Loren Evers, Senior Manufacturing Engineer, Medtronic, St Paul, MN, USA
Moulding of silicone rubber parts is commonplace, but moulding as an industry is today undergoing dramatic change. Large segments of the moulding business are moving to countries with inexpensive labour rates, and the parts of the industry that are still in the West are refocusing their resources and priorities. Mould presses that once ran 24 hours a day are now standing idle. The game has changed. As the moulding industry gets smaller, so are the parts we are being asked to manufacture. The moulding of small parts is becoming more frequent. Over the last two years, every time I get a call from a design engineer asking me to work on a new moulded part, it is always smaller and usually more complex than the one before. I work in the biomedical world, and I can tell you with confidence that moulded parts in my field are not getting bigger!
Traditionally, design engineers created a part drawing, and then “threw it over the wall” to the moulding department. “Make a tool and get me some parts,” was the cry. If this sounds familiar, don’t worry. Even in large and well organised companies, this was the standard mode of operation, and it would still work if we were making large parts with simple geometries. My design engineer colleagues, however, know that the game has changed. Complex geometries and micro sized parts require that existing technologies are reworked. Design cycles and time constraints are becoming tighter. Cooperation between engineering disciplines is now vital to ensure that a company stays competitive and brings products to market more quickly. I am finding that many of the tools and techniques that served me well for so long in the moulding industry simply don’t work anymore.
Manufacturing Conditions
Traditional manufacturing conditions may not apply to modern moulding businesses. For example, moulded parts for the medical industry may now require moulding equipment to be housed in a controlled environment. Human implantable parts most often are manufactured in a Class 10,000 or ISO Class 7 clean room, and particle sizes and counts are controlled as well as temperature and humidity. Manufacturing spaces with controlled environment conditions are challenging and expensive to operate, but may now be a necessity. Whether implanted in the human body or not, contamination control is of the utmost importance when moulding very small parts.
Another challenging manufacturing condition is that of regulation. Biomedical manufacturing in the United States, for example, falls under governmental guidelines set by the Food and Drug Administration (FDA). Quality system regulations outlined in the ISO standards set the course of manufacturing for most other parts of the world, so even if you are not making biomedical grade parts, good manufacturing practices (GMPs) are a necessity.
Clean environments and governmental regulation are only two examples of the many possible conditions under which moulding operations may be required to operate. Although they are not directly part of the manufacturing operation, they do become necessary in many of today’s advanced manufacturing systems.
Materials
There is a large variety of materials that can be selected for moulding. Every time I chat with my material science friends, or open a moulding trade magazine, I learn about the new materials that are available. Sadly, I am unable to use many of these exciting new materials in the biomedical moulding world. Any material used in medical devices — and especially human implantable devices — must undergo rigorous testing to become qualified for production use. The material I know most about — and use almost exclusively — is silicone rubber.
All silicones are not created equal. I am constrained by my list of “qualified” materials, which have undergone all the necessary testing to become qualified. This is an internal company process and can take quite some time, as well as consuming large sums of money. When I hear about a new and improved material, I realise that even though it may solve a potential engineering challenge, it could be years before I can make it available for production use.
As a moulding engineer that makes human implantable parts, silicone rubber is my material of choice, even though it is considered a legacy material by many material scientists. Although it has been around a long time, however, don’t sell it short. Silicone rubber is one of the most biocompatible materials known to man, and its wonderful inert properties make it the most widely used material in human implantable medical devices. Traditionally silicone rubber materials came to us in a gumstock form, usually in large blocks. This material is usually transfer or compression moulded in presses using either hot or cold methods. Before use it requires milling or mixing of the two parts. During the moulding process the material needs to be vulcanised to fully crosslink the polymer. This heating process often occurs during moulding, although sometimes the parts must undergo time in an oven to fully complete the process. These gumstock materials served the industry well in the past, and are still used today in certain applications.
The advent of liquid silicone rubber materials (LSR), however, changed the rubber moulding industry forever. Although it is called “liquid”, this is a misnomer. LSR flows much more readily than gumstock rubber, but it almost has the consistency of peanut butter. Liquid silicone rubber comes in pails or drums, and unlike the gumstock requires no pre-processing. It still comes in two parts, but it can be pumped out of the barrels and directly into the injection system, and mixing of the two parts can happen on the fly. Micro moulding and complex geometries are much easier to attain when using the liquid silicones. Material changeover, or addition of additives such as colour, become trivial tasks as they can be mixed in as a third stream through the injection moulding equipment. Liquid silicone rubber moulding is therefore very close to a truly continuous process.
Equipment
Moulding equipment can be found everywhere these days. Old transfer presses can easily be purchased from used equipment vendors, and you can even find moulding presses on e-Bay! I am sorry to tell you, however, that most of these presses will not work for modern micro moulding applications. These machines will work fine if your business is moulding large parts, but there isn’t a lot of that happening these days in the biomedical field. We are in the micro moulding business, and therefore we require low tonnage presses. A 5 ton press is generally more than adequate, although some of the production units can be up to 20 tons. Finding small moulding presses that meet your requirements is not an easy task, as there are not a lot of vendors out there making them. Different styles of moulding equipment have different utility requirements. As I stated earlier, for example, if manufacturing is done in a cleanroom, an all-electric moulding press may be preferable.
