I Can't Believe It's A Stamping!™

Case Study #2: Stamping Titanium - A Cost Effective Solution

angioLINK, a Taunton, MA provider of innovative medical devices for vascular access management, was looking to penetrate the rapidly growing vascular sealing marketplace with a new product that would quickly close vessels with the pull of a trigger. The device inserts a titanium staple, only 3mm in diameter, which first expands to maximize the area requiring sealing, then closes.

The titanium’s small size, as seen here with a scalpel blade, demonstrates the precision capability of stamped components

The manufacture of the staple is very challenging because of the forming characteristics of titanium, the small size and complex shape of the part, and the tolerances on key dimensions. The initial manufacturing concept was machining, but this would be very costly for the volumes that angioLINK anticipated. They decided to explore alternative, more cost effective manufacturing methods and worked with OKAY, a contract manufacturer of complex metal stamped components and sub-assemblies based in New Britain, CT, to develop the staple as a metal stamped solution.

Prototyping can play a critical role in determining if a titanium stamped solution will work for your device. The primary reason for prototyping is verification that the component or assembly will function as intended in its final application. However, for metal stampings, it is critical that you go beyond prototyping for fit, form and function and think ahead to manufacturing. Prototyping of metal stampings will allow you to determine if the stamping process is stable and capable, if cost expectations can be achieved and to identify opportunities for improvements in both cost and quality of the component or assembly. “Titanium behaves differently in stamping, bending, and forming operations than other types of metals such as stainless and carbon steels that many stampers are familiar with. Due to its crystalline structure, it is more strain rate sensitive,” says John Schmidt, Director of Special Metals at Ulbrich Speciality Strip Mill.

3D modeling of stamped components facilities prototyping development

OKAY’s Production Proven Prototyping™ process utilizes the same tooling concepts, sequence of operations, and grain direction that will be used in the production tool. Replicating the stamping manufacturing process during the prototype stage can minimize costly changes later in the project that would not have been identified using machining or other prototyping methods. Component strength, cracking, surface finish, burrs/edge condition, and feature tolerance capability are a few of the concerns that need to be assessed. The dissimilarities between metal injection molding, machining, and stamping are all affected by these factors.

The titanium material specified by angioLINK is .013” (.38 mm) thick 3AL-2.5V annealed Titanium. Titanium is very cold workable but is extremely abrasive, soft and gummy, and has a higher resistance to forming than other annealed low carbon metals and requires more attention to spring-back in forming when compared to other materials. “Titanium exhibits about 25% more “spring back” due to its lower elastic modulus, and it tends to gall more easily than steels, so tooling designs need to be modified appropriately,” says Schmidt.

One of the biggest challenges of stamping Titanium is its susceptibility to gluing itself to the tooling surfaces. “Sometimes this has an effect like sandpaper where it just abrades and causes minute gritty particles to collect in the tools,” says Shawn Russell, Vice President of R&D and Engineering at OKAY. Other times pickup on the tools will actually grip the material being bent, stretching the features of the staple out of tolerance. These conditions are caused by heat and friction.

To mitigate these problems, tools were developed with very generous wipe radii on the form dies and punches. Surface finishes were held to 5 micro or less. In addition, die lubrication needs to be critically applied to create a barrier between the titanium part and the die material.

Due to the precision of the gutting and forming stations in high precision stampings, the tooling modules may be self-guided with the upper punch holders allowed to float.

Another challenge with stamping the staple is its size. For example, the short legs are gutted all around with a slot in the middle leaving the width of the blank just slightly larger than material thickness. Typically, stamping operations require blanking areas that are greater than material thickness. This requires special holding and guiding techniques in the tool. The staple tips are very critical to hold, guide, and support. The tips of the staple are held to +/- 0.0002” (.005 mm) maximum spherical radius, an almost perfect point. “Many people believe that stampings must maintain material thickness throughout the component. However, coining, swaging, and extruding operations can be used to form complex features in a stamped product,” says Russell.

The shape of the staple creates a unique forming challenge while trying to maintain dimensional stability. The shape of the staple is distinctive in that all eight legs are interconnected at the top and the bend radius of each leg has no relief around it where it connects to the main body. This results in a portion of each bend radius to fall partially inside the top flat area, which causes interference with the bend radius of neighboring legs. This causes a lack of isolation and control of the previously formed long legs when forming the four short legs. This was overcome by a sequence of rotary forming tools, which set each bend independently.

The staple has over bent-legs that posed a challenge for designing the progressive die. A transfer station system was designed to form and eject the part from the tooling. It transfers the part to and from 6 tooling stations and part ejection.

In progressive die stamping, a ribbon is used to transfer the part formation throughout the tooling.

Nearly all medical applications for stamped titanium components require them to be free of any scratches, nicks and burrs. “In most cases, refined progressive-die tooling can be designed to minimize or eliminate these conditions,” says Russell. One example is the use of coining. Coining uses a punch and die to compress the burr into the edge of the material which eliminates the need for secondary deburring operations. Vibratory or barrel tumbling or an electropolish process can also be utilized to both eliminate burrs and enhance surface quality.

Since the staple is an implantable device, the parts require cleaning. OKAY developed a proprietary method of removing the staples directly from the progressive die tooling, sending them through a “flush” system, and then automatically loading into the packaging. The packaging is then incorporated directly into the customer’s assembly process.

“OKAY provided angioLINK with the proper expertise and guidance to meet the challenge of stamping our unique staple from the early prototype stage to final production tooling. Their staff is extremely knowledgeable, professional and committed to the customer / vendor team concept for problem solving,” says Bob LaFerrara, Director of Operations at angioLINK. Understanding the challenges and design guidelines of stamping titanium can lead to a cost effective alternative for many higher volume medical device applications. Metal stamping provides a capable and stable process for high volume, tight-tolerance components. Stamping is worth a look for OEMs looking for a more cost effective alternative manufacturing process for titanium components.