MICRO MOULDING | ARTICLE
are the processes of handling, control of micro components, mould maintenance and ambient conditions in the industrial environment.
These internal and external factors in the process are vitally important for the end result of a process such as micro injection. In brief, this is an injection taken to its extreme, wherein the normal ‘rules’ do not work and the rheological behaviour of the materials, mechanics of the moulds, reproducibility of the machines and control of product and environment have to be carried out from a different angle, on another scale and forgetting about the idea of ‘negligibility’.
Materials
The contribution or effects of polymers on micro injection are the result of two very different facets:
1. Characteristics of the piece: this facet is linked to the design process, where one must be very aware that because of the extremely small sizes it will be difficult to balance the mechanical, thermal and even chemical properties (surface attacks) through geometrical changes. Consequently, selecting materials which clearly have a ‘surplus’ of the desired properties for the piece should be borne in mind — even to a greater cost for the material and greater complexity in the mould.
2. Process and rheological behaviour: given the great variety of materials and suppliers of such, one might imagine it would be easy to find a material that fulfils the characteristics of the piece, even when considering the points mentioned above. However, reality shows that certain materials are not plasticised sufficiently in the micro injection machines (potentially leading to an irregular process and/or non-compliant pieces). A further point related to the material and rheology is the additives and their effects on the micro moulds (excess residue deposited, chemical attack, etc.) — the difficulty in filling or ejecting them.
Micro moulds
The chief, most critical part of the whole micro injection process is the design and construction of the mould. Here, one must remember that generally much lower tolerance is required than in the classic precision injection.
Further matters to be considered are the design of the channels and injection sites; and the use or otherwise of hot channels and nozzles — which, given the low weights to be injected, should be used with greater care and micro injection-specific designs.
The choice of steels is even more critical than in the classic injection as tiny edges or cavities with very limited thickness of steel must resist very high pressures.
14 | commercial micro manufacturing international Vol 7 No.1
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