Gumstock materials require a transfer press. These simple machines use a ram and piston assembly to forcefully push the rubber into the mould to form parts. It doesn’t get much simpler than this. You control the speed of the transfer and the pressure of the transfer along with the clamp pressure on the mould. Generally the mould platens are heated and the vulcanisation of the rubber occurs during the moulding process. Some of the cold transfer operations may require a heat press to vulcanise the rubber after the mould is filled at a room temperature state. Moulding of micro parts can be quite challenging in the transfer moulding arena, and the ability to get this material to properly fill the mould and give you a complete part is not always an easy task.
A press that uses liquid silicones is a much more complex animal than the transfer press. With the complexity, though, comes the ability to do amazing things, especially in the world of micro moulding. As stated in the materials section of this article, liquid silicone rubber comes in buckets or drums, and it is necessary to pump this material from the bucket and deliver it to the injection part of the press. Some systems are able to do this in one operation, but many use a pumping station to deliver the material at lower pressures to the injection head, where the material is mixed and brought up to the high pressures necessary to inject into the mould. Different vendors use different mixing and injection methods, and in a future article I will discuss the pros and cons of these methods.
Options on machines allow for control of the injection pressure, rate, and even the shot size. There is no right or wrong way to inject the rubber, only different ways, and it really depends on the part size and the material used in the application as to the method used.
Liquid silicone machines shine in the micro moulding arena. This material flows easily into the tool, and will fill out the part regardless of the size and feature complexity. This bold statement of course is true only if you have a well designed mould. Mould design is fundamentally different when using gumstock rubber, or a liquid silicone rubber. The gates, runners and part geometry considerations can vary greatly, and this brings us to the magic of the toolmaker.
Tooling
In my opinion, the future of micro moulding lies in the tools or moulds used to make the parts. When the rubber hits the road (yes, pun intended) you can have the most sophisticated injection press in the world, but if you don’t have a good mould you will never get parts.
Metalworking technologies have made huge advances in the last 10 years. When I was an undergraduate I worked in a small machine shop on campus. I spent countless hours in front of my Bridgeport mill making chips with a 0.5” (13 mm) end mill. I knew all about feeds and speeds. “Slow that spindle down!” the shop supervisor would warn me, “You’ll wear out the cutter, those things don’t grow on trees!” If someone had told me then that in 10 years people would be operating mills with 0.00197” (0.05 mm) cutters that turned at 30,000 rpm cutting Rockwell 52 hardened stainless steel I would have laughed out loud. Milling hardened steel at those spindle speeds would have been deemed impossible. Needless to say, I can walk down to my tool room right now and see two of these fine machines cutting away at hardened stainless steel.
Wire Electro Discharge Machining (EDM) is in a similarly advanced state. In the “old days”, wires pulled through the steel were 0.010” (0.254 mm). A small wire was 0.004” (0.1016 mm). The toolmaker might be able to deal with 0.002” (0.0508 mm) under special conditions. Now toolmakers regularly use 0.002” (0.0508 mm) wire, and some of these machines are capable of running 0.0008” (0.02032 mm) wire.
I won’t even talk about the capabilities of the next generation EDM sinker machines. They aren’t even called sinkers anymore, but instead are termed micro erosion machines. I will leave the topic of micro erosion to your own research, and trust me, it is fascinating. I have found there is one word to describe these futuristic machines, and that word is “wow”! The technology is so amazing, I am not yet sure how I will utilise it in my future mould builds.
A well designed tool starts with a high quality part design. Most high end shops use 3D modeling software packages. This allows the tool designer to import the part model directly into the tool design process. The tool designer must work closely with the part design engineer to insure a robust tool is designed. Tool makers do not work independently in the modern world. Micro moulded parts may require tooling tolerances to the 4th decimal point (0.0001” or 0.00254 mm) in order to meet the required part dimensions.
The world of micro moulding is an amazing field. Micro moulding silicone rubber adds to the challenges. Imagine, have you ever tried to measure a rubber band? The bottlenecks in the micro moulding field seem to be moving around. A few years ago if I wanted a 0.005” (0.127 mm) gate on a part, the toolmaker would tell me that I couldn’t do it. Now with small wire EDM, toolmakers can make such features. I have learned to be careful what I ask for, as these days, I might get it! Now that I have my 0.005” (0.127 mm) gate, I am wondering if I can reliably push my material through a hole that small. Due to modern tool making technologies my once unreasonable requests are now being answered.
Over the wall part design is a thing of the past. Design engineering, manufacturing, materials and tool making must work together as a team. Toolmakers need to understand the injection process, and material science engineers and design engineers need to understand how metal is cut to make a mould. A successful tool build is a team effort.
Loren Evers is a Senior Manufacturing Engineer at Medtronic. He is currently performing manufacturing support for Medtronic’s internal silicone moulding operation as well as doing consulting on moulding and tool design for other business units throughout the company. His area of expertise is silicone rubber moulding, with an emphasis in micro moulding and tool design. He has a Bachelors Degree in Mechanical Engineering and a Masters Degree in Manufacturing Systems. He is currently completing a certification for post-secondary teaching.
