MOLDING MICRO SURFACE FEATURES
9/19/13 Here’s something that always causes a few eyebrows to be raised, and which exemplifies the progress that has been made in micro molding in recent months and years.
Micro molding per se is now obviously an established technology, and we at Micro Engineering Solutions have been involved with hundreds of successful projects with numerous OEMs over the years. But sometimes even we sit back and go “Wow!!”.
The picture you see with this blog is of a silicone microfluidic device with injection molded features that are 3 microns deep. 84 million such structures can fit onto a microscope slide. Until relatively recently, this would have been impossible, but now that we are able to create such tiny surface features with such accuracy and repeatability, the possibilities that are opened up across industry are endless.
Let’s start with the basics, however. Not everyone can manufacture parts with such micro surface features. To achieve such precision requires the combination of significant experience in micro molding, and an innate understanding of the specific contingencies that affect manufacturing at such a scale.
The key focus, quite obviously, is on tooling. Gate location, for example, can help to eliminate hesitation problems, where surface defects occur due to stagnation of polymer melt flow over thin-sectioned areas. Don’t forget also that gates in micro molding are often as small as 75 microns, which has a huge impact on shear heat.
Feature definition can also be significantly enhanced through vacuum venting which eliminates gas entrapment. As in common with many areas of micro manufacturing, however, vacuum venting is not always as simple as it may seem. While in terms of feature definition it typically produces positive effects, depth ratio can be affected significantly by material wettability and melt viscosity, and processing conditions such as injection velocity and mold temperature.
Experience quickly teaches you that when dealing with such tiny surface features, positive features are much easier to replicate than negative features. Also, retarding heat transfer significantly enhances feature replication. In some instances, especially when using semi-crystalline polymers, replication is optimised through the use of gas assisted micro injection molding.
For any micro feature in a tool, it is vital that the full depth of the feature is replicated precisely, and to achieve this requires a forensic understanding of material viscosity characteristics and solidification times. Replication is sensitive to flow direction, and can be improved by increasing cavity pressure and temperature within a certain range. Success is only achieved when the feature is replicated perfectly thousands and thousands of times. There is no room for half measures.
The critical nature of the process is exacerbated when it is considered where components and parts with such micro surface features are used. They are typically found in micro- and nanofluidic devices such as micro-pumps, valves, and flow sensors; micro-filters and micro-reactors; and nano-filters, channels, and membranes.
Very often, such products are used in the medical device sector, which in many ways is driving the requirement for smaller and smaller surface features, often in the area of microfluidics. Some of these components are in direct contact with blood and other body fluids, and there are also numerous optical and dental applications. In addition, the use of micro parts with micro surface features in an array of bioresorbable devices and permanently implanted devices is growing exponentially. So perfect and reliable replication from part to part is of paramount importance. It really can be a matter of life or death!
The tooling for parts with such small features can be made using a variety of technologies in various materials. For example, CNC machining, micro milling, and micro wire EDM are most often used to make steel tools. Electroforming techniques can be used to make nickel alloy tools, and UV, EUV, and E-beam lithography can make tools from silicon or glass.
Once again, the equipment used for such tooling processes is not commonly available to all, and requires that OEMs access it and harness its potential— and the necessary expertise to use it — via qualified and respected sub-contractors such as MES.
At MES, we offer a service that works closely with clients at every stage of the design to manufacture process. It is becoming increasingly the case that OEMs now see the value in “partnering” with sub-contract companies such as us in order to benefit from the 360 degree view we have of the process of developing such micro parts, components, and features.
For example, due to the enormous manufacturing challenges touched upon above, it is very rarely if ever the case that we are approached with a design that does not require reiteration to allow it to be manufactured at the size required or in one of a range of innovative materials that are often used in medical applications. It is only by tapping into a company such as MES, that such input is possible, and it obviously more cost-effective to engage us early in the design stage in order to avoid unnecessary reworking. The focus always comes back to one thing — design for manufacture.
So, what about our 84 million structures on a microscope slide? Well, obviously, the ability to able to manufacture with such micro surface details is central to a number of new innovations in a number of industry sectors. Key to the possibilities opened up, for example, is the way that such features add to the surface area of the part, and therefore increase it adhesive properties exponentially.
So, whether you are concerned with such cutting-edge micro molding developments, or something a little less ground-breaking, be sure to discuss your project with MES as early as possible. It is almost certainly the case that we will have ideas that you have not considered, and can assist you at the early design stage to ensure success in mass manufacture and replication.
Donna Bibber, President of MES would be delighted to discuss your requirements, and can be contacted on t. +1 (774) 230-3459 or e. donna@microengineeringsolutions.com
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