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Fewinvestigations have examined the effect of the asymmetric cooling sys-temtemperature on products [3–5,13–15]. This study, through analysisand experiments, observes the effect of the filmheat retardation duringthe IMR process on product warpage and the temperature variation ofthe contact surface between the hot melt and mold, and performs anasymmetric cooling system temperature design for the core and cavity.The impact on product crystallinity, tensile strength, and product sur-face roughness is also investigated, given that supporting industrialneeds is the motivation and purpose of this research.2. Experimental work2.1. MaterialsIn this study, the plastic material is polypropylene (Globalene 7533,Taiwan Polypropylene Co., Ltd) with a density of 897 kg/m3, a heat- deformation temperature of 93 °C, a thermal conductivity of 0.255W/mK,and a specific heat of 3100 J/kgK. The material of the plastic film is PC(Bayer DE1-1) with a thickness of 0.175 mm, a thermal conductivity of0.197 W/mK, a specific heat of 2093 J/kgK, and a density of 1200 kg/m3.The mold was made of P20 steel with a density of 7750 kg/m3,athermalconductivity of 36.5 W/mK, and a specific heat of 462 J/kgK.2.2. In-mold roller injection molding technique (IMR)There are two parts to this research. In the first part, the influence ofthe IMR process on product warpage is investigated. Themold used is a100mm× 100 mm× 1.2mmsquare plate, as shown in Fig. 1(a). In thecentral surface of the core insert and cavity insert, a temperature sensor(Type 4003B from PRIAMUS) with a diameter of 1 mm is embedded.There are two stages of the injection process. Each stage has its own pa-rameters, but thematerial temperatures are both fixed at 230 °C. In thefirst stage, a symmetric cooling systemdesign is adopted,with the tem-perature of the core and cavity both set at 50 °C to investigate the effectof the film on the product warpage. In the second stage, an asymmetriccooling system design is adopted: the temperature of the cavity is fixedat 50 °C, and the temperature of the core is changed to 65 °C to investi-gate the impact of this asymmetric cooling system design on theproductwarpage during the IMR process (Fig. 2).
The PC filmis attachedto the cavity. The measuring points of product warpage are shown inFig. 1(a). In the second part of this research, the surface crystallization,roughness, and tensile strength of the products manufactured viathe IMR process of each stage described in the first part are inspected.In order to perform the tensile strength test, the mold is cut into115 mm × 19 mm × 1.2 mm dumbbell-shaped specimens, shown inFig. 1(b). In this research a mold temperature controller (BYCW-021410FS) and an injection molding machine (KM50-160C2) withmaximum injection rate of 63 cm3/s and maximum injection pressureof 2500 bar are used.2.3. Measuring method2.3.1. Measurement of temperature fieldTo measuring the temperature, 1-mm-diameter temperature sen-sors are embedded in the central surface of the core insert and cavity in-sert, and the mold temperature controller is adjusted to allow theinjection to be executed only when the temperatures of the core andcavity reach their own settings. During the injection process, the tem-peratures of the two contact surfaces are measured at one location between the film and the mold, and another between the hot melt andmold. The measuring points and their designations are shown in Fig. 3.2.3.2. Measurement of warpage deformationThe measurement of product warpage is conducted by a 2-dimensional optical meter. The central point of the product is fixed,and then, through image projection, the warpage from the centralpoint to its two sides can be measured, as shown in Fig. 1(a). Whenthe injection conditions become stable, for each condition, ten productsare injected as specimens, their warpage is measured, and then fiveproximate data are elicited to calculate the mean value.2.3.3. Measurement with grazing incident X-ray diffractometer (XRD)Crystalline phase analysis for products is performed by scanningspecimens with a grazing incident X-ray diffractometer. Based on themeasured X-ray diffraction peaks (XRD peaks), the crystallite size and crystallinity of the surface of the products injected under conditionswith/without film attached are then calculated. The x-ray source isCuKαwith awave length of 1.5416 Å, an incident angle (θ) of 1°, a scan-ning pitch of 0.02°, and a scanning scope (2θ)of5°–50°. Furthermore,the crystallinity and the crystallite size are calculated by the followingformula [16,17].Crystallinity ¼ AcAa þ Acð1Þwhere Ac is the peak area for crystallized surface, and Aa is the peak areafor non-crystallized surface.t ¼ 0:9λB cos θð2Þwhere t is the average crystallite size; λ is wavelength of CuKα(1.5416 Å); B is the broadening width at half the maximum reflectionintensity; and θ is the degree of maximum reflection. 