With a basic knowledge of plating types, moldmakers can make an educated coating choice that will

improve mold performance.

by Steve Deisher

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.

General   Advantages Obtained Through Plating

•   Enhances the releases chanracteristics over tool steel or stainless steel
  • Provides corrosion protection for the mold, in some cases superior to   stainless steel
  • Improves wear resistance from galling and scratching
  • Salvages worn, damaged or incorrectly machined details
  • Extends useful life of the mold and reduces downtime
  • Helps eliminate the need for release agents on mold surfaces
  • Helps prevent handling and storage damage
  • Helps enhance material flow during the molding operation
  • Decreases cycle time
  • Improves finish
  • Conserves material (plastic)
  • Preserves textured surfaces
  *Note: These particular advantages are dependent upon the particular coating   choice.

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.

Identifying Problems

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.

Specific Component Advantages Obtained through Plating

•Barrels,   screws, nozzle assembly: Increases flow, reduces wear and corrosion, resists   straining and reduces purging time and material.

•Leader  pins: Reduces or eliminates lubes and protects against corrosion

•Runner  plates: Increases flow

•Parting lines: Helps maintain sharp and clean edges, and reduces flash

•A&B plates: Reduces buildup and adds corrosion protection. Protects water lines   from corrosion mineral buildup, and often outperforms   stainless steel ( Which is subject to pitting corrosion)

•Gates,   slides: Reduces or elimiinates wear. Adds corrosion protection and reduces   cleanup.Back up plates, ejector plates,   support column rails.

•Mold   areas: Improves release and material flow, adds corrosion resistance and can   reduce the amount of the resin used.

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.


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