Mold Surface Temperature Research Articles

Simulation of Shrinkage and Warpage of Rotationally Moulded Polymer Parts

The rotomoulding industry is impeded by long manufacturing cycle times, due to insufficient time required for the materials to achieve a stable state on cooling. Thermal residual stresses are induced in rotomoulded parts due to uncontrolled heat transfer, producing shrinkage and warpage in parts. There is little information about quantifying shrinkage and warpage due to a lack of available experimental data. The main aim of this work is to develop simulation model for shrinkage and warpage occurrence in rotomoulded polymer parts. In this work, the Abaqus FE modelling tool is used for obtaining the thermal coefficients data required in shrinkage model. The simulation results obtained by cooling of the molten polymer fuel tank part has provided information about heat transfer coefficient data. It is expected that once model is developed taking into account the effect of crystallization in polymers on shrinkage, it will be used to simulate shrinkage and warpage in actual rotomoulded parts. However, in this work only preliminary results are discussed. This simulation work provides a scientific understanding needed to achieve desired optimum mould surface temperature for improved cycle time, whilst minimizing shrinkage and warpage occurrence.

Minimizing Warpage on Injection molding through Response Surface Methodology (RSM)

Cosmetic defects can occur in injection molding. The aim of this study is avoidance of warpage on an injection moulding parts. The selected parts is an aesthetics part in automotive industry This study is conducted on moulded parts on the plating jigs, where warpage cannot be compensated through the fixing of parts on jigs entering plating baths where the temperature of the etching baths is around 65deg.Analysis of variance is used to determine how inputs affect outputs. The inputs used are mould surface temperature, raw material melting temperature, filling pressure and filling time. These parameters are varied to get an optimal process parameters setting to avoid the warpage of the parts by using response surface methodology.It is very difficult to understand why the warpage on the parts happens.There is lots of factors which can contribute for warpage and to identify them is a huge task and this can be due to numerous things and to sequester the role of each parameters is an uphill task. One of the parameters is the holding pressure which decides how much the part is packed which, in turn, is the cause for the stress in the part. This induced stress when gets relieved over a period of time creates warpage in the part. Warpage can also be dependent on the sequence of filling the part with various gates. For example, if the part is filled from the centre & the end gates are opened last, it may cause over packing of the part at the end & cause more warpage only at the ends locally. However, usually parts are filled in this fashion only to avoid warpage in the centre.

Numerical and experimental investigations of a conformally cooled maraging steel injection molding tool fabricated by direct metal printing

Costs and time are correlated greatly in the precision mold industries. To improve the productivity in series production, significant reduction in the cooling time is required. The profiled conformal cooling channel (PCCC) is a unique approach to reduce cooling time in the injection molding process compared with a conventional straight drilled cooling channel. In this study, a maraging steel injection molding tool with PCCC was manufactured by direct metal printing. Cooling performances of the injection molding tool with different methods were investigated via low-pressure wax injection molding. The part cooling time, part temperature difference, mold surface temperature difference, and product total deformation were modeled numerically using Moldex3D simulation software. It was found that the product cooling time for the injection molding tool with PCCC was reduced greatly. The product cooling time for the injection molding tool with PCCC, with gas cooling, and without cooling channels are 27 s, 94 s, and 543 s, respectively. The molding cycle time for the injection molding tool with PCCC about 93% can be saved compared with the injection molding tool without cooling channels. The molding cycle time of the injection molding tool with PCCC about 63.2% can be saved compared with the injection molding tool with gas cooling.

Design of Conformal Porous Structures for the Cooling System of an Injection Mold Fabricated by Additive Manufacturing Process

Abstract The cooling system of plastic injection mold plays a critical role during the injection molding process. It not only affects part quality but also its cycle time. Traditionally, due to the limitations of conventional drilling methods, the cooling system of the injection mold usually consists of simple paralleled straight channels. It seriously limits the mobility of cooling fluid, which leads to the low cooling efficiency for the parts with complex free-form surfaces. In this research, an innovative design method for the cooling system of an injection mold is proposed by using conformal porous structures. The size and shape of each cell in the conformal porous structure are varied according to the shape of an injection molded part. Design cases are provided at the end of this paper to further illustrate the efficiency of the proposed method. Compared with those existing design methods for the uniform porous structures, the proposed method can further reduce the nonuniformity of the mold surface temperature distribution and decrease the pressure drop of the cooling system.

