For years we have been hearing about the virtues of aluminum molds for injection parts, and MSI Mold Builders has built its fair share—mostly for special-purpose applications that did not require many actions. For smaller molds using specialty aluminum has its benefits; and, if cycle times are critical, you can justify the high cost. However, our experience has led us to a different set of solutions: hybrid molds, P-20 cavities and inexpensive aluminum cores.
To review some facts, aluminum molds have been used for low-pressure molding for more than 40 years. Aluminum molds are not cheap molds, only molds made from a different material. They can, have and do use all the same features available in a conventional injection mold. However, when you get to the point where lifters and cams require extra work, it is probably not as good a payback as with a P-20 mold. Aluminum molds are built with guided ejection, parting line locks, etc. Cheap aluminum molds are ones that are built for the wrong reasons or were designed to run 50 to 100 parts, but are now at 3,000 cycles for which they were not designed. Now enters the hybrid mold.
The Hybrid Mold
There are six facts to understand about hybrids: (1) they are not the cure for all programs; (2) have a limited market, but larger than the all-aluminum mold; (3) make for a good transition mold to overcome any fear of plunging into an all-aluminum mold; (4) solve much of the issues all-aluminum molds cannot; (5) offer many of benefits of the aluminum mold with a lot less risk; and, (6) fill a gap for programs that don’t have high-volume, but require high cosmetics on the cavities.
There are basically two different types of hybrid molds:
1. P-20 with full aluminum core. In this mold, the core block is aluminum (pick your grade—cost implications as well as volume driven) with a steel ejector box. Some people believe that you need steel from the parting line through the aluminum core block to the riser, but we have not found that to be the case unless special circumstances exist. This option also limits potential water locations and can increase the cost.
2. P-20 cavity with aluminum core inserted into P-20 support plate. Unless you are making a core insert because of part requirements, this improves mold life and gives you a steel parting line on both sides as well as thermal conductivity and ease of machining. Cost savings are less than P-20 with full aluminum core, but better than an all-P-20 mold. If you are inserting just to have an aluminum core, review the part configuration to see if it makes sense. Oftentimes the act of inserting drives the cost up, but you will still see an improvement in cycle times.
Applications and Benefits
Q: Why would we suggest a hybrid and what is the value to the molder and the OEM? A: Thermal conductivity. Aluminum’s thermal conductivity rate is five times that of P-20. It is basically the same as beryllium copper (BeCu).
You insert beryllium copper to enhance heat removal from areas of a P-20 mold; consider that now your entire core is as thermally conductive as beryllium copper. You also get a rise in surface temperature at injection, which in many cases will improve the flow characteristics and still cool the part faster than P-20. Some materials will not respond favorably to this temperature change, but these molds are not for every application.
One of the reasons a hybrid mold is so exciting is that you can obtain and maintain the same finish as a P-20 mold since the cavity is P-20. We have been successful in certain materials with texturing or applying a high finish to an aluminum mold, but the likelihood of it washing out or fading is increased. However, if you are only going for a few pieces (fewer than 5,000), you may be successful in an all-aluminum mold.
The material itself is significantly less expensive than P-20. We generally use a cast material, and throughout 15 years of using this material we have not seen one example of pits, inclusions, etc. Keep in mind that this is also the core side; it machines and polishes in about a third of the time it takes P-20—if we go slowly. Generally, the core has the most time associated with it.
EDMing ribs and features in a core is very expensive and we are limited in P-20 to approximately five to 10 times the diameter of the cutter when cutting ribs. In aluminum, we are routinely limited by the available cutters with enough flute length.
Polishing ribs and features in P-20 is very expensive and time consuming. Through our experiences with aluminum, we estimate an 80-percent savings over P-20. On most cores today we are only polishing for pull, so only the ribs and other features are getting polished. We can apply fillet radii when polishing rather than valuable machine time. Also we typically can drop off one electrode for each burn location.
High-speed machining makes short work of aluminum. All the machining operation times are decreased significantly over P-20. Currently, thick section P-20 materials are a little difficult to come by, which means increased material costs. As those prices keep going up, aluminum becomes a better value.
As noted, thermal conductivity is similar to beryllium copper and with a complete core side in aluminum, the expectation is significantly reduced cycle times. None of our customers have been willing to make a set of duplicate molds (one in aluminum and one in P-20 for a side by side test), but simulations have projected a significant cycle reduction and the actual cycle time reduction experiences were greater than the simulations predicted.
Typically, the core side of the mold has the most detail, so if you can decrease the hours associated with building the core, leadtime can decrease. Since we decrease polishing, EDM, etc., we will decrease the total leadtime of the mold.
We have experienced some challenges with thermal expansion differences between a manifold and an aluminum cavity, which can be dealt with, but are frequently a challenge. With the P-20 cavity and the manifold we avoid these issues and it still performs similar to a standard P-20 mold.
Some mention the potential for galvanic reaction between the P-20 and aluminum, and while we have not experienced this, it is recommended that the mold be sprayed with a good grade of mold preservative prior to storage.
A consideration of any mold is cost-effective repair/revisions, and with a hybrid mold cavity repair and revision is no different than that of a conventional P-20 mold. On the other hand, the aluminum core is easy to weld or insert similar to a P-20 mold.
Another advantage of the aluminum core is that in the P-20 mold the cavity is the half likely to see damage from flash as an unsupported surface. In the hybrid mold, it is more likely the core, which makes it easier to repair without cosmetic damage. Inserting the aluminum core is similar to P-20, but less consideration needs to be placed on cooling because of the thermal conductivity of aluminum.
Hybrid molds will not withstand high pressure in the same way a complete P-20 mold will. Care must be taken to use the appropriate amount of clamp pressure to mold the part and not much more. Cranking up the clamp pressure to cure a flash problem rather than determining the cause and repairing it, will most likely result in damage. However, that is true with a P-20 mold as well. Gas assist molds, both external and internal, are really great applications for hybrid molds as the core sees much less injection pressure than a conventional injection mold.
Applying good molding practices is paramount for both types of molds and being sure to identify the mold material is important for a mold setter. Many of our customers paint an aluminum or hybrid mold in a different color to advise mold setters that this is a “different” type of mold.
Hybrid molds offer options to the OEM and molder that can decrease mold costs as well as cycle times. On average a part volume of fewer than 20,000 parts per year and 100,000 life would be a very good candidate for a hybrid mold; although examples of EAU over 50,000 and 300,000 lifetime are possible.
Part configurations will be a significant factor in choosing a hybrid mold over a traditional P-20 mold. Intricate molds with a high number of mold actions would likely not be a good candidate unless the part volumes are low. Using a mold manufacturer with experience in hybrid molds is highly recommended to improve your success factor.
Roger Klouda is president of MSI Mold Builders.
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