Important tips and information about mold coatings to help you achieve the level of production that you

and your customers desire.

                          By Steven J. Bales

There’s an awful lot to know these days about molding plastic and how to get the very best performance

from the valuable tools you build or run. This guide has been written to provide important tips and information about mold coatings. After reading this, you should have a very good idea of what coatings—from the very traditional to the very latest—will help you to achieve the level of production you

and your customers desire. After all, these tools are an investment and they need to be protected for the

life of the products they mold.

The Key Role of Coatings

Before introducing you to the wide range of coatings on the market today (see Chart 1), it’s important to

note the role coatings can play in an effective preventive maintenance (PM) program.

PM is really the key to protecting your tooling, your investment. Why? Because it saves time and money.

Once you invest in a mold coating to improve tool performance, then a PM program is always a good

idea to ensure you get the maximum benefit. These two steps should be a given in any shop.

Nickel-boron nitride is uniformly deposited and used where exceptional release is required. It also has

corrosion protection and an obtainable hardness of 67 RC. Photos courtesy of Bales Mold Service.

Remember, no coating lasts forever, and producing substandard parts from a mold with a worn coating is

no way to win customers and stay profitable. PM is probably the most cost-effective strategy you can put

in place. The key is to educate your personnel on how mold coatings wear during production. Every

coating is different, so it’s of benefit to have employees learn how to tell when the coating is showing

deterioration, especially in high-wear areas such as gates and runners.

For example, wear in and around gate areas plated with hard chrome is the first sign that your mold

needs servicing. How can you tell there is wear? The chrome coating is approximately 20 RC points

harder than the base steel, so exposed steel will wear much faster than the coated surfaces surrounding

it, causing a slight or pronounced edge or “step” on the surface.

Conversely, nickel will wear almost evenly, causing a kind of feathering effect, making it more difficult to

recognize wear. A more identifiable difference will be the color because when nickel coating wears, it

produces a shadow or halo effect on the steel. No step or edge will be evident. The steel also will have a

more silver appearance compared to the somewhat tarnished look of the nickel coating.

This knowledge makes pulling a mold for maintenance before the coating wears through an ultra

important aspect of a PM program. To miss important wear signals means more costly repairs and

additional polishing expense.

Coating

Rockwell Hardness

Coefficient Of Friction

Applied Temp.

Ideal Use

Molding
  Applications

Diamond-Chrome

85 + R.C.

0.15 or less

130 Deg. F

Excellent
  Abrasion Resistance

Moving Components Slides, Rotating   Cores,Locks, etc.

Hard Chrome
  QQ-C-320

72 R.C.

0.20 or less

130 Deg. F

Good
  Abrasion Resistance

Abrasion Resistance
  Glass filled Materials 
  (Not Recomended for PVC)

E.N. Boron-Nitride

54 R.C. as plated

67 R.C. After H.T.

0.05 or less

185 Deg. F

Superior
  Lubricity &
  Excellent Abrasion

Where Superior
  Release & High Wear or High Temperature
  are an Issue

Nickel-Cobalt

62 R.C.

0.24 or less

185 Deg. F

Good
  Abrasion &
  Corrosion Resistance

Uniform Deposit for
  Complex Details &
  Good Chrome
  Alternative

Electroless Nickel
  AMS-2404-C
  AMS-2405-B

50 R.C.

0.45 or less

185 Deg. F

Moderate 
  Abrasion &
  Excellent
  Corrosion Resistance

Excellent Chemical 
  Resistance &
  Uniform Deposit
  Recommended for PVC Molding

Nickel-P.T.F.E.

45 R.C.

0.10 or less

185 Deg. F

Excellent
  Corrosion
  Resistance &
  Superior Lubricity

Excellent Release
  Properties for Deep
  Ribs, “No Draft” Cores, Textured Surfaces & Difficult to Eject Polymers


Sulfamate Nickel
  QQ-N-290

40 R.C.

0.45 or less

140 Deg. F

Pitt Free
  Heavy Deposits

Isolated Deposits
  for Dimensional
  Corrections


Engineered coatings   and finishes. Chart courtesy of Bales Mold Service














Measuring Wear

A recommended tool for measuring the wear level of any coating is an electronic thickness gauge that

uses a combination of magnetism and eddy current to accurately measure surface thickness. When the

mold first arrives in your plant, take the time to measure the surface thickness—especially in high-wear

areas—using this determined that the finish is wearing to a critical level, pull the mold and send it out for

maintenance.specialized tool. As you run production on the mold, occasionally pause to re-measure those areas. When you have determined that the finish is wearing to a critical level, pull the mold and

send it out for maintenance.

