Dominykas Turcinskas, commercial manager, Micromolds, part of UAB Technoprojektai
As most modern devices are either getting smaller and/or requiring small parts, the demand for micromoulding continues to grow. Micromoulding is a highly specialised process for producing ultra-small, thermoplastic parts that have micron- or even submicron-scale tolerances. First, tooling experts use CNC machining or electrical discharge machining (EDM) to produce a microstructured aluminium or steel mould that has a cavity in the shape of the part. A micromoulding machine is then used to inject a thermoplastic resin into the mould’s cavity to create the part. Typically, a micromoulded part weighs a fraction of a gram and its micro features range from 5 to 50 µm or less.
The main differences between micromoulding and conventional moulding are the shot volume and precision offered. Micromoulding machines can inject a fraction of a gram of material at high precision because they have higher resolution feed options and these result in even pressure distribution inside the cavity. Another, albeit obvious, difference is that micromoulding uses smaller moulds that have smaller cavities and features inside.
Finally, micromoulding demands that special attention is paid to packaging and quality control, since the parts are very small, but conventional moulding tends to view these as secondary operations.
Micromoulding in place of conventional moulding
Sometimes, micromoulding can be used to produce small- but not strictly micro-sized parts. There is significant demand for such parts, that are small enough to fit in a micromoulding projected area (for example, ⌀~100 mm circle perimeter) and do not exceed micromoulding shot volume (for example, ~15–30 cm3). Moreover, innovating companies often seek a high-resilience and low-risk market entrance involving pilot launches of up to 100,000 pc manufacturing volumes.
In these circumstances, micromoulding is often the best solution. It can offer significant cost and time reductions over conventional moulding, attributable to factors such as:
- lower machine operating costs because machines are smaller and lower clamping forces are exerted;
- faster and cheaper machining because the moulds have fewer cavities and, in some instances, they are produced in aluminium;
- reduced waste because shorter runners are needed to fill in the cavities and therefore there are significantly less cut and disposed runners; and
- modification being easier and more flexible because machining is faster and cheaper.
It is possible to save up to 3–4x on costs and for finished products to enter the market in less than 21 days.
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The break-even point of micromoulding in terms of costs associated with the quantity produced.
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Minimising production costs through micromoulding.
Micromoulding usage across industries
MedicalMost medical procedures demand extreme accuracy and therefore, in many cases, the instruments used must be small and highly sophisticated. Micromoulding is widely used in the manufacture of drug delivery devices, catheters, diagnostic systems, hearing aids, etc. It is especially suitable for the instruments used in minimally invasive surgeries, such as aortic stressed that treatments and neurosurgeries.
Moreover, micromoulding is increasingly used for parts of the new types of microfluidic systems that are becoming increasingly popular and widely applicable in various medical applications, including point-of-care (PoC).
According to a report by Mordor Intelligence, the medical industry accounts for approximately a quarter of the global micromoulding market share1.
Micromoulding in medical device manufacturing.
Electronics
As modern electronic devices are getting smaller, there is a growing need for high precision and complexity. The benefits of micromoulding are exploited in the manufacture of various electronic components. A prime example is microoptics for laser-based devices, lenses, smart phones, etc. Micromoulding is also used to produce components such as connectors, integrated circuits (ICs), plugs and switches for communication and music devices, etc.
Microelectromechanical systems (MEMS) are often micromoulded too. In fact, demand for innovative micromoulding processes in this field is increasing. For instance, biomedical microelectromechanical systems (BioMEMS) are now being widely investigated and next-generation sequencing (NGS) and PoC diagnosing opportunities are already applicable.
The electronics industry accounts for a little more than a fifth of the global micromoulding market share1.
Overmoulding in electronics manufacturing.
Automotive
Micromoulding is widely used for manufacturing the small and light components required for automobiles, namely under the hood (for example, engine or break) components and various others such as those used in the door locking mechanism, as well as buttons, clips, switches, washers and even gears.
The automotive industry accounts for most, namely almost a third, of the global micromoulding market share1.
The micromoulding market share distribution among different industries.
The limits of thin-wall moulding
Firstly, to discuss thin-wall moulding, the concept itself should be clarified. Thin-wall moulding can be classified according to the ratio of flow length and wall thickness: L/T ratio. As different plastics have different flow rates their maximums of the ratios will vary accordingly. Table 1 shows the maximum L/T ratios for 10 of the most widely used thermoplastics.
Table 1: Maximum L/T ratios for 10 of the most widely used thermoplastics.
The quality of a micromoulded part is highly dependent on the correct design of its wall thickness. This means choosing compatible ranges of wall thickness for various thermoplastics and maintaining similar aspect ratios throughout the design for manufacturing (DfM) process. Failing to do these things may result in:
- irregular cycles, as thicker walls takes longer to cool than thinner ones;
- too thin walls that could prove too fragile as well as cause flow rate (the speed of cavity filling) errors, the latter potentially resulting in voids if the material does not fill all the features before it cools; and
- uneven walls that cool and solidify differently, which may cause permanent warps or sink marks on the surfaces of the part.
