Replication of surface micro‐features using variothermal injection molding: Application to micro‐fluidics

Abstract

We study injection molding of mesoscale items with µm‐scale surface features, namely micro‐fluidic channels, relating replication quality to process conditions. Using variothermal molding, the variables are the pre‐heat temperature of the cavity insert surface before melt injection and the mold cooling start time. Surface temperatures and in‐cavity melt pressures are continuously measured. Rounded upper corners of the micro‐channels are used as an index of replication quality. For polymethylmethacrylate the thickness of a layer with solid‐like properties (below 124°C) is calculated and used with pressures to interpret results. It is shown how improved replication correlates with low layer thickness at the end of the compression phase when pressures are at a maximum, and the necessity of properly timed cooling to lock in replication before melt pressures fall. Results show how the inter‐relationship of layer thickness and melt pressure is controlled by pre‐heat temperature and cooling switch‐on time. Delayed cooling can result in poorer replication, due to a retraction effect of the plastic. Too early cooling also reduces replication of parallel‐to‐flow features downstream of transverse features, and of the replication of transverse features on their downstream side. Good replication of 100 and 70 µm channels requires lower pre‐heat than 400 µm ones. POLYM. ENG. SCI., 58:1726–1738, 2018. © 2017 Society of Plastics Engineers

Injection of a microtextured polymer part surface: Influence of experimental parameters on the replication rate

“…Is also measured the shift in pressure at switchover between both sensors, ΔP, and the distance, Δx, between the in cavity sensors. [19] In the scenario of a flow through a rectangular section, it is possible to determine the shear stress, τ, according to the Poiseuille flow, using Equation (5) and the shear rate, _ γ, adjusted by the Rabinovitsch expression for a non-Newtonian fluid, for a rectangular section (calculation of equivalent cross section between the two sensors [see Figure 2, Equation (6)]). The viscosity, η, is determined using the ratio η = τ=_ γ:…”

“…In the case of a hot-cold cycle (variotherm process), they also allow changes in temperature to be controlled and managed. [13,19] For amorphous materials, a mold temperature close to, or often above the glass transition temperature, T g , gives better replication results. [9,20] However, this variotherm technique causes repeated thermal shocks to the mold, which in time leads to a drastically shorter production lifetime (by number of cycles), as well as longer cycle times than a conventional molding cycle.…”

Improvement in nano‐pattern replication of injection molding by polyamide/dendrimer blend

“…[ 5,14 ] In general, pattern structures, such as pattern size, shape, and aspect ratio, and molding conditions, such as melting temperature, mold temperature, injection pressure, and injection speed, affect the precision of replication. [ 15–19 ] Improved replication precision has been reported by adjusting molding methods such as rapid heating of mold surface, [ 20,21 ] vacuum molding, [ 22 ] and injection compression molding. [ 23,24 ]…”

“…[5,14] In general, pattern structures, such as pattern size, shape, and aspect ratio, and molding conditions, such as melting temperature, mold temperature, injection pressure, and injection speed, affect the precision of replication. [15][16][17][18][19] Improved replication precision has been reported by adjusting molding methods such as rapid heating of mold surface, [20,21] vacuum molding, [22] and injection compression molding. [23,24] Polymer resin, a material used in injection molding, is injected into the mold by an injection molding machine resulting in solidification of the resin followed by rapid decrease in its flowability, which makes it difficult for it to fill the nanostructure of the nano-pattern.…”

Mold replication in injection molding of high density polyethylene

“…[ 19 ] Mold preheating before injection is an efficient technical solution also to improve replication of microfeatures. [ 20 ] In fact, experimental results revealed that mold Ra does resist the cavity filling of polymer melt in microinjection molding. [ 6 ] In order to improve mold replication in micromolding, an effort is necessary to fully understand the mechanisms at the basis of this interaction between mold and polymer.…”