Mold Insert Research Articles

Efficient cooling of hot spots in injection molding. A biomimetic cooling channel versus a heat‐conductive mold material and a heat conductive plastics

This study aimed at optimizing the injection molding process of an automotive oil filter housing made from PA6.6. Mass accumulations in its design intensely increased the cooling time. In a first successful approach, a copper alloy mold insert (λ = 106 Wm−1 K−1) that contains two externally cooled heat‐conducting copper pins (λ = 310 Wm−1 K−1) was installed. We hypothesized that a biomimetic cooling channel structure in a steel mold insert would even perform superior. Using simulation software Sigmasoft® v5.0, the mold insert materials (steel Bohler X20Cr13, Ampcoloy® 83), the plastics grade PA66‐GF35 (λ = 0.27 Wm−1 K−1 or λ = 0.40 Wm−1 K−1), and three cooling designs were evaluated for their impact on cooling the hot spots in the part: the copper pin‐system, a conformal cooling channel, and blood‐vessel like channels. In the latter, the major artery branches into two sub‐arteries, which further divide into two capillary tubes each. The capillaries merge into two sub‐veins and those fuse in the major vein again. As expected, the plastics heat conductivity dominates the cooling. The biomimetic (blood‐vessel) channels, as hypothesized, cooled the major hot spot more efficient than the conformal channel and the copper pin system do. In detail, compared to the heat‐conductive insert, the cycle time may be reduced by 10 s, in spite of the lower heat conductivity λ (23.5 Wm−1 K−1) of the steel insert. Using the biomimetic (blood‐vessel) structure in a heat‐conductive mold insert would reduce the cycle time by a further second. However, raising the heat conductivity of the plastics would save another 15s. Experimental tests proved the cooling efficiency. Nevertheless, cooling channels only perform well, if they are close enough to the mass accumulation. In conclusion, transferring biologic cooling structures to injection molding seems to be promising. Ongoing advances in Additive Manufacturing will help to efficiently implement these structures into mold inserts for injection molding. POLYM. ENG. SCI., 59:E180–E188, 2019. © 2018 The Authors. Polymer Engineering & Science published by Wiley Periodicals, Inc. on behalf of Society of Plastics Engineers.

Fabrication of micro piezoelectric rod array using metallic mold system for mass production

LIGA process was carried out to fabricate the copper mold insert for a micro piezoelectric array. X-ray mask was fabricated using UV lithography and PMMA which is the X-ray photoresist was bonded to the titanium substrate done lapping. The bonded PMMA was exposed to X-ray exposure with the total amount of 4.0 kJ/cm3. Copper was then electroplated above the PMMA. After the post-processing, the copper mold insert with the pattern size 400 μm, the height 924 μm, was fabricated. To replicate the patterned micro rod array, PIM process was used. The entire optimized PIM process corresponding to the previous research was applied. The fabricated copper mold insert was used in injection molding process. The ejection pins were also fabricated using the same LIGA process for metal mold and used to extract the injection-molded specimens. After injection molding, two-step debinding and sintering processes were conducted and finally, micro piezoelectric rod array with a pattern size of 322 μm was fabricated without any defects such as collapse and break. The dimension accuracy was ensured through the SEM image analysis and dilatometry results. The physical and piezoelectric properties of sintered part were measured to confirm the part performance made by PIM process. Results indicated the possibility of using the metal mold for mass production system to produce the micro piezoelectric rod array.

Nanostructured Free-Form Objects via a Synergy of 3D Printing and Thermal Nanoimprinting.

High‐resolution surface patterning has garnered interests as a nonchemical‐based surface engineering approach for creating functional surfaces. Applications in consumer products, parts for transportation vehicles, optics, and biomedical technologies demand topographic patterning on 3D net shape objects. Through a hybrid approach, high‐resolution surface texture is incorporated onto 3D‐printed polymers via direct thermal nanoimprinting process. The synergy of geometry design freedom in 3D printing and the high spatial resolution in nanoimprinting is demonstrated to be a versatile fabrication of high‐fidelity surface pattern (from 2 µm to 200 nm resolution) on convex, concave semicylindrical, and hemispherical objects spanning a range of surface curvatures. The novel hybrid fabrication is further extended to achieve a high‐resolution curved mold insert for rapid prototyping via injection molding. The versatility of the fabrication strategies reported here not only provides a post‐3D printing process that enhances the surface properties of 3D‐printed objects but also opens a new pathway to enable future study on the effects of combining microscale and nanoscale surface texture with macroscopic curvature. Both have been known, individually, as an effective approach to tune surface functionalities.

