CONFORMALLY COOLED STEEL MOLDS VERSUS ALUMINUM MOLDS FOR PLASTIC INJECTION MOLDING

14 Jan CONFORMALLY COOLED STEEL MOLDS VERSUS ALUMINUM MOLDS FOR PLASTIC INJECTION MOLDING

ADDITIVE MANUFACTURING ENABLES BETTER CONFORMAL COOLING

Additive Manufacturing (also known as 3D Printing among other names) has made great strides in the last few years in terms of both technical advancements and manufacturing production implementation. One application that has been proven and implemented to some degree is designing and generating conformal cooling channels for steel molds for plastic injection molding. AM permits the cooling channels to be produced in close proximity to the contours of the internal cavity walls of the steel mold. 

Additive Manufacturing (AM) facilitates the design and construction of conformal cooling channels that are virtually impossible to machine using more conventional subtractive machining. Using AM to build unique conformal cooling channels provides the ability to generate serpentine channels next to the mold cavities, permitting precise cooling of the molten plastic. This can increase typical injection molding cycles up to 50% (depending upon the complexity of the mold and other issues) over standard molds constructed of the same or similar alloy steel. However, a 30% cycle time improvement is more typical.

In addition to reduced cycle times, AM-generated conformal cooling channels funnel the liquid coolant at a constant distance from the injected plastic melt pool. This produces a better part without the warpage, sink marks, or other defects characteristic of standard steel mold production. The bottom line is that the insertion of proper conformal cooling channels in a steel plastic injection mold will produce better parts faster, permitting the molder to be more competitive and make more money, if the production run is long enough to recover the upfront costs. (Note: there are many renderings of conformal cooling channels on the web if the reader wishes to view such; however, we did not have permission to reproduce them herein.)

DRAWBACKS TO AM GENERATED CONFORMAL COOLING CHANNELS

There are upfront costs that must be considered when generating conformal cooling channels using AM. First, the mold builder must possess or have ready access to a proper AM machine. Not all AM machines are created equal; there are a wide variety of types of AM machines with widely varying performance parameters. In addition to the machine, one must have operators skilled in the operation of the AM equipment including the software. 

Another essential capability is to own or have access to some type of Computational Fluid Dynamic (CFD) simulation software such as Moldflow. If flow through the cooling channels is not properly modeled, differential cooling can result in products that are actually worse than if they were produced in standard steel molds. Even if product quality does not suffer, cycle times very well may. 

An additional upfront cost is the time and money to perform the CFD analysis. Depending upon the complexity of the part, it is not unusual for a CFD simulation to take weeks to complete, which must be completed before one can start cutting and/or adding metal. The CFD analysis must properly represent the unique characteristics of the conformal cooling channels within the CFD software to conduct a proper analysis. 

Once the mold is put into production, there are other concerns and costs. The cooling channels are now mere millimeters below the molding cavity surface, so extra care needs to be taken in handling the mold, especially during cleaning or other affiliated handling.

Another production concern is more rapid buildup of sediment within the internal cooling passageways. This is caused by the extra twisting of the cooling channels to better cause the coolant to achieve a greater level of desired turbulent flow (in opposition to laminar flow in standard molds with straight-through channels). The buildup can be mitigated to a great degree, however, by flushing the inserts with acid solution as well as using a closed-loop filtration system.

THE ALUMINUM ALTERNATIVE TO AM CONFORMAL COOLING 

As an alternative to employing this rather complex approach to lowering production costs and improving part quality, one should consider using a standard aluminum mold. Tests conducted over the years demonstrate that aluminum molds can achieve similar production rates and part quality improvements equivalent to that gained by AM-generated conformal cooling in steel molds. Many production shops have confirmed what independent tests show to be true.

It would not be prudent to displace all steel molds with aluminum molds. There are many applications where steel molds are the preferred choice; in these cases, AM-generated conformal cooling should also be considered as a means to increase production rates and provide improved part quality. At the same time, aluminum molds are far more capable of achieving long-term production than is often thought. As one recent article pointed out, for some time now there have been a lot of shops using aluminum molds and enjoying the benefits thereof, which will be enumerated later in this article.  Some of these shops do not often admit to using aluminum molds for fear of being shunned by the user community that is so dedicated to steel molds. 

As most of the readers of this article are aware, there are several different grades of aluminum used for plastic injection molds including: large cast plate like M1or Duramold 5; medium grade aluminums such as 6061; and very high strength grades like 7075-T651, and modern alloys with improved properties like Alumold,  Hokotol, and QC-10, among others. This is no different than having a large variety of steels available for making plastic injection molds. There are at least nineteen different steels typically used for plastic injection molds, with some others employed for specialized applications. In addition, other aluminum and steel alloys for injection and other type molds are continually being developed.