Part Counts

Be sure to record the measurements taken with the thickness gauge and use the notes to create a history

of maintenance requirements for the tool. A cycle counter installed on the mold will allow your tooling

engineer to record wear levels as compared to piece part counts, thereby doubling the effectiveness of

your PM program. Part counts are a great way to determine maintenance needs, especially with

high-volume molding projects.

From the very first time you run the mold, keep an accurate piece count until it is ready for its first

maintenance work. Use that count as a gauge for when the next maintenance is due. Because you know

approximately when the mold will be ready to be refurbished, you can arrange the service in advance

with your coating vendor. This not only gives him ample time to schedule your mold maintenance, but it

also allows you to optimize the use of the mold and the machine that’s running it.

Coating Challenges

Even today, there are those who question the benefits of using fancy—sometimes more

expensive—coatings to prolong tooling life or enhance performance. To some, the tried and true hard

chrome or electroless nickel are all they’ll ever need to accomplish those goals. But we all know that

today’s engineered plastic materials can be pretty rough on injection molds.

Challenges to mold maintenance extend beyond glass- and mineral-fillers to include rice hulls, wood

fibers, metal powders, flame retardants and other additives—not to mention the resins themselves. In

addition, outgassing and moisture acidity often accompany abrasive wear, taking an even bigger toll o

expensive tooling.

In addition, growing complexity in mold design involves tinier, more intricate flow passages and more frequent.

use of moving cores and slides. All of these circumstances have prompted the development of a wider variety of mold coatings that can keep molds operating longer between repairs.

New Coating Science

If you are molding highly intricate parts using glass-filled materials, you might think using hard chrome

will be sufficient because it is a classic, reliable way to protect your mold from both corrosion and

abrasion. However, hard chrome, for all its benefits, does not tend to plate uniformly in detailed areas like

ibs and bosses. There is a newer solution—a nickel-cobalt alloy coating that can overcome that mitation.

Nickel Cobalt

Nickel-cobalt can be an economical alternative to hard chrome. Hard chrome requires construction of a conforming anode to coat the mold. The more detail in the mold, the more time it takes to build the anode

and the more expensive the process becomes. This nickel-cobalt alloy coating requires no anode, and because of its electroless properties, it plates much more uniformly.

Complex details, such as deep ribs, bosses, and textured surfaces, can be coated and will enhance the

release capabilities when using nickel-PTFE.

The cobalt gives it good abrasion resistance, but its hardness is 62 RC, 10 points lower than hard chrome.

Is it worth paying extra for hard chrome’s superior wear protection? You have to consider the material

being run in the mold. What’s the percentage of glass? Is corrosion a greater concern than abrasion?

Diamond Chrome

Hard chrome and a nickel-cobalt alloy coating offer two very good solutions for abrasion resistance, but

for very high-wear conditions, an even newer product called diamond-chrome offers exceptional protection.

It has an RC rating greater than 85 and is a chromium-matrix composite coating with a dispersion of

nanometer-size, spherical diamond particles. Since diamonds are unmatched for hardness, this coating

offers protection beyond the norm. Though their Rockwell ratings are comparable, diamond-chrome

outperforms titanium nitride (TiN) coating because it won’t compromise the dimensional integrity of the

plated tool. The difference is that it is applied at only about 130oF while TiN requires application temperatures of 800oF or higher.

Diamond-chrome can plate prehardened, heat-treated or nitrided steel and other base materials such as

aluminum, beryllium-copper, brass and stainless steel. Recommended uses include cores, cavities,

slides, ejector sleeves, and rotating and unscrewing cores. Its anti-galling properties are advantageous

on moving cores and slides.

