Medical Device Link.

MANUFACTURING

Connection issues

Polymers are increasingly being used to replace metal parts in a variety of applications. Yet, metallic or ceramic components are still required for certain functions in a product and must be joined to the polymer parts with high precision and strength. Until now, connecting these materials has been achieved with glueing, screwed fastening or the mould-in technique. The capabilities of each joining process define its area of application and each has its demands and limitations.

Clamped or screwed joints enable a detachable connection. Positive fit connections such as glueing or moulding are able to transmit high power free from play, which provides good positioning accuracy. Glueing is employed for joining large areas. Complex preparation of the components is necessary for all mechanical connections.

The most commonly used connection technique for plastics with metal is the mould-in technique during injection moulding. The mechanical component is placed in an adapted tool prior to the injection moulding process. An optimal result requires tight tolerances of the tool and high precision components and part handling is difficult; a plastic–metal joint is not possible with this process. To obtain it, a postmoulding process is required. This is primarily used for thread-inserts, which are heated by induction and then pressed into the plastic component. All the components must be heated; however, ceramics cannot be processed. In addition, the positioning of the inductor is often difficult and the heat input is insufficient for small structure sizes.

The laser fusion technique

Figure 1: Process steps (F = force, S = stroke). (click image to enlarge)

A laser induced fusion technology has been developed that is based on the fact that all thermoplastic polymers are transparent or at least translucent in their natural condition. This technique is already successfully employed in laser polymer welding. However, the technique has been developed further and the absorbing partner in the join is not the polymer, but the metallic or ceramic part, which is heated through the transparent component by laser radiation. An important factor in the process is the use of a material that has a higher heat resistance than the bonding plastic. That material is heated briefly by absorption of laser radiation to a temperature above the flow point of the plastic employed. Possible choices for the heat resistant material are essentially metals and ceramics, but could also include heat resistant plastics such as Teflon. The laser radiation penetrates through a transparent sapphire or fused silica tool, then through the transparent polymer part being joined and is then absorbed by the metal or ceramic part. By using transparent clamping tool inserts, heating by laser can take place during the forming phase in the closed tool. Figure 1 shows the different process steps involved:

• positioning and applying pressure
• heating the metal part through the plastic component with laser radiation
• penetration into the plastic component after exceeding the glass transition temperature
• cooling down and creation of positive locking.

Figure 3 (below): The various combinations of materials used with the laser fusion technology.

Metallic thread insert Materials: Steel, PMMA Thickness of polymer plate: 10 mm Diameter of the screw: M4 Positive fit by the use of thickness increase.

Polymer–ceramic joint Materials: Zirconium Oxide, PMMA Thickness of polymer plate: 10 mm Diameter of cylinder: 8 mm Positive fit by use of surface roughness. Additional axial bore to avoid material ejections.

Polymer–polymer joint Materials: Teflon, PMMA Thickness of polymer plate: 10 mm Diameter of cylinder: 10 mm Positive fit by use of a notch Additional axial bore to avoid material ejections.

Polymer–silicon joint Materials: Silicon, PMMA/PC Thickness of polymer plate: 3 mm Diameter of silicon plate: 7 mm Thickness of silicon plate: 0.3 mm

Figure 2: Direct irradiation of the insert when using a nontransparent polymer. (click image to enlarge)

If plastics that are not transparent to laser radiation are used, the process can be adapted so that the radiation does not pass through the plastic, but instead the laser beam is guided laterally and directly at the metal component, see Figure 2.

The laser-irradiated, and therefore heated, component is pushed against the plastic part by mechanical pressure. In doing so, the plastic heats up by thermal conduction to a temperature above its glass transition temperature. The application of further mechanical pressure makes the component penetrate into the plastic. By selecting suitable component geometry, a strong positive joint is obtained after cooling. This geometry can be a thickened region around which the plastic flows on penetration, or a notch or bore through which the plastic can flow to establish a form-fitting joint. The material displaced during the impression operation results in undesired bulges on the surface of the plastic. These can be avoided or at least minimised when hollow cavities into which the plastic can flow are present in the component. Preliminary bores in the plastic part serve the same purpose.

Process parameters

The most important process parameter is the temperature of the more heat resistant material. If the temperature is too high, bubbles form between the two parts and sometimes discolouration occurs in the plastic. If the joining temperature is too low, cracks occur in the plastic because of stress and the more heat resistant material will be deformed. Compared with heating concepts already in use today such as inductive heating, heating by means of laser radiation is mainly independent of the thermal and electrical conductivity of the material in which the part is to be pushed. This effect can also be used when ceramic parts are employed instead of metals. Hybrid components then combine the properties of the ceramic insert such as great mechanical strength (hardness), high resistance to wear and good thermal stability with the low weight and the design freedom of the polymer. The high energy density of laser radiation enables rapid, selective heating of the insert component without the whole part being heated. If the thermal radiation emitted by the parts is measured by a pyrometer, a temperature related signal can be generated as a function of the material used and the applied laser power. The pyrometer signal, together with the desired temperature, is processed in a controller and a corresponding signal setting equivalent to the laser power is passed onto the laser. In this way, stress free insertion of the parts is possible without harmful overheating of the components.

Applications

The laser fusion joining technology described here can be used when plastic and metal or plastic and ceramic have to be joined to combine their specific properties for enhanced performance of a product. Figure 3 shows various combinations of materials that have already been investigated. Cuboidal thermoplastics such as poly(methyl methacrylate) (PMMA) and polycarbonate (PC) with thicknesses of between 3 and 10 mm have been used, together with metal, ceramic, silicon and Teflon of differing geometries as the more heat resistant material.

The fields of application for this technology are expanding and there is great potential for medical device applications. Some examples follow.

• Reinforcement of plastic components: When the components are reinforced with metal inserts, the mechanical loads are absorbed by the metal component and as a result the parts can withstand much greater mechanical stresses.
• Joining plastic spectacle lenses to the frame: Apart from increasing strength and securing against loosen-ing, the technology allows new design elements and the components can be joined directly at the opticians using smart machinery, which still has to be developed; no pre- or postprocesses such as drilling holes for screws are needed.
• Use in mobile telephone technology: Metal pins in hinges frequently subjected to load in the telephone’s plastic shell can prolong hinge service life; in addition, the joint imparts a higher quality image.

Jens Holtkamp is Project Manager and Alexander Olowinsky is Group Leader Micro Joining at Fraunhofer Institute for Laser Technology ILT, Steinbachstrasse 15, D-52074 Aachen, Germany, tel.+49 241 89 06 491, e-mail: alexander.olowinsky@ilt.fraunhofer.de, www.ilt.fraunhofer.de