The need for tiny, even micron-scale parts has increased over the past few years, and thus the relevance of micro technologies is increasing due to the drive toward miniaturization. Because of microinjection molding capacity for mass production and relatively cheap production costs, it may be considered one of the fundamental technologies for mass micro manufacturing (replication).
Micro injection molding involves shaping micron-level geometries to a plastic product with the help of molds which have cavities and cores. The process starts with moving of material, which is in the form of pallets, from a hopper into a plasticizing unit where it melts and liquefies. The molten polymer is then injected, under pressure, into a mold cavity and core, where it is held under pressure for a certain amount of time to account for material contraction. When the melt cools down inside the mold's shape, the component is ejected, and the process is repeated. When technology is adapted, this cyclic operation enables mass replication of micro parts.
Micro-injection molding is not a traditional molding
Even though, microinjection moulding seems to be no different from traditional injection molding in its nature, the challenges lurk in the details. When molded components or their features gets small variety of challenges arises: demoldability (ejection), high aspect ratios (HAR), melt flow penetrability, capillary effects, advanced venting solutions, extreme mold and injection temperatures, hesitation effects, mold manufacturing (micro-machining) problems (tool breaking, positioning, inspection), material selection, optical quality checking and packaging/handling of micro parts.
Micro-injection molding hesitation effects and penetrability problems
The huge surface to volume ratio of many micro parts results in quick cooling times of the injected material inside the tools. It is crucial to take this into account when designing the mold. Despite the fact that polymers often exhibit a "self-isolating" effect and have low heat conductivity, the injected materials quickly cool down on the cavity and core walls, making it impossible to completely penetrate into the micro-cavities. Micro components have thin walls and large surfaces compared to their volume, which causes the melt's temperature to quickly equalize to the mold’s, thus it is always good to minimise this factor early in the design phase.
Micro mould advanced venting solutions
In order to avoid faults in the molded part caused by compressed air inside the cavity, the proper venting has to be assured. It is another main factor in determining the quality of the micro component that is molded. A special built mechanism to evacuate the air from the microcavities is needed if the micro geometries are too small to be vented normally through the parting line of the molds or traditional venting channels. Another good solution to this problem is vacuum the cavity before the injection. However, this technology still requires adaptation and research for a reliable use.
Changing micro machined inserts not the molds
Another common application in the micro injection molding is the use of inserts. For example, to mold microfluidic channels electroplated nickel inserts can be used. They can be placed and replaced inside the mold to expand the micromachining capabilities when tooling. The key benefit of employing a mold with interchangeable inserts is the opportunity to test various micro-part geometries without changing the fundamental structure of the mould. In a process where the finalized mould design is developed through a number of iterative steps in which parts are injected and the mould design is revised, the usage of moulds with inserts lowers the overall cost of process setup. However, this benefit comes with a certain cost – different mold and insert materials have different deformations at varying temperatures which may cause misalignments or even tool damage.
Micro-injection molding material selection
The experimental results have been impacted by the use of various polymeric materials in the production of micro parts. The use of materials with high shear thinning rheology is recommended because it enables mold filling with the least amount of injection pressure. Determining the best material for each application without testing it under various conditions is difficult due to the interplay between the type of polymer used and the moulded component.
Viscosity in micro-cavities
The efficiency of molding is influenced by the chosen plastic's characteristics, such as viscosity, specific heat coefficient, and thermal expansion. In recent studies, measurements of melt viscosity in small-dimension geometries were made utilizing high-fluidity amorphous ABS and PS resins, high-low density PE resins, and high crystallinity POM resin. It is feasible to determine the viscosity values from the recorded pressure drop received from pressure sensors and melt volumetric flow rate. When compared to information provided from a conventional capillary rheometer, it was discovered that ABS, PS and POM viscosity increases as micro-channel size reduces.
Wall-slip effect
When melt flows through micro-channels, wall-slip effect occurs. Wall-slip effect results in a higher viscosity reduction as micro-feature size decreases. Also, when melt temperature rises, the wall-slip effect becomes more pronounced. The ratio of slip velocity to mean melt velocity and the percentage reduction in viscosity within the micro cavities rise with decreasing micro-channel size. It seems that the wall-slip effect plays a dominant role in viscosity reduction.
Micromolding parameters for high quality micro-parts
Throughout the years of experience, it has been found that the following are the primary injection molding variables influencing the part quality during the cycle time of molding:
Injection pressure
Cooling time
Mould temperature
Holding time
Holding pressure
Melt temperature
Injection speed
There might be a variety of good combinations of those parameters to deliver required quality of the part, however, there is no single formula which would lead to the general solution. In most cases the play with these variables lead to empirical parameters combination settings which are individual for every project. This burden sits on micro moulder’s shoulders and only his proficiency determines the right settings.
Micro moulding in the scope of micro-manufacturing
For the production of polymeric micro-components, the micro injection molding technology is gaining significant relevance nowadays. This process has the potential to play a key role in sustaining the demand for micro components in biomedical, optical, and electronics fields. By discovering new materials, process controls, simulation techniques, and methods for quality checking, field of micro injection molding is advancing quickly and appears to be able to surpass most of the current technological constraints.
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