Diamond-chrome also is very strippable and has no adverse effect on the base material, saving time and

money when maintenance is needed. TiN is strippable as well, but it can take up to several days to

remove with a peroxide-based solution. Diamond-chrome can be stripped in minutes using reverse

electrolysis in a caustic solution.

In addition, diamond-chrome can be deposited at any controlled thickness from 20 millionths of an inch to

0.001 in. TiN is generally only applied in thin deposits of a few millionths of an inch. Diamond-chrome can

coat complex details, while TiN has very limited coverage of complex details. While TiN is very lubricious,  

with a coefficient of friction (COF) of 0.4 (against steel), diamond-chrome has a COF of 0.15—nearly

three times more lubricious.

Nickel-Boron Nitride

When it comes to molders’ needs for a specialty coating that offers excellent release properties and high

resistance to wear, heat, and corrosion, an electroless nickel-phosphorus matrix containing boron nitride

particles should be considered.

It has a very low COF (0.05 against steel) and an RC hardness of 54, which can be increased to 67 RC

after heat treating—a unique characteristic. Nickel-boron nitride can be applied to any substrate at only

185oF and can be easily stripped without compromising the base material. Though it is approximately 20 percent more expensive than nickel-PTFE, this coating will outperform nickel-PTFE at up to 1250oF, which far surpasses the 500oF maximum limit for all PTFE-based coatings.

Because applying nickel-boron nitride is an autocatalytic process, it requires no anode, therefore saving time and money. In addition, it will not compromise thermal conductivity of the mold. Applications include unscrewing cores for closures, where reduced cycle times are essential.

Where lubricity is needed for better release from deep ribs, zero-draft cores, textured surfaces and

“sticky” polymers, a nickel-PTFE composite will greatly improve part release and enhance resin flow by

as much as 4 to 8 percent for shorter cycle times. COF is 0.10 against steel.

It should be noted that applying pure PTFE to the mold adds high lubricity, but only a very short-term

benefit. PTFE by itself has no hardness, so it won’t last. But a dispersion of 25 percent PTFE by volume

in a co-deposit with nickel results in 45 RC hardness for added wear and corrosion protection.

Tried and True

Despite the new coating science, we cannot throw out the old, reliable coatings such as like hard chrome

or electroless nickel just yet. There’s no question that they still have their uses.

Hard Chrome

For example, hard chrome’s top advantage is that it has a hardness of 72 Rockwell C (RC) and is applied

at the low temperature of 130oF. When applied in its purest form, it allows you to achieve any SPI finish

on your tooling.

Hard chrome is often a good choice for electrical circuit-breaker molds since they use materials

containing as much as 40 percent glass. To help combat erosion and prevent severely damaging gates

and surrounding mold areas, it is usually recommend to use a high-diamond polish, followed by a hard-chrome coating of 0.0003 to 0.0005 inches for added protection.

The downside can be cost, since chrome plating is limited to areas accessible by an anode. If your mold

has complex details, it could require extra conforming anode construction and that adds time and expense to the project. Another possible drawback is chrome’s environmental impact—chromium is a  carcinogen. Some companies are attempting to develop better, cleaner alternatives, but so far nothing matches hard chrome’s benefits from a tooling perspective.

Electroless Nickel

Like hard chrome, electroless nickel has been used successfully for years, particularly to protect molds

where corrosive off-gassing is created by materials such as PVC or halogenated fire retardants. It is not

uncommon to see such resins produce an orange rust, corroding the unprotected mold almost right

before your eyes. Products molded of such materials for the electronic or medical industry often can not

tolerate the presence of any oxidation by products.

Electroless nickel does an excellent job of resisting oxidation because it plates very uniformly in thin

deposits of 0.0002 to 0.0003 inches. Even in tight areas of detailed parts, electroless nickel at 50 RC

hardness is ideal for corrosion protection. It can be deposited in very accurate thicknesses of 0.002 to

0.003 inches and can be ground or EDM’ed. Thus, electroless nickel often is used for dimensional

build-ups under flash chrome and for enlarging threaded cores and inserts or precisely sizing cavities. It

also works very well on entire mold bases, A and B plates, ejector-base housings, pin plates and pillar

supports, providing years of low-maintenance, rust-free operation.

