolding ex-ceeds 100MPa, a thousand times higher than the at-mospheric pressure, resulting in high shear rates ap-proaching 104s−1. Recent studies1−3on high-speedmicromolding further revealed the likely existenceof extremely high shear rates exceeding 105s−1.Notethat this high shear rate, like the thermal field, is alsodistributed spatially in the thin gap. Since the poly-mer undergoes phase transitions under a gradientthermomechanical field in liquid, rubbery, and solidstates, the structural andmorphological heterogene-ity in the molded part is unavoidable.Most of the aforementioned part defects may beeliminated or at least alleviated if an elevated moldtemperature close to or even above the polymer tran-sition temperature is used. This elevated mold tem-perature, however, substantially increases the cycletime, thus lowering the productivity to a great ex-tent. The ideal molding condition is to have a hotmold during the filling stage and a cold mold dur-ing the cooling stage.4,5In this case, the hot moldprevents the polymer from freezing during the fill-ing stage, whereas the cold mold during the cool-ing stage maintains a short cycle time. In reality, asingle mold is used in injection molding; therefore,means for rapid temperature change of the samemold are required to approximate this ideal mold-ing condition. Despite the advantages of the differ-ential mold temperature setup, an injection moldtypically presents a large thermal mass and is diffi-cult to heat and cool rapidlywithin the normal injec-tionmolding cycle. Furthermore, anymodifiedmoldshould possess similar mechanical performance asa standard mold to endure the harsh environmentin injection molding. Mold rapid heating and cool-ing is vastly different from other industrial heat-ing and cooling problems, and it presents a uniqueset of issues needing to be addressed for successfulapplications.Although most investigations on mold rapidheating and cooling appeared after 1990, someearlier endeavors6−8date back to the early 1960s.At that time, reciprocating screw injection moldingmachines started to replace ram injection moldingmachines and soon became widely adopted in theplastics industry. In particular, Johnson8revealedthat heated molds allowed flow-induced molecularorientations to relax once the mold cavity is filled.This very early study demonstrated the capabilityof improving the part quality but in a rather imprac-tical way since it involved time-consuming heatingand cooling of a large portion of the mold system.The number of studies on mold rapid heating andcooling has drastically increased in recent years,especially after the 1990s. The primary driver stemsfrom the growing need of precision parts, opticalparts, and partswithmicrofeatures in the electronicsand biomedical industries. Without an elevatedmold temperature during the filling stage, it is diffi-cult tomold a thin and long partwithout short shots,a precision part with minimal residual stress andthus acceptable warpage and dimensional stability,an optical part with a low level of birefringence, andamicrostructured partwith high aspect ratio surfacemicrofeatures. Different from the earlier work, mostof the more recent work focused on selectively heat-ing only the surface portion of the mold. This notonly enhances the heating efficiency but also reducesthe burden for cooling. Different terminologies formold rapid heating and cooling have been coined inthe literature; the authors intended to differentiatea particular technology from others. Examples arelow thermal inertiamolding,4,5variotherminjectionmolding,9−12momentary mold surface heatingprocess,13−17rapid thermal response molding,18−20rapid thermal processing,21and dynamic moldsurface temperature control22−25. A number ofinnovative approaches for rapidly heating only thesurface portion of the mold have been presented,including methods such as resistive heating of athin conductive layer,26−30convective heating usinghot fluid,31−34heating using condensing vapor,35induction heating,9−12,22−25,36−38high-frequencyproximity heating,39,40infrared heating,21,41−47heating and cooling using a volume-controlledvariable conductance heat pipe,48,49heating andcooling using thermoelectric Peltier modules,2,50passive heating by the incoming polymer,5,51−61microwave heating,62,63contact heating,64,65flameheating,13−17etc.Despite the large body of literature available,mold rapid heating and cooling does not representa well-developed area of expertise. Development ofcapable techniques for rapidly heating and coolingthemold surface portion is a difficult task because ofthe constraints set by the heat transfer process andthe endurance limits set by the thermal, mechanical,and sometimes electrical properties of the materialsused for the mold. At present, rapidly heating andcooling a large surface area mold remains a majorchallenge in the polymer molding industry. Themost successful area of applications for mold rapid heating and cooling is most likely in microinjectionmolding in which a relatively small mold size isinvolved. However, for a small mold, for example,with dimensions of a couple of centimeters long, acentimeter wide, and less than a centimeter thick,special techniques for mold rapid heating andcooling may not be needed. Such a small mold maybe heated and cooled just using conventional meth-ods (e.g., electrical cartridge heating) to achievethe required fast thermal response. The thermalperformance of these small molds can be furtherimproved by judicious engineering design of themold.66,67This article attempts to offer, to some extent, aconstructive overview on the state of the art inmoldrapid heating and cooling, aiming at explaining theworkingmechanisms and providing an unbiased ac-count of the pros and cons of existing processes andtechniques. Successful applications of existing pro-cesses are highlighted and potential improvementsto these processes are suggested. The article furtherintends to provide a fundamental understanding ofthe elementary building blocks needed in a work-able mold with a rapid heating and cooling capabil-ity. An increased understanding of these elementswill facilitate the development of more capable newprocesses.