Design of an Automated Laser Welding Workstation for Medical Manufacturing

J. Pischlar, J. McNulty, B. Holm, M. Vohl, D. Foley, L. Stradins. University of Wisconsin-Stout

General DisclaimerThe following information regarding American Medical Systems products is in no way a criticism or an accusation of negligence towards safety, quality or reliability of the processes studied. The sole purpose of the research was to alleviate a number of the manufacturing inconveniences of the exceptional and life changing products created and produced by a world class medical device manufacturing company.

BackgroundAmerican Medical Systems (AMS) is the world's leading independent company focused on developing, manufacturing and marketing medical devices that restore male and female pelvic health. During the fall of 2010 academic semester at the University of Wisconsin-Stout, AMS partnered with a group of manufacturing engineering students to devise a solution to a current manufacturing issue on one of their products. The team of engineering students was tasked with investigating the primary source of variability in an assembly step of the manufacturing process of this product.

The assembly step examined was a process where a polypropylene (PP) mesh is inserted into a polyethylene (PE) sheath and bonded together. AMS currently utilizes an operator intensive hot melt adhesive dispensing system to achieve this bond. Being that both the materials, PP and PE, are chemically and physically inert this results in a high degree of variability in the bonds strength*. One of the requirements of this product is that the bond strength be greater than 8lbf so that during the implantation of the device the mesh does not separate from the sheath and cause damage to the surrounding tissue. In order to mitigate this risk, AMS has discovered that by providing additional consistent clamping pressure during the adhesives cooling or “curing” time, the bond will meet the required specification. Though the clamping step can produce higher bond strength, it is highly operator dependent due to the fact that it requires the operator to apply a consistent pressure for a consistent time, beginning at a consistent temperature.

Once the team had received the product specifications and restrictions from AMS, the members discussed the various methods used for joining dissimilar materials; the list of methods the group was familiar with is as follows:

• Adhesives• Mechanical Fasteners• Ultrasonic Welding• Laser Welding

The team then performed a literature search focused on the various methods of joining to create a pro/con list for each.  *Bond strength – The force required to separate the polypropylene mesh from the polyethylene sheath utilizing an Instron tensile tester.

Joining Methods Available Implementing NIR Absorbers• Diode or Nd:YAG lasers, which emit light in the 400 – 1064nm

wavelength range, are the only feasible, commercially available option for the laser welding of transparent thermoplastics.

• While the light emitted by such lasers naturally passes through transparent thermoplastics, third party materials called Near Infrared Absorbers (NIR Absorbers or IR Absorbers) can be introduced at the interface of the two layers to absorb the energy.

• While different variations of NIR absorbers exist, the most common product is Clearweld, which was invented and patented by The Welding Institute (TWI) and is currently being commercialized by Gentex Corporation .

• Clearweld can be compounded with resin to create a substrate with uniform absorber throughout (additive method) or can be dissolved in a solvent for dispensing or dipping application (coating method). Prior to interacting with laser energy, Clearweld has a slight green color. After being exposed to laser energy, Clearweld becomes colorless and mostly transmissive. This property allows multiple layers to be welded and reduces the possibility of thermal degradation .

Process Fundamentals

1. Two thermoplastic materials are positioned in a manner such that a layer permitting the light emitted by the laser to pass through is on top of a layer than can absorb it.

2. The bottom layer absorbs the laser energy and heat begins to generate. As a result, the bottom layer begins to flow.

3. Heat transfers from the bottom layer to the top layer, which also begins to flow. With the polymer chains of the two layers flowing, they begin to intertwine; a chemical bond results.

4. The workpieces cool and a joint is made.

Process Challenges

For medical device manufacturers such as AMS, the Clearweld process offers many benefits: • Biocompatibility • Non-contact processing; eliminating surface damage• Production of clean, optically clear joints• Hermetic seals• Fast cycle times; less than 1 second per weld is possible• No particulate generation with little or no flash• No third body as with adhesives• Low residual stress; thin, flexible substrates can be joined without

distortion

Automated Workstation Design

Product Overview

Close-up photograph of bond location:

• Both components are transparent and thus allow light in the visible and NIR spectrums to pass through them without effect.

• This phenomenon is an asset for welding a clear component to a colored component, but adds difficulty when welding two clear components together.

• The diagram below outlines the level of difficulty associated with the laser welding of an array of thermoplastic material configurations:

Traditional Cutting Edge

AdhesivesUltrasonic Welding

Mechanical Fastening

Laser Welding

• Impedes new product engineering and design abilities

• Restricts manufacturing parameters

• Limits high-speed or high volume capabilities

• Mars the finished products’ aesthetics

• Increases automation and manufacturing productivity

• Reduces production costs

• Superior joint quality and performance

• Unprecedented finished product appearance

Increasing Level of Diffic

ultyPP MeshPE Sheath Bond LocationBond Location

Hot Melt Adhesive Movable XY Table

Laser Beam Out

Workpiece Goes Here

Laser Beam In

Focusing Optics