Development of an Injection Mold for Liquid Silicone Rubber Using Rapid Tooling Technology

ABSTRACTRapid tooling (RT) technology can reduce the time to market for injection molded products compared to standard machining methods. Aluminum-filled epoxy resin molds can be used in place of conventional steel molds for small batch injection molding production. Liquid silicone rubber (LSR) parts are stable and used in many applications. To compress time to market for new injection molded products it is necessary to develop a mold swiftly and effectively. In this study, RT technology was used to make a mold with a heating element for LSR injection molding. The mold was fabricated from aluminum-filled epoxy resin and injection could be done after a mold warm-up time of 35 min – to stabilize mold surface temperature. The optimal process parameters were found to be: injection speed 50 mm/s, packing pressure 14 MPa, mold temperature 180°C, and packing time 12 s.

Hierarchical Structure of iPP During Injection Molding Process with Fast Mold Temperature Evolution.

Mold surface temperature strongly influences the molecular orientation and morphology developed in injection molded samples. In this work, an isotactic polypropylene was injected into a rectangular mold, in which the cavity surface temperature was properly modulated during the process by an electrical heating device. The induced thermo-mechanical histories strongly influenced the morphology developed in the injection molded parts. Polarized optical microscope and atomic force microscope were adopted for morphological investigations. The combination of flow field and cooling rate experienced by the polymer determined the hierarchical structure. Under strong flow fields and high temperatures, a tightly packed structure, called shish-kebab, aligned along the flow direction, was observed. Under weak flow fields, the formation of β-phase, as cylindrites form, was observed. The formation of each morphological structure was analyzed and discussed on the bases of the flow and temperature fields, experienced by the polymer during each stage of the injection molding process.

Design and analysis of composites manufacturing tooling for rapid heating and cooling

Simple analytical models for designing rapid heating (via electric resistance or fluid flow in conformal channels) and cooling (via fluid flow in conformal channels) capabilities for composites manufacturing tooling are presented and demonstrated. The electric resistance heating and conformal channel cooling models are implemented for design purposes through spreadsheets and experimentally validated for a test mold designed for the Specialized Elastomeric Tooling out-of-autoclave process. Considering the modeling assumptions made, the rapid heating model provides very reasonable estimates of time versus mold surface temperature behavior. However, the rapid cooling model over-predicts the cooling time, although sensitivity to flow conditions (laminar vs. turbulent) is quite evident. Both models allow a designer to quickly vary design parameters to determine how production performance (e.g., cycle time) is affected. Finally, the model for rapid heating via conformal channels is demonstrated for a test mold. The high flow rates of heating oil required in this example as predicted by the model demonstrate the compromises a designer must make in tooling development.

Temperature and pressure evolution in fast heat cycle injection molding

ABSTRACTAn accurate mold temperature control during injection molding processes allows obtaining objects with better surface finishing, more accurate surface replication, and reduced frozen-in orientation. The control of these properties is particularly important when the thickness of the molded part is very small as in the case of microinjection. In this work, thin multilayer heaters are adopted to obtain a very fast mold surface temperature evolution during the process of an isotactic polypropylene (iPP). The effect of mold temperature history on the pressure developed inside the cavity is analyzed and correlated to both gate solidification and mold deformation. Results confirmed that, with a fast control of the cavity surface temperature, a reduction of the injection pressure is achieved, without affecting the cycle time. The understanding of the phenomena that occur during the fast temperature evolution on the cavity surface can allow the microstructural calibration of the molded parts.

