Mold Insert Research Articles

Upscaling the fabrication routine of bioreplicated rose petal light harvesting layers for photovoltaic modules

The hierarchical micro-/nanotextures adorning the petal surfaces of certain flower species exhibit outstanding sunlight harvesting properties, which can be exploited for photovoltaic (PV) applications via a direct replication approach into polymeric cover layers. This route has been so far hampered by the restricted size of the original bio-template and by the limited number of replication cycles when a polymeric mold is used. Here, we therefore introduce an upscaling strategy allowing the fabrication of mechanically stable, temperature resistant and large area nickel mold inserts which can be employed for hot embossing lithography, and ultimately for the mass production of bioreplicated films that improve light management in PV modules. As a proof-of-concept, we laminate the thus produced textured foils, here corresponding to rose petal replicas, onto glass encapsulated copper indium gallium diselenide (CIGS) solar modules with a surface of 100 cm2. We demonstrate an increase of the power output of 5.4% with respect to a device with an uncoated glass cover layer (measured in outdoor operating conditions). This improvement is notably attributed to the excellent light in-coupling properties of the replicated texture at high oblique incidence angles.

Microinjection Molding of Out-of-Plane Bistable Mechanisms.

We present a novel fabrication technique of a miniaturized out-of-plane compliant bistable mechanism (OBM) by microinjection molding (MM) and assembling. OBMs are mostly in-plane monolithic devices containing delicate elastic elements fabricated in metal, plastic, or by a microelectromechanical system (MEMS) process. The proposed technique is based on stacking two out-of-plane V-beam structures obtained by mold fabrication and MM of thermoplastic polyacetal resin (POM) and joining their centers and outer frames to construct a double V-beam structure. A copper alloy mold insert was machined with the sectional dimensions of the V-beam cavities. Next, the insert was re-machined to reduce dimensional errors caused by part shrinkage. The V-beam structure was injection-molded at a high temperature. Gradually elongated short-shots were obtained by increasing pressure, showing the symmetrical melt filling through the V-beam cavities. The as-molded structure was buckled elastically by an external-force load but showed a monostable behavior because of a higher unconstrained buckling mode. The double V-beam device assembled with two single-molded structures shows clear bistability. The experimental force-displacement curve of the molded structure is presented for examination. This work can potentially contribute to the fabrication of architected materials with periodic assembly of the plastic bistable mechanism for diverse functionalities, such as energy absorption and shape morphing.

Design, fabrication of honeycomb-shaped 1–3 connectivity piezoelectric micropillar arrays for 2D ultrasound transducer application

As a core component of 2D ultrasound transducers, honeycomb-shaped 1–3 connectivity piezoelectric micropillar arrays have attracted enormous attention due to their unique performance and functionality. In this paper, honeycomb-shaped 1–3 connectivity piezoelectric micropillar arrays with a high aspect-ratio were designed and fabricated by means of deep X-ray lithography and powder injection molding in six steps: preparation of lost mold, powder-binder mixing, injection molding and demolding, removal of binders, and densification of powder. A polymer-based lost mold insert was generated by a synchrotron X-ray exposure and development process. The optimal volumetric ratio between the piezoelectric powder and binders was determined by torque rheology behavior, then they were homogeneously mixed with a twin extruder mixer. To fully fill in the micro-cavities of the lost mold, rheological properties of the mixture were analyzed with a capillary rheometer using different shear rates (50–5000 s−1) and temperatures (140 °C, 150 °C, and 160 °C). After the mixture was completely injected, the lost mold was chemically dissolved in acetone and rinsed in methanol without bending or clustering of the micropillar arrays during evaporation. The binders in the injection molded portion were thermally decomposed using a continuous heating schedule of 200 °C, 390 °C, and 600 °C in argon gas under atmospheric conditions. Finally, the particles in the sample were densified into a coherent, solid mass by eliminating pores at 1300 °C. Based on the proposed micro-manufacturing process, defect-free honeycomb-shaped 1–3 connectivity piezoelectric micropillar arrays with a pattern dimension of 42 μm and aspect-ratio of 5 were successfully produced.

Perception of One's Own Voice After Hearing-Aid Fitting for Naive Hearing-Aid Users and Hearing-Aid Refitting for Experienced Hearing-Aid Users.