The most common “knock” against using aluminum for plastic injection molds is the stigma that aluminum is a soft alloy. Compared to hardened steels, this is true, but with respect to many plastic injection mold applications, aluminum produces better parts than a steel mold. Because aluminum is a “soft” material, it tends to “even out” the microscopic molecular “hills and valleys” over time while in use; instead of galling, aluminum surfaces slide. If properly used, it is a fact that aluminum molds have been documented to produce more than 2 million shots. 

Before deciding to use an aluminum mold, one should ask several questions, such as:

1. Is the resin conducive for aluminum?
2. What is the geometry of the part to be molded?
3. What is the quantity?
4. What is the desired cycle time?
5. How long does the mold need to last?

Of course, these are the same type of questions that should be asked regarding the use of steel when evaluating the possible use of a low cost “soft” steel versus a more exotic and expensive “hardened” grade. 

ADVANTAGES OF ALUMINUM MOLDS

The proper use of aluminum for plastic injection molds can result in many advantages including those gained using conformal cooling, such as allowing for longer material flow distances with less injection pressure thus permitting molds to fill faster and more efficiently. As will be further presented below, aluminum molds have a number of additional advantages, even over steel molds with AM-generated conformal cooling. 

 Documented testing has shown that the use of aluminum molds can result in the following improvements:

• An increase in productivity because the thermal conductivity of aluminum is four to five times better than steel. As such, the position and number of coolant lines is less critical than for steel molds. While a 20% to 30% increase in productivity is typical, some customers have experienced an 80% increase, and more, as documented by the Honda study referenced later in this article. 

In addition, as shown below, coolant lines are typically straight drilled through holes or right-angle intersecting lines resulting in more rapid and lower cost manufacturing to gain similar or better injection part production results.

(Graphic renderings provided by Tooling Software Technology [TST], LLC using VISI software)

• The reduction in the number of coolant lines can often allow placement of additional ejector pins if needed.
• Finished parts, even with thin cross-sections, have minimal warp with better dimensional stability than standard steel molds because the heat dissipation across an aluminum mold is very uniform.  The result is a significantly decreased tendency to distort the part due to shrink differences.
• The even heat distribution across an aluminum mold reduces residual stresses within the part, thus significantly reducing the tendency of part to fail due to residual internal stresses ultimately being released.
• A reduction in scrap that is caused by warping, cracking, short shots, and distortion during processing typical of steel molds.
• Long production runs – over 2 million parts in some cases. 200,000-300,000 shots are not unusual even for high mold temperatures (300°F and above) and glass-filled resins such as 30% glass-reinforced polysulfone resins which requires a melt temperature over 600°F. 
• At 1/3 the weight of a steel mold, there is less wear and tear on the molding machinery.
• The even temperature distribution of aluminum molds allows parts to be run at lower temperatures and clamp pressures, reducing energy costs. 
• Lower weight molds running at lower temperatures and pressures permits, or even demands to some degree, that parts be run on smaller machines with smaller footprints and lower operating costs, including less energy usage.
• Lower weight molds in smaller machines enhances operator comfort and safety, although one must be careful not to get careless because of these factors. 
• While hotrunners are generally not required with aluminum molds, when needed or desired, existing hotrunner technology has been successfully employed many times. 
• Manufacturing lead-times to produce aluminum molds (from mold design to initial operation) are reduced using aluminum molds over a similarly designed steel mold. Using American Quality Molds’ (AQM) standard 7075-T651 mold bases can reduce lead times even further since AQM strives to keep their standard mold bases and Rapid Insert Dies (RIDsTM / MUDsTM) “on-the-shelf” and ready to finish, assemble, and ship within 1 to 2 business days. In some cases, mold bases and RIDsTM that have been ordered in the morning were shipped out to the customer that afternoon. Even special sized bases and modifications can be finish fabricated and shipped within a few days to a week, although there are exceptional cases that require more time. Production of finished molds requires more time, of course, but are still typically produced much more quickly than a similar steel mold. 