Conformal cooling with heat-conducting inserts by direct metal laser sintering

With the development of layer manufacturing technologies injection mold inserts with conformal cooling channels can be manufactured. If the cooling channels can be placed along the geometry, the heat removal is uniform and effective. In tight mold regions, formation of cooling channel is not possible or not efficient. The combination of conformal cooling and heat conductive insert can be an ideal solution for the effective cooling.

Effect of Hybrid Carbon Fillers on the Electrical and Morphological Properties of Polystyrene Nanocomposites in Microinjection Molding

The effect of hybrid carbon fillers of multi-walled carbon nanotubes (CNT) and carbon black (CB) on the electrical and morphological properties of polystyrene (PS) nanocomposites were systematically investigated in microinjection molding (μIM). The polymer nanocomposites with three different filler concentrations (i.e., 3, 5 and 10 wt %) at various weight ratios of CNT/CB (100/0, 30/70, 50/50, 70/30, 0/100) were prepared by melt blending, then followed by μIM under a defined set of processing conditions. A rectangular mold insert which has three consecutive zones with decreasing thickness along the flow direction was adopted to study abrupt changes in mold geometry on the properties of resultant microparts. The distribution of carbon fillers within microparts was observed by scanning electron microscopy, which was correlated with electrical conductivity measurements. Results indicated that there is a flow-induced orientation of incorporated carbon fillers and this orientation increased with increasing shearing effect along the flow direction. High structure CB is found to be more effective than CNT in terms of enhancing the electrical conductivity, which was attributed to the good dispersion of CB in PS and their ability to form conductive networks via self-assembly. Morphology observations indicated that there is a shear-induced depletion of CB particles in the shear layer, which is due to the marked difference of shear rates between the shear and core layers of the molded microparts. Moreover, an annealing treatment is beneficial to enhance the electrical conductivity of CNT-containing microparts.

Value chain and production cost optimization by integrating additive manufacturing in injection molding process chain

Additive manufacturing (AM) can create considerable value when integrated into conventional manufacturing process chains. Tooling for new molded product development as pilot case of integration of AM in the injection molding process chain is investigated. The study shows that injection molding based on AM for fabrication of polymer tool inserts is economically advantageous allowing 80–90% production costs reduction as compared with a conventional tooling process chain based on machining. Fabrication of soft tools with AM results in a production lead time reduction in the range of 60–70% compared to the time required to machine mold inserts in brass or aluminum.

Microfabrication of Nonplanar Polymeric Microfluidics.

For four decades, microfluidics technology has been used in exciting, state-of-the-art applications. This paper reports on a novel fabrication approach in which micromachining is used to create nonplanar, three-dimensional microfluidic chips for experiments. Several parameters of micromachining were examined to enhance the smoothness and definition of surface contours in the nonplanar poly(methyl methacrylate) (PMMA) mold inserts. A nonplanar PMMA/PMMA chip and a nonplanar polydimethylsiloxane (PDMS)/PMMA chip were fabricated to demonstrate the efficacy of the proposed approach. In the first case, a S-shape microchannel was fabricated on the nonplanar PMMA substrate and sealed with another nonplanar PMMA via solvent bonding. In the second case, a PDMS membrane was casted from two nonplanar PMMA substrates and bonded on hemispherical PMMA substrate via solvent bonding for use as a microlens array (MLAs). These examples demonstrate the effectiveness of micromachining in the fabrication of nonplanar microfluidic chips directly on a polymeric substrate, as well as in the manufacture of nonplanar mold inserts for use in creating PDMS/PMMA microfluidic chips. This technique facilitates the creation of nonplanar microfluidic chips for applications requiring a three-dimensional space for in vitro characterization.

Flexible micro manufacturing platform for the fabrication of PMMA microfluidic devices

In this work we present a micro manufacturing platform for the production of polymeric microfluidic devices on a mass scale, based on the integration of microinjection moulding and femtosecond laser (fs-laser) micromachining technologies. A mould prototype was designed for the fabrication of polymeric thin plates characterized by simplified microfeatures representative of typical Lab-on-a-Chip (LoC) devices. The injection moulding master tool includes replaceable metallic inserts, which were fabricated by exploiting the extreme flexibility and accuracy of fs-laser milling. Here, the laser process parameters have been studied and properly adjusted to meet the target geometry and surface quality of the mould inserts, which were subsequently characterized by confocal and SEM microscopy. The micro injection moulding (μIM) process parameters for the device production have been defined by complete three-dimensional filling and packing process simulations. Finally, the micro-injection mould with reconfigurable inserts was employed for the production of thin plates with simplified microfeatures using PMMA. The ability to reproduce these microfeatures via μIM is an essential step to approach to the mass-production of a polymeric LoC and the use of replaceable micro-inserts fabricated by direct fs-laser ablation promises high flexibility in the design and manufacturing of such devices.

