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Solving molding problems with beryllium copper: moldmaking.

Solving Molding Problems With Beryllium Copper

Beryllium copper has been used as a mold material for plastics since 1935. It provides high thermal conductivity, strength, hardness, and wear resistance; it resists corrosion caused by corrosive plastics like PVC, provides excellent mold life, and reduces molding cycle times for thermoplastics. Blowmolders and injectiong molders utilize beryllium copper for inserts, solid molds, injection nozzles, core pins, and other mold components. Beryllium copper molds can range in size from several thousand kilograms to the smallest of molds.

The article provides the plastics molder with information necessary to weigh the costs and benefits of berylium copper against those of aluminum, tool steel, and other mold materials. Material property and selection criteria are reviewed, and examples of cycle time reductions, mold life increases, and cost reductions are provided. In addition, the choice of cast versus machined beryllium copper is examined.

Beryllium Copper Alloys

There are two basic families of beryllium copper alloys: high strength alloys and high conductivity alloys. Both families are available in wrought and cast forms. Table I compares the alloys' properties with those of other common mold materials.

High conductivity Protherm has a thermal conductivity that is approximately 10 times greater than those of stainless and tool steels, double that of aluminum Alloy 7075, and 20% higher than that of AMPCO 940. It has higher harness and strength than aluminum or any copper alloy of equivalent conductivity, and it provides good wear resistance and accurate replication of detail when cast. This alloy is available in the wrought form as Protherm and the cast form as Alloy 3C and is primarily used for foamed plastics molds, injection blowmolds, and hot runner systems.

The high-strength cast Alloys 275C (C82800), 245C (C82600), 20C (C82500), 21C (C82510), and wrought Moldmax plate have peak hardnesses greater than those of most common steels but lower than that of hardened tool steel. They have thermal conductivities three to four times greater than those of stainless and tool steels and they have very good wear resistance. When cast, the high strength beryllium copper alloys also provide very accurate replication of fine detail. Well-maintained wrought or cast beryllium coppermolds reportedly provide the same long mold life as steel molds undr normal operating conditions; some molds have already provided up to 30 years of usage. These alloys are used primarily for injection molds and extrusion blowmolds, as well as hot runner and hot manifold systems.

Cycle Time Reduction

The rate of heat removal from a mold is dependent upon the thermal conductivity of the mold materials. Users of beryllium copper molds and mold inserts have reported reductions as great as 75% in total cycle times resulting from such usage. For example, the replacement of a tool steel core with a water-cooled Moldmax beryllium copper core reduced the cycle time for a 152-mm injection molded cartridge case by 70%, from 3 min to 55 sec. In addition, the elimination of distortion problems with the finished part enabled molding yield to go from 75% to 99.5%. When Moldmax beryllium copper is substituted for tool or stainless steels, reductions of 30% to 50% in injection molding cycle time are not unusual.

One blowmold manufacturer conducted a test to compare the difference between the use of steel and Moldmax for pinch-offs and neck rings in aluminum blowmolds for 1-qt bottles: the molding speed was gradually increased for both molds until defective parts were produced. It was found that the mold with beryllium copper inserts ran three times faster without producing defective parts than the mold with steel inserts.

Shorter cycle times can result in considerable savings. Using the findings above, the potential cost saving for a 6-cavity bottle mold was calculated: molding cost was 3.5 cents per bottle when using beryllium copper and 10.5 cents per bottle when using steel. The difference in raw material cost per mild was approximately $200 and the annual cost saving was $70,000. These molds are expected to produce 10 million bottles over a service life of 10 years.

Blowmold manufacturers report that their customers normally have improvements of 15% to 75% when beryllium copper is substituted for steel inserts. This difference in cycle time is possible because beryllium copper's thermal conductivity is 3 to 10 times greater than that of most commonly used tool or stainless steels. Since the potential improvement in cycle time depends upon the molding machine, mold design, and the component produced, one can expect cycle time improvements with beryllium copper. However, one cannot assume a 75% reduction in cycle time for every application.

The large difference in heat flow rates among the beryllium copper, steel, and aluminum alloys discussed above is illustrated in the Figure. This chart illustrates the rate of heat removal from a mold versus the difference between the plastic injection and ejection temperatures. For example, if it is necessary to reduce the plastic temperature by 200 [degree] C, the rate of heat removal with Protherm (Alloy 3 in the Figure) would be 40,000 J/s. Aluminum Alloy 7075 and beryllium copper Moldmax (Alloy 25 in the Figure) would have a heat removal rate of approximately 18,000 J/s, and the steel alloys would have a heat removal rate of 4000 J/s. Beryllium copper Alloy 25 and aluminum would remove heat 4.5 times faster than steel; beryllium copper Alloy 3 would remove heat 10 times faster.

