As the last post in our series, we explore when aluminum might be the right material for your next injection mold. Aluminum is a material that carries many different opinions, depending on whom you talk to. There are also molding process that lend themselves well to aluminum molds, some of which contradict and challenge some of the points below. As an injection molder and mold maker with over 60 years of experience building aluminum molds, we feel it’s important both the pros and cons of aluminum molds are discussed.
How many parts are expected to be molded?
There are several grades of aluminum readily available for the construction of injection molds. These grades range from general purpose 2024 (soft) up to high-strength aluminum alloys like QC-10 (Alcoa). Due to its alloy make up, materials like these are considered to be one of the “strongest” aluminum at just about 20RC. Generally speaking, aluminum molds are used in lower volume productions, not exceeding a few thousand parts. In the past, aluminum molds became prevalent for low volume production molds due to the fact that they were easy to machine and less expensive than pre-hard steel. Today, that advantage is gone thanks to the latest cutting technology, which treats aluminum and hardened steel equally. In addition, the cost per pound for higher grade aluminums has exceeded that of pre-hard steel, leaving pre-hard steel as the optimal choice for even low volume production molds. With the cost of higher grades of aluminum being as high as it is today, many injection molders and mold makers have turned to the softer grades of aluminum, as it offers a cost savings over steel materials. It is important to understand that while there may be a cost savings, there are implications related to part quality and surface finish.
What surface finish requirements are expected of the molded part?
Most grades of aluminum machine, finish and accept texture proficiently. Most high-end aluminums have the ability to accept a polished surface, often ranging between a B-1 and A-3 polish. A large concern with aluminum is porosity and inclusions. More common among softer/less expensive grades, porosity and inclusions occur when the alloying process changes and results in a greater level of impurities. This can becomes an issue during the machining process (CNC or EDM). When a cutter or electrode encounters a pocket of porosity or an inclusion, a pit is created leaving an imperfection on the surface of the aluminum. The material then has to be repaired either in the mold (which can be difficult due the welding difficulties of aluminum) or on the part itself with the use of filler and further part finishing (such as paint). Because of this, aluminum may not be the best choice for external and cosmetic production parts. However, because extra finishing cost is seldom an issue and the focus is on up front tool cost, aluminum can be a great choice for prototype tools.
What metal conditions exist as a result of the part design and are they conducive of excess wear?
In its untreated state, aluminum is a very soft material, especially when compared to the variety of steels available. Because of this, aluminum is susceptible to quick and excessive wear, especially on wiping surfaces. Aluminum molds tend to need more maintenance compared to steel, too. Parting lines can wear quickly, creating flash, and need routine maintenance. Aluminum also has very low (poor) compression strength. If you happen to close the mold on a single part or insert, the aluminum will “hob” and become damaged, requiring expensive repair. For these reasons, aluminum is not well suited for any type of insert or over molding. Lastly, you should consider the dimensional stability. The amount of residual stress in a piece of aluminum can vary quite a bit depending on which area of the plate the material you are machining came from. These stresses can release during the machining process causing significant warpage of the block. It’s best practice to rough machine the block and allow the material to sit for 24-48 hours to relieve the stress prior to machining in detail. Taking the above into consideration, and because of the high pressures and heat associated with most injection molding processes, aluminum molds (especially softer grades) are prone to breakage, and extreme caution must be used when molding parts from this material. You should take caution when trying to mold very tall and thin features under high pressure. This is especially related to the pressures required to help create cosmetically pleasing parts, like sink free.
What type of plastic resin will be used on the molded part?
Aluminum is very vulnerable to fast wear and fatigue when high heat or highly abrasive materials are being molded, and should not be considered for any type of production molding. However, it can still be a great choice for extreme low volume (truly prototype) and non-cosmetic parts, even when running such materials. With “high heat” materials, the heat will actually begin to degrade the aluminum and mechanical properties of the aluminum will fall off at around 300 degrees Fahrenheit. This will often result in not being able to achieve the properties of the plastic resin was intended for.
One process that lends itself well to aluminum molds, and is not so restricted when considering total expected volume, is structural foam molding. Structural foam molding is an injection molding process that utilizes very minimal pressures during the process, and results in low levels of stress on the mold itself. Here, the chances of mold failure are greatly reduced and pitting if seldom an issue. Structural foam parts already require post molding finishing and coating (like primer, filler, and paint) in order to achieve a cosmetic surface. Structural foam molding is typically done at an increased nominal wall when compared to conventional molding (typically .250”). Because of this, long cycle times are often needed. Aluminums has 4-5 times better thermal heat transfer than steels, making it an ideal candidate for structural foam molding and allowing for slightly lower cycle times as a result.
A wide variety of mold materials are readily available for the construction of injection molds. By considering these 4 basic questions, you can make a more educated choice as to what the correct material is for you. While choosing the best option for your mold is not complicated once you understand the pros and cons of each material, it is a critical part of the process that can affect the success of your project. Research done prior to mold construction, and more importantly vendor selection, can help ensure you receive the performance you expect from your next mold.