Dissatisfaction with the sound of one’s own voice is common among hearing-aid users. Little is known regarding how hearing impairment and hearing aids separately affect own-voice perception. This study examined own-voice perception and associated issues before and after a hearing-aid fitting for new hearing-aid users and refitting for experienced users to investigate whether it was possible to differentiate between the effect of (unaided) hearing impairment and hearing aids. Further aims were to investigate whether First-Time and Experienced users as well as users with dome and mold inserts differed in the severity of own-voice problems. The study had a cohort design with three groups: First-Time hearing-aid users going from unaided to aided hearing (n = 70), Experienced hearing-aid users replacing their old hearing aids (n = 70), and an unaided control group (n = 70). The control group was surveyed once and the hearing-aid users twice; once before hearing-aid fitting/refitting and once after. The results demonstrated that own-voice problems are common among both First-Time and Experienced hearing-aid users with either dome- or mold-type fittings, while people with near-normal hearing and not using hearing aids report few problems. Hearing aids increased ratings of own-voice problems among First-Time users, particularly those with mold inserts. The results suggest that altered auditory feedback through unaided hearing impairment or through hearing aids is likely both to change own-voice perception and complicate regulation of vocal intensity, but hearing aids are the primary reason for poor perceived sound quality of one’s own voice.

Thermodynamic analysis and injection molding of hierarchical superhydrophobic polypropylene surfaces

Abstract In this study, thermodynamic analysis of the hierarchical structure of the 3D cylinder-textured surface model was performed. The wetting states at different penetration depths, the effects of three components on the wetting properties, and all equilibrium contact angle of the hierarchical structure were investigated. It was found that the interaction between micropillars and nanopillars can affect the transition energy barrier and the transition pitch in the wetting-state transition process. This showed that all components would play a key role in enhancing the surface hydrophobicity. Polypropylene (PP) surfaces with mono micropillars and hierarchical structures were both fabricated by injection molding. Mold inserts for hierarchical structures were obtained by the combination of a punching plate and an anodized aluminum alloy plate. The static contact angle (CA) and the roll-off angle of injection-molded PP surfaces were measured and analyzed from the perspective of thermodynamic analysis. With the hierarchical structures, a static CA of about 163° as well as a roll-off angle of about 5° was approached. Compared with a mono micropillar-structured PP surface, the hierarchical-structured PP surface has a larger static CA and a smaller roll-off angle. The work demonstrates an inexpensive and reproducible technique to fabricate function-designed controlled hierarchical structures on PP material.

An investigation of the surface waviness features of ground surface in parallel grinding process

The high precision tungsten carbide (WC) and silicon carbide (SiC) optical mold inserts used in glass molding are usually machined by ultra-precision grinding, but the residual surface waviness error is difficult to be eliminated. This paper mainly focused on the surface waviness features related to the grinding wheel run-out error and the non-integer rotation-speed-ratio (RSR) in parallel grinding process. After the integrate consideration of grinding kinematics and microscopic material removal, the generation mechanism and distribution trend of surface waviness features were revealed. In order to simulate the surface waviness topography of parallel ground components, a mathematical model was developed and experimentally proved, which was appropriate for both integer and non-integer RSR. Besides, the mechanism of surface waviness reduction was explained when RSR was non-integer, which was due to the material removal again by the secondary grinding zone of grinding wheels. Then prediction models for the surface-waviness-period-feature (SWPF) and surface-waviness-amplitude-feature (SWAF) was built and experimentally verified, which was based on the waviness profiles overlapping effect. It found that the wave-shift resulted from the non-integer RSR had significant influence upon the surface waviness features. In the end, a convenient method was proposed to reduce the surface waviness error of parallel ground components through proper adjustment of grinding wheel rotation speed to achieve proper non-integer RSR and wave-shift, which was proved practically feasible and effective for both plane components and Fresnel components.

Effect of mould roughness on injection moulded poly (methyl methacrylate) surfaces: Roughness and wettability

Lab on a chip technology requires exquisite control over key surface properties using mass manufacturing strategies. Here, we investigate the relationship between the surface roughness of a micromilled mould insert and that of an injection moulded poly (methyl methacrylate), PMMA, part. The choice of milling tool, feed rate, and radial depth of cut dimension was varied. The smallest roughness (arithmetic mean height) was ∼50 nm for the mould insert and ∼20 nm for the moulded PMMA part. The latter is comparable with cast PMMA and appears to be a lower limit for the conditions studied, irrespective of the insert roughness. Wettability measurements on the PMMA surfaces report a slight increase in hydrophilicity with increased roughness factor, qualitatively consistent with Wenzel’s prediction. Scanning electron microscopy and atomic force microscopy show that the PMMA surface roughness is dominated by replication of larger milling features, with very smooth interstitial regions. It is shown that well-chosen milling parameters for insert manufacture can reduce machine time without compromising the surface quality of the injection moulded parts.

