What is Micro Moulding? As an engineer involved in moulding I spend quite a bit of time in the tool room. I do not consider myself a toolmaker, or even a machinist. The time that I worked with machine tools would probably be considered the Dark Ages by most of the younger toolmakers in the shop at my company. Nevertheless, I am a moulding engineer, and the moulds made by these artisans I call toolmakers are a critical part of my job. It is in my best interest to understand the machines, the people and the techniques used to build these moulds. Introduction
I attended a session at a recent conference, and the discussion centred on micro moulding. Everyone in attendance agreed that there is no hard and fast definition of a ‘micro’ part, but everyone knows one when they see it.

Donna Bibber of Micro Engineering Solutions is a renowned expert in the field of micro parts. She stated in her talk that with a good mould you are 85% of the way to success. Her comment was met with nods of agreement throughout the audience.

The two days of the conference were filled with discussions about everything small: micro parts, micro assemblies, and micro moulds. After I returned to my regular job, I thought further about what I had learned during this event. Two main points stuck with me.

First, the largest barrier to successful micro moulding is a good mould, and second, finding a skilled tool shop to make the mould needed for producing quality micro parts is a difficult task.

The Art of Cutting Metal
Metalworking has a rich history. Some would argue that the origins lie in the Bronze Age over 5000 years ago, and were continued by the Romans 2000 years ago. I love history, but let’s not go back that far. We will take a look at the machines past and present in order to understand where we stand today. I will discuss two basic methods of metal removal, mechanical (milling) and electrical (EDM).

Milling
Bridgeport has been making milling machines since the late 1930s. Although the machines have changed over the years, the concept of the spindle holding a turning cutter is still the same. The largest advancements until recent times were in the control of the cutter movement. The birth of the CNC (computer numerical controlled) mill revolutionised the machine tool industry.

Fast-forward to today. High-speed mills routinely turn cutters at 30,000 rpm with precise motion through advanced spindle design and complex computer control. The toolmaker no longer worries about the hardness of the metal. The game has changed. The physics of cutting metal with the cutter turning at high speed (20,000 rpm+) is vastly different than using the Bridgeport with a spindle turning at 120 rpm. Today, 0.5 mm carbide ball end mills with exotic coatings easily machine Rockwell 52 hardened stainless steel.

EDM
The concept of EDM (Electrical Discharge Machining) goes back to the late 1700s in England with Joseph Priestly. Some may disagree, but the work Priestly did to increase the understanding of conductivity between materials and interactions between them set the stage for future research. This was a long road, even though the process was well understood in the 1940s, the ability to move the electrode in a precise way prevented the advancement of this method of metal removal. Once the proper control systems were developed, ‘cutting metal’ with electricity was fully commercialised in the mid-1970s.

Micro Erosion or Micro EDM is an extension of EDM machining. The concept is the same, but the miniaturisation of the components has allowed very small features to be created in the metal. The advances in position control systems allow this tiny electrode to cut three-dimensional features into steel. The electrode, because of its small size, is actually dressed by the machine itself. There are a few manufacturers of these devices. I will leave it to you as homework to find out the exact specifications offered by each manufacturer.

Wire EDM is yet another type of EDM machining. This is a process whereby a wire is drawn through the metal. Traditionally, wire EDM has been used to create tapered gates and other circular features in moulds. Similar to the previous metalworking technologies, the advancements have been amazing. Early wire machines used 0.010 inch diameter wires to cut the metal. The battle for miniaturisation took the industry down to 0.002 inch wire. The latest offerings are pulling a 0.0008 inch diameter tungsten wire with 0.0004 inch on the horizon. Just for reference, a human hair is generally 0.003 inches in diameter. Again, the advances in the control systems have made it possible to use such small wires.

The Barriers to Micro Mould Making
Understanding the basics of the machines that make moulds is important, but as we move into the world of micro moulding this understanding is much more important. The machines and methods that have served the mould making industry well even five years ago may prove to be inadequate. The mould maker needs access to the proper tools to create the ever shrinking features that engineers require in part designs.

Some equipment in the tool room is adequate for construction of micro moulds, other machines are not. This is the point at which it is important to understand the technology. Speak with the mould makers and take inventory of their technology needs.

Milling. The only limitations of modern high-speed milling machines in my opinion is the size of the cutter. If the feature is physically too small the cutter just won’t fit. Sometimes, in this case, it is necessary to move to another technology, such as EDM. Do not underestimate the ability of the toolmaker to manipulate a 0.5 mm ball end mill. The world of high-speed precision milling is vastly different than that of the old Bridgeport. The physics behind the ability to mill-hardened steel are nothing short of amazing. To compete in the world of micro moulding it is critical to be able to use the smallest tooling available. Commercially available precision CNC mills can run this small tooling, but the ability to machine hardened steel is highly desired. The old Bridgeport Mill will need to be moved into the corner of the shop and a high-speed CNC milling machine will have to take its place.

EDM. Traditional EDM using copper or graphite electrodes is commonplace in mould making. The challenge is placing the tiny features required on the part into the mould. Modern EDM sinker machines are highly capable of holding micro tolerances and will fare well, as long as the tool shop has the ability to create the fine detail that will be necessary in the electrodes.

Wire EDM machines have undergone many changes in the last few years. What was considered to be a state-of-the-art machine just a few years ago may not make it in the micro mould making arena. Micro mould making requires pulling ever smaller diameter wires. A machine that does not have the ability to use 0.002 inch diameter wire may not be sufficient. As I stated before, do not underestimate the ability of the toolmaker to “tweak” the machine and make it perform admirably, but there are limitations. I have observed the use of a 0.0008 inch diameter wire in my organisation, and was told by the toolmaker that in this particular instance there was no way that a larger diameter wire would work. Micro erosion machines did not exist when I first got into the field of engineering. If your tool room has one I am certain that it is a fairly new acquisition. This technology is so new to my organisation that I am still trying to figure out how we will use this fascinating machine. Even though I am still learning the capabilities of micro erosion, I know enough to understand that it will have a prominent position in our future micro mould making endeavours.

Conclusion
Together we have taken a long journey from the past to the present. We have looked at some of the technologies used by the toolmakers to create moulds, namely EDM and milling. My main purpose of this overview was to give you a chance to take inventory of the technologies you currently have, and understand the steps you need to take to compete in the world of micro moulding. Some of the legacy equipment can be utilised to make micro mould tooling and some cannot. It is critical that you know where you stand.

When I speak to people about the capability of an organisation to participate in the world of micro mould making, I always use my racing car analogy. You can have the best racing car in the world. It may be built to compete in the Indy 500. However, it is useless without a highly skilled driver. Likewise, the tool shop not only needs the best technology with regards to equipment, it must also have highly skilled toolmakers.

Photographs in this article were taken by Megan LeMieux.

Loren Evers is a Principal Manufacturing Engineer at Medtronic. His area of expertise is silicone rubber moulding, with an emphasis on micro moulding and tool design. He supports the commercial side of the silicone moulding operation, as well as being highly involved in moulding R&D throughout the company. He has a Bachelors Degree in Mechanical Engineering and a Masters Degree in Manufacturing Systems. Loren is a certified Post Secondary Teacher and he spends time educating engineers and managers on the technology of micro moulding.