Replicative manufacturing of metal moulds for low surface roughness polymer replication

Abstract

Tool based manufacturing processes like injection moulding allow fast and high-quality mass-market production, but for optical polymer components the production of the necessary tools is time-consuming and expensive. In this paper a process to fabricate metal-inserts for tool based manufacturing with smooth surfaces via a casting and replication process from fused silica templates is presented. Bronze, brass and cobalt-chromium could be successfully replicated from shaped fused silica replications achieving a surface roughnesses of R_q 8 nm and microstructures in the range of 5 µm. Injection moulding was successfully performed, using a commercially available injection moulding system, with thousands of replicas generated from the same tool. In addition, three-dimensional bodies in metal could be realised with 3D-Printing of fused silica casting moulds. This work thus represents an approach to high-quality moulding tools via a scalable facile and cost-effective route surpassing the currently employed cost-, labour- and equipment-intensive machining techniques.

Fig. 1. Process from the master structure to the metal replication.

a The master (positive) structure is fabricated using 2-photon-polymerisation before being copied into polydimethylsiloxane (PDMS) via casting (negative) (scale bar: 5 mm, magnified view scale bar: 500 µm). b Fused silica part (positive) fabrication, by casting silica nanocomposite onto the created PDMS-Replication mould and curing it using UV-Light (scale bar: 5 mm, magnified view scale bar: 500 µm). c After debinding and sintering, a fully-dense and transparent fused silica replication structure is obtained (positive) (scale bar: 4 mm, magnified view scale bar: 400 µm). d Casting of metals against the sintered fused silica replication structure using bronze metal (negative) (scale bar: 4 mm, magnified view scale bar: 400 µm).

Fig. 2. Characterization of the replicated metal moulds.

a White-light interferometry measurement of the generated metal inserts for bronze (red), brass (yellow), and cobalt-chromium (green). b Optical lens master structure which was used to characterize the overall shrinkage during the process. c Fused silica replication of the optical lens. d Resulting cast bronze metal lens (negative). e AFM-Measurement of an unstructured casted bronze insert with a surface roughness of only R_q 8.0 nm. f Comparison of Vickers hardness values of manufactured samples of bronze (error bar standard deviation n = ±4 HV), brass (error bar standard deviation n = ±5 HV for casted and n = ±11 HV for nickel plated) and cobalt-chromium (error bar standard deviation n = ±9 HV for casted and n = ±13 HV for nickel plated) in pristine form and after nickel electroplating. The error bars were determined using the standard deviation of measured data, 10 measurements were carried out in each case.

Fig. 3. Polymer injection moulding using casted metal moulds.

a Schematic representation of the manufacturing process of a metal insert and its use in injection moulding. b Close-up of the injection mould which was used as an insert (scale bar: 10 mm). The inset shows a magnification of the dot matrix structure (scale bar: 500 µm). c Close-up of an injection-moulded polymethylmethacrylate (PMMA) component replicated from the metal insert (scale bar: 10 mm). The inset shows a magnification of the structure (scale bar: 500 µm). d White-light interferometry image of the 2000^th PMMA component produced from the mould (IM-Part 2000) e Comparison of the cross-section measured using WLI of the first polymer replicated PMMA component (IM-Part 1, red) and the of the 2000^th component (IM-Part 2000, blue) created using the metal insert (black).

Fig. 4. Various examples from nature and technology for the applicability of the described metal replication process.

a Cicada wing made of a cobalt-chromium alloy (scale bar: 10 mm, magnified view scale bar: 500 µm). b Metal replication of a human fingerprint in brass (scale bar: 10 mm, magnified view scale bar: 500 µm). c Microlens array in brass with a lens diameter of 30 µm (scale bar: 10 mm, magnified view scale bar: 200 µm). d Bronze metal replication of different lines-and-space structures in the range of 5–25 µm in bronze showing interference effects (scale bar: 10 mm, magnified view scale bar: 100 µm). e Function test of a polymeric component replicated form the structure in d showing the expected diffractive far-field pattern (scale bar: 25 cm). f Replicated metal inserts with a mirror surface finish in bronze, brass and cobalt-chromium (scale bar: 10 mm). g Schematic representation of the production process of 3D-Printed Glassomer moulds for direct metal casting.  h Metal figurines in bronze, brass and cobalt-chromium, produced using a 3D-Printed Glassomer mould (scale bar: 10 mm). i Detailed view of the face of one figure, as brass metal replica (scale bar: 1000 µm). j Top view of the one figure, cobalt-chromium metal replica (scale bar: 5 mm). Original Sphinx design (Thing # 1404323) by Perry Engel from thingiverse.com (2016), adapted by author.