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Micromolding - in Depth Insights

As most of modern devices are either getting smaller or requiring tinier components, the demand for plastic micro molding continues growing. Thus it is easy to guess - this article is going to dive us in peculiarities of micro injection molding technology – analysis of its features as well as materials used with it, packaging challenges, design for manufacturability and the future horizons of micro molding.

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What is plastic micro molding and what are its features?

Micro molding is a highly specialized process where micro-structured steel or aluminum molds are CNC and EDM machined within micron or even submicron scale tolerances. Usually, when molded part weighs fraction of a gram or its micro features range from 50µm to 5µm or less in largest side micro molding world reveals.

The main difference between micro molding and traditional molding technologies is the shot size and the precision of injection machines. Micromolding machines can inject fraction of a gram with high precision as they have higher resolution feed options which results in even pressure distribution inside the cavity. In micro injection molding smaller molds are used too. Micro molds are machined with smaller cores and cavities and micro features inside with precision CNC and EDM tools. In conventional molding things like packaging and quality management can be viewed as secondary operations, however, micro molding process demands extensive attention to packaging and quality control details, since molded parts are very small.

Could micro injection molding replace conventional injection molding?

The answer is: YES. Micromolding sometimes can be ‘small’ but not ‘micro’. In a vast variety of demanded plastic parts many of them might be small enough to fit in micro mold projected area (e. g. of ⌀~100mm circle perimeter) and not to exceed micro injection shot volume (e. g. ~15-30cm3). Moreover, innovating companies often seek for resilience and low-risk market entrance with pilot launches with manufacturing volumes up to 100k pcs.

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In these conditions there will be no better way than using micromolding technology. A significant cost and time reduction is possible compared to traditional injection molding. It is possible to save up to 3-4x on tooling costs and enter the market with finished products in less than 3 weeks:

Low machine operating expenses, since there are smaller machines used and lower clamping force is exerted.
Fewer mold cavities and aluminum used results in faster and cheaper machining.
Waste minimization due to shorter runner systems required. Since there are shorter runners needed to fill in the cavities, there is a dramatic difference in the volumes of cut and disposed runners, in comparison to traditional injection molding.
Easy and flexible modification is possible due to the fast and low cost mold machining.

How extensively µIM is used across industries?

Medical and Healthcare industries

Undoubtedly, the field of medicine as such requires an extreme accuracy in most of the processes. Therefore, in many cases, the medical instruments used must be small and highly sophisticated. Thus, micro molding is widely used in medical devices manufacturing: drug delivery devices, catheters, diagnostic systems, optical and hearing aid components, etc. and is highly applicable for the instruments of minimally invasive surgeries. For instance, neurosurgeries, aortic treatments, etc.

It might also be stressed that new types of microfluidic systems are becoming more and more popular and widely applicable in various medical performances (including Point-Of-Care applications). No surprise that medical industries capture approximately a quarter of the global market share of micro injection molding, according to 'Mordor Intelligence'

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Electronic industries

Since modern electronic devices are getting smaller, there is a growing need of high precision and complexity for this sector, too. Micro molding benefits may be exploited in various electronics components manufacturing. Micro-optics might be one of some examples (e.g., manufacturing laser-based devices, smart phones, lenses, prisms, etc.). As well as microelectronic components: such as connectors, switches, plugs, computer chips, etc. for computers, communication technologies, musical devices and other microelectronics fields.

Microelectromechanical systems (MEMS) often require micro molding manufacturing, too. Since the industry itself is in the growth stage, the demand for innovative micro molding in manufacturing processes is increasing as well. For instance, BioMEMS (Biomedical Micro-Electro-Mechanical Systems) are now being widely investigated and potential Next Generation Sequencing (NGS) and Point-of-Care diagnosing opportunities already applied which significantly increases the demand for MEMS.

The rapid development of modern technologies leads to a dramatic growth in electronic industries and this might be represented by the fact that electronic sector holds a little bit more than a fifth of the global micro injection molding market share (ibid).

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Automotive industries

Micro injection molding is quite widely used for manufacturing automobiles’ components which frequently require light and small components. Micro molding is used for under the hood parts (e.g., engine or breaks) of a car and for various other components relevant for automotive industry, such as different clips, washers, door locking mechanism parts, various buttons, switches and even for micro plastic gear manufacturing. Since the whole automotive industry is huge and requires many micro parts, no wonder why this sector captures the most value (almost a third) from micro molding (ibid).