Study of the Effect of Mold Corner Shape on the Initial Solidification Behavior of Molten Steel Using Mold Simulator

The chamfered mold with a typical corner shape (angle between the chamfered face and hot face is 45 deg) was applied to the mold simulator study in this paper, and the results were compared with the previous results from a well-developed right-angle mold simulator system. The results suggested that the designed chamfered structure would increase the thermal resistance and weaken the two-dimensional heat transfer around the mold corner, causing the homogeneity of the mold surface temperatures and heat fluxes. In addition, the chamfered structure can decrease the fluctuation of the steel level and the liquid slag flow around the meniscus at mold corner. The cooling intensities at different longitudinal sections of shell are close to each other due to the similar time-average solidification factors, which are 2.392 mm/s1/2 (section A-A: chamfered center), 2.372 mm/s1/2 (section B-B: 135 deg corner), and 2.380 mm/s1/2 (section D-D: face), respectively. For the same oscillation mark (OM), the heights of OM roots at different positions (profile L1 (face), profile L2 (135 deg corner), and profile L3 (chamfered center)) are very close to each other. The average value of height difference (HD) between two OMs roots for L1 and L2 is 0.22 mm, and for L2 and L3 is 0.38 mm. Finally, with the help of metallographic examination, the shapes of different hooks were also discussed.

Improvement of heat transfer analytical models for thermoplastic injection molding and comparison with experiments

The aim of analytical models presented in this paper was to provide a first set of data for choosing thermal conditions during thermoplastic injection molding. Two different models allowed the quick determination of the cooling/solidifying time, the mold surface temperature variation and the heat flux densities exchanged between the polymer and the mold. The first model specific to amorphous polymers was based on the 1D heat conduction equation. The second model dedicated to semi-crystalline polymer was based on the adaptation of the solution of the Stefan problem for part with a finite thickness. For each case, the influence of the thermal contact resistance between polymer and the mold on the cooling/solidifying time was highlighted. Then, the description of heat transfer in the mold allowed to determine the time needed to produce reliable parts when the mold was in periodic steady state. The attenuation of the periodic temperature variation through the thickness of the mold was evaluated through the penetration depth. Parameters calculated from these analytical models were compared with experimental results obtained with an instrumented mold or with data computed with a coupled model. The good agreement between them validated the interest to these models to get quickly reliable characteristic parameters of injection molding.

Optimisation of mould surface temperature and bottle residence time in mould for the carbonated soft drink PET containers

Polyethylene terephthalate (PET) bottles, which are usually produced by injection stretch blow moulding (ISBM) are widely used for carbonated soft drinks (CSD) storage and transportation. Stretch rod movement, blow pressure, preform temperature profile, mould surface temperature and material properties are among the most important factors affecting the final product's quality in terms of the thickness distribution, burst pressure and top-load resistance of the bottles. However, the residence time of the blown bottle inside the mould is also an important factor affecting its final properties. Especially in PET bottle production for hot fillings, the residence time is a very important factor because the longer the residence time the better the crystalline structure of the PET. In this production, the lid section is desired to have a fully crystalline form so that it can withstand hot fluids. In this study, the aim was to optimise the mould surface temperature and the blown bottle's residence time inside the mould for 1 L soft drink PET bottle production based on the final properties using the ECHIP 7 design of experiment (DOE) program. The method employed through this program was a quadratic one. Optimum process parameters were determined by the response surface method (RSM) and the process settings ensuring maximum top-load, burst pressure, Tg and degree of crystallinity were regarded to be optimum. It was found that the optimum mould surface temperature and blown bottle residence time inside the mould were 10 °C and 20 s, respectively.

Determination of interface heat transfer coefficient between aluminum casting and graphite mold