Disposable Optical Stretcher Fabricated by Microinjection Moulding.

Microinjection moulding combined with the use of removable inserts is one of the most promising manufacturing processes for microfluidic devices, such as lab-on-chip, that have the potential to revolutionize the healthcare and diagnosis systems. In this work, we have designed, fabricated and tested a compact and disposable plastic optical stretcher. To produce the mould inserts, two micro manufacturing technologies have been used. Micro electro discharge machining (µEDM) was used to reproduce the inverse of the capillary tube connection characterized by elevated aspect ratio. The high accuracy of femtosecond laser micromachining (FLM) was exploited to manufacture the insert with perfectly aligned microfluidic channels and fibre slots, facilitating the final composition of the optical manipulation device. The optical stretcher operation was tested using microbeads and red blood cells solutions. The prototype presented in this work demonstrates the feasibility of this approach, which should guarantee real mass production of ready-to-use lab-on-chip devices.

Injection Compression Molded Microlens Arrays for Hyperspectral Imaging.

In this work, a polymer microlens array (MLA) for a hyperspectral imaging (HSI) system is produced by means of ultraprecision milling (UP-milling) and injection compression molding. Due to the large number of over 12,000 microlenses on less than 2 cm², the fabrication process is challenging and requires full process control. The study evaluates the process chain and optimizes the single process steps to achieve high quality polymer MLAs. Furthermore, design elements like mounting features are included to facilitate the integration into the final HSI system. The mold insert was produced using ultraprecision milling with a diamond cutting tool. The machining time was optimized to avoid temperature drifts and enable high accuracy. Therefore, single immersions of the diamond tool at a defined angle was used to fabricate each microlens. The MLAs were replicated using injection compression molding. For this process, an injection compression molding tool with moveable frame plate was designed and fabricated. The structured mold insert was used to generate the compression movement, resulting in a homogeneous pressure distribution. The characterization of the MLAs showed high form accuracy of the microlenses and the mounting features. The functionality of the molded optical part could be demonstrated in an HIS system by focusing light spectrums onto a CCD image sensor.

Enhancing Thermal Simulations for Prototype Molds

Our goal was the thermal analysis of epoxy acrylate-based prototype molds with numerical simulations, and to compare and analyze the measured values and calculated results. The difference between the thermal calculations and the measured values is significant; the actual temperature of the mold is higher than the calculated values. Based on the numerical simulations, we found that in the case of epoxy acrylate-based mold inserts, temperature results can be made significantly more accurate by changing the heat transfer coefficient between the surface of the mold insert and the melt. We proved that in the case of small-series epoxy acrylate-based molds, the temperature dependence of the thermal properties of the mold material, and the temperature and pressure dependence of the heat transfer coefficient need to be taken into account for accurate temperature results. We proved that the heat transfer coefficient between the mold surface and the melt is considerably lower than in the case of metal molds, due to lower cavity pressure and a lower temperature difference between the mold surface and the melt.

Comparisons on microstructure, mechanical and corrosion resistant property of S136 mold steel processed by selective laser melting from two pre-alloy powders with trace element differences

Selective laser melting (SLM) method has a great potential for fabricating injection mold with complex structure. In this study, two S136 mold steel pre-alloy powders with trace element difference were fabricated by SLM, and then comparison of samples on microstructure, mechanical and corrosion resistant property was investigated. Results showed that the sample fabricated with a higher Si+Mn content exhibited the lower mechanical properties, such as micro-hardness of 48.73 HRC, ultimate tensile strength of 1186.7 MPa and elongation of 10.6% due to a few of pores observed on the cross surface. For the SLM sample produced by another powder with a lower Si+Mn content, it showed equiaxed grains and cellular dendrite grains coexisted and exhibited the superior mechanical properties (50.31 HRC, 1467.9 MPa and 11.1%) combined with well corrosion resistance. Then the injection mold insert with conformal cooling channel was successfully processed by this powder. The cooling time was reduced by 30% during the injection cycle. Therefore, it can be seen that trace element differences have an important effect on the SLM S136 mold steel. The results suggest that it is essential for successful fabricating components with a suitable composition of alloy during SLM process.

