How to Expand the Use of Aluminum Injection Molds

Managing thermal expansion differences is crucial in high-temperature injection molding. Steel guide pins and bushings expand less than the surrounding aluminum mold, risking misalignment and mold damage.
Source: TST – Tooling Software Technology LLC

After inventing the first successful electric light bulb, someone asked Edison how he dealt with 1,000 failures before succeeding. Among the other answers quoted as a response was, “Sir, I did not fail 1,000 times. I merely found 1,000 ways that would not work.” I can only hope that my intended journey to expand the capabilities of aluminum molds for plastic injection molding does not require discovering 1,000 ways my quest will not work.

As noted in previous articles written by me and others, the use of aluminum molds can produce significant cost savings — even for long production runs — over steel molds. However, that does not mean there are no legitimate issues concerning the use of aluminum versus steel for injection molds. For example, wear and consequential mold deterioration for some of the newer, more abrasive injection materials. While 7075-T6 aluminum provides better wear than 6061 or cast aluminum, newer aluminum alloys provide even better wear properties, although generally at a higher material cost than cast, 6061 or even 7075-T6 plate.

The following factors must be addressed to broaden the application of aluminum molds:

1. One must follow the proper steps to set up a molding machine to run an aluminum mold for maximum throughput and longevity. There is little doubt that if one runs an aluminum mold exactly like one runs a steel mold, the life of the aluminum mold will be severely compromised, hence the widely held notion that aluminum molds are only for prototyping and very limited production.
2. Treat the tool with a hard surface coating. At first blush, this seems reasonable, but there are potential issues with each coating. For example, will a certain coating hold up over the long haul without deterioration or cracking under heavy pressures at elevated temperatures? For coatings to become a widely viable solution, successful applications over long runs need to be demonstrated.
3. If various coatings demonstrate they are viable, then determine if lower cost aluminum (such as 6061 or even cast) can compete with the higher priced, more robust aluminum if they are coated, thereby offsetting the additional costs of the coatings while enhancing performance and extending life at the same time.
4. While demand for bioresins (also known as bioplastics or biopolymers) is growing because they are more friendly to the environment, they are not friendly to injection molds. They are more thermally sensitive and caustic than plastics that are used in everyday injection molding. Bioresins also require a very narrow process temperature gradient, not high temperatures but a narrow temperature range. So, the thermal characteristics of aluminum make it potentially advantageous for processing bioresins. At the same time, the viscosity of many bioresins demands higher pressures for injection and tend to be more corrosive than standard resins, so you need to determine if coatings can address these issues as well.
5. Glass-filled, high-modulus plastics can replace steel in demanding applications such as large construction equipment. However, they have an abrasive nature and require high injection process temperatures (as much as 500°F, approaching the annealing temperature of high-strength aluminum) and high injection pressures. The efficacy of coatings for these applications has yet to be determined.
6. One major issue that must be addressed when significantly higher injection temperatures are required is the difference in the coefficient of thermal expansion between the steel guide pins, bushings and the aluminum mold. As the metals reach these higher operating temperatures, their expansion rates are so diverse that the guide pins do not expand as much as the aluminum block into which the guide pins are fitted, which can destroy the mold’s integrity.

Can these issues be resolved? Answering this question may involve discovering a thousand ways it will not work. However, I do have some ideas that I believe will effectively address those issues, so let the experimentation begin!