Saddling Up CIM for Medical Devices

INJECTION MOLDING

Fabricating parts at the micron level is a challenge for medical device manufacturers. Such parts must be both sterilizable and resistant to wear. In addition, these tiny components often incorporate complex shapes. To obtain all the desired characteristics, device makers are increasingly turning to ceramics, and specifically to the process of ceramic injection molding (CIM). This article covers the CIM basics and provides examples of how the technology is being used for medical, dental, and laboratory device production.

CIM History and Processes

The earliest recorded use of CIM technology was in the 1930s when, for only a short time, an early version of the technique was used to make spark plugs. However, most experts agree that modern CIM processing began in the late 1960s when it was found to be valuable for manufacturing semiconductor capillaries used in wire bonding.

CIM can be looked at most accurately as a marriage of three technologies. It blends powder science, injection molding, and sintering technology.

Although formulas can be customized for desired results, the process always begins with a mix of ceramic powders and plastics. The ceramic powder particles are less than 1 µm in size. These powder particles are blended with a plastic media that acts as a binder system.

The initial mixture that goes into the injection molding equipment is roughly two-thirds ceramic and one-third plastic. The plastic acts as a carrier to transport the tiny ceramic particles into a mold. After the blend is molded, the part needs to go through further processing to make it useable.

The part that comes out of the molding machine is considered a green part. The green component is put through a thermal process (~250°C) called debinding. This process basically removes the majority of the plastic material, or binders, by burning it off.

After debinding, the part is put into a high-temperature sintering kiln at ~1500°C. This is known as the sintering step, in which the part shrinks, densifies, and takes its final shape. The end result is a device made of an incredibly hard material (typically ~1550 Knoop, which is equivalent to 2000 on the Vickers hardness scale, compared with stainless steel, which is approximately 150 Vickers) with a smooth, porosity-free finish.

Although some elements today are similar to those used decades ago, there have been great advances in process steps and materials since the early electronics applications first appeared. Both the morphology (distribution and shape) of the powder and the binder systems have changed dramatically. For example, the powders now come in various shapes, including spherical, oval, or angular, that inform molding and sintering behavior. And new plastic materials are exceptionally clean during the burn-off or removal step compared with older carriers. Also, new materials are continually introduced in different grades and blends.

Current Technology and Usage

Today, much of the focus for medical devices centers on alumina (A1203) and zirconia (Zr02) materials. These ceramics materials impart hardness and compressive strength as well as wear and corrosion resistance. In addition, the materials are biocompatible and have dimensional stability over a wide range of temperatures. They can also withstand high working temperatures. They have good electrical insulation and dielectric properties (see Table I). Both alumina and zirconia materials have advanced in terms of purity, grain size, grain structure, and mechanical properties.

To decide which material to use, medical device engineers might start by determining of the level of hardness and toughness required by the application. Broadly speaking, alumina and zirconia are on opposite ends of the scale when it comes to hardness and flexural strength (i.e., one measure of toughness). Figures 1 and 2 provide comparisons of three material blends in terms of flexural strength and hardness.

In a wear environment, the ratio of fracture toughness to hardness often determines the wear performance for oxide ceramics. Therefore, so-called softer materials such as zirconia may outperform the harder alumina because of its higher fracture resistance.

It should be noted that because of the nature of the powder, zirconia uses a different plastic binder formulation than alumina and, therefore, has a different shrink rate.

Moving Into Medical Devices

As design engineers have gained experience with the properties of ceramics, CIM has become a more attractive processing method.

Because of the physical properties achieved using CIM, the technology allows medical device manufacturers to experiment with new types of product development. With CIM, physical properties include extremely small sizes, complex shapes, clinically clean nonporous finishes, and extreme strength. Such properties can provide alternatives for medical device design and development.

The key characteristics of ceramics are their hardness and their inertness. They don't interact with anything that may pass through or around them. They are physically stable and biocompatible. Ceramics give a very smooth surface finish, around 0.2 µm Ra (Ra = average roughness), or approximately 0.000008 in. rms. In certain cases, injection-molded ceramic can replace metals, machined ceramics, or plastic components. Ceramics can be an acceptable replacement in surgical instruments and diagnostic equipment, as well as orthodontic and dental tools.

For example, minimally invasive electrosurgical devices were originally formed with plastic as an insulator. However, because this insulator deforms over time, some manufacturers have turned to ceramic material for electrical insulation and surface finish. The stiffness of the material enables designers to reduce overall device dimensions and eliminate some supporting metal components. Cutting blades are often made from molded ceramics because of the material's extreme hardness and its ability to hold a sharp edge.

In flow cytometry systems, a miniature ceramic nozzle can be used to replace an electroformed gold nozzle. Users often find that the gold nozzle does not withstand the wear that occurs during the process and thus requires frequent replacement. The ceramic part presents a smooth surface that does not wear or pit from the continual fluid contact that is integral to this type of system. CIM is also a suitable process for creating biopsy markers that have biocompatibility and visibility under magnetic resonance imaging systems.

