One of the major problems with injection molded plastic and rubber products is burrs. The problem is attributed to wear or wound in the Parting Lines (PL) of the mold. These minute wear or wound form a small gap and plastic or rubber can be squeezed into the gap, thus forming burrs. The conventional repair technique is welding over the worn/damaged areas followed by finishing.

The overlay method in PL is usually done by metal-inert-gas (MIG) and/or tungsten-inert-gas (TIG) techniques. However, use of the welding technique when making repairs has various shortcomings, such as distortion and cracking due to partial melting of the mold and introduction of tensile residual stress upon cooling. Also, tensile stress develops in the weld causing a pulling action toward the middle of the bead, and this, in turn, produces shrinkage in the die that is commonly called the "under-cut".

The ideal repair technique to eliminate these problems would be a low heat input overlay process. An electro-spark deposition technique meets these requirements because of its very low heat input. The process is particularly suitable for overlaying on a small (low mass) mold and die because it produces no distortion, cracking or under-cut. Also, even very large molds or dies can be repaired locally without dismantling. On-site repair work is drastically possible to reduce down time and increase productivity.

ESD (Electro Spark Deposition) system uses electronic circuits similar to the one found in EDM (Electric Discharge Machining). ESD was invented in the ex-Soviet Union and was used for coating thin tungsten carbide on mold surfaces to prevent wear and scuffing and for protection from heat. However, the thickness of the coating was limited to no more than 30 µm, and it could not be used for thick alloy overlays. ESD has gone through various improvements that overcame these shortcomings. The movement of the consumable electrode in the original invention was merely vibration, back and forth on the application surface. The present system employs a rotating electrode and argon gas shield that prevents the formation of oxide and/or nitride in the deposit. The composition of the consumable electrode has been modified to obtain a thick deposit with suitable properties for overlay application on various molds and dies.

Some injection mold users do not have in-house capability for repairing the molds. When burrs on the molded products are found, the worn mold is shipped to an independent repair shop where welding, grinding and EDM machines are available. Often the injection work is stopped while the mold is being repaired which may cause a serious delivery problem. And also mold users have to pay for repair fees.

However, the ESD overlay process solves many of the repair-related problems by eliminating the distortion, under-cut and excess finishing time. Finishing an ESD overlay is easily accomplished using micro grinders, files and/or oilstones. This is not only faster and safer but also very cost effective when compared with using weld-repair when restoring the PL in injection molds.

2-1. Principle

The ESD process uses the stored energy in condensers that is transferred to a consumable electrode for a very short duration of 10^-6 to 10^-5 sec for every 10^-3 to 10^-1 sec. The temperature at the tip of the electrode is about 8,000 to 25,000ºC and ionized material is transferred to the mold surface producing an alloyed layer of deposit as shown in (Section A) Figure 1. A diffusion layer (Section B) is produced at the interface of the substrate and overlay providing excellent bonding. Also, diffusion bonding occurs among the overlapped layers.

Figure 1. Schematic Diagrams of Coating and Overlay

Figure 2. Concept in Pulse-Spark Relationships. Pulse Time (pt):10-6 to 10-5 sec., Interval Time (It): 10-3 to 10-1 sec. PowerOutput(A):Capacitance(µF), Frequency (Hz): Sparking/sec

Figure 3. Schematic View of Diffusion Bonding Mechanism

2-2. Reason for Low Heat Input

ESD provides a very low heat input during the overlay process because the Pulse Time (pt) is extremely shorter than the Interval Time (It) as shown in Figure 2. Diffusion bonding is accomplished during the pt and essentially no heat buildup takes place in the deposit although the temperature at the tip of the electrode instantaneously reaches above 1000 ºC while the substrate temperature is kept at the ambient.

2-3. Why Strong Bonding is Possible

Strong bonding in the ESD deposit is possible even though the heat input is low. Formation of a diffusion layer occurs upon sparking when the heated-ionized electrode transfers to the work surface. (See Section B, Figure 1.) Because of the diffusion bonding the deposit strongly adheres to the substrate. Figure 3 shows a schematic view of the bonding mechanism.

4-1.Overlay by ESD

Photo 1 shows overlay work using the Depo series ESD units. The test piece is a tool steel plate of SKD-61, 50mm x 80mm x 20mm, showing overlay on the tip of a corner tip, edge, hole and pinhole. The top and middle are before and after overlay, respectively. The bottom is after finishing. (The tip of a white arrow)

Photos 1,2,3,4,5 and 6

4-2. Comparison of Overlay Processes

Historically, the most commonly used process for repairing dies and molds is argon shielding welding (TIG or MIG) followed by spot welding. The most recent development is the ESD process that has been used since 1995. However, a laser welding technique has been developed in Germany; it is not widely used in the world due to its high cost. Table 1 show the differences in the characteristics among several overlay processes and compared with ESD, particularly those of the Depo series offered by the TechnoCoat Corporation.

Table 1. Comparison of ESD against other Overlay Processes on Die and Mold

5. Applicable Base Metal

The following mold and die metals can be treated with ESD: low carbon steel, medium carbon steel, tool steel, mold alloy, cast iron, cast steel, stainless steel, Al alloy, copper alloy, electro-formed nickel-copper mold and most metals and alloys with sufficient electrical conductivities.