The demand for minimally invasive procedures is growing, as patients seek to spend less time on the operating table and in the recovery room. Micro-molding has emerged as the solution for medical device manufacturers, allowing smaller devices to change an open surgery into a minimally invasive procedure. Such small devices can be highly complex and difficult to make. Additionally, many designs call for the integration of dissimilar materials, such as silicones, thermoplastics, and biocompatible metals. This often requires complex secondary assemblies or over-molding, which can create many problems for ill-equipped manufacturers. Only experienced molders who understand the tool design and processing complexities can meet these requirements. How do you find these experienced molders and what questions should you be asking to determine the right fit?

Fig. 1 - A micro-molding machine plant is used to produce liquid silicone parts

In recent years, there has been an ever-growing push to produce smaller molded components with thinner walls and minute features for medical devices. These components are only producible using advanced micro-molding methods. Micro-scaled plastic components are critical for companies to keep their devices at the forefront of medical technology. How do you choose a capable molder for these essential components? Business approach, tooling knowledge, micro-molding expertise, and materials experience should all be considered. Read on to discover critical considerations for choosing a vendor capable of producing the highest quality micro-devices, and thus, ensuring the success of your micro-molded project.

Business Approach

Success with micro-devices depends heavily on partnering with a company that has the right business approach and experience. In recent years, there has been significant buzz around the term “micro-” within the molding industry, and more and more molding companies are targeting these types of projects to capture a piece of the “micro pie.” It is important to avoid companies that claim to have experience in micro-molding without the proper business acumen to back it up. Many of these molders do not have the proper teams, equipment, manufacturing methods, and tooling and processing expertise to produce high quality micro-parts. (See Figure 1)

When choosing a micro-molder, consider their market niche and how it aligns with the specific needs of your medical project. Scale, volume, longevity, and other technical aspects should be carefully considered. For instance, many micro-molded components are long-term, unrestricted FDA Class VI medical devices made from implantable grades of polyether ether ketone (PEEK), silicone, polyurethane, or resorbable materials. Once in full production, these projects tend to be low- to mid-volume runs, ranging from thousands to hundreds of thousands of parts per year. They also tend to require a higher degree of validation than most general molded devices and heed full Installation Qualification (IQ), Operational Qualification (OQ), Performance Qualification (PQ) validations and scientific molding approaches. These molders must have specific knowledge on how to store, handle, and process implantable materials and finished devices. For micro-molding projects, it may be advantageous to select a “boutique” molder who specializes in such applications and who is able to dedicate the proper engineering and technical resources to your project.

In general, keep in mind that just because a company has micro-molding capabilities, it does not mean that they will be the right fit for your project. As with traditional injection molding companies, each micro-molding company tends to focus their work around specific product volumes within a market or specific niches within an industry. It is important to consider all aspects of your project and how they align with the skill set of the micro-molders you are considering for your micro-molding projects.

Questions to ask regarding business approach:

  • When did you begin micro-molding?
  • What is your market niche?
  • What experience do you have with my specific industry?

Tooling Knowledge and Molding Expertise

Fig. 2 – A micro-molded machine Peek component.

Arguably, the most critical consideration in micro-molding projects is the approach your vendor takes when constructing molds for micro-sized parts. The construction of mold tooling for micro-molded components requires an advanced level of accuracy and different skill set than that of traditional injection molds. Consider the tolerances required for micro-molded parts. Traditional injection molded parts can often have tolerances that are in the range of +/-0.005" or more. Micro-molded parts can often have features that are as small as—or even smaller— than this. Likewise, venting and tooling mismatch can often be in the range of 0.002-0.003" for typical plastic parts, which can result in a parting witness line of a few thousandths of an inch. While this is not typically an issue for traditional molded components, a witness line of this magnitude can be as large as some part features on micro-components, which can result in parts that simply will not function with mating devices and assemblies.

Injection molding equipment designed specifically for micro-molded components does not rely on a screw and check ring system like traditional injection molding machines, but instead utilizes a screw and plunger system. These systems tend to have much smaller volumes then traditional presses, which lead to significantly lower material residence times, and, as a result, much less material degradation. These systems also limit the issues related to the traditional check ring systems that are known to cause material “dead spots.” The result is cleaner, stronger, high-quality micro-molded parts. (See Figure 2)

Vendors must employ the newest, most technologically advanced machines and techniques to keep up with the ever-advancing requirements of smaller micro-devices. The technology behind many traditional mold-making methods such as computer numerical control (CNC) machining and electric discharge machining (EDM) have come a long way in recent years and today can provide the accuracy required for most projects. However, it is important to understand the tolerances that micro-mold builders are able to hold using their equipment so that the end dimensions and tolerances of the molded part can be accurately achieved. Additionally, your mold vendor must have a thorough understanding of how to polish micro-mold cavities, how to construct the molds for proper heating/cooling considerations and how to handle and support near paper-thin steel inserts.

Questions to ask regarding tooling and molding experience:

  • How do you design tooling specifically for micro-molding?
  • What kind of system does your molding machinery employ (screw and check ring or screw and plunger)?
  • What experience do you have in polishing micro-mold cavities?
  • How do you address heating/cooling concerns?

Materials Experience

Fig. 3 – MRPC created this two-material, micro-molded part by over-molding liquid silicone rubber onto PEEK.

Adding to the complexity of micro-mold construction is the fact that many materials used to make micro-molded parts tend to be high-temperature, high-performance materials such as PEEK, bio-resorbables, and liquid-crystal polymers (LCP). These materials, especially when utilized for implantable medical devices, require that the tooling be made from non-corrosive materials like stainless steel, because traditional tool steels may not hold up to the high molding temperatures involved and can corrode at the surface. This impacts the biocompatibility of the molded implants and the life of the mold.

Making this process even more complicated is the fact that many features within micro-molds can be measured in mere thousandths of inches, meaning that thin-steel conditions are not just an exception, but are indeed the norm. These thin-steel tooling inserts can easily become super-heated, especially when molding high temperature materials, which can lead to quality issues such as part degradation, voiding, weak knit lines and so on. The high injection pressures and speeds required in micro-molding can also easily cause damage to these steel features. It is critical to control not only the molding process itself, but also the heat of the material, the temperature of the mold and the sheer heat created by this demanding process to accurately and repeatedly produce quality micro-molded devices.

Consider the experience needed for success with multi-material molding an over-molded micro-device in which a combination of polymers is used, or in which a plastic is molded over a metallic insert. (See Figure 3) For this device, it is important to find a partner with extensive experience in both over-molding and micro-molding, as this project will require intimate knowledge in the bonding and compatibility of differing materials, as well as all of the technical aspects in processing, tooling and handling of micro-devices.

Questions to ask regarding materials experience:

  • How do you ensure accuracy with micro-parts?
  • What experience do you have with multi-material molding?


Medical micro-molding projects can be both exciting and challenging. As these devices become more commonplace through technological innovation, micro-molding vendor selection becomes even more important. When choosing a capable vendor to help tackle a micro-molded project, it is critical to understand the key differentiators between those who say they can mold micro-components and those who actually can. Not all vendors are truly capable, and even within the micro-molding industry, vendors have industry-specific capabilities and leverage different technologies and business approaches. A vendor’s capabilities may or may not be ideally suited for your medical micro-molding project. Having the foresight to ask the right questions about your prospective vendors’ business, equipment, tooling and processing expertise can help you develop the right partnership and ensure the success of your medical micro-molding project.

This article was written by Ken Kostecki, Southeast Territory Sales Manager for MRPC, Butler, WI. For more information, Click Here .