Creating microfluidic chips is a difficult and precise process. Manufacturers have several options available for creating small channels, holes, and V-grooves. While some use chip making techniques like embossing, photolithography, or micro 3D printing for these purposes, micro milling microfluidic chips is still the preferred option for prototyping, though not for serial production.
However, micro milling is useful for much more than making chips. Manufacturers can also use it for making the tools used in the injection molding process. However, the main use of micro milling is that it allows the rapid creation of chips for prototyping. This article explains what micro milling is and the issues manufacturers must consider when using this technique.
What Is Micro Milling?
Micro milling is a subtractive manufacturing process that leverages cutting tools to remove material from a part. Manufacturers create a 3D model of the intended part, such as a chip, using computer-aided design (CAD) software. With the design in place, the micro milling process uses rotating cutting tools to remove material until the part matches the design.
With micro milling, manufacturers can narrow features down to micron-level making it a flexible process ideal for creating micro components that have complex shapes. The process’s speed and adaptability make it ideal for rapid prototyping of microfluidic chips.
The Benefits of Micro Milling Microfluidic Chips
There are several advantages to micro milling microfluidic chips:
· The process requires no complex tooling, which saves enormous amounts of time for manufacturers. As a result, micro milling is the ideal option for rapid chip prototyping, allowing manufacturers to test their designs before beginning a full production run.
· Having the ability to cut various types of materials using micro milling make it a viable option for quickly testing various materials for microfluidic devices.
· Micro milling allows for the fabrication of multi-height features, which is difficult to achieve with photo-lithography.
The Limitations of Micro Milling
Though micro milling has many applications, there are some limitations manufacturers must consider before using the process.
Aspect Ratio
Endmills and drills are rarely good options when the manufacturer needs to create small features that have high aspect ratios. For example, 200µm diameter endmills typically achieve a top aspect ratio of 3:1, which may be not enough for the part.
Furthermore, increasing the diameter of an endmill leads to higher aspect ratios being required due to the tool becoming more rigid. As such, manufacturers may find it more difficult to fabricate high-aspect ratio features, such as narrow and deep trenches, with micro milling.
Run-Out
Run out is a discrepancy of the cutting path and tool diameter at a certain spot along the outer edge of the cut. When tool rotates it is crucial that each flute would hit at the exact same point along the cutting path.
In the research paper “A Study of Surface Roughness in the Micro-End-Milling Process,” researchers from UC Berkely examined the effects of run-out on the micro milling process.
They discovered that run-out plays a significant role in the surface quality achieved when micro milling parts. Using 6061 aluminum, they found that the dominant cutting marks on the material had a period of twice the chip load. This means that one cutting edge makes a
deeper cut than the other, leading to an uneven cut.
Still, the study concluded that micro milling can produce a high surface quality if the manufacturer accounts for run out.
Surface Roughness and Resolution
Surface roughness is the measurement of the smoothness of a surface’s profile. It’s typically calculated by measuring microscopic variations from the peak and valley of the surface.
In milling, surface roughness influences how a part interacts with its surrounding environment. Visible machining lines, which micro milling may produce, can affect how a microfluidic chip interacts with the parts that surround and support it.
Burr Formation
Burrs come into play in both polymer and metal manufacturing. The term refers to the formation of rough ridges or edges on a piece. The presence of burrs reduces part safety, creates additional stress during part operation, and makes the part more susceptible to corrosion.
Unfortunately, micro milling is one of several machining processes that create burrs. As such, manufacturers must use a micro-deburring technique when micro milling microfluidic chips. Examples of this include the following:
· Hand deburring involves special technicians removing burrs while examining the part under a high-resolution microscope.
· Waterjet deburring uses highly focused and pressurized water streams to remove burrs.
· Thermal deburring burns off the burrs using an explosive gas mixture to create thermal energy.
· Electromechanical deburring combines a salt solution with electricity to dissolve burrs without affecting the surrounding material.
Internal Radius
In micro milling, any machinery a manufacturer uses has limitations in terms of it’s the angles, curves, and contours it can achieve. These machines typically have a corner radius, which is a term that refers to the internal radius of the corners of the machined part.
Due to rotational nature of the milling the tools are round and thus they have some radius which is unavoidable. This means that while milling internal pockets the corners will be with radiuses which will be equal half the diameter of the tool. However, this is not valid for outer corners.
Alternatives to Micro Milling
Micro milling isn’t the only option available to manufacturers when creating microfluidic chips.
Laser Machining/Ablation
Laser machining uses laser pulses to create structures and cut holes. It works in a similarly subtractive way to micro milling. However, in this case, the process involves using laser light to vaporize unwanted material. This creates a clean process and allows for greater flexibility in designs. On the other hand, the process is relatively slow and thus very expensive. On average 1 cubic centimeter can be ablated in one full day for the metal material.
Photolithography
Photolithography involves coating the material with a photoresist layer before exposing it to a precise pattern of intense ultraviolet light. This process allows for the creation of extremely small features. However, it’s only suitable when used with flat substrates. Plus, the process is far more expensive than micro milling and even laser ablation.
Selective Laser Etching
This technique allows the manufacturing of complex features by laser affected and later etched out material, which often makes it useful when creating biomedical devices. Selective laser etching also offers a short time-to-market and offers few constraints when designing part geometry.
Consider Micro Milling for Microfluidic Chips
Micro milling microfluidic chips is an effective technique because micro milling offers short lead times and excellent flexibility. This combination allows for the rapid creation of complex geometries, making it ideal for prototyping and production processes that require fast turnarounds.
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