Copper Alloy Molds:
Why Didn't Someone Tell Me Sooner?

Copper Applications in Innovative Technology

By Ralph Willett

As a manufacturer of injection molded parts, it might catch your attention if one of your colleagues suggested that decreasing your injection molding cycle time by 20% could be accomplished easily. Of even more interest to your business and customers might be that, having taken advantage of this "easy accomplishment," your injection molded parts would experience less warpage and show finer detail. Then again, talk is cheap, and what has your colleague got that proves these generous statements?

Until a few years ago the answer to the above question was, "It�s been my observation". With the support of the Copper Development Association (CDA), along with some copper alloy producers, steps have been taken to prove what previously only made common sense. It only made common sense that if the speed and uniformity of cooling could be improved in the injection molding process, cycle time would decrease and the warpage of parts would improve significantly. And if a material was available with suitable corrosion resistance, wear resistance and strength, along with substantially higher thermal conductivity than stainless steel, all the improvements talked about could be achieved. A miracle material? No, simply a metal almost ignored in the injection molding industry. Copper! But enough said. Read on for the results of some innovative work done by the people at Western Michigan University.

Researchers at Western Michigan investigated the changes taking place in the injection molding industry and noted that there was pressure to move industry practice from what was perceived to be an art, to a science. For instance, they saw that presses and peripheral equipment and controls, polymer blends and monitoring equipment had all gotten significant attention and improved production efficiencies had been achieved. Meanwhile, largely ignored during this era of improvement was the mold itself. For instance, it was not unusual to see mold designs developed in the 1960�s still in use, with no design improvements. The means of achieving efficient cooling were little understood and virtually under no control. True, some shops had moved toward an understanding of these factors, but most had not.

First, during the injection molding process, the only coolant that is of any consequence is that closest to the heat source: the plastic part being formed. Water cooling lines and channels should be placed where needed, not where easiest to drill. Second, materials with high thermal conductivity will cool parts faster than mold materials of lower thermal conductivity. Why weren't these simple truths being acted upon? The answer, as mentioned above, is that while there was theory and speculation there was little available data. Western Michigan University Noticing this paucity of data engineers at Western Michigan University, along with the Copper Development Association, set out to remedy the situation.

The objective of the University�s study was to understand the relationship between temperature distribution in various areas of the mold and its affect on cycle time and part warpage. The experimental plan first examined whether consistent data could be collected using production molds, an assessment of experimental error. The results of the replication indicated that the individual tests results could be compared. No unacceptable deviation in the data was detected between three individual experimental runs. Having verified the validity of the test setup, and assured that the data collected would be meaningful, the experimental design was completed as follows: Five mold materials would be tested. These are shown below along with some nominal properties.

Naturally, the same injection mold process variables were applied to each of the different molds to achieve a valid comparison. These process conditions included: coolant temperature, water flow rate, resin type, melt temperature, and pressure. While holding all process variables constant, the data collected to compare the mold materials were:

1. The weight and dimensions of the plastic part.
2. Mold core and cavity temperatures.
3. Cavity pressure.

(For readers not familiar with what copper molds for plastic injection molding look like, pictures of some cores as well as overall views of molds are shown at the end of this document.)

The test part was a 33-mm-diameter bottle cap with a top thickness of 1.5 mm and wall thickness of 1.7 mm. The cap dimensions, the factor used to assess warpage, were determined after exposing the formed cap for 48 hours to a controlled environment, a guideline suggested by the American Society for Testing and Materials. In addition, the gage used to measure warpage was subjected to a repeatability and reproducibility study (R & R). Such studies quantify the measurement error resulting from the gage itself as well as the error attributable to the operator. The R&R study results showed the gage and operators to be accurate within acceptable limits, and the test program results dramatically demonstrated the advantages of using copper as an injection mold material.

First, consider part warpage: In a bottle cap, the under surface of the part is placed on a flat fixture. Warpage is determined on the outside surface of the cap and is the variation in height from the center surface to the edge of the cap top. The results are shown in Figure 1. The flat portion of the warpage curve is the minimum warpage the part achieved. This is also considered the region of part stability. It is clear that, at cycle times below 9.5 seconds, the performance of the steel cores with respect to maintaining part stability deteriorates rapidly. Meanwhile, part stability using copper cores is achieved at 7.75 seconds. At this time, 7.75 seconds, the warpage with a steel core use is more than 4 times that with copper. Of equal interest is the fact that there is essentially no difference in the performance among the copper alloy cores used or among the steel cores. Therefore, the Western Michigan tests have defined two generic categories for core materials: Steels and Coppers . The cycle time for the Coppers is seen to be approximately 18% faster than for of the Steels !

The cycle time/warpage results in Figure 1 can be readily explained by analyzing the core temperatures of the two materials. Analyses of cavity temperatures showed that the majority of heat removal in this mold design was through the core. Examine the core temperature curves, Figure 2. Again the core materials have separated into two material categories. The copper cores clearly distribute and release heat more efficiently as evidenced by their lower temperature.

There was another important result derived from the test program. Prior to conducting the experiments the researchers at Western Michigan University applied a novel three-dimensional mold cooling analysis to the mold design used for the test described above. Their objective was to predict the temperature of mold components and, of even more importance, the temperature uniformity of the mold. It is a well known fact that part integrity and warpage are better when a mold that does not vary in overall temperature. The actual temperatures of cavities and cores described in the experiment correlated well with the predicted values derived from the analysis. This gives mold designers and users another tool to more intelligently pick mold materials and create mold designs without having to resort to costly and time consuming experimental runs.

In summary, the use of copper alloys as injection mold material can lead to substantially faster cycle times compared to steel materials. While the initial cost of the copper material is higher than steel, the faster cycle time and resultant manufacturing cost savings quickly eliminate the cost difference. Potential users of copper materials should examine the possibility of applying three-dimensional analysis to demonstrate ahead of time the many advantages that are available to them by using copper alloys for their molds. There are a variety of copper alloys available with unique properties that can be adopted for different types of molds and plastic parts. CDA has published two Application Data Sheets for copper alloy molds:

1. Copper Alloy Molds - The Plastic Industry�s Best Kept Secret
2. Whirlpool Uses Copper Alloy Mold for Dishwasher Console Part

Ralph Willett
Willett Technical Services
Bloomingdale IL 60108

Also in this Issue:

• Direct Exchange Thermal Systems:
  The Wave Of The Future
• Copper Alloy Molds:
  Why Didn't Someone Tell Me Sooner?
• The Metallurgy of Copper Wire

Archives: