As medical devices become smaller, the need for precision microminiature components increases. This provides several challenges for the medical device manufacturer as they seek cost-effective parts meeting the exacting quality specifications that play a critical role in the overall performance of the medical device.
A large percentage of surgical and medical devices contain precision engineered miniature and micro metal components produced from a wide variety of expensive — and often precious — raw materials. Manufacturers are focused on reducing these raw material costs, and at the same time looking for ways to save labor-intensive assembly costs while improving yield and component reliability.
Quality, as a whole, encompasses making parts precisely and consistently to spec, regardless of throughput volume and speed. But medical device designers also seek lower costs and high volumes, and when addressing those issues, traditional manufacturing methods such as screw machining simply can’t do both well.
All of these considerations create a growing demand for efficiency and cost-effective manufacturing techniques that machining can’t provide, while retaining or even improving repeatable quality. As a result, many medical device manufacturers are now turning to cold forming as a solution.
This article will discuss how medical manufacturers can achieve improved quality at reduced cost through cold forming versus other traditional manufacturing methods. It will provide a fundamental description of the cold forming manufacturing technique, address its features and benefits, and review the importance of manufacturer and supplier collaboration to achieve cost reduction, added part strength, increased production speed, and consistent quality at high volumes. A product case study is included.
What is Cold Forming and How Does it Differ from Machining?
Cold forming is the application of force with a punch to a metal blank staged in a die. The force exceeds the alloy’s elastic limit, causing plastic flow until the metal blank assumes the shape bound by the punch and the die. As the name implies, this method of forming is achieved by force alone, forgoing the application of additional heat or cutting and shearing as seen in other traditional manufacturing methods (Figure 1).
This is not a new technology for manufacturing micro-precision components, yet it is still not well known or understood among new product development (NPD) teams. One of the reasons for this is a widespread lack of related course work in engineering schools. Yet, cold forming is a sound, fundamental manufacturing method that has been used in many industries for decades with widespread success. Here are some attributes that make it particularly attractive for precision micro/miniature components where a combination of cost savings, quality, and production speed are essential.
Cost savings through reduced material scrap. Cold forming is a net shape solution. During the process, wire is transformed by a sequence of die blows into a specific shape, with the material flowing to fill the part geometry and dimensional tolerances defined by the tooling engineer. So there is virtually no waste created. Without scrap to deal with, there is little to no recycling cost associated with the process, less lubricant to reclaim, and minimal labor to handle it all. In general, the wire feedstock is less expensive than the bar used for machining.
With all forms of screw machining, including single- and multi-spindle and Escomatic processes, scrap is not only unavoidable, it is a significant by-product of the process, often equivalent to 50% of the final part’s mass. Since scrap contributes to cost, the less the better.
High throughput reduces per-part overhead costs. With an optimized part formation progression for a complex component, cold heading delivers yields at a rate of 90-300 parts/minute (PPM) standard. Generally, yields for a similar design produced from a multi-spindle screw machine will be in the 6-20 PPM range. Cold forming is an order of magnitude faster.
Since the cost of each part must absorb a proportionate share of manufacturing overhead, it becomes clear very quickly that a cold-headed part can amortize that cost by many multiples over that of a screw-machined part. At the same time, the man-to-machine attendance rate favors cold forming by a factor of 2 or more.
Consistent quality at high speeds and volumes. Because the wire forms that are composed into shapes using contained dies allow the material no escape route, there is very little opportunity for a part to be malformed. Through technical collaboration, critical dimensions are defined, a first part approval is achieved, and the tooling is made and prototypes are approved. When production is started, a cold heading machine will faithfully reproduce exactly what it has been set up to do, virtually unattended, 24/7, over many hundreds of thousands, or even millions of impressions. Additionally, the manufacturer’s quality team performs routine inspections to ensure that the last part made is always identical to the first part approved.
Improved part strength. Cold forming is a process in which the native tensile strength of the material is increased through work hardening instead of decreased through material removal. For every 1% of area reduction or increased surface area of a part’s cross-section due to cold forming, its tensile strength increases by a factor of ~0.6 to 1.5, depending on the alloy. This physical property is known as the work hardening rate of the material. The work hardening rate varies depending on starting tensile strength and material composition. No process that removes material from the native shape, such as screw machining, can achieve this.
Cold forming and rolling are ideal for many part types. Cold forming is an ideal method for creating symmetrical, full-radii tapers; undercuts on solid material; and tubular parts with or without a head, splines, and more. Through secondary operations, knurls; single or multiple cross threads; barbs for one-way inserts; and clips, detents, or custom shapes for retaining two parts can be obtained. Cold forming is a preferred manufacturing method for projects that have mission-critical applications, require speed and high quantity without sacrificing quality, and require increased part strength due to past part failures, and for NPD projects needing custom micro-miniature parts.
Collaborative Design for Cost Reduction and Quality
Every manufacturing technique has advantages and disadvantages. Machining, for instance, is extremely precise, but per-piece costs on large quantities can be burdensome. Working with the engineering team at the part supplier to understand cold forming’s strengths and weaknesses will reveal how to design to its advantages and avoid its flaws. This sounds upside down from a designer’s perspective, since product design should not be a servant to any specific manufacturing technique. But when a certain technique offers compelling cost advantages that lead to a more affordable part, as long as no sacrifice is made to its function, then designing to its strengths can offer real-world solutions that benefit everyone.
Quality can be associated with how well a finished part meets specification and functions within the medical device. This challenge is the responsibility of both the engineering team at the medical device manufacturer, and the engineering and production teams at the part supplier. Utilizing a collaborative approach paired with a DFMA process or similar design verification process can ensure the part meets, and often exceeds, expectations of cost savings, part strength, speed, and consistent quality. This question-and-answer approach helps determine the most cost-effective and efficient assembly method, manufacturing process, and materials for a particular part or product. The steps in the collaborative and DFMA processes are shown in Figure 2.