Precise fabrication of microtextured stainless steel surfaces using metal injection moulding

In this study, we show a new, cost effective and straightforward method to fabricate stainless steel (SS) microtextures with various geometries. We were able to design and texture a durable Ni mould insert by a microworking robot technique. Furthermore, the obtained microtextures were replicated onto stainless steel surfaces by metal injection moulding. A computer-controlled program enabled precise control of the spacing between the textures giving a broad choice for surface design. Surface geometries, such as micropillars and micropits with different dimensions, were produced on planar and curved surfaces. These results introduce a new platform for mass production of microtextured stainless steel surfaces with high surface control. The obtained surfaces have potential applications by meeting the demands of reducing surface contact and providing surface protection from mechanical damage.

Fabrication of curved diffractive optical elements by means of laser direct writing, electroplating, and injection compression molding

In this work, a new process chain for the fabrication of curved micro-structured optical elements by injection compression molding is investigated. The fabrication is demonstrated for the example of a curved diffractive optical element (DOE). In a first process step a master substrate is fabricated using laser direct writing on a curved glass substrate. Blazed diffractive micro-structures with lateral feature sizes down to 5 μm and height of 1.6 μm are created. A subsequent electroplating process is applied to create a nickel stamper to be used as a tool insert for the molding process. During electroplating, a 3 mm nickel layer is formed, transferring the diffractive structures from the master substrate into a solid mold insert. The nickel stamper shows an accurate reproduction of the micro-structures. The mold insert is integrated into an injection compression molding tool to replicate the optical elements. Results show that the blazed diffractive structures are replicated with a high quality. Tests of the components within a chromatic-confocal measurement setup confirm that they can potentially replace expensive conventional elements.

Effect of 2024 Al Alloy Insert on the Grain Refinement of a 2024 Al Alloy Prepared via Insert Mold Casting

In this study, an insert mold casting was fabricated by inserting 2024 Al extruded rods into a 2024 Al melt. The molds were kept at a 2024 Al melt for different times. The 2024 Al extruded rods were used to refine the 2024 Al alloy grains because the advantage of this method is that it is contamination free compared with other grain refiners. Moreover, we investigated the macro and microstructure of the ingots. Further, we analyzed the refinement mechanism of the 2024 Al rod on the 2024 Al alloy. Our result showed that when the immersion time of the 2024 Al insert was 0 s, a metallurgical bonding was partly formed between the 2024 Al insert and the 2024 Al alloy mold cast. When the immersion time of the 2024 Al insert increased to 5 s, the 2024 solid insert was dissolved in the liquid; the coarse dendritic grains were replaced by fine equiaxed grains. The refinement mechanism for the insertion of a 2024 Al rod on the 2024 Al alloy was to melt the 2024 Al insert and have it decrease the degree of the liquid superheat, which thus increased the cooling rate and provided a large number of small particles that acted as the nucleus of heterogeneous nucleation. However, these particles were melted gradually in the high-temperature liquid after an increase of immersion time. Thus, the refinement effect of 2024 Al insert on the solidified structure was weakened.

Multi-metal additive manufacturing process chain for optical quality mold generation

Abstract This paper introduces an innovative process chain for the generation of mold elements based on multi-metal additive manufacturing. Starting from an indirect tooling approach for the fabrication of a mold master with optical surface quality, three different additive processes, namely electroforming, thermal spraying and selective laser melting (SLM), are combined to manufacture a test mold element. In order to maintain the targeted optical quality of the mold surface, SLM process parameters optimization has been carried out through numerical simulations. The approach is experimentally verified by generating prototype mold inserts and validated in injection molding trials, showing no signs of degradation of the mold insert after 1000 injection molding cycles.

Carbide-bonded graphene-based Joule heating for embossing fine microstructures on optical glass

This paper reports the fabrication of high-quality microstructures on optical glass via hot embossing using carbide-bonded graphene (CBG) based Joule heating for the first time. In this study, a tailor-made micro hot embossing tool equipped with a modified CBG-based Joule heating system was designed and developed for transferring microstructures (e.g., microlens arrays, MLAs for short) from the CBG-coated silicon mold insert into the optical glass (P-SK57). Initially, the surface topographies of a typical 3 × 3 MLA from a bare silicon mold to an embossed glass replica were compared for evaluating form errors brought in during different preparation stages. The feasibility of the CBG-based Joule heating technique for the non-equilibrium thermal imprinting of optical glass was evaluated by the surface integrity and replication fidelity of the embossed MLA features. Thermally induced residual stress and imaging performance of the embossed glass lens were assessed as well. Experimental results indicate that the proposed CBG-assisted hot embossing technique, in combination with single point diamond turning technology, has the capability of producing high-quality glass MLAs. Moreover, this technique allows notably fine replication of surface shapes at the microscale, as well as roughness information at the nanoscale. Consequently, the embossed MLA replica shows satisfactory imaging performance.