To produce castings of titanium, nickel, copper, aluminum, and zinc alloys, graphite molds can be used, which makes it possible to provide a high cooling rate. No die coating and lubricant are required in this case. Computer simulation of casting into graphite molds requires knowledge of the thermal properties of the poured alloy and graphite. In addition, in order to attain adequate simulation results, a series of boundary conditions such as heat transfer coefficients should be determined. The most important ones are the interface heat transfer coefficient between the casting and the mold, the heat transfer coefficient between the mold parts, and the interface heat transfer coefficient into the environment. In this study, the interface heat transfer coefficient h between the cylindrical aluminum (99.99%) casting and the mold made of block graphite of the GMZ (low ash graphite) grade was determined. The mold was produced by milling using a CNC milling machine. The interface heat transfer coefficient was found by minimizing the error function reflecting the difference between the experimental and simulated temperatures in a mold and in a casting during pouring, solidification, and cooling of the casting. The dependences of the interface heat transfer coefficient between aluminum and graphite on the casting surface temperature and time passed from the beginning of pouring are obtained. It is established that, at temperatures of the metal surface contacting with a mold of 1000, 660, 619, and 190°C, the h is 1100, 4700, 700, and 100 W/(m2 K), respectively; i.e., when cooling the melt from 1000°C (pouring temperature) to 660°C (aluminum melting point), the h rises from 1100 to 4700 W/(m2 K), and after forming the metal solid skin on the mold surface and decreasing its temperature, the h decreases. In our opinion, a decrease in the interface heat transfer coefficient at casting surface temperatures lower than 660°C is associated with the air gap formation between the surfaces of the mold and the casting because of the linear shrinkage of the latter. The heat transfer coefficient between mold parts (graphite–graphite) is constant, being 1000 W/(m2 K). The heat transfer coefficient of graphite into air is 12 W/(m2 K) at a mold surface temperature up to 600°C.

Effect of Heating and Mold Temperature on the Mechanical Properties and Microstructure of B1500HS Boron Steel

Abstract To know the effect of the heating temperature and the temperature of mold surface on the mechanical properties and microstructure of B1500HS steel, and supply some practical guides for the components with the distributed microstructure and mechanical properties, B1500HS steel sheets, heated at different austenitization temperatures (870°C, 900°C, 930°C, and 960°C), were formed in hot stamping tools with the different mold temperatures (100°C, 200°C, 300°C, and 400°C). The strength of hot stamping parts was tested by the uniaxial tensile experiment. Moreover, the microstructure, fractography, and micro-hardness of hot stamping parts were measured to evaluate the effect of austenitization temperature and mold temperature on the microstructure and mechanical performance of UHSS steel. The results show that when the heating temperature is 900°C or above, the original microstructure of B1500HS steel can be fully austenitized. Variation of austenitization temperature in the range of 900°C ∼ 960°C has no obvious effect on the mechanical properties of hot stamping parts cooled at the same mold temperature. The mold temperature has a significant effect on the mechanical properties; the structural components with distributed properties can be manufactured by controlling the mold temperature. The austenitization and mold temperatures have no obvious effect on the elongation of parts except the part with the subcritical quenching. The austenitization temperature of 900°C and mold temperature of 100°C are the better parameters of hot stamping for the parts of B1500HS steel required with higher hardness and strength.

DETERMINATION OF HEAT TRANSFER COEFFICIENT BETWEEN ALUMINUM CASTING AND GRAPHITE MOLD

Graphite molds can be used to produce titanium, nickel, copper, aluminum and zinc castings. Using of the graphite molds provides a high cooling rate. Moreover no die coatings and lubricants are required. To get appropriate results of the casting process simulation in graphite molds it is necessary to know thermophysical properties of materials and boundary conditions such as interface heat transfer coefficients, but they are still unknown. The most important properties are heat transfer coefficient between casting and mold, and between mold parts and between mold and environment. The heat transfer coefficient h (iHTC – interface Heat Transfer Coefficient) was determined between cylindrical aluminium (99,99 % Al) casting and mold made of low-ash graphite. The mold was produced by milling graphite blocks on the CNC machine. The heat transfer coefficient was determined by minimizing the error function, representing the difference between the experimental and obtained by simulation temperature in the mold during pouring, solidification and cooling of the casting. The dependences of the iHTC between aluminium and graphite on the temperature of the casting surface and time elapsed from the start of pouring of the casting. Determined values of the heat transfer coefficient at metal temperatures 1000, 660, 619 and 190 °С are 1100, 4700, 700 and 100 W/(m2 ·К) respectively. Therefore, with decreasing of the melt temperature from 1000 °C (pouring temperature) to 660 °C (aluminium melting point), the heat transfer coefficient increases. After casting surface forming and lowering its temperature, the heat transfer coefficient decreases. Decrease of the iHTC at temperatures below 660 °C (lower the melting point) is associated primarily with the appearance of an air gap between the mold surface and casting surface as a result of linear shrinkage. The iHTC between the mold parts (graphite– graphite) is constant 1000 W/(m2 ·К). The heat transfer coefficient between graphite and the air environment is 12 W/(m2 ·К) at the mold surface temperature up to 600 °C.