2.3.4. Tensile testThe test standard is ASTM-D638,where the clamping length of sam-ple gauge is 25 mm, and the rate of the clamping head is 50 mm/min.The maximum tensile strength can be obtained from the test results.2.3.5. SEM testA Topcon Sm-300 scanning electronmicroscope (SEM) is used to ob-serve the microstructure of the surface of products injected under con-ditions with/without film attached.2.3.6. Surface roughness testA 3-dimensional lasermicroscope (VK850, KEYENCE Corp., Japan) isused tomeasure the surface roughness of products injected under con-ditions with/without film attached.2.4. Experimental analysisIn this research, the productwarpage problemcaused by the heat re-tardation effect of filmduring the IMR process is investigated. Using thenumerical simulation and analysis performed on ANSYS software, ap-propriatemodels are built via computer simulation to observe the influ-ence of the film heat retardation effect on the temperature distributionof the mold's contact surface and its impact on product warpage. Thecomputer is used to simulate the influence of the asymmetric tempera-ture design of the cooling system on product warpage. The moldingconditions set for simulation are the same as those for the experiments.3. Results and discussionsFig. 4 shows the simulation temperature profiles for Tp-f,Tp-m,andTf-m when the melt temperature is 230 °C and mold temperature is50 °C for PC films with thickness of 0.175 mm. The temperature of thecontact surface between the mold and the film increases because ofthe plastic film attached to the cavity surface during the IMR process.The temperature difference between the core and cavity is larger be-cause the heat-conduction coefficient of plastic is lower than that ofthe steel (P20), causing higher interface thermal impedance, and conse-quently creating a greater heat retardation effect. In addition, the heat inthe hot melt contacting the mold can easily be conducted out becausethe contact interface has a lower thermal impedance resulting fromthe better heat-conduction coefficient of the steelmold (P20). The tem-perature distribution diagramof the contact surface shows that the IMRinjection molding process can easily cause product warpage problemsbecause the film it uses induces heat retardation and makes the coretemperature differ from the cavity temperature.Fig. 5 shows the simulated and measured temperature profiles forTp-m,andTf-m at various times. The results of both the experiment andsimulation exhibit a similar trend. Fig. 6 shows the simulated andmea-sured warpage for the simulated and measured IMR process (withinserted film) and conventional injection molding (without film). The film used in the IMR process can cause heat retardation on the productsurface, inducing an asymmetric temperature distribution of the moldin the direction of varying product thickness, causing different shrink-age levels and consequently leading to greater product warpage.Through the time-history analysis of the mold temperature field, acooling system with asymmetric temperature function is designed, inwhich the cavity mold temperature is fixed at 50 °C, and the coremold temperature is set at 65 °C. Then a simulation and analysis ofboth the original process (symmetric temperature condition) and theprocess with the asymmetric temperature conditions just describedare performed. Fig. 7 gives the simulation temperature profiles for Tp-fand Tp-m for the asymmetric cavity and core cooling system design. Itshows that the asymmetric cooling system design can reduce the tem-perature difference between the core and the cavity. Fig. 8 presentsthe simulated and measured warpage for the asymmetric cooling sys-tem design and same mold temperature setting (cavity and core). Itshows the asymmetric design can effectively reduce product warpage.The results of the simulation show a reduction of 63%, while the exper-imental results yield a reduction of 53%. The results of both the experi-ment and simulation demonstrate a similar trend.Fig. 9