Plastics molders, operating both captive and job shops, are frequently plagued with downtime due to repairs that must be made on damaged or worn mold components. Typical repairs include damaged cavities, worn gate areas, and parting lines that cause flashing of molded parts.
Conventional machine shop practices commonly employed to repair plastics molds share a serious drawback—they all require that the mold be removed to carry out the needed repair. For larger repairs, the part must often be sent to an facility that uses tank electroplating. In-situ selective plating allows the repair of molds, reducing downtime substantially.
Selective Plating Process. Mold/die costs vary widely depending on size, materials and origin. With costs ranging from tens of thousands of dollars up to millions for large automotive part dies, the last thing a producer wants to do is buy new tooling. Just the cost of removing a mold from the machine for outside repair is substantial, and the cost of lost production is greater still. This makes typical in-situ plating repairs for a one-day job a very attractive option. These costs vary depending on the size and depth of the mold area needing repair.
The general approach for selective plating repairs is:
- “Dish out” the repair area to create the appropriate depth-to-diameter ratio (typically this is about a 1:7 ratio to get rid of any sharp corners)
- Apply cleaning and surface treatment chemicals to prepare the localized area for plating
- Use “selective plating” to fill the depression with soft metal material (often copper)
- Then selectively plate with a harder metal that adheres well to the mold and fill material
- Have a mold repair technician do final dimensioning for the required finish and sizing
As a general rule, defect depths of up to about 0.060” are good candidates for this type of repair. Greater depths may be possible if there are cracks and crevices that can first be repaired through other techniques such as welding and preliminary grinding. However, use of those techniques with the mold in the machine may or may not be practical.
If a mold has a lot of corrosion, scratches and other small surface flaws that result in a poor finish on the molded part, the temptation may be to remove the tooling and send it out for tank electroplating. However, selective plating can take care of these flaws in place, without extensive masking, and is significantly faster than tank electroplating.
What’s more, selective plating equipment is portable, enabling the operator to meet plating and build-up requirements wherever they arise. This flexibility is an important cost and time saver. It minimizes dismantling, downtime, turnaround delays, and shipping charges usually associated with sending parts out to be plated.
Dozens of pure metal and alloy solutions are available for use in brush plating to accommodate different mold metals and finishes. These solutions typically have higher metal content than those used in tank plating. Deposits can be produced that are hard, fine grained, very low in porosity, have low residual stress, and are not prone to hydrogen embrittlement. Electropolishing and application of wear-resistant coatings to the mold surface can also be accomplished using the same techniques. Examples of plastic mold plating applications are listed in Table 1.
Table 1. Examples of Brush Plating Applications for Plastic Molds
|Application||Metal Deposited||Capping Metal|
|Mold Cavity Damage||Nickel or Copper Fill||Nickel Semibright, Nickel-Tungsten, or other Nickel Alloys|
|Resize core pins and bushings 1||Nickel or cobalt||Not applicable|
a) Molds for PVC
b) Mold storage
c) Water-cooled molds
a) Hydraulic cylinder repairs
b) Resizing shafts, journal areas, and bearing fits4
|Copper, Nickel or Cobalt||Nickel Semibright, Tin or Zinc|
| || || |
1. Can resize to correct dimension without subsequent machining
2. Gold resists hydrocloric acid that might be a by-product of PVC moulding; cadmium prevents rusting of steel molds and is effective in water channels and back plates
3. No need to strip chrome
4. Shear-fit to resize
As shown in Table 1, selective plating can provide an effective repair on a variety of mold components. Nickel and cobalt are excellent choices for selective deposition to resize core pins and bushings, often without finish machining. These two materials have good toughness characteristics, in addition to excellent wear resistance. If a harder surface is required, nickel-tungsten, nickel-cobalt, and cobalt-tungsten alloys may be selectively deposited in a hardness range of Rockwell C 47 to 60.
The preparatory process used depends on the base material of the mold and any plating from prior processing that may be present. This process can include solutions for cleaning, etching, desmutting, activating, and preplating. In all cases, preparatory and plating solutions are applied with handheld plating tools wrapped with an absorbent cover material saturated with the solution. The mold is attached to the ground lead of a DC power supply, and the plating tool is connected to the positive lead to complete the circuit.
The molds used in many high-end products such as automobiles have become increasingly expensive and have long lead times due to offshore manufacturing. Therefore, it becomes critical to be able to quickly repair defective components, rather than replacing them or delaying repair by removing them from the machine, shipping them out for repairs and waiting for their return. Taking a mold out of service for an extended time is often not an option.
In addition to in-place defect repairs, there are several other applications in which selective plating provides a quick and lower-cost means of returning critical components to service.
Resizing Core Pins and Bushings. As wear occurs and pin and bushing dimensions deviate from accepted tolerances, it becomes necessary to discard and replace core pins and core bushings. With the accurately controlled selective plating process, these parts can be quickly plated back to size and put in service, often without finish machining. Nickel and Cobalt are excellent choices for selective deposition in these applications, since they have good toughness characteristics, in addition to excellent wear resistance. If a harder surface is required, nickel-tungsten, nickel-cobalt, and cobalt-tungsten alloys may be selectively deposited in a hardness range of Rockwell C 60 to 68.
Flash Correction. Worn gate areas or damaged parting lines may be repaired to eliminate flashing by the use of selective plating. After the amount of wear in the gate area is measured, the digital ampere-hour meter is used to control the amount of metal deposit needed to resize the gate precisely without the need for subsequent machining. Chipped or damaged parting lines are repaired in similar fashion.
Machine Maintenance. Many plastics molders who utilize the selective plating process for mold repair have found the process equally useful in press or machine maintenance. Scores or scratches in chromed hydraulic actuating cylinders are filled with copper and then capped with nickel or cobalt (see Figure 2). Again, this repair is performed without disassembling the cylinder from the press and without the need to strip and re-plate chrome. Worn shafts and journal areas may be resized rapidly by the selective plating process. Totally round and concentric bearing fits are obtainable by selective plating.
Selective plating is a cost-effective mold component repair technique for a broad array of applications. It can often be accomplished without removing parts from production machinery, and can be completed in a matter of hours.
ABOUT THE AUTHORS
Robert Burfield is a Market Manager at SIFCO Applied Surface Concepts in Independence, Ohio. He was educated at the University of Akron and has nearly 30 years of experience in tooling, selective plating and quality assurance.