Development of Micropatterns on Curved Surfaces Using Two-Step Ultrasonic Forming.

Nanoimprint lithography (NIL) is a micro/nanoscale patterning technology on thermoplastic polymer films, and has been widely used to fabricate functional micro/nanoscale patterns. NIL was also used to develop micro/nanoscale patterns on curved surfaces by employing flexible polymer stamps or micropatterned metal molds with macroscopic curvatures. In this study, two-step ultrasonic forming was used to develop micropatterns on a curved surface out of a flat metal stamp, by connecting ultrasonic imprinting and stretching processes. Ultrasonic imprinting was used to replicate functional micropatterns on a flat polymer film, using a flat ultrasonic horn and micropatterned metal stamps with prism and dot micropatterns. An ultrasonic stretching process was then used to form a curvature on the patterned film using a curved ultrasonic horn and a soft mold insert, to avoid damage to the pre-developed micropatterns. The ultrasonic horn was designed to have three different tip radii, and the resulting forming depth and curvature formation were investigated experimentally. As a result, three different curved surfaces containing two different micropatterns were obtained. The developed curved films containing micropatterns were then evaluated optically, and showed different optical diffusion and illumination characteristics according to the film curvature and micropattern type. These results indicate that the proposed technology can extend the functionality of conventional micropatterned products by imposing appropriate curvatures.

Molecular Dynamics Simulation on the Influences of Nanostructure Shape, Interfacial Adhesion Energy, and Mold Insert Material on the Demolding Process of Micro-Injection Molding

In micro-injection molding, the interaction between the polymer and the mold insert has an important effect on demolding quality of nanostructure. An all-atom molecular dynamics simulation method was performed to study the effect of nanostructure shape, interfacial adhesion energy, and mold insert material on demolding quality of nanostructures. The deformation behaviors of nanostructures were analyzed by calculating the non-bonded interaction energies, the density distributions, the radii of gyration, the potential energies, and the snapshots of the demolding stage. The nanostructure shape had a direct impact on demolding quality. When the contact areas were the same, the nanostructure shape did not affect the non-bonded interaction energy at PP-Ni interface. During the demolding process, the radii of gyration of molecular chains were greatly increased, and the overall density was decreased significantly. After assuming that the mold insert surface was coated with an anti-stick coating, the surface burrs, the necking, and the stretching of nanostructures were significantly reduced after demolding. The deformation of nanostructures in the Ni and Cu mold inserts were more serious than that of the Al2O3 and Si mold inserts. In general, this study would provide theoretical guidance for the design of nanostructure shape and the selection of mold insert material.

Laser-assisted surface processing for functionalization of polymers on micro- and nano-scale

Rapid manufacturing of functionalized polymers enable a cost-efficient method for many applications in the biological, and medical sector. Laser structuring techniques differ for fast fabrication processes of thermoplastics. Here we report on direct UV laser ablation of polystyrene and polydimethylsiloxane to control wettability and bio-compatibility. For this purpose contact angle measurements and L929 cell growth experiments were performed as function of laser and process parameters. This work also describes the potential of laser-assisted hot embossing which has been introduced to replicate pre-fabricated micro- and nano-sized structures onto polymers proving it to be a cost-efficient and flexible method for surface functionalization. Therefore, pre-fabricated laser-induced periodic surface structures have been manufactured as mould inserts using ultrafast laser radiation near the ablation threshold. These mould inserts can also be made of nickel, silicon or glass proving laser-assisted hot embossing to be a versatile tool for the functionalization of thermoplastics and meet the requirements for modern manufacturing with regard to quality and production flexibility.

Molecular Dynamics Simulations on the Demolding Process for Nanostructures in Injection Molding.

Injection molding is one of the most potential techniques for fabricating polymeric products in large numbers. The filling process, but also the demolding process, influence the quality of injection-molded nanostructures. In this study, nano-cavities with different depth-to-width ratios (D/W) were built and molecular dynamics simulations on the demolding process were conducted. Conformation change and density distribution were analyzed. Interfacial adhesion was utilized to investigate the interaction mechanism between polypropylene (PP) and nickel mold insert. The results show that the separation would first happen at the shoulder of the nanostructures. Nanostructures and the whole PP layer are both stretched, resulting in a sharp decrease in average density after demolding. The largest increase in the radius of gyration and lowest velocity can be observed in 3:1 nanostructure during the separation. Deformation on nanostructure occurs, but nevertheless the whole structure is still in good shape. The adhesion energy gets higher with the increase of D/W. The demolding force increases quickly to the peak point and then gradually decreases to zero. The majority of the force comes from the adhesion and friction on the nanostructure due to the interfacial interaction.

