Aluminum vs. steel tooling: Which material is right, and how to design and maintain?

By Clare Goldsberry
Published: August 29th, 2012

With OEMs hammering their mold suppliers for ways to reduce cycle times, aluminum tooling offers one answer. With its ability to transfer heat from the mold much faster, many OEMs are choosing aluminum. Another benefit is faster machining times, which can reduce the mold-build time by 10-15% in some instances, according to Darcy King, president of Unique Tool & Gauge Inc. (Windsor, ON).

However, there are a number of criteria to consider when determining if an aluminum mold is the right tool for the job. According to King, the first thing that Unique looks at is whether or not the molder that will be producing the parts has experience with aluminum molds. "If the molder is experienced in running aluminum molds, that means an aluminum mold won't be challenging," King told PlasticsToday. "If they haven't had any experience with aluminum-only steel molds-they need some education in the process."

Al Standaert Darcy King
Al Standaert, Technical Sales Manager (Left) and Darcy King, President (Right) of Unique Tool & Gauge.

David Myers, VP of Sales for DRS Industries Inc., a Holland, OH-based mold manufacturer, concurred. "There's a learning curve to producing parts at production facilities that are only used for molding parts from steel molds," Myers explained to PlasticsToday. "Not only in how you process the parts but how you treat an aluminum mold versus a steel mold.  

"It's not a significant difference, but enough that it can cause problems. Your savings on the cycle time can be offset by the risk element inherent in the learning curve. We find ourselves supporting OEMs with aluminum tools and bringing in the production molder to educate them on aluminum tools," Myers added.

Unique primarily builds steel molds for the automotive industry, but has a long history of building aluminum molds as well. King said that the company also builds 'hybrid' molds that may be primarily steel but might incorporate aluminum inserts in a standing or deep-draw piece, or an area where it's difficult to get water resulting in a hot area that will affect cycle or hurt quality.

Aluminum or steel?
King explained there are several criteria in that will determine if an aluminum mold is suitable for an application. Part design/configuration is a major consideration. "A part has to be adequately designed for the molding process," said King. "We've built aluminum tools with lots of actions and/or poor shut-offs with no problem, but we might use steel shut-offs in an aluminum tool."

Unique Tool & Guage Inc. Part volume is also a critical factor when deciding whether to use aluminum.  "If you have an extremely high volume part, we might lean toward steel and/or at least have specific areas inserts made in steel," said King. "Sometimes we take the hybrid approach in a steel mold and, because aluminum is cost effective and conductive, we'll use aluminum in certain areas."

DRS Industries began life 30 years ago as a pattern shop but has evolved over the years to specialize in aluminum prototype and bridge tooling for both automotive and non-automotive applications. "Because we specialize solely an aluminum tooling for low-volume parts, everyone here is immersed in the aluminum world," explained Myers. "Our longest-lead items fall at about six weeks or less. Our customers can use that time savings to perform design iterations, make engineering changes or fast-track a bridge tool. They then rely on the aluminum tool for production parts, which means they can hold off on the steel tool until they've got the part validated, verified and learned the lessons from the aluminum bridge mold. Then, after all part design concerns have been alleviate, they can cut loose the steel production steel mold."

Aluminum mold criteria
Advancements in the various types of aluminum material suitable for mold manufacturing have gone a long way toward making it a more popular choice in many mold applications. Myers noted that when the company first started many years ago, QC7 by Alcoa was about the only suitable mold aluminum available. "Now there are three primary aluminum types we use today for any of the production molds that we build today, including Hokotol, Alumold 500, and QC-10 by Alcoa that replaced QC-7," Myers said.

Myers said that there are four things DRS looks at  when considering the suitability of an aluminum mold:

  1. The molded part material. Will it be a mild material like PP or TPO, or a more aggressive material such as an ASA or glass-filled Nylon, or maybe a higher-heat material such as Ultem? "We look at the material because it has an impact on the life expectancy of the tool," said Myers.
  2. Part geometry and cosmetics. "We look at surface-finish requirements - will it be grained, polished, etc. We also look at part features that will require tall, deep, thin details in the tooling. If so, we may insert those areas with steel," said Myers, noting that "each part is thoroughly evaluated and if the part geometry warrants the majority of the mold to be inserted with steel, it's probably not a good candidate for aluminum."
  3. Part volumes.  How many parts annually will be molded and what is the life of the program?  "We do stretch way outside what others do in aluminum in terms of volume," Myers commented.  "We produce aluminum tools that are grained or have highly-polished cavities and cores, which realize higher volumes. We haven't experienced any grain washout or polish issues but again you have to know how to process an aluminum mold properly. We've experimented with a lot of complex designs and new materials, and constantly push the envelope in that regard. There's a misconception that you're limited to just PP and TPO in aluminum tools. At DRS, we've had significant success with more abrasive materials."
  4. Who is going to run the tooling?  "Does the customer intend for us to design, build and run the mold or will the customer take it to an outside vendor? If they take it outside, does that company have experience with running aluminum molds? If not, we'll send tool and die person, and a process engineer to the molder to teach them how to process the tool and maintain it." DRS's molding division has eight injection molding presses ranging from 95-730 tons clamping force.

Critical maintenance issues
While there are not major differences in maintaining an aluminum mold vs. a steel mold, both Myers and King agree that there are some particularly critical maintenance issues with an aluminum mold. The parting lines should have particular attention paid to them.  "We tell the molders that certain cleaners should be used, and once-a-shift wipe down the parting lines so you don't get gas build-up," King noted. 

Recently, Unique Tool brought an automotive mold back in-house after it was taken out of production after just under a million shots. "It was the very first aluminum tool Honda ever built with us. We wanted to do more testing in our R&D lab," King explained.  "When we inspected this tool, the parting line looked like it was less than 6 months old instead of several years old. The resilience of aluminum is amazing. It shows you what aluminum is capable of if everything is engineered right, built right, molded right and maintained properly. You'll get some really good success with it."

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