As micro injection molding continues to take a fair share of the manufacturing market, it becomes clear that micro mold manufacturing is a critical step to guarantee high-precision and high-quality results. The higher the precision and the quality of the micro mold manufacturing, the higher precision and quality you can get from micro injection molding.
This guide covers all the details regarding the micro mold manufacturing process, how they are machined to the tightest tolerance possible for higher precision, what are the types of micro machining that can be applied, and the technologies behind the micro machining process.
Table of contents
What is micro machining?
How are micro molds machined?
Types of micro machining methods
Non-mechanical methods
Mechanical methods
How to select the best alternative for micro mold manufacturing
What is micro machining?
Micro machining can be defined as the manufacturing process used to create 3D parts at a micro scale level. In other words, it means machining with tools with diameters that are smaller than 400 µm and can be as small as 1/3 the diameter of a human hair.
Although it has existed since the late 1990s, the most recent developments in micro machining technology and tooling materials have made it possible to see machines with sufficient spindle speed and strong long-lasting cutting tools that can meet the repeatability and strength to run at high speeds as required for micro mold manufacturing.
How micro molds are machined?
When machining molds for micro injection molding, it is important to pay attention to three essential aspects:
Part size
Feature size
Dimensional tolerances
Another important aspect to keep in mind to obtain the best results when machining micro molds is that there is a direct relationship between the material of the mold, the cutting tools and the machining process itself, so making sure they are in synchrony is vital.
Of course, there are several different micro machining processes, so it is necessary to select the most suitable option.
Types of micro machining methods
The types of micro machining methods for micro mold manufacturing can be classified into two categories: non-mechanical methods, and mechanical methods.
Non-mechanical methods for micro mold manufacturing
Non-mechanical methods for micro mold manufacturing include chemical processes such as wet and dry etching are more frequently used to produce molds for very specialized applications, especially within the optical and biology fields. However, the geometries that can be achieved with these processes are limited.
On the other hand, among the non-mechanical methods for micro mold manufacturing, there is one that is worth highlighting: Micro EDM.
Micro EDM for micro mold manufacturing
EDM stands for Electrical Discharge Machining, so micro EDM is the process that applies traditional electrical discharge machining but at the micro scale level.
Micro EDM is a great alternative for micro mold manufacturing because it allows the possibility of achieving both concave and convex microstructures, including the most complex 3D microstructures with high aspect ratio that may be required by several micro injection molding applications.
Micro EDM consists in taking advantage of the erosive action of an electrical discharge between a conductive tool (electrode) and the workpiece to achieve material removal. This is normally done in one of two forms:
The electrode is made to the desired shape of the cavity that is required. This electrode is then fed vertically over the workpiece, thus eroding the reverse shape into it.
Using a very thin wire electrode with a diameter within the micro scale, the desired shape is eroded as the electrode follows the path that has been programmed into the special CNC machine.
When using micro EDM for micro mold manufacturing, there are three factors that need to be taken into account in order to achieve the best performance. These factors are:
Melting point of the materials
Thermal conductivity of the materials
Electrical conductivity of the materials
According to an experimental study on impulse discharge machinability performed in 2018 by Quanpeng He, Jin Xie, Ruibin Guo, Peixin Ma and Yanjun Lu,
“A low melting point and electrical conductivity result in a good micro-machined shape with a low relative wear rate. High electrical conductivity and a low melting point produce low surface roughness, high micro-removal rate, and high discharge energy efficiency. Low thermal conductivity leads to a high aspect ratio and low micro-removal rate”.
Micro EDM technology for micro mold manufacturing
Micro EDM technology has been widely developed, and the options for micro mold manufacturing include highly capable micro EDM machining centers that feature:
Twin axis processing.
Combining processes such as Micro EDM Drilling, Micro EDM Sinking, Wire EDM Electrode grinding, and 3D Micro EDM Milling.
Tools integration.
8-axis control.
Possibility to change between electrodes with different diameters
Mechanical methods for micro mold manufacturing
While micro EDM is definitely a very cost-effective process that can achieve micro mold manufacturing with high quality and precision, there are some micro injection molding applications that require molds with even more geometric freedom and lower surface roughness. And here is where the mechanical methods for micro mold manufacturing excel.
The main idea of using mechanical methods for micro mold manufacturing is taking advantage of the most modern tooling developments for micro machining which use diamond to achieve surfaces with precision to the micro scale level without the need for post processing.
Of course, this type of high precision machining can only be possible thanks to the current CNC technologies, which allow experience manufactures to produce microfeatures as precise that can be measured to values below the 5µm. An important achievement of these technologies is the high speed the spindle can reach, which allows to avoid chip build up and heat concentration in micro machining applications.
Among the most common CNC high precision processes applied for micro mold manufacturing there are micro turning, micro milling, and micro grinding. And these three are usually performed in combination to achieve the desired results.
Micro machining technology for micro mold manufacturing
The most common micro machining technology used in the present for micro mold manufacturing is high-speed micro milling. In micro milling processes, normally a diamond tool rotates on a spindle and moves along the surface of the fixed workpiece. During milling operations, the tool rotates along the axis perpendicular to the workpiece. At least three numerically controlled axes are used in this process.
Common factors that affect the efficiency and the quality of high-speed micro milling for micro mold manufacturing include:
The corner radius of the milling tool.
Optimization of the tool path.
Tool wear.
Angle of the cutter axis. This should always have an inclination to avoid a cutting point with a speed value of 0.
Of course, there are other technologies that can be used such as single-point diamond turning, fly cutting, and vibration assisted cutting technologies. However, they are mostly used for highly specialized applications such as optics.
How to select the best alternative for micro mold manufacturing
For a micro injection molding manufacturer to select the best alternative among the available processes for micro mold manufacturing, it is necessary to consider the specific requirements of the part that will be molded and possible constraints, costs, the number of times the mold will be used (durability or life span), and geometrical complexity.
For example, when it comes to costs, a higher removal rate increases production rate and a higher degree of automation reduces labor costs. So, the production rate of the manufacturing process and the degree of automation it requires will be critical factors to analyze.
Regarding the mold durability, it usually depends on the material and how it resists high-temperature and high-pressure cycles. For example, aluminum molds may last up to 200 cycles while being made two times faster and cheaper than steel molds.
When it comes to geometrical complexity, the higher the complexity the lower the micro milling machinability, so other methods like micro EDM may be required.
There are some studies that have provided good information such as tables for complexity index, and a set of rules to select between micro milling machining and micro EDM. However, it is important to remember that they are not completely infallible.
Based on all these factors, making the best selection usually requires a certain degree of expertise. Many times, the best solution is to combine both micro milling with micro EDM to obtain the micro mold manufacturing result needed for the micro injection molding application, but this should only be used when the increased cost and extra processing time is justified.
Conclusion
As a conclusion, the best course of action when deciding between high-speed micro machining and micro EDM to obtain the most suitable mold for the micro injection molding application in hand is to leave it in the hands of an expert.