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 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 solutions.
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 required.
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).
|Engineered Coatings and Finishes|
|Ideal Use||Molding Applications|
54 R.C. as Plated
67 R.C. After H.T.
0.05 or Less
|Superior Lubricity and Excellent Abrasion||Where Superior Release and High Wear or High Temperature are an Issue|
0.45 or Less
|Moderate Abrasion and Excellent Corrosion Resistance||Excellent Chemical
Resistance and Uniform
Deposit Recommended for PVC Molding
0.10 or Less
|Excellent Release Properties for Deep Ribs. “No Draft” Cores, Textured Surfaces and Difficult to Eject Polymers|
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.