Powder injection molding process for fabrication of piezoelectric 2D array ultrasound transducer

Powder injection molding process has been developed for a cost-effective fabrication of micro PZT array in diagnostic 2D array ultrasound transducer. The developed process consists six main steps; mold insert fabrication, mixing of powder and binder, injection molding, demolding of sacrificial mold insert, solvent and thermal debinding, densification of PZT ceramics. 1–3 type PZT/polymer composite fabricated by developed process shows 0.67 of electromechanical coupling property in plain mode and 0.56 in rod array mode which are 99% and 96% of reference property. Based on high productivity and shape complexity of ceramic injection molding, micro-manufacturing process for micro-scale PZT transducer has been demonstrated with according piezoelectric performance characterization.

Electrical Resistivity of Wire Arc Sprayed Zn and Cu Coatings for In-Mold-Metal-Spraying

Electrical functionalities can be integrated into plastic parts by integrating thermally sprayed metal coatings into the non-conductive base material. Thermally sprayed conducting tracks for power and signal transmission are one example. In this case, the electrical resistance or resistivity of the coatings should be investigated. Therefore, the electrical resistivity of wire arc sprayed Zn and Cu coatings has been investigated. In case of Zn coatings, spray distance, gas pressure and wire diameter could be identified as significant influencing parameters on the electrical resistivity. In contrast, process gas, gas pressure and voltage do have a significant influence on the electrical resistivity of Cu coatings. Through the use of the In-Mold-Metal-Spraying method (IMMS), thermal degradation can be avoided by transferring thermally sprayed coating from a mold insert onto the plastic part. Therefore, the influence of the transfer process on the electrical resistance of the coatings has also been investigated.

Investigation of index change in compression molding of As40Se50S10 chalcogenide glass.

Chalcogenide glasses are emerging as alternative materials for low-cost and high-volume glass molding processes for infrared optics. In precision glass molding, it is well documented that the refractive index variation in the molded elements can lead to substantial amounts of aberrations. The variation has such a significant effect that the optical designs with molded lenses need to be carefully considered and compensated for index variation to achieve targeted optical performance. This research is aimed to evaluate the refractive index change of a chalcogenide glass during the molding process by both finite element method-based simulation and optical experiment. First, a set of mold inserts was designed and machined by high-speed single-point diamond milling. The structure of the lower mold insert was semiclosed and detachable, which facilitated the molded infrared prisms' release from the mold. Second, finite element method simulation was implemented to predict the refractive index change during the cooling phase by using the Tool-Narayanaswamy-Moynihan model for structural relaxation behavior. It was confirmed that refractive index variation occurred inside the molded wedge due to rapid thermal cycling. However, the amount of variation in the molded element indicates that the refractive index change during the molding process was not uniform. Finally, the refractive index of the molded wedge was measured by an optical setup. The results showed that the index shift is approximately -0.0226 for As40Se50S10, which matched the numerical result by simulation. Compared with oxide glass materials, the index drop of As40Se50S10 has a significant effect on optical performance of molded optics, and the postmolding refractive index should be taken into account in the optical design. In summary, the results presented in this article provided reliable references for refractive index change of As40Se50S10 glass, crucial for precision glass molding or similar applications.

Fabrication of microfluidic chip mold based on through-mask electrochemical etching and micro electroforming process

During the fabrication of nickel microfluidic chip mold insert, the problem of poor interfacial adhesion strength between electroformed layer and substrate often appeared which seriously reduced the service lifetime of the micro mold. In this paper, a novel pretreatment method of pickling and through-mask electrochemical etching was proposed to improve the interfacial adhesion strength of nickel microfluidic chip molds. Micro beams were used for exploring the effect of different pretreatment parameters on improving the adhesion strength. The adhesion strength was measured through a micro shearing test. The test results show that the mean interfacial adhesion strength without pretreatment is 52.1 MPa, and the interfacial adhesion strengths of other three groups with different pretreatment parameters are 103.4 MPa, 158.2 MPa and 229.7 MPa, respectively. The result indicated that pickling and electrochemical etching can effectively improve the adhesion strength between electroformed layer and substrate. Besides, in order to find the most suitable fabrication parameters for nickel mold insert, lateral corrosion, etching depth and etching surface area were also analyzed. The measured results show that the adhesion strength almost increases linearly with the increase of etching surface area in a scope of 1.50 mm2 to 2.07 mm2. Overall consideration, when a double cross-shaped microfluidic chip mold insert was fabricated by pretreatment of pickling for 20 s and electrochemical etching for 5 min, the micro mold insert has higher interfacial adhesion strength and the substrate has a moderate lateral corrosion, which contributes to extending the service life of microfluidic chip mold.