The thermal conductivity and rate of thermal expansion of the high strength beryllium copper closely match those of aluminum. Beryllium copper inserts in aluminum will allow uniform heat dissipation and thus avoid alignment shifts that result from thermal expansion mismatch, which can bea problem with steel inserts.

The beryllium copper mold's more uniform heat removal also minimizes residual stresses in the molded plastics. On the other hand, the residual stresses in steel and aluminum can cause distortion and warpage problems for solid molds. When compared with steel molds, beryllium copper molds reduce or eliminate these problems; they also reduce post-molding shrinkage of the molded plastic.

Because of their high thermal conductivity and corrosion resistance, Protherm molds are used for injection molding hard-to-cool foams. Protherm provides twice the thermal conductivity of aluminum Alloy 7075 and 10 times the thermal conductivity of steel alloys, and has better strength and wear resistance than aluminum. When it is substituted for aluminum in foam molds, its greater thermal conductivity can reduce cycle time by as much as 50%.

Water cooling lines can sometimes be reduced or eliminated in beryllium copper molds while obtaining faster cooling rates and reducing machining costs. Beryllium copper is ideal for mold sections or cores where it is impossible to drill cooling lines.

Thermal cycling can cause minute cracks to develop in steel molds' areas of heat concentration, such as corners, projections, and feeding gates. These thermal fatigue cracks cause product defects and require frequent mold repair or replacement. Beryllium copper's high thermal conductivity and high strength are two safeguards against thermal cracking, a phenomenon that other molds are prone to.

Corrosion Resistance

Wrought beryllium copper's superior corrosion resistance makes it ideal for any mold application in which corrosive plastics are used. Users of beryllium copper molds report that it provides excellent resistance to the hydrochloric acid by-product of PVC molding. Most of the large PVC bottle manufacturers use beryllium copper exclusively because it reduces or eliminates corrosion problems encountered in the past with aluminum and steel molds. Although mold life varies with the application and mold maintenance, users have fund beryllium copper molds to last far longer than steel or aluminum molds in PVC applications. All steels, including stainless steels, suffer severe pitting when subjected to hydrochloric acid; aluminum alloys are also sevrely attacked. But corrosive acids will eventually attack all materials, including beryllium copper, that are exposed for an extended period of time. Therefore, beryllium copper molds should be cleaned prior to storage and during extended periods of inactivity. Although cast beryllium copper molds can be used successfully, wrought Moldmax and Protherm have been found to be more resistant to corrosive attack than cast molds or cast billet.

Structural foams that release carbonic acid are another example of a potential corrosion problem. Users have found that while beryllium copper works well in this environment, tool steels suffer severe attack. On large production orders, the difference in mold life provided by a corrosion-resistant mold can mean savings to the molder in inspection and repair time, downtime, and replacement mold costs.

If so required, beryllium copper molds can be easily plated with nickel or chrome for additional corrosion resistance. According to users, the combined corrosion resistance of protective plating and a beryllium copper mold provides better mold life than does plated steel or aluminum.

In addition, beryllium copper exhibits good corrosion resistance to the factory environment: no rust forms on the mold surface during storage or processing Because beryllium copper does not rust, cooling channels that may become clogged with corrosion products in steel molds are maintenance free for beryllium copper molds.

Wear Resistance

Most mold manufacturers and users equate high hardness with wear resistance. Several types of wear problems are encountered by the molder: galling that is caused by sliding mold components like ejector pins; abrasion that is caused by filled resins; and erosion that is cause by flowing plastic. When heat-treated to the harnesses shown in Table 1, beryllium copper provides excellent resistance to all three forms of mold wear.

Table 2 shows Armco data that compare, by means of the Taber Met-Abrader test, the abrasion resistance of high strength Alloy 20C with that of several commonly used tool and stainless steels. The test measures the weight loss of samples rubbed against each other. Although Alloy 20C has a lower peak hardness than many of the other materials tested, it has a high abrasion resistance rating. Beryllium copper's abrasion resistance is due, in part, to its natural surface lubricity and its high thermal conductivity which prevents heat buildup from friction.

