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- 7 -Cost Savings for Mold Materials: Consider the DetailsWays to reduce costs that are not related to the price per pound of the mold material. By Patricia MillerChoosing the right tooling material for plastic molding is becoming ever more difficult. In light of intense competition, as well as the cost of raw materials, which is driving the price of materials for molds up, it becomes increasingly more important to be selective in the choice of mold materials. There are things that can be done to help the moldmaker make the best selections for the application at hand, and these things are not simply looking at the price per pound. But in order to do this, the moldmaker must consider other factors. This starts at the beginning of the moldmaking cycle, as the part to be manufactured is being proposed. The major areas to be considered in a mold design include: Size and complexity of the part or mold Production quantity required Type of plastic molding material required and its impact on the molding environment Mechanical requirements for the mold Physical property requirements for the mold (thermal conductivity, stiffness, thermal expansion) Stability requirement of mold during operation Assembly issues (mating material criteria, coatability) Design features (sharp corners, thin sections, sealing methods) Surface condition requirements (polishing, texturing demands) Manufacturing methods (electro dis-charge machining, hard milling) Cost savings can occur in all these areas, but for this discussion four areas of alloys will be addressed: 1. New high hardness matrix alloys 2. Thermally conductive alloys 3. Corrosion resistant materials 4. Prehardened alloys In each case, the cost of the mold material is significantly outweighed by the benefits these alloys bring. Figure 1. A mold for plastic injection molded electric motor rotors, made of 30 percent glass-filled Polyamide. Mold material has, to date, increased the life of the mold over 20 percent, eliminating mold repairs and refurbishment. Figures courtesy of Bohler-Uddeholm Corporation.1. High Hardness Matrix AlloysIn an exciting development for mold materials, there are now tool steel grades available that can replace S7, H13 and with coating replace A2, D2 and M2 types where wear resistance is required. But the advantage that these grades bring is that they can be used from 50 HRC up to 62 HRC, and are weldable, polishable to high levels, texturable and are coatable when even higher wear resistance is required. Figure 2. The relationship of highly conductive alloys, thermal conductivity versus hardness. The new Cu-Ni alloy, having hardness levels like that of P20, has higher thermal conductivity than other copper alloys and aluminum.Very tough grades, these materials also provide an added advantage that thermal conductivity exceeds that of H13, hence cycle times can be reduced. These grades replace past cold work grades that could only achieve their mechanical properties by low temperature tempers, which did not permit good weldability, nitriding or PVD coating above 400oF. Their high polishability and texturability is due to the excellent uniformity of the matrix from the use of high technology remelting processes.An example of this is shown in Figure 1. An injection mold insert made of Polyamide plus 30 percent glass fiber, was manufactured from a chromium-molybdenum-vanadium alloyed tool steel. At 54-56 HRC, it has been running for over 7,700 pieces and is still running well. Premium H13 at 50-52 HRC began to wear and plastically deform at 6,000 pieces. 