The presented method aims at manufacturing complex surface with micro features by a soft-tooling process, i.e., to use polymer additive manufacturing to produce inserts for polymer injection molding. In other words, polymer parts with functional surfaces are injection molded by polymer inserts.
Surface functionalities can be realized by micro features; for instance, Doan et al.1 and Luchetta et al.2 demonstrate functional surfaces in the field of cell biology, and Hu et al.3 shows an example of optical elements, etc. One type of surface feature, micro pillars, has been investigated intensively for promoting cell proliferation. They are able to enhance the bonding between the proliferated tissues and the surface if the micro pillars are patterned in certain ways4,5.
Polymer replications of micro features have been intensively studied, and precise molding can be achieved by many processes6. For instance, Metwally et al. has reported high fidelity between the molded parts and mold for replication of micro and sub-micro features on flat surfaces7.
There are many protocols for manufacturing micro pillars or features; however, most of them can only be applied on flat surfaces or surfaces with constant curvature. For instance, Nian et al.8 showed that micro features could be achieved by hot embossing on a curved surface. Those protocols are not suitable for complex shapes with three-dimensional surfaces, which are required by most real-life devices. Apparently, the fabrication of a three dimensional cavity with micro features on the surface challenges current protocols; meanwhile, ejection of the parts may fail for high aspect ratio pillars on a complex surface if they are non-perpendicular to the demolding direction. Bissacco et al.9 used 3D mold inserts and obtained sub-micro features by injection molding; in their study, specific sub-micro features with a low aspect ratio were generated by an aluminum anodizing process and successfully replicated by polymer on a complex component.
Researchers have attempted to involve additive manufacturing in the process chains for polymer replication in order to achieve the designed surface texture. Lantada et al. described a process chain that starts from AM prototypes, and then employs coating techniques for obtaining metallic mold inserts for injection molding10. Polymer parts produced by AM have been proved to be directly applicable as mold inserts11,12, which is the soft tooling process presented in this protocol.
In our previous work, we demonstrated that Ø4 µm PEEK pillars (2 µm high) were successfully demolded on a vertical wall by injection molding13. In this protocol, the investigated product is a ring with four tines, each characterized by having an angle of 60°. This product has been studied by Zhang et al.14, where micro features were introduced by implementing pre-fabricated nickel plates, and micro pillars (Ø4 µm) with aspect ratio of 0.5 were obtained by silicone rubber injection molding on the tines.
In the presented method, micro features can be obtained on complex surfaces created by a soft tooling process chain. Steel mold cavities are replaced by a set of inserts made by vat photo-polymerization-based AM. Compared to metal AM, photopolymer based AM technology is able to achieve higher precision15. Moreover, the machining time and cost is reduced significantly compared to conventional tooling processes based on CNC machining. According to a recent case study16, by using soft-tooling for thermoforming, the cost is reduced by 91%, while the consumed time is reduced by 93%. This protocol is suitable for products requiring a high design flexibility and intermediate production volume. It has been proved that inserts manufactured from carbon fiber reinforced photopolymers can withstand up to 2500 injection molding cycles for polyethylene before noticeable mold deterioration17. The choice of injection-molded material is limited by the thermal properties of the chosen photopolymer for inserts. Polymers with high melting temperatures may not be applied in a photopolymer cavity. In this study, polyethylene (PE) was chosen to conduct the injection molding test.