Minimization of variation in volumetric shrinkage and deflection on injection molding of Bi-aspheric lens using numerical simulation

The profile of a bi-aspheric lens is such a way that the thickness narrows down from center to periphery (convex). Injection molding of these profiles has high shrinkage in localized areas, which results in internal voids or sink marks when the part gets cool down to room temperature. This paper deals with the influence of injection molding process parameters such as mold surface temperature, melt temperature, injection time, V/P Switch over by percentage volume filled, packing pressure, and packing duration on the volumetric shrinkage and deflection. The optimal molding parameters for minimum variation in volumetric shrinkage and deflection of bi-aspheric lens have been determined with the application of computer numerical simulation integrated with optimization. The real experimental work carried out with optimal molding parameters and found to have a shallow and steep surface profile accuracy of 0.14 and 1.57 mm, 21.38-45.66 and 12.28-26.90 μm, 41.56-157.33 and 41.56-157.33 nm towards Radii of curvatures (RoC), surface roughness (Ra) and waviness of the surface profiles (profile error Pt), respectively.

Heat transfer analysis at high cooling rate on the surface of thermoplastic parts

Heat transfer simulation in thermoplastics processing is crucial to predict the quality of parts in terms of geometry and mechanical properties. The results accuracy depends on the knowledge of thermo-physical properties, crystallization kinetics and boundary conditions, which have to be determined in representative process conditions. In this work, we focus on the impact of high cooling rate on heat transfer and crystallization at the surface of a thermoplastic part. For this purpose, a home-made device named “Lagardère apparatus” is presented. It can reproduce cooling rate encountered in injection moulding by the sudden contact between a cold surface and a molten polymer. The first objective of this paper is the direct measurement of the thermal contact resistance used to model the imperfect contact between the polymer and the mould. The mould surface temperature and the heat flux at the interface were provided by a heat flux sensor. The specificity of this methodology was to obtain the polymer surface temperature with an optical fibre associated with a photodetector. A particular attention was made to avoid the invasiveness of the sensors on heat transfer. The second objective of the paper is the crystallization study at the surface of the polypropylene part. The experimental heat flux was compared with computed ones for which crystallization kinetics comes from Fast Scanning Calorimetry measurement and a standard model commonly used in the literature. It seemed that a very fast crystallization occurred at the surface of the part with a lower released enthalpy compared to the bulk.

Parameter optimization of cooling system in U-shape hot stamping mold for high strength steel sheet based on MOPSO

This work aims to simultaneously optimize the cooling efficiency, cooling uniformity, and mold service life of the cooling system in U-shape hot stamping mold. And three indicators of the average temperature (Tave) of the hot stamping mold surface, the standard deviation of the temperature (σT) of mold surface, and the maximum equivalent stress (\( {\overline{\sigma}}_{\max } \)) of mold were adopted to represent the cooling efficiency, cooling uniformity, and mold service life, respectively. Such three indicators are greatly influenced and even can be adjusted by the mold structure parameters, such as the diameter of cooling channels (D), the number of cooling channels, the center distance of adjacent cooling channels (L), and the distance from the center of cooling channel to the mold surface (H). Based on the Box-Behnken design method and finite element simulation results simulated by the MSC. Marc software, the mold structure parameters D, L, and H were selected as the three independent variables, and the three response surface models of Tave, σT, and \( {\overline{\sigma}}_{\max } \) were, respectively, established. The applicability and reliability of the response surface models were checked by analysis of variance. And then the mold structure parameters (D, L, and H) were optimized by a multi-objective particle swarm optimization to simultaneously minimize the indicators of Tave, σT, and \( {\overline{\sigma}}_{\max } \). After optimization, the instantaneous cooling rates of steel sheet are higher than the critical cooling rate of martensite transformation before MS, which indicate that full martensite can be obtained in the steel sheet. The temperature uniformity of mold surface was improved, and a smaller \( {\overline{\sigma}}_{\max } \) of mold was obtained. Finally, a series of hot stamping experiments according to the optimizing scheme were conducted by a four-column hydraulic press. The experimental results verified the evolution of the mold temperature field simulated by finite element software and the reliability of the improved U-shape hot stamping mold.