2.Thermally Conductive AlloysCopper alloys have been available for several years to address the need for cycle time reduction and part reproducibility, and have been used particularly for cores where plastic residing times are highest. Copper-beryllium alloys are available in hardness ranges of 30-40 HRC. In addition, there is a copper-nickel alloy that can achieve hardness of 30 HRC, which is in the range of a typical P20. The advantage this grade brings is that its thermal conductivity exceeds that of other copper alloys in this hardness range, and also that of aluminum. This grade also is antigalling and corrosion resistant (see Figure 2).Figure 3. Rough milling of the cavity; premium H13, 45 HRC.3. Design and Stainless Alloys It is time to reconsider stainless alloys. There are new stainless alloys whose benefits are clear: high polishability, with toughness levels in the range of Premium H13, up to 50-52 HRC. The fact that these alloys will maintain their corrosion resistance with minimal need for rework or repolishing over the life of the mold, and still provide a durable, high mechanical strength mold, is worth careful consideration.Figure 4. Drilling of cooling channels; premium H13, 45 HRCOne area which has limited the life of stainless molds in the past has been the use of tapered pipe plugs. Heat treatment limitations, machining issues with devel-oping the threads, stresses generated in the threads following torquing, along with the corrosive conditions of dead zoneswhich create pitting attack on the stainlesscan lead to cracking in these regions. New stainless alloys can minimize susceptibility to this along with plug designs that are available to handle hydraulic sealing issues without machining threads into the mold material. 4. Prehardened Mold MaterialsLooking away from the steel cost to manufacturing technique, we now see the development of machining practices, which permit the customer to use grades that are prehardened to higher hardness ranges. It is now possible to machine grades like H13 at hardness levels in the range of 44-46 HRC, and in many cases even harder. The advantage this brings is that the steel can be prehardened, in a method that gives excellent properties because the cooling rates can be faster when less detail is in the mold, and cracking susceptibility is less. The integrity of the steel increases, while the need for rough machining, stress relieving and prefinish machining is eliminated. This saves time and money, when usually at the stage when heat treatment is performed, time constraints are high. With these time constraints, corners get cut and heat treatment is not always done to optimize the property of the steel. Tempers may not all get done, and cooling rates are slowed down to permit less stock to be left on, because the moldmaker has less time to remove the extra stock needed for the movement that will take place from a good, rapid quench. Figure 5. Finishing milling of cavity; premium H13, 45 HRC.Some examples of how to machine a hardened H13 are given in Figures 3, 4 and 5.ConclusionThere are many ways to reduce cost that are not related to the price per pound of the mold material. With the ultimate goal to provide the customer what they need in terms of part integrity and reliability, manufacturing a mold that will provide all of these things in a reasonable way requires a thorough review of the design criteria, manufacturing processes and production demands. New materials and methods are available that were not there the last time the mold was made, that can help minimize the overall cost of the mold.附录B专业外文翻译节省模具材料费用细则降低成本的方式涉及的不光是每磅模具材料的价格。作者：帕特丽夏 米勒正确的选择塑料模具材料变得越来越重要，在竞争激烈的今天，原材料的成本升高使得模具的价格上涨，模具的材料的选择就本的日益重要。有些东西可以帮助模具制造者们最好的选择模具材料，并且这些东西不就简单的降低每磅模具材料的价格。但是为了节省成本，模具制造商们肯定也会考虑到其他因素。首先要考虑的是模具制造的周期，当要制造一个零件的时候，模具的主要设计的部分包括：l 模具尺寸和复杂的模具型腔l 产品质量要求l 塑料成型的类型和成型因素的影响l 模具制造机械设备的要求l 模具物理因素要求（传热性、硬度、热膨胀）l 生产过程中的稳定性装配问题（符合装配原则，防绣）l 设计原则（锐角转角、避免薄壁、密封性好）l 符合表面技术要求（抛光、粗糙度要求）l 制造方式（电镀、电火花加工、磨削加工）每个阶段都可以作到节省成本，但是有四个方面最为突出：1. 新型高硬度铋铅锡锑合金2. 热导性良好的合金3. 抗腐蚀材料4. 预硬合金无论以上那一种材料，作为模具材料，在节省成本方面的价值都是超过其他材料的方案。例1 电机转子注塑模添加了30的玻璃填充物聚酰胺，使得模具的寿命延长了20，减少模具修理和抛光工序。本例由Bohler-Uddeholm公司提供。1 高硬度铋铅锡锑合金模具材料的发展令人乐观，出现的许多新型的工具，钢取代了S7，H13并且表面处理钢代替了A2，D2，M2。这些新型的高级钢的优势在于他们的硬度达到HRC50HRC60，并且能够焊接、精密磨削，组织致密和良好的耐磨性。例2 高热导性钢的导热性与硬度的联系。新型的铜镍合金有着高硬度，达P20，比其他铜合金和铝合金有着更好的导热性。那些超硬的材料，他们的优势在于热导性超过H13。因此生产周期缩短。这些高级钢替代了过去冷作模具钢需要通过低温回火才能达到所需机械性能，而且冷作模具钢还不能焊接，挤压或者400oF的PVD表面处理。高硬材料的优良的表面特性来自于高技术的熔化处理得到的均匀的内部晶体结构。例1能够说明以上观点。注塑模加入30的聚酰胺玻璃纤维，被制造成铬钼钒合金工具钢。硬度在5456HRC，它能够工作7700次而毫物损伤，Premium H13的硬度在5052HRC，在6000次的工作之后，塑件开始发生变形。2 热导合金钢铜合金的使用已经有很多年了，一直以来都在努力缩短铜合金重新
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