Tooling Corner: Options for restoring mold surfacesTooling Corner: Options for restoring mold surfaces

Editor?s note: Steve Roy is Nd:YAG product manager at Trumpf Inc., and Michael Francoeur is president of Joining Technologies.

Plastics injection molds usually require some form of surface restoration during their service lives. Most often, specialty welding is used because of the precision required and the value of the mold. A primary difficulty in restoring surfaces on delicate molds is limiting the heat to the part, so that welding does not alter the properties of the mold. Secondly, downtime must be minimized and the mold returned to operation as soon as possible.

Gas Tungsten ARC Welding

Gas tungsten arc welding (GTAW) or tungsten inert gas (TIG) welding has been the process of choice for more than 40 years. GTAW has evolved into a highly refined thermal application process for surface restoration on tool steel. Today?s advanced, constant-current power supplies equipped with microprocessor controls provide the highly skilled welding specialist with an extraordinary tool for microwelding and surface cladding.

The GTAW process evolved from fundamental electrical theory. First, an electrical circuit must be completed?the workpiece must be conductive material and create a good path for electrical energy. Electrons travel from the cathode through the electrode and ionize gas to form a bridge of highly charged ions that flow to the anode. The plasma gas cloud surrounding the electrode can create operating temperatures up to 10,000F. The second heat-creating phenomenon is derived from electrical resistance between the anode and the cathode. The weld current (amperage) is related to the electrical resistance and arc voltage, this electrical resistance creates additional heat.

The welding specialist first starts an arc on the metal surface by carefully holding a close-tolerance gap. As a melt zone appears, wire filler is dabbed into the molten pool. Although the micro-arc process can be performed at very low amperage, heat is quickly produced and a high degree of skill is required. Micro-arc welding requires skill to manipulate the arc gap, weld pool, and filler wire while viewing through arc glare and working on a part preheated to 400F.

There are only a handful of highly qualified jobshops responsible for restoring delicate molds in this capacity. The skill required for precision micro TIG or GTAW welding is considerable, and the cost is commensurate to that skill.

The Laser Alternative

Nd:YAG lasers offer some distinct advantages over gas tungsten welding methods for tool-steel resurfacing. Because the average power used in the laser repair process is low (less than 20W) and the peak power/laser pulse is high (multikilowatts), the tool steel can be melted and filler wire added with concentrated heat input. The laser?s focused energy input allows most tool steel to be welded with minimal preheating, reducing the repair time and minimizing distortion.

High-power laser pulses are typically between 5 to 10 msec in duration, resulting in low or minimal annealing of the base material. Base metal hardness can be maintained or exceeded depending on the composition of the filler wire. The combination of the proper filler and appropriate laser parameters can tailor hardness to the specific application. If the laser pulse is shortened, the cooling time also is reduced, resulting in increased hardness.

By limiting the size of the melt zone, rapid solidification occurs in the individual weld pulses, resulting in negligible drawing of base material into the weld zone which greatly reduces or eliminates ?sinking.? The focused spot size of the laser beam is typically between .3 and .6 mm in diameter.

During the resurfacing of worn parts, filler wire is positioned over the area to be processed, and the laser beam operator visually traces the wire. The laser energy rapidly coalesces both the wire and substrate. This process ensures that no more of the mold is exposed to thermal energy than necessary. Since the heat is concentrated, preheating is seldom necessary?thus eliminating the potential for distortion of the part. In order to achieve a specified dimensional requirement, several passes are layered until the desired dimension is attained. Inert shielding gas is used to blanket the weld zone to minimize oxidation.

The laser beam delivery optics are typically 5 to 6 inches away from the workpiece, and positioning is done via motorized work-stage and joystick control.

Since lasers are high-energy light beams, more energy is needed for reflective material. For example, aluminum and copper need the most energy because of their high reflectivity and thermal conductivity, while most tool steels work well with lasers and are easily processed at lower power levels.

The amount of filler material needed also affects the laser power. Larger repairs require larger diameter wire filler. The larger the wire, the greater the focused laser beam needs to be to attain the proper power density for melting, accordingly increasing the power needed for processing.

Laser Equipment Requirements

Pulsed laser power supplies can be easily integrated into the versatile workstation via fiber optic cable and range from 50 to 200W of output power. One version comes with a motorized top that rises up and out of the way to ease loading and unloading of the workpiece. To accommodate heavy parts, the motion system rides on an air-bearing plate. This plate detaches from the interior of the workstation and slides out onto an accessory table that fits right up against the front of the workstation. Parts weighing up to 330 lb can slide into the workstation on the air-bearing plate and then be reattached for processing. The motion system platform can travel 10 inches along the x- and y-axes and is controlled with a speed-sensitive joystick. The z-axis can move 11.8 inches via a footswitch, to accommodate processing large parts.

The system includes gas shielding, a video monitor, soot removal, and an internal parameter toggle for quick parameter changes. The workstation has a 16X binocular used to view the part and target the beam?s location. The work station also incorporates a programmable CNC control used to trace complex profiles.