Areal Surface Texture Filtering Selection in High-Precision Micro Injection Moulding
Selecting areal surface texture filter cutoffs under ISO 25178 matches transmission bandwidth to moulding replication limits, preventing false tooling rejects.
Micro-Fluidic Substrate
Meaning
Injection moulded plastic sheets featuring networks of microscopic channels and chambers handle nanolitre volumes of fluids in medical diagnostics. This component, known as a micro-fluidic substrate, contains patterns of channels with typical widths of fifty to two hundred micrometres. It is manufactured under high-precision conditions to ensure that the fluidic paths are free from blockages and dimensional deviations that would alter flow behavior.
The surface chemistry must also be uniform to prevent the unintended adsorption of biological molecules.
Resin Selection
Thermoplastic resins are selected based on their optical clarity and high melt flow index. Cyclic olefin copolymers and polymers are frequently selected for a micro-fluidic substrate due to their low autofluorescence and high transmission of ultraviolet light. Polystyrene is also used for less demanding diagnostic cards.
The high melt flow index of these resins enables the filling of long, thin channels without premature freezing of the polymer.
Channel Replication
Moulding microscale channels requires careful thermal management during the injection cycle. Rapid cooling of the polymer melt can lead to incomplete filling, resulting in rounded channel profiles rather than the square profiles required for consistent capillary action. Using variothermal moulding, the cavity is heated above the polymer’s glass transition temperature before injection.
This technique guarantees high replication fidelity of the micro-structures.
Sealing Requirement
Completed substrates must be sealed with a cover lid to form closed channels. Thermoplastic bonding methods like ultrasonic welding and thermal diffusion are used to secure the cover sheet without clogging the fluid paths. Leakage occurs if the bonding is non-uniform or if the substrate features high flatness deviations.