TYPICAL DRAWBACKS OF ALUMINUM MOLDS

• Weld repair is perceived as being more difficult than similar repairs on steel molds. However, just like steel molds, some grades of aluminum are more difficult to weld repair than others. 7075-T6 is certainly more difficult than 6061-T651 to weld; some will argue, however, that while welding aluminum is different than welding steel, it’s not more difficult once one learns how. 
  • Certainly, consideration should be given to the fact that the use of filler metals required for proper weld repair weakens the metal in the weld area as the filler metal is usually an aluminum alloy with lower properties. This is especially true with respect to high-strength aluminums such as 7075-T651. 
  • A successful aluminum weld requires that the weld operator be properly trained to perform the required weld procedure; it will differ from those procedures used to weld steel.
  • Careful attention to the weld process is required to prevent a color mismatch after welding; this can become an issue, especially if the tool is to be textured.
• Press sizes and associated operating parameters are critical for aluminum molds just as they are for steel molds. Whereas the danger with respect to press sizes for steel molds is too small of a press, the danger for aluminum molds is often a large press running at the high clamp pressures typically needed for a steel mold. Attempting to operate at higher than necessary clamp pressures can result in the shot size becoming a problem within the barrel capacity. Also, typical press cycle parameters for steel would involve longer fill, pack, and hold times than that required for aluminum, resulting in some pin push as well as drag or stress marks. As noted by one processor, “If that happened with a steel tool, you’d probably lengthen the molding cycle, thinking that would alleviate the problem. In aluminum it would make things worse.” 
• In regard to the item above, it is recommended that aluminum tools be run on smaller machines with reduced clamp tonnage. It is important not to over-clamp and close up all the parting lines or vents. Instead, it is important that the aluminum tools be allowed to “breath” to remove built-up gases. 
• Aluminum molds should be opened and wiped down after each shift to remove any built-up residue that might occur due to the gases. While this is extremely important for aluminum molds, it is not uncommon for this to be recommended for steel molds as well.

While aluminum molds have shown excellent wear resistance for a wide variety of resins (including glass-filled), the author has some concern regarding molding some of the newer resins being developed and entering the market. These new fiber-infused resins are intended to provide greater part toughness and durability over current resins but they have already shown that they generate significant wear on steel molds. Articles demonstrating the advantages of hard coatings to reduce wear, and even restore, steel molds are regularly being published because these newer filled resins are so abrasive.

Aluminum molds can be successfully hardcoat anodized to provide a Rc65 surface. However, consideration must be given to the fact that the hardcoat anodizing finish is approximately 0.002 inches above the finished aluminum substrate (plus another 0.002 inches embedded in the substrate). In addition to the thickness consideration, hardcoat anodizing can “chip away” from corners that are 90 degrees or less. Once this happens, repair involves stripping the anodized surface and re-anodizing.  Currently, this author is working with some others in the industry to conduct a further study on various potential surface treatments to address the wear issues involving the use of aluminum molds for the newer more abrasive resins.

NOTED STUDIES DOCUMENTING ALUMINUM MOLD ADVANTAGES

IBM (International Business Machines – yes, that IBM)

In late 1991, IBM (International Business Machines) formally announced the completion of a five-year study using aluminum molds for high volume production. The study demonstrated that compared to identically designed and built tool steel molds, aluminum molds cost 50 percent less, were delivered in half the amount of time, and produced higher quality products during cycle times that were reduced by 25 to 40 percent. This data was compiled for a number of molds and various product designs, using a variety of plastic molding compounds; some with glass loadings as high as 40 percent. 

Honda of America Manufacturing

Around the turn of the millennium, Honda invited several hundred mold shops to a “co-management” meeting to develop testing and production procedures to verify the results of a study, conducted earlier by Honda, on the benefits of aluminum molds versus steel molds. Among other benefits, as outlined above, the results of the verification testing demonstrated:

• a 30% reduction in pricing of the molds;
• as much as a 50% to 80% reduction in cycle times
• long production runs, sometimes over 600,000 shots
• improved part quality

FlowFront’s Moldflow Analysis

A 2005 article written in the Moldflow publication FlowFront looked at computer simulation of cycle time and cooling versus actual molding. After carrying out simulations on 12 parts which had very different characteristics in terms of shape, size, and plastic materials, it was concluded that significant savings in total cycle time could be realized by using aluminum instead of steel molds. Cycle time savings of 10-20% were seen in cases where there were no critical tolerances linked to the deformation of the part due to the effect of the heat. However, as stated by the article regarding this study, “savings of 60-200% were seen in cases where heat deformation affected critical design tolerance levels.”

CONCLUSION

    While conformal coolant lines for steel plastic injection molds using additive manufacturing/3D printing can reduce production costs while improving part quality, similar results and more can be achieved at lower cost in significantly faster time, from concept to production, using aluminum molds. Purchasing 7075-T651 standard mold bases from American Quality Molds can often provide even faster, more cost effective starts to producing an aluminum mold. 

ABOUT THE AUTHOR

Don Shrader is currently the Vice President of American Quality Molds (AQM) (www.aqmolds.com). A former Air Force Master instructor pilot, Mr. Shrader spent many years after his military service as a mechanical automation manufacturing engineer, conceiving and designing automated manufacturing equipment for many different companies. Prior to his involvement in AQM, he spent over 25 years as a consultant to Alcoa, Inc.’s Defense Sector working with Alcoa technical personnel to develop and implement improved aluminum alloy solutions in military aircraft that reduced weight while improving performance. Aircraft ranged from the aging KC-135 and C-5 to the F-22 and the new F-35.