Selecting Plating Process for Plastic Molds
There are many causes of wear and corrosion that contribute to reducing life in molds for plastics.
When choosing a plating for your mold or mold components, you must first identify the problem to be solved or, better yet, the problem to be prevented.
Causes of Mold Damage
The first question you should ask is, "Is this problem the result of corrosion, or wear?"
If the component has previously been plated, this may be difficult to determine. A good plating supplier can determine if a component has been plated before and, if so, with what kind of plating.
Abrasive wear can occur on compression molds that use mineral or glass-filled materials. This wear can cause a scouring action on the mold surface.
In transfer and injection molding of thermosetting materials, wear is often detected in high flow areas such as 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 often, damage results from continuing to run a mold after flashing occurs. However, there are often other sources of damage such as when water contacts unplated surfaces and corrosion ensues.
Water forms in molds from condensation, seepage through porous metals, 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 leaves water on the mold surface.
Corrosion is progressive. 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 is attack from acids that form when some thermoplastics decompose from overheating. Overheating can happen in the plasticizing cylinder, the hot runner system, or in the mold cavities because gates are too small, venting is inadequate, or cooling is inadequate. During the molding of PVC, a small amount of hydrochloric acid is formed, which is extremely corrosive to the mold cavity.
Many of these problems can be prevented by the proper choice of the coating and the application process.
Many mold makers and molders are very successful with one particular coating, and use it for all of their applications. Unfortunately, there is not a single magical coating out there that solves all problems.
Benefits of Plating Plastics Molds
The correct coatings, properly applied on mold components:
- Enhance mold release characteristics.
- Provide corrosion protection for the mold, in some cases superior to stainless steel.
- Improve wear resistance from galling and scratching.
- Salvage worn, damaged or incorrectly machined details.
- Extend the useful life of the mold and reduces downtime.
- Reduce the need for release agents on mold surfaces.
- Protect against handling and storage damage.
- Enhance material flow during the molding operation.
- Decrease cycle time.
- Improve finish.
- Help conserve material.
- Preserve textured surfaces.
For molders and mold makers to take advantage of these benefits, a familiarity with coatings and the plating process is appropriate.
There are two basic types of plating, electrolytic and electroless. Electrolytic plating sends electrical charges through a solution of the coating to create the plating action.
The process relies on both a positive charge, and a negative charge. The negative charge flows through the part to be plated, which becomes known as the cathode. The positive charge flows through a conductive metal, referred to as the anode.
To illustrate, hard chrome plating is one of the oldest types of electrolytic plating processes. A chromic acid solution is the medium from which the coating emanates. When the plating process occurs, the negative and positive ions transfer in the solution, causing a metal (chrome) to reduce onto the base metal being plated -- the cathode.
Because the process requires electricity, the plater is constantly fighting against the laws of nature. Electricity travels in straight lines and goes to the closest point. On sharp corners, there will be a heavy buildup of plating, and, in the recesses, the plating will be thin.
Electroless plating, such as in electroless nickel plating, uses additives in the solution to take the place of the electricity. These additives are known as reducing agents.
All metals have a natural charge. And, when the reducing agents detect the charge in the metal to be coated in the plating bath, they start to react. The metal in the bath solution -- in this case, nickel -- reduces to a plating onto the base material of the component being plated. No electrical current is required. Wherever the plating solution touches the base material, the plating will adhere. This gives the coated part a very uniform deposit, and the plating thickness can be controlled within 0.00005 to 0.0001 inch, even on complex shapes.
Electroless nickel plating on aluminum has advanced from a rather complicated technology to a very consistent, easily reproducible procedure. It can extend the use of aluminum to applications where it could not be considered otherwise.
As a consequence, electroless nickel is being relied upon more as a finish of choice by design engineers looking for lightweight, high strength materials in the manufacture of their products.
In the mold industry, electroless nickel plays a big part in increasing mold life by providing a surface hardness from 46 to 48 Rockwell C, and improving release characteristics.
What to Choose
Within these two families you have several types of deposits, including composite and alloy deposits. As a rule, if you need a perfectly uniform deposit in a complex shape, electroless nickel deposits are best. Hardness of the coating as-plated can vary by modifying the chemistry of the plating solution -- as hardness increases, corrosion resistance decreases. Also, it can be plated to size often eliminating costly grinding operations generally needed after plating. One drawback is that it is generally more difficult to apply in a thickness over 0.005 to 0.007 inch; although there are exceptions.
Electroless nickel, by its very nature, is an excellent corrosion barrier for most mineral acids, whereas chrome is attacked by these materials. Stainless steels can also be susceptible to attack by chlorine or fluorine-containing plastics, which can lead to pitting or stress corrosion cracking. This can be eliminated with a high phosphorous electroless nickel deposit over the stainless steel.
Hard chromium is the hardest deposit and exhibits excellent mold release characteristics. It offers superior wear resistance in an as-plated condition (65 to 72 Rockwell C). And it doesn't have to be plated very thick to be effective. Just a few ten thousandths of an inch can dramatically improve wear resistance. Hard chrome can be plated up to 1/8th of an inch thick or more, with special care during processing, to salvage worn or mis-machined parts.
There are more than eight plated coatings for molds available from Techmetals, Inc. alone. Many of them were specially engineered. With so many coatings, so many different types of molding, and even more types of molding materials, there is no simple answer to which coating will most enhance performance.
There will be a plating solution for your molding operation if there is a corrosion, erosion, material flow, or release problem. Evaluation of individual coatings and application processes, on a case by case basis, is the key to success.
Before you make a critical decision, seek advice from an industry leading plater like Techmetals, Inc. The company has developed many special coatings and processes to solve problems. Besides hard chrome and nickel plating, Techmetals is skilled in machining, grinding, polishing, brush plating, welding, metalization, blasting, degreasing, and heat treating. The firm can handle parts up to 10,000 pounds and 10 feet long in its modern 80,000 square foot facilities.
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