Why is aluminum and not steel used to build prototype molds for molding medical silicone parts?

Question: Why is aluminum and not steel used to build prototype molds for molding medical silicone parts?

Answer: Aluminum is used to build prototype molds because an aluminum mold is easier to machine than a steel mold. As a result, the aluminum mold can be built faster than the steel mold which leads to a lower cost in building prototype molds and a shorter lead time. Due to the fact that the aluminum mold doesn’t have as long tool life as the steel mold does, the prototype molds don’t require the longevity of tool life due to prototyping purposes. Therefore, the aluminum mold is generally preferred over the steel mold for molding medical silicone parts.

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About Hean

Prior to joining Albright Technologies, Hean was an intern and a part-time employee at Berry Plastics. After starting at Albright Technologies in May 2011 as an intern, he recently became a full-time employee as a Project Engineer in Training. Currently his responsibilities include, designing molds, programming codes for tools, inspecting articles, setting up a LIM machine and anything else that helps the team move forward. Hean received his Associates degree in Liberal Arts and a concentration in Physical Science from Middlesex Community College in 2008, and a Bachelors degree in Plastics Engineering with a business minor from UMass Lowell in 2011.

  • Bschmidt

    That may be fine for soft silicone. But I don’t agree you save any time versus cutting in appropriate steel. Aluminum has many drawbacks though. Significantly higher thermal expansion than steel…so dimensions must be compensated especially for very tight toilerance parts. Completely different release characteristics versus steel, so mold release must be compensated. Completely different surface finish so prototypes won’t necessarily represent production parts. If you treat mold surface to achieve a particular surface texture or roughness…won’t be the same as production mold.
    Typically I would build a single cavity prototype mold in 1 day out of pre-hard or even 440C ss for most any high precision highly engineered elastomer part.
    With all this in mind why would you ever consider aluminum for the prototype unless the production mold was also going to be made from aluminum?

  • Bschmidt

    Also…overmolding an insert that is made of steel or even brass or even most highly engineered plastics cannot be accomplished with an aluminum mold…to soft and teh insert damages the mold firt shot.

  • Jaygalvin

    We use Alcoa’s QC-10 for a lot of the molds we build for in-house use. It’s much faster than steel to fabricate and our cycle times are often faster. We use them for “hard” plastics as well as elastomers as long as there isn’t any abrasive filler. We have several molds that have over 100K cycles and still have crisp partinglines.

  • Hean

    Thanks you for your comments. First, it is important to clarify that the prototype molds discussed in my original post are manufactured to make tangible parts for low volume, R&D, and/or testing purposes. These prototype molds are not meant to represent the production mold as you probably thought they are. Also, let me address your concerns point by point for clarification.

    Based on my experience, machining an aluminum mold is a faster cutting process when compared to a steel mold. Both feed rate and step percents for aluminum are typically higher than for steel; significant time saving is more noticeable when using small cutters. According to the Aluminum Association, Inc., machining rates for aluminum are three to ten times faster than steel. (http://www.moldmakingtechnology.com/articles/steel-wars-aluminum-versus-steel). The faster cutting process for the aluminum mold leads to a cost savings that can be passed on to the customers.

    I entirely agreed with some of your points regarding compensation on the mold, release characteristics, and surface finish. Dimensions on an aluminum mold must always be compensated differently from a steel mold in order to produce silicone parts that meet the required specifications. However, we generally have ±0.005” tolerance on the finished articles here at Albright. An aluminum mold can be machined to mold silicone articles that have a tight tolerance as small as ±.0005” per customer’s requirement. Similarly, the release characteristics of aluminum would be different from the steel since aluminum and steel are two different alloys. Nevertheless, we rarely have any substantial problem with the release characteristics with the aluminum mold when molding medical silicone parts. Also, surface finish can be more easily machined onto the aluminum mold than the steel.

    Despite the different properties between aluminum and steel, the aluminum can be machined to produce medical silicone parts with high quality.

    Here at Albright, hundreds of overmolded medical silicone parts (silicone to steel inserts) are successfully produced via a single aluminum mold. While the quantity of the overmolded parts would be higher if the overmolding was done with a steel mold, it isn’t necessary to use the steel mold because the quantity is generally low in prototyping.

    Since the aluminum can be used to reduce the cost of producing the medical silicone parts without sacrificing the quality of the parts, this is a major reason the aluminum mold is a preferable material for prototyping comparing to the steel mold.