CIM is commonly used to form orthodontic brackets for teeth. The ceramic can be translucent, which makes it more aesthetically pleasing than metal while maintaining durability. Dental devices such as screws and implants can also be made by CIM. Another type of device is a dental abutment. A dental abutment is a critical piece that fits between an implanted post and a porcelain or zirconia crown in a patient's mouth. It is commonly manufactured using CIM technology.

The technology is also used in various miniature dimension devices, such as catheter tips, pacemaker feedthroughs, cochlear implants, insulators, and neurostimulator housings.

The hardness of ceramic material ensures long-lasting wear resistance. And because of the smooth surface, ceramic devices are easy to clean and are compatible with any sterilization chemical or process, including autoclaves.

Manufacturers may be hesitant to adopt ceramics in their process because of the high cost associated with the material. Advanced ceramics in general are so hard and durable that shaping them into something complex could require final machining using diamond tooling. This step adds cost and, for some complex shapes, such refinishing is very difficult if not impossible. However, CIM processing enables design freedom because the parts are molded rather than machined. The process for complex parts is less expensive than it would be for machined ceramics.

Also, CIM can reduce costs because it can replace machining and assembly of multiple separate parts or enable several parts to be combined into one.

Critical Process Steps

During part development using this technology, it is critical to know where to start to ensure that the device will shrink to the exact size and dimensions required. The first step is to perform a volumetric calculation of how much binder (plastic material) is needed in relation to ceramic for the starting mixture (often called feedstock). Ceramic injection molders often use proprietary calculations for this step. From such a calculation, engineers can estimate the density of the end product (once all the binder is burned off). This end density presents an accurate idea of how oversized the initial part must be in the mold. To achieve the desired end density, the feedstock must be meticulously prepared and must be as uniform as possible.

Even with a perfect mixture, there is a danger that certain features will shrink differently than other features of the same part. Drag forces during sintering or shrinking can influence the final part.

Another concern is that a part could develop a protruding point, or arm, caused by gravity. Experience is often the only way to prevent these deformities. A manufacturer of components using CIM technology must have experience in the application to have successful runs.

For example, specially made tooling is required that can withstand the abrasive ceramic that is being molded. The process steps for the desired result, as well as the ability to adjust these steps, take time to learn.

Careful attention to debinding and sintering cycles is critical as well. Controlled temperature distribution and minimal temperature gradients within the processing equipment help result in the most consistent output.

The Limits of CIM

When considering CIM, manufacturers should consider the complexity of the part and the quantity needed. If a user only requires a simple tubular shape, for example, there are other processes that are less expensive and more efficient. Extrusion or uniaxial pressing techniques are perfectly suitable for simple shapes. If the end result part is a rectangular block of ceramic, it is better to machine and grind it than to use injection molding. However, if that block needs an array of intersecting holes, threads, odd features, or curves, then injection molding might be more suitable.

Size is another consideration. Very tiny holes, measuring up to 25 µm, are attainable with CIM, but tiny holes even for a simple shape are very difficult to achieve with machining processes. Once features shrink down to microns in size, CIM is a much better option (see Figure 3).

Manufacturers should also take quantity into consideration. Most CIM applications should be in quantities of 10,000 pieces per month, or more. However, most high-end molding facilities also have demanding projects with extremely intricate design requirements for only a few thousand pieces per month, or even per year. Such projects are the exception though, and most injection molding providers agree that if the requirement is only 10–20 pieces per month, CIM is not the right process unless the part cannot possibly be manufactured using any other method. Also, the end use of the device must be critical enough so that the overall monetary return on the end product exceeds the cost to manufacture that specific part. A good example would be a critical miniature ceramic device used in a full laboratory cell separation and cell analysis system.

Another consideration is whether changing to a CIM process would also require that the manufacturer make a material change. Molded prototypes of part shapes can indicate whether a material change is necessary. This allows the manufacturer of the device to evaluate the performance of the material change before making a final decision. If the medical device is still in the design stage and a question exists as to whether a ceramic is the right material to use, a fabricated prototype may provide the answer. This could be accomplished by using a different technology first, such as microelectrical discharge machining (µEDM). Prototyping allows small quantities to be made out of the ceramic or alternative materials to compare all materials and methods.

Conclusion

From devices implanted in the body to equipment used in hospitals and laboratories, ceramic injection molding offers a good alternative in both material and process. The biocompatibility and biostability of ceramic makes it an ideal choice for many medical device applications.

Devices and their parts are becoming smaller and smaller. Holes that measure 25 µm in diameter were unheard of five years ago, but are now common. For micron-sized devices with intricate designs, CIM may be an appropriate process. Often, it may be the only way to actually develop some of these tiny designs. For medical device manufacturers, CIM offers a whole new spectrum of design potential.

Travis Ayers is general manager for Small Precision Tools (Petaluma, CA).

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