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Does aluminum alloy hardness matter?

May 15, 2001

3 Min Read
Does aluminum alloy hardness matter?

It cycles faster and is easier to machine than tool steel. Now newer, harder, premium aluminum alloys are touted as tough enough for production tools. An experienced aluminum toolbuilder and supplier, Rafael Kilim of Almo (London, U.K.) believes that proper design is more important than designer-label hardness when it comes to getting best results. 

Kilim says he has found four standard aluminum alloys suitable for injection molding (specified here by their international codes)—2014A and 2024, which are copper alloyed, and 7075 and 7022, which are zinc alloyed. All of these are used as aerospace grades with very good consistency, Kilim reports. They are also hard enough to give good machinability, he says. 

"In the hardness range of 145 to 150 Brunel, excluding the 6000 Series, which is in the 90 Brunel range, all of these alloys can be readily obtained from aerospace aluminum wholesalers at vastly reduced costs," he says. 

Though some of the specially tempered brand-name alloys have a small improvement in hardness, Kilim says a small decrease in elongation occurs, which increases the tendency of the material to develop cracks. 

Still, he says he has found that any of these four basic alloys is suitable for moldmaking. "I feel that an unnecessary premium is attached to the trade names," he adds. "To obtain the best results using aluminum tooling, an understanding of how to design in aluminum plays a far greater role than any slight improvements in alloy hardness." 

The Case For Tailored Performance 
However, consultant and aluminum tooling missionary David G. Bank (Geneseo, NY) argues, "All aluminum alloys are not created equal." Bank says even better results can be achieved when moldmakers settle for something better than a standard, off-the-rack aluminum. 

Designer-label alloys are usually hybrids tailored to a particular market application, like injection molds, he says. A mold experiences continuous heating and cooling cycles, variable pressures, and continuous use with a variety of plastics. High machinability, thermal conductivity, and corrosion and fracture resistance play important roles, as do stability, weldability, polishability, and availability. 

There is much more than just comparing hardness when it comes to selecting the best aluminum tooling alloy, he says. There also is something to be said for the quality and consistency of brand-name products, according to Bank. In his former life as a rapid toolbuilder, he says he tried them all—including 2024, 6061, and 7075—but settled on Alcoa's QC-7 and used it exclusively for years. 

"Today, everyone is searching for ways to reduce cost, beat their competitors, shorten cycle time, and improve their bottom line," Bank summarizes. "Considering overall mold costs, saving pennies on buying your mold materials is not the answer." 

Though they may sometimes differ, both Bank and Kilim agree that significantly reduced mold manufacturing time and improved cycle times can be achieved through the use of aluminum molds. 

Design Tips for Aluminum Tools 
Almo's Rafael Kilim offers the following guidelines for designing aluminum molds:

  • Aluminum cools the plastic material rapidly, so the gate size can be increased, enabling processors to reduce injection pressure and in turn reduce clamping pressure.

  • Clearance for ejector pins should be slightly larger than for steel molds as aluminum tends to bind.

  • Any slide movement inside the mold should be guided with different material, such as aluminum bronze or hardened steel. Aluminum on aluminum is not advised.

  • Cooling channels can be placed further from the cavity; during molding, the mold should not be overcooled.

  • All shutoffs should have generous taper angles or a different material should be inserted into these areas.

  • Aluminum should not be used in tools processing a high-temperature material; aluminum mold performance degrades under abnormally high temperatures. 

In the end, Kilim recommends that mold designers and moldmakers rely on their experience, taking into account elements such as the number of cavities, planned production quantities, plastic part design, plastic material used, where the mold will run (in-house or with a contract molder), and customer expectations.


Contact information
Almo
London, U.K.
Rafael Kilim
Phone: +44 (208) 459-7477
Fax: +44 (208) 459-7433
Web: www.almo.uk.com

David Bank Assoc.
Geneseo, NY
David G. Bank
Phone: (716) 243-9298
Fax: (716) 243-5567
E-mail: [email protected]

 

 

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