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How far could thin wall molding go?

Firstly, to discuss thin wall molding, the concept itself should be clarified. Thin wall molding can be classified according 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. Here are the maximums of L/t ratios for 10 of the most widely used thermoplastics:

ABS: 170/1
SAN: 120/1
PA: 150/1
PC: 100/1
HDPE: 225/1
LDPE: 275/1
PP: 250/1
PMMA: 130/1
POM: 150/1
PS: 200/1

The quality of the molded part is highly dependent on correct design of wall thickness. By highlighting ‘correct’ design it is meant to choose compatible ranges of wall thickness for various thermoplastics and to maintain similar aspect ratios throughout whole part design process. Failing in this design for manufacturability stage may lead to:

Timely cycles, since thicker walls cool longer than the thin ones;

Too thin a wall might be too fragile and in addition, may cause flow rate (the speed of flowing into cavities) errors. The latter issue may result in voids if material does not fill all the features before it cools;
Uneven walls cool and solidify differently, and this factor is usually a reason why there might exist any unintended permanent warps or sink marks on the molded part surfaces.

Since thin wall molding is primarily dependent on resins choice it is good to refer on some experimental data. The table below demonstrates the most widely used plastic materials with minimum and maximum wall ranges for injection molding:

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MATERIAL

ABS

ACETAL

ACRYLIC

NYLON (POLYAMIDE)

POLYCARBONATE

POLYESTER

POLYETHYLENE

POLYPROPYLENE

POLYSTYRENE

POLYURETHANE

APPLICATION

Mostly for plumbing or automotive industry

May replace some parts that used to be metallic Mostly replacing glass for beauty, fashion or even art industries

Various industrial and mechanical uses

Used in a wide range of markets

Perfect for disposable and recyclable products

Various application possibilities, however, frequently used in food industries, since it does not leach chemicals

Applicable in various industries

WALL THICKNESS

0.143 mm – 3.556 mm 0.762 mm – 3.048 mm 0.635 mm – 12.70 mm

0.762 mm – 2.921 mm 1.016 mm – 3.810 mm

0.635 mm – 3.175 mm 0.762 mm – 5.080 mm

0.635 mm – 3.810 mm

0.889 mm – 3.810 mm

2.032 mm – 19.05 mm

When resin material is chosen some other requirements must be met for thin wall molding. Since thin walls cool faster than thick walls, thin wall molding requires the higher speed of cavity-filling (fill time indicates the time required for the material to flow into cavities). For instance, a 25% drop in wall thickness needs a 50% drop in time of injection. Thin wall manufacturing requires specialized machinery to process higher speed and pressure. Even though modern technologies allow standard machinery to fill thinner and thinner parts, the tiniest parts require more advanced machines for both, injection and clamping, cycles.

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What materials are the best for micro injection molding?

There is a high variety of materials that may be used in micro molding. However, there definitely are some crucial constraints not to forget while choosing materials, such as: mechanical properties (what are expected operating environment, high-heat situations, hygroscopic properties?) compatibility (contact with other biological bodies, cosmetic appearance and price. Some of the most popular materials for micro molding are shown in the table below.

MATERIAL

LCP (liquid crystal polymer)

PMMA (polymethylmethacrylate)

COCs (cyclic olefin copolymers)

PEEK (polyether ether ketone)

PLA (polylactic acid)

PGA (polyglycolic acid)

Polyethylene

Polypropylene

Polycarbonate

APPLICATION

High temperature tolerance; Great chemical and weathering resistance; Stress cracking resistance

Great transparency; Ultraviolet radiation resistance; Scratching resistance

Great flowability; Heat, chemical, moisture resistance; High clarity

High chemical resistance; Great heat and pressure tolerance; Stress-crack resistance and high strength

Biodegradability; High transparency; Great compatibility

Biodegradability; High strength; High abrasion and solvent resistance

Great chemical resistance; High strength and surface hardness; Abrasion resistance

Great chemical and heat resistance; High flexural strength and fatigue resistance; Electrical insulation

High transparency and high dimensional stability; Rigidity and toughness; Moisture and chemical resistance

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It should also be highlighted that with the rapid technological development and the growth of demand, there is an increase in the use of bioabsorbable polymers in micro injection molding. Bioabsorbable materials are widely applicable in modern healthcare. Since these polymers may be absorbed and dissolved by a human organism, the use of them lowers the number of surgical interventions needed for specific (most usually, orthopedic) treatments. Along with the innovations, grows the demand for the applications of these materials and this is where modern micro molding technologies are used, too.