Ceramic Injection Moulding using 3D-Printed Mould Inserts

In general, micromilled mould inserts made of steel, aluminum or brass are used today for ceramic injection moulding (CIM) processes. However, tool making via mechanical subtractive manufacturing processes as micromilling is time- and cost-effective and the use of 3D printed mould inserts becomes an attractive alternative to metal mould inserts. In this paper, we report about the use of 3D printed mould inserts for CIM of alumina microreactor parts. It was observed that mould inserts printed using the Polyjet technology were very well suited for functional prototyping via CIM. The mould inserts surface was found without visible thermally introduced damage after twenty CIM process cycles. In contrast to the high quality of mould inserts printed using the PolyJet technology, mould inserts made via fused deposition modeling (FDM) technology revealed as not applicable for the purpose of this study. The mould inserts manufactured using FDM-printer exhibit significantly higher surface roughness, larger longitudinal deflection and manufacturing-related undercuts along the edges of the 3D printed microstructure.

Direct polymer additive tooling – effect of additive manufactured polymer tools on part material properties for injection moulding

PurposeThis study aims to investigate the influence of additive manufactured polymer injection moulds on the mechanical properties of moulded parts. Therefore, polymer moulds are used to inject standard specimens to compare material properties to specimens produced using a conventional aluminium tool.Design/methodology/approachPolyJet technology is used to three-dimensional (3D)-print a mould insert in Digital ABS and selective laser sintering (SLS) technology is used to 3D-print a mould insert in polyamide (PA) 3200 GF. A conventionally aluminium milled tool serves as reference. Standard specimens are produced to compare resulting mechanical properties, shrinkage behaviour and morphology.FindingsThe determined material characteristics of the manufactured prototypes from the additive manufactured tools show differences in terms of mechanical behaviour to those from the aluminium reference tool. The most significant differences are an up to 25 per cent lower tensile elongation and an up to 63 per cent lower elongation at break resulting in an embrittlement of the specimens produced. These differences seem to be mainly due to the different morphological structure caused by the lower thermal conductivity and greater surface roughness of the polymer tools.Research limitations/implicationsThe determined differences in mechanical behaviour can partly be assigned to differences in surface roughness and morphological structure of the resulting parts. The exact extend of either cause, however, cannot be clearly determined.Originality/valueThis study provides a comparison between the part material properties from conventionally milled aluminium tools and polymer inserts manufactured via additive tooling.

Functionality characterization of injection moulded micro-structured surfaces

Micro-structured surfaces are increasingly being used on parts and products to embed functional properties that could be optical, electrical, thermal, or likewise. In this work, directional optical properties were achieved on plastic with a microstructure composed of a close array of ridges, defined by a constant width and by the angle with respect to the normal of the generative surface. Under constrained lighting, the reflectance was maximized from a certain viewing angle and direction, and minimized from its horizontally orthogonal position. The purpose was the generation of quick response codes that could be easily scanned by means of commercially available software, e.g. smartphone applications, or professional equipment for identification or embedding specific information within the sample parts. To evaluate the functionality, defined as the generated light contrast from contingent micro ridges, the replicates were characterized by means of a robot assisted vision system, provided with a light source and a camera, used as a gonioreflectometer. The contrast was then correlated to the replication quality, i.e. the deviation of three defining parameters of the structures from the mould insert, thus determining the best processing conditions. The results showed that high injection speed, 60 °C mould temperature and 100 MPa packing pressure were required to achieve optimal replication and generated contrast: large variations in the surface functional behaviour were present even for small differences between the parts. However, an empirical approach highlighted that even for processing parameters that were less likely to promote replication, sufficient information decoding was achieved.

Thermal analysis based method development for novel rapid tooling applications

The use of soft molds manufactured by additive manufacturing is gaining popularity in small-series production, thanks to the development of additive technologies and their materials. The thermal properties of soft molds, however, differ considerably from those of conventional, metal molds, and this greatly affects the conditions in the mold during manufacturing. We investigated the thermal properties and cooling conditions of epoxy acrylate-based, directly manufactured soft prototype molds. We used four different mold inserts, two with conformal cooling, one with conventional cooling and one without cooling. We found that due to the poor thermal properties of the mold material, the thermal conditions in the insert with conventional cooling and the insert without cooling were almost identical, this proves that in this case, conventional cooling cannot remove heat effectively. The use of conformal cooling, however, can reduce the cyclic heat load of mold inserts by about 70%, which increases the lifetime of the mold.