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Optimizing Aluminum Tools
Plating aluminum molds for longer production life.
By Steven J. Bales
Aluminum molds is a hot topic with molders and moldmakers alike discussing and debating the advantages of
using aluminum as a cost-effective way to become more competitive. Because the advantages are very valid
ones—e.g., shorter leadtimes on the tool build due to faster machining times, which also reduces costs, plus
better thermal conductivity that can mean greatly reduced cycle times, etc.—a discussion about plating aluminum
molds is appropriate for those considering their use.
History and Applications
Using aluminum tooling for injection molding is not an entirely new concept. Initially, prototype molds were most
commonly made out of aluminum, although the automotive industry has used it for years and it has slowly been
gaining popularity with companies outside the automotive realm.
More and more customers have asked how to extend the life of these tools so they can be used for limited
production. As this trend progressed, those same customers began approaching aluminum tooling as bonafide
production tools, and even more questions were asked, such as:
•What is the final finish required so that the tool may be plated for better part release?
•Does it require a paper finish, a diamond finish or perhaps a light bead blast, for example?
•What is required to prevent corrosion and wear?
These are all good questions that should be answered prior to treating the tool.
Aluminum tooling is also becoming more popular in such applications as blow molds, R.I.M. molding, rubber
molds, structural foam molds and R.T.M. molds due to new technologies and the development of aluminum
mold plate, specifically designed for molding plastic. It is gaining in popularity and believers in the material say it
is, in fact, underutilized, though it may not be appropriate for every application.
With aluminum tooling gaining popularity as bona fide production tools, it is important to learn how to protect
them and improve their performance through the use of traditional and engineered coating materials. Photo
courtesy of Aluminum Injection Molding.
With traditional tool steels, everyone is looking for extended production life and so plating the tooling with hard
chrome or nickel—or perhaps more specialized, engineered coatings—is done to stave off wear or corrosion, or
to promote better release. With aluminum tooling, the same goals are sought after and there are practical
If in addition to extended tool life the manufacturer also wishes to maintain a certain gloss level on the aluminum
tooling due to molding decorative parts, electroless nickel is recommended, as it will help maintain the surface
finish longer, producing decorative parts with relative ease.
Because aluminum is soft, if left uncoated abrasion from the plastic can break it down and alter the resulting
gloss level of the molded parts. Electroless nickel adds a 50 Rockwell hardness that will protect and extend the
gloss or texture on the mold’s surface.
What is more, electroless nickel can obtain a much better quality surface finish than aluminum can by itself, but it
must be pointed out here that there is some surface preparation required before the tooling can be plated. For
example, to get a lens quality finish, it is recommended that one first brings the aluminum up to an SPI A-3
surface finish, and then apply 0.0003 – 0.0005 high phosphorous electroless nickel before further polishing it to
achieve a diamond quality finish.
This process also offers a tremendous time and cost savings in other ways. Often, aluminum brings with it
various imperfections that are not always visible with the naked eye, but can be very detectable on molded parts,
resulting in wasted material and back-to-the-bench time to analyze and correct the problem. Electroless nickel
will help to smooth over and minimize these imperfections before the tool is put into production.
Because electroless nickel is deposited uniformly on all surfaces, it will fully envelope the part and include all
tapped holes, dowel pin holes, etc., and will actually improve the structural integrity of the aluminum tooling.
Another plus is that an electroless nickel application will not compromise the aluminum properties because it is
applied at a low temperature of 180oF.
Other coatings work equally well on aluminum tooling, depending on what production characteristics are
David Bank, president of Aluminum Injection Mold Co., one of the nation’s most outspoken aluminum tooling
advocates, likes using nickel boron nitride coating on molds built using aluminum mold plate alloy.
“There are two reasons I use the nickel boron nitride coating, one is for abrasion resistance when I build molds
to run low percentage of glass filled materials and for corrosion protection when building molds for running
material like PVC,” says Bank. “In both cases, I have had great success having coated several molds. The nickel
boron nitride application process is very aluminum friendly and it is strippable to allow for changes when
needed. No matter your reason for choosing a coating, nickel boron nitride is a very affordable insurance policy.”
Corrosion Protection and Waterlines
If corrosion is a concern, Nickel-P.T.F.E., nickel-boron-nitride and electroless nickel can all aid in its prevention.
There should be no need to use a mold spray or rust preventative during mold shut-downs, if one of these
engineered coatings is used.
Waterlines can also benefit by electroless nickel plating of aluminum tooling. If used, there is no need for
concern about constriction of the lines or the white, scale-like coating that can slow cycle times because the
plating material can virtually eliminate these problems. When the entire tooling block is plated, the waterlines are
also coated as long as the plugs are taken out of the tool prior to application.
At 50 Rockwell, straight electroless nickel offers general abrasion protection and is the best protection from PVC
gasses; Nickel-P.T.F.E., at 45 Rockwell, is good for moderate abrasion protection, but offers improved lubricity
as well as good corrosion protection; and nickel-boron-nitride, at 54 Rockwell, is great for abrasion protection
and also offers excellent release and corrosion protection (see Chart).
It should also be noted that there are different grades of aluminum that require different methods of pre-treatment to ensure proper
adhesion of any plating material. So it’s always beneficial to know your base material, or find a plating vendor that is equipped to
analyze it for you. This will almost guarantee the best adhesion on slides, shut-offs, parting lines and other tooling components.
There’s no doubt that this trend toward the use of aluminum tooling for long-term production molding will continue, and there will
undoubtedly be a number of interesting aluminum alloys developed to accommodate demand and meet various molding
application requirements. No matter what comes down the line, there is an engineered coating available that can improve
production quality and prolong the life of the tooling—all it takes is a little homework and the services of a plating vendor that has
the experience and resources to help make it happen.