What is the future of micro injection molding technology?

Plastic injection molding is used in a majority of industries across the world. Old manufacturing technologies are replaced or upgraded by the new ones and the industry 4.0 is catalysing all of it. Micromolding is not an exception and thus must to remain innovative and to adapt to the new market demands where components are getting smaller and smaller. For this reason, new technologies are being developed to improve micromolding:

Significant progress in substance control. The most visible progress is that companies are trying to research the recycling of polymers and this research is associated with environmental considerations;
New innovations depend on customer needs, it’s because sometimes they require something that companies cannot create. This demand puts a lot of pressure on manufacturer and for this reason new technologies are being produced, for example, extreme thin wall molding, 2-shot micro molding and automatic insert molding, are direct results of the market demands;
New micro molding sensors have been specially adapted to the mould, previously the sensors were too large. New sensors are very compact, easily installed, save significant space in the mold and are designed to monitor temperature, pressure, warpage, shrinkage and others processes;
One of the latest innovations are CNC machine tools and micro sinker EDM. These devices allow molders to inject shots of less than 1 gram with minimal damage and very high shot accuracy. Advances in shrinking pressure and temperature sensors hardware allow for better control and real-time monitoring of the process.

Runnerless or reduced flow path molds are designed to save expensive materials and it will allow machine manufacturers redesign to achieve high accuracy and ultra-small shot sizes. New advances include non-standard material designs, improved reduced wall thickness filling options, stress removal and mold annealing, improved mold and material monitoring systems.

Challenges in micromolding: micro assembly and packaging

Packaging and micro assembly cost is a big portion of the overall cost of any micro-scaled product and it is an important part of the development of a microscopic product. Efficient packaging and assembly is a key for success products in the marketplace.

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The main reason for the cost of packaging and assembly of micro-scaled products is the lack of automation in both of these operations. Most micro-assembly requires the use of operators to manually select and insert small parts using powerful microscopes and micro-tweezers. Manual assembly is extremely expensive and takes a lot of time. Operators who are assembling such a micro-scaled parts, suffer from the tension on the eye strain, have strict requirements for the final product, but must also achieve the required reliability of its quality.

To make micro-assembly easier and much quicker, several specific tools and equipment must be available for this process:

Visual system with high-performance stereo microscope, long-lasting distance and high resolution camera and monitor. The latter is used to provide instructions and feedback during and after assembly;
Micro-positioner with a resolution of 40 nm for workpiece control, microgripper and position management;
Real-time computer vision for controlling servo mechanisms and motors and assemble parts within micron level accuracy;

High resolution, high precision transfer tool for handling parts and components.

If you are making a micro-scaled product and you don’t want to assemble it under the microscope, there are some methods that helps to combine different parts together at the design stage:

Two-shot micro molding. This method let to inject two different materials into a mold at two different or in the same place.
Ultrasonic welding. It is affective when joining thermoplastics and compatible metals;
Laser welding. This is usually used for joining micro components, when 3D geometry cannot be combined through overmolding. Laser welding also can be used to clean and disassemble materials such as wires quickly and without breaking them;
Staking. This is a very cheap way for assemble polymer and metal by using folding of one material into another;
Solvent bonding is known as cheaper and faster way for joining micro-scaled components. Typically, it is combining different materials and solvents, using micro and nano pipettes. Those two components, must be bonded together, especially if this combination will be used as an implant.

Packaging of micro components is as important as micro-assembly. Each micro-scaled part must be delivered to the customer safely. When sending small, sharp or friction and vibration sensitive parts, packaging can be a very difficult process, it has to be well thought out. Micro-packaging requires components to be individually packed in special packages or pallets. When dealing with clean room requirements or ISO 13485 quality assurance it is also very important to ensure an appropriate temperature of machines and airflow around it. Usually it is a must to have fans generating filtered airstreams to prevent air contamination and dust attaching to the molded parts until they are packed.

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