Development of a rapid surface temperature variation system and application to micro-injection molding

In conventional injection molding, the mold temperature control is obtained by a continuous cooling method, in which a coolant with constant temperature is circulated in the cooling channels to cool the mold and the polymer melt. During the filling stage, this causes an abrupt polymer solidification close to the mold surface, which reduces the section open to flow and, due to the viscosity increase, causes a decrease of the ability of the polymer melt to fill the cavity. This issue is particularly significant for micro-injected parts in which high aspect ratios are precluded because of premature solidification. In this work, a system for rapid surface temperature control was designed, built and applied to a cavity for micro-injection molding. The system consists in an electrical resistive thin component and an insulation layer and can increase the mold surface temperature of some tenths of a degree Celsius in a time of the order of one second. The system is versatile enough to allow the control of thermal histories during the whole process and at different positions inside the cavity. Injection molding tests were then carried out with this system by using a general purpose isotactic polypropylene and a cavity 200mm thick in order to check the effect of surface heating on reachable flow length and morphology of the molded parts. The effect of mold temperature on the flow length was as expected dramatic on both the flow length and the obtained morphology: the samples molded with a high mold temperature presented a spherulitic morphology in the whole cross-section, while with a low surface temperature the spherulitic morphology was detectable only at the positions close to the midplane whereas the layer closer to the surface present a very oriented structure due to flow taking place at low temperature.

Investigation of the influence of vacuum venting on mould surface temperature in micro injection moulding

The application of vacuum venting for the removal of air from mould cavity has been introduced in injection moulding with the intent to enhance micro/nano features replication and definition. The technique is adopted to remove air pockets trapped in the micro-features, which are out of reach for conventional venting technologies and can create considerable resistance to the melt filling flow. Nonetheless, several studies have revealed a negative effect on replication that could possibly arise from the application of vacuum venting. Although the incomplete filling of micro-scale features has often been attributed to poor venting, the limited research examining the application of vacuum venting has produced mixed results. In this work, the effect of air evacuation was experimentally investigated, monitoring mould and polymer temperature evolution during the micro injection moulding process by means of a high-speed infrared camera and a sapphire window, which forms part of the mould wall. The results show that air evacuation removes a mould surface heating effect caused by rapid compression of the air ahead of the flow front and subsequent conduction of that heat into the mould surface. Hence, with the increase of the surface-to-volume ratio in micro-cavities, air evacuation has a detrimental effect on the cavity filling with polymers that are sensitive to changes of the mould temperature.

Analysis of asymmetric morphology evolutions in iPP molded samples induced by uneven temperature field

Mold surface temperature has a strong effect on the amount of molecular orientation and morphology developed in a non‐isothermal flowing polymer melt. In this work, a well‐characterized isotactic polypropylene was injected in a rectangular mold cavity asymmetrically conditioned by a thin electric heater specifically designed. The cavity surface was heated at temperatures ranging from 80 to 160°C for different times (0.5, 8, and 18 s) after the first contact with the polymer. Asymmetrical thermal conditions have a strong influence on the melt flow, by changing its distribution along the cavity thickness, and final part deformation. The morphology distribution of the molded samples was found strongly asymmetric with complex and peculiar features. Optical and Electron microscopy confirmed the complete reorganization of the crystalline structures along the sample thickness. X‐rays analysis reveals that molecular orientation of the sample surface decreases with the mold temperature and the heating time. © 2016 American Institute of Chemical Engineers AIChE J, 62: 2699–2712, 2016