Many moldmakers have had great success with one particular coating and have used it for all of their applications. Unfortunately, there is not a magical coating out there that works in every situation. Many times, excellent coatings are used in the wrong applications and one of the major challenges is correcting those problems. When plating is used properly, the positive results can be stunning. There are many causes of wear and corrosion in plastic molds that can contribute to greatly reducing mold life. Plating can solve many of these situations and can improve mold performance in many other ways. Because there are more than eight different plated coatings for molds, a basic knowledge of plating is necessary to make an educated choice for properly solving your particular problems.
Types of Plating
There are two basic types of plating: electrolytic and electroless. Electrolytic plating requires electricity to make the process occur. When the plating process occurs, the metal (chrome) reduces onto the base metal of the part being plated. Because the process uses electricity, the plater is constantly fighting against the laws of nature. On sharp corners there will be a heavy buildup of plating. In the recesses, the plating will tend to be thinner.
Electroless plating (such as electroless nickel) is just the opposite of electrolytic plating. Wherever the plating solution touches the base material, the plating will adhere. This gives the plated part a very uniform deposit and the plating thickness can be controlled within 0.00005 to 0.0001—even on complex shapes.
Within these two families you have several types of deposits, including composite and alloy. Generally, if you need a perfectly uniform deposit in a complex shape, electroless nickel deposits are best; however, hard chromium is the hardest deposit and has excellent release.
When choosing plating for your mold or mold components, you must first identify the problem to be solved or the problem to be prevented. The first question you should ask is ÒWhat is the cause of this problem? Is this problem the result of corrosion or wear?Ó If the component has previously been plated, this may be difficult to determine. Have your plating vendor look at the component to determine if it has been plated corrosion and if so, what type of plating.
Abrasive wear can occur on compression molds that use mineral or glass-filled materials. These materials can cause a scouring action on the mold surface. In transfer and injection molding of thermosetting materials, wear is often detected in the high flow areas such as in the sprues, runners, gates, and portions of the cavities and cores that are directly opposite the gates. In injection molds for thermoplastics, wear most commonly appears on the surface opposite the gate.
Most damage results from continuing to run the mold after flashing occurs. However, there are other sources of damage of which to be wary. These sources can include water contacting unplated surfaces causing corrosion; water condensing in the molds; seepage through porous metals; and, leaky pipe fittings and O-rings. Where chillers are used for mold temperature control, condensation of moisture on the mold surfaces can sometimes occur even while they are in full operation. Careless handling of hoses and feed lines during hookup can leave water on the mold surface. Corrosion is progressive and even if the molds are stored after being sprayed with an antioxidant, a few drops of water or condensation can cause tremendous and costly damage.
Another source of damage are attacks from acids. These acids may form after exposure to corrosive materials generated by thermoplastics decomposition (often due to overheating). Overheating can occur in the platicizing cylinder, the hot runner system or in the mold cavities as the result of too small gates, or inadequate venting or cooling systems. During the molding of PVC, a small amount of hydrochloric acid is formed which is extremely corrosive to the mold cavity. Electroless nickel, by its very nature, is an excellent corrosion barrier for most mineral acids, whereas these acids will attack chrome. Stainless steels can also be susceptible to attack by chlorine or fluorine containing plastics. This can lead to pitting or stress corrosion cracking, which can be eliminated with a high phosphorous electroless nickel deposit over the stainless steel.
Enhancing Your Mold's Performance
With so many different types of molding and even more types of materials available, there is no easy answer as to which coating will best enhance performance. An investigation into these coatings is worthwhile because if your molding operation is left unprotected, it can create corrosion, erosion, materials flow or release problems. The proper plating can make all the difference. Before you make that critical decision, call your plating vendor of choice and ask which coating will work best for your application.