At a Rockwell hardness of C53, H13 tool steel lost 22 mg/1000 cycles in the Taber test. In comparison, Alloy 20C, with a Rockwell hardness of C40, lost only 3 mg/1000 cycles. Type 420 stainless steel with a Rockwell hardness of C46 experienced a weight loss of 170 mg/1000 cycles. Of the materials listed in Table 2, only D2 tool steel and chrome plate had better abrasion resistance than Alloy 20C; these differences were small in comparison with those involving the other materials listed. Moldmax, a wrought product, will perform as well as 20C. The higher beryllium cast Alloys 245C and 275C will have even better wear resistance.

Users also found beryllium copper to have better galling resistance. This improved galling resistance, which prevents moving components from seizing, has encouraged the use of beryllium copper in ejector pins and other moving components. In order for beryllium copper to produce the benefit of its anti-galling characteristics, it need only be present in one part of the couple. It has thus proven to be a relatively low-cost solution to a high-cost problem. It should also be noted that beryllium copper is commonly used as a bushing and bearing material in demanding applications like aircraft landing gear.

Alloy Selection

In addition to choosing between the high strength and high conductivity beryllium alloys, molders can choose from a range of mechanical properties available through the alloy's temper specification. Most machined beryllium copper molds employ prehardened plate that has been heat-treated by the supplier; soft-temper, annealed product is also available. For maximum wear resistance, the molder can order heat-treated plate with peak strength. If additional machinability or ductility is required, the molder can also specify a slightly lower strength. In order to raise the strength and conductivity of the alloy, annealed beryllium copper must be age-hardened (heat-treated) before service.

Cas beryllium copper molds achieve maximum strength, wear resistance, and hardness with a two-step solution anneal and age-hardening treatment. Cast molds that have been age-hardened without the separate anneal tend to have lower wear resistance. It is noteworthy, however, that cast-to-shape and aged molds have been used successfully in many applications. If one does not use protective atmospheres, the scaling that can occur during solution annealing can damage cast-in fine detail. One can maintain fine detail and attain good mold properties by simply casting and aging.

Beryllium copper has the strength and hardness to replace steel in blowmold pinch-offs and neck rings. In most injection mold applications, solid beryllium copper molds or inserts provide the same mold life as steel. In both applications, the resulting cycle time reductions provide cost savings.

Cast Versus Machined


Beryllium copper molds are normally cast to shape or machines from cast or wrought plate, bar, or rod. When deciding whether to use a machined or cast beryllium copper mold, the purchaser should consider the properties required, the configuration of the mold, the moldmaker's resources, and the number of molds to be purchased. Both mold construction methods can produce excellent molds and are worthy of consideration. In many cases, cast beryllium copper molds are less expensive and, in large quantities, have faster turnaround times than machined molds. When one is purchasing cast molds, it is important to select an experienced vendor. An experienced foundry will be aware of proper casting and safe handling techniques for beryllium copper. Table 3 outlines the advantages of machined and cast molds.

Fine-Grained Casting


"Orange peel" is a term used to describe surface irregularities that appear in a cast mold after heat treating. With standard beryllium copper casting alloys, a slow cooling rate of 50 [degrees] C/min can cause the formation of large grains known as columnar grains. The orange peel effect is the result of the intersection of the ends of columnar grains with the polished surface of the mold.

The use of beryllium copper alloys that contain titanium for grain refinement is one way to reduce grain size. These alloys have the designation "CT" (275CT, 245CT, and 20CT). However, titanium is subject to fade (loss by oxidation) during long holding periods and from agitation in melting furnaces. As a solution to this problem, better fine-grained beryllium copper alloys have been developed: their commercial designations are Alloys 21C, 245CF, and 275CF. These alloys, which do not fade, provide finer grain size than the CT alloys. Their castings' grain size is comparable with that of wrought beryllium copper plate; their properties are equivalent to those of Alloys 20C, 245C, and 275C.


Both aluminum and steel molds lose hardness and strength during weld repair. If properly repaired, beryllium copper molds will not lose significant amounts of strength or hardness. Beryllium copper is also reportedly easier to weld-repair than steel or aluminum, and thus shortens downtime. Purchasers should consider such repairs, which substantially lengthen the lives of beryllium copper molds, when selecting molds for long-run applications.

Beryllium copper is also easily machined. Moldmakers generally find that beryllium copper requires 10% to 25% less time to machine than common mold steels.
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Author:Houska, Catherine
Publication:Plastics Engineering
Date:Jan 1, 1990
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