Fabrication of an infrared Shack–Hartmann sensor by combining high-speed single-point diamond milling and precision compression molding processes

A novel fabrication method by combining high-speed single-point diamond milling and precision compression molding processes for fabrication of discontinuous freeform microlens arrays was proposed. Compared with slow tool servo diamond broaching, high-speed single-point diamond milling was selected for its flexibility in the fabrication of true 3D optical surfaces with discontinuous features. The advantage of single-point diamond milling is that the surface features can be constructed sequentially by spacing the axes of a virtual spindle at arbitrary positions based on the combination of rotational and translational motions of both the high-speed spindle and linear slides. By employing this method, each micro-lenslet was regarded as a microstructure cell by passing the axis of the virtual spindle through the vertex of each cell. An optimization arithmetic based on minimum-area fabrication was introduced to the machining process to further increase the machining efficiency. After the mold insert was machined, it was employed to replicate the microlens array onto chalcogenide glass. In the ensuing optical measurement, the self-built Shack-Hartmann wavefront sensor was proven to be accurate in detecting an infrared wavefront by both experiments and numerical simulation. The combined results showed that precision compression molding of chalcogenide glasses could be an economic and precision optical fabrication technology for high-volume production of infrared optics.

On the application of micro hot embossing for mass fabrication of template-based dielectric resonator antenna arrays

In this article, application of micro-replication of polymers in the field of next generation mm-wave antennas is presented for the first time. The concept is applied to recently proposed template-based dielectric resonator antenna arrays fabricated by deep X-ray lithography. In this work, the array templates are replicated by hot embossing of polymethyl methacrylate (PMMA) layers using a nickel mold insert fabricated from the direct X-ray exposed PMMA master (LIGA process). Transfer quality of the master pattern is examined through measurements by optical microscope. Under controlled conditions, a number of templates are replicated demonstrating repeatable quality and consistent performance among the devices. A four element dielectric resonator antenna array is fabricated and measured for the impedance bandwidth response. This proposed methodology has the potential to enable low cost and mass utilization of dielectric resonator antennas and arrays in future millimeter-wave applications.

Experimental analysis for fabrication of high-aspect-ratio piezoelectric ceramic structure by micro-powder injection molding process

Aspect ratio effects in the micro-powder injection molding process were experimentally analyzed for fabrication of high-aspect-ratio piezoelectric ceramic structure. The mechanisms of critical defects have been studied according to individual manufacturing steps. In the molding process, incomplete filling phenomenon determines the critical aspect ratios of a micro pattern. According to mold temperature, an incomplete filling phenomenon has been analyzed with respect to different pattern sizes and aspect ratio. In demolding and drying process, the capillary behavior of sacrificial polymeric mold insert determines the critical aspect ratio of a micro pattern. With respect to pattern dimensions, slumping behavior has been analyzed. Based on our current systems, micro PZT feature has stability when it has lower aspect ratio than 5. Under optimized processing conditions, 20 μm and 40 μm ceramic rod array feature which has 5 of aspect ratio were successfully fabricated by the developed process. Further modification points to fabricate the smaller and higher feature were specifically addressed.

Molecular dynamics simulation and experimental investigation of the geometrical morphology development of injection-molded nanopillars on polymethylmethacrylate surface

Injection molding is one of the most promising techniques to fabricate polymeric parts with functional surface nanostructures in large numbers. The replication fidelity of nanostructures directly affects their efficiency. In this study, molecular dynamics (MD) simulation and experimental trial for injection molding nanopillars on polymethylmethacrylate (PMMA) surface were constructed in both injection and packing phases. The filling process was investigated by analyzing the snapshots, densities and gyration radiuses. Influences of packing parameters on geometrical morphology were studied. Results showed that the central area of nanopillars was almost voided during the injection phase and polymer melt could barely flow into nano-cavity. Only the edge area was filled in injection phase and circular structure was observed on the top surface in the experiments. Complete filling was mainly achieved in packing phase. PMMA molecules were stretched along the sidewall and gradually enriched due to the nonbonded interactions. Molecules near the cavity surface were highly oriented along the flow direction. The circular structures were increasingly obscure with a higher packing. PMMA melt was adsorbed on cavity surface because of the lyophilicity of nickel mold insert. Experimental study provided consistent regulation with MD simulation in geometrical morphology development of injection-molded nanopillars on PMMA surface.