Successful thin-wall moulding is primarily dependent on the choice of material, therefore it is a good idea to refer to some experimental data. Table 2 shows the applications and minimum and maximum wall thickness ranges for some of the most widely used materials.
Table 2: Applications and minimum and maximum wall thickness ranges for some of the most widely used materials.
After the material has been chosen, other requirements must be met for thin-wall moulding. Since thin walls cool faster than thick walls, thin-wall moulding requires higher speed cavity-filling (fill time indicates the time required for the material to flow into cavities). For instance, a 25 percent drop in wall thickness necessitates a 50 percent drop in injection time. Thin-wall moulding requires specialised machinery to ensure higher speed and pressure cavity filling. Even though modern technologies allow standard machines to fill thinner and thinner parts, the tiniest parts require more advanced machines for both injection and clamping cycles.
The preferred materials for micromoulding
A wide range of materials can be used in micromoulding. However, there are some crucial constraints not to forget when choosing a material, such as mechanical properties, compatibility, cosmetic appearance and price. Table 3 shows the properties of some of the most popular materials.
Plastic grade materials.
It should also be highlighted that rapid technological development and growth in demand have seen an increase in the use of bioabsorbable materials in micromoulding. Bioabsorbable materials can be absorbed and dissolved by the body and their use lowers the number of surgical interventions needed for specific (usually orthopaedic) treatments.
Assembly and packaging challenges
Assembly and packaging account for much of the overall cost of micromoulded parts. The main reason for this is the lack of automation in both operations.
Micropackaging
Most assembly processes require operators to manually select and insert small parts using powerful microscopes and microtweezers. This takes a lot of time and is extremely expensive. The operators assembling the parts are likely to suffer from eye strain and are under pressure to ensure the final product meets strict reliability requirements.
To make assembly easier and quicker, certain equipment and capabilities must be made available, namely:
- a visual system featuring high-performance stereo microscope, long-lasting distance and high-resolution camera and monitor, the latter being used to provide instructions and feedback during and after assembly;
- a micropositioner affording a resolution of 40 nm for workpiece control, microgripper and position management;
- real-time computer vision for controlling servo mechanisms and motors and assembling parts to within micron-level accuracy; and
- a high-resolution, high-precision transfer tool for handling parts.
Optical, quality controlled microassembly.
Moreover, to avoid having to assemble parts under the microscope, there are processes that can be implemented at the DfM design stage to produce them combined or separately in-situ at the same time, namely:
- two-shot micromoulding for the injection of two different materials into the mould in the same place or in two different places;
- ultrasonic welding for joining thermoplastics and compatible metals;
- laser welding for joining parts if 3D geometry cannot be combined through overmoulding (laser welding also can be used to clean and disassemble materials such as wires quickly and without breaking them);
- staking for the cheap assembly of polymer and metal by folding one material into another; and
- solvent bonding for cheaper and faster joining of parts (typically, this involves using micro and nano pipettes to combine different materials and solvents, thus bonding the parts together, an essential process if the parts are to be used as an implant).
Packaging is as important as assembly. Each part must be delivered to the customer safely. If sending small, sharp or friction- and vibration-sensitive parts, packaging can be a very difficult process that has to be well thought out. It requires parts to be individually packed in special packages or pallets. If needing to meet cleanroom or ISO 13485 medical device quality management system (QMS) standard requirements, it is very important to ensure an appropriate temperature in the machines and airflow around the parts. Usually, it is a must to have fans generating filtered airstreams to prevent air contamination and dust attaching to the parts until they are packed.
Efficient assembly and packaging is key to the success of micromoulded parts in the marketplace.
The latest micromoulding developmentsIndustry 4.0 is a catalyst for manufacturing technologies being constantly upgraded or replaced by new ones. Micromoulding is no exception and must remain innovative and adapt to meet market demand for smaller and smaller parts. Recent developments in micromoulding include:
- mould sensors that are very compact, easily installed, save significant space in the mould and allow for real-time monitoring of pressure, temperature, shrinkage, warpage and other factors;
- CNC and micro sinker electrical discharge machined tools that allow moulders to inject shots of less than 1 gram with very high accuracy and minimal damage;
- runnerless or reduced flow path moulds that reduce expensive material waste and will see micromoulding machines redesigned to achieve higher accuracy and ultra-small shot sizes; and
- micromoulding machine advancements including allowance of non-standard material designs, reduced wall thickness filling options, stress removal, mould annealing and improved material and mould monitoring.
Innovations depend on customer demand, a significant majority of customers requiring something that companies are unable to produce. This demand puts a lot of pressure on the manufacturer to deliver, resulting in the development of new technologies. Micromoulding developments such as two-shot micromoulding, automatic insert moulding and extremely thin wall moulding are all the direct results of customer demand.
Micromolds, part of UAB Technoprojektai
www.micromolds.eu / https://technoprojektai.lt/
Reference1Thermoplastic micro molding market, growth, trends, COVID-19 impact and forecasts (2021–2026) [industry report]. Mordor Intelligence.Available at: http://bit.ly/3pCKGt0