Know Your Mold Finishes

Before determining what coating to use—if one is needed—the mold finish must be taken into account

because, as noted earlier, certain finishes may actually increase the need for a mold coating, and some

combinations work extremely well together improving lubricity and release properties.

There are four standard SPI finishes: diamond, stone, paper and blast. Each gives the molding surface a

different appearance, from a glossy, mirror-like surface (A-1 Diamond) to a fairly rough, gritty texture

(from blasting with glass beads or aluminum oxide). Each of the four finishes has three grades as well.

Diamond

The A-1 Diamond finish is the most perfect finish available, which means it has the lowest RA value (roughness average). There are no high or low ridges. For example, a paper scratch on steel can rate a 2

to 4 RA finish, whereas an A-1 Diamond is lens-quality smoothness, generally 1 RA or less. Roughness

is almost immeasurable.

But a number of plastic materials tend to stick like glue to the flawless, mirror-like finish, making such

perfect smoothness almost detrimental in many molding applications. One good example is molding

polystyrene on a polished straight-wall core with 1d or less draft. Streak or drag lines can appear on the

parts. This can be solved by flash-chrome plating the core, which creates a surface with micro-cracks.

Impregnating those cracks with PTFE and then re-establishing the A-1 Diamond finish solves the problem in more than 95 percent of cases.

Thin-wall container molding adds another dimension to the use of a diamond finish. Molders of sour

cream, yogurt and cottage cheese containers find their products are more attractive to the customer if

they have a slight sheen. To obtain that effect, these parts require a high-diamond polish with a slightly

interrupted gloss adjustment so that the slight sheen will occur. This finish adjustment also allows for much better release of the parts.

In thin-wall molding applications such as these, a light bead-blast finish is applied—just enough to very

slightly interrupt the flawless A-2 Diamond surface. The surface is buffed again, leaving just a bit of

almost invisible stipple. This finish plus a coating of nickel-PTFE will greatly improve part release and

enhance mold filling.

Phenolics and other thermosets almost demand a perfect polish and work extraordinarily well with a

diamond finish. Combine that with a hard, protective coating like chrome or diamond-chrome, and you

will strengthen the mold’s surface and optimize release.

Again, using a topical PTFE coating would be of minimal benefit because it will not last long. Successful

application of straight PTFE depends on having a sufficient grain structure in the mold finish to hold onto

the coating. Since molding thermosets requires a perfect finish on the tool, PTFE by itself will have

limited adhesion to the surface and therefore will fail relatively quickly.

Texture and Release

There are many textured surfaces today, including faux leather for automobile dashboards, woodgrains,

geometric patterns and stipple patterns found on pagers, cell phones and computer components. A

plated mold coating is often essential to obtaining a textured surface with adequate lubricity.

Textured surfaces require protection. The peaks of the textured surfaces are the first areas of mold detail

to experience wear, making it very important to check the mold periodically with a profilometer to

measure grain depth and peak counts. Mold coatings help decrease the frequency of repairs and

refurbishment by maintaining the integrity of the textured surface.

If a diamond finish presents release problems, a blast finish can be the answer—particularly when

molding textured parts using materials such as silicone rubber, flexible PVC, TPEs and some soft

polypropylenes. These products tend to cling to a polished finish, but breaking up the surface with a light

blasting improves release. Add a coating of nickel-PTFE and you get even better release.

Hard chrome and electroless nickel plating will help protect textured surfaces, as will a nickel-cobalt

coating. Unlike hard chrome, electroless nickel-cobalt plates uniformly, which makes it ideal for very

detailed molds with deep ribs and bosses. It combines the corrosion protection and lubricity of electroless

nickel with the strength of cobalt.

Summary

If you’re looking for enhanced performance in your molds, the proper combination of surface treatment, and finish can provide additional benefit by extending production times between preventive maintenance.  

Your coatings vendor can be a valuable resource for educating your personnel on how coatings you use

will wear over time, as well as a way to reduce downtime and cut costs.


2017年02月15日

How to Choose the Right Plated Coatings for Improved Mold Performance
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