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Cost Savings for Mold Materials: Consider the Details
Ways to reduce costs that are not related to the price per pound of the mold material.
By Patricia Miller
Choosing 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 Alloys
In 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 Alloys
Copper 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 HRC
One 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 zones—which create pitting attack on the stainless—can 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 Materials
Looking 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.
Conclusion
There 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.
附錄B
專業(yè)外文翻譯
節(jié)省模具材料費用細則
降低成本的方式涉及的不光是每磅模具材料的價格。
作者：帕特麗夏 米勒
正確的選擇塑料模具材料變得越來越重要，在競爭激烈的今天，原材料的成本升高使得模具的價格上漲，模具的材料的選擇就本的日益重要。有些東西可以幫助模具制造者們最好的選擇模具材料，并且這些東西不就簡單的降低每磅模具材料的價格。
但是為了節(jié)省成本，模具制造商們肯定也會考慮到其他因素。首先要考慮的是模具制造的周期，當要制造一個零件的時候，模具的主要設計的部分包括：
l 模具尺寸和復雜的模具型腔
l 產(chǎn)品質(zhì)量要求
l 塑料成型的類型和成型因素的影響
l 模具制造機械設備的要求
l 模具物理因素要求（傳熱性、硬度、熱膨脹）
l 生產(chǎn)過程中的穩(wěn)定性
裝配問題（符合裝配原則，防繡）
l 設計原則（銳角轉(zhuǎn)角、避免薄壁、密封性好）
l 符合表面技術(shù)要求（拋光、粗糙度要求）
l 制造方式（電鍍、電火花加工、磨削加工）
每個階段都可以作到節(jié)省成本，但是有四個方面最為突出：
1. 新型高硬度鉍鉛錫銻合金
2. 熱導性良好的合金
3. 抗腐蝕材料
4. 預硬合金
無論以上那一種材料，作為模具材料，在節(jié)省成本方面的價值都是超過其他材料的方案。
例1． 電機轉(zhuǎn)子注塑模添加了30％的玻璃填充物聚酰胺，使得模具的壽命延長了20％，減少模具修理和拋光工序。本例由Bohler-Uddeholm公司提供。
1． 高硬度鉍鉛錫銻合金
模具材料的發(fā)展令人樂觀，出現(xiàn)的許多新型的工具，鋼取代了S7，H13并且表面處理鋼代替了A2，D2，M2。這些新型的高級鋼的優(yōu)勢在于他們的硬度達到HRC50～HRC60，并且能夠焊接、精密磨削，組織致密和良好的耐磨性。
例2． 高熱導性鋼的導熱性與硬度的聯(lián)系。新型的銅鎳合金有著高硬度，達P20，比其他銅合金和鋁合金有著更好的導熱性。
那些超硬的材料，他們的優(yōu)勢在于熱導性超過H13。因此生產(chǎn)周期縮短。這些高級鋼替代了過去冷作模具鋼需要通過低溫回火才能達到所需機械性能，而且冷作模具鋼還不能焊接，擠壓或者400oF的PVD表面處理。高硬材料的優(yōu)良的表面特性來自于高技術(shù)的熔化處理得到的均勻的內(nèi)部晶體結(jié)構(gòu)。
例1能夠說明以上觀點。注塑模加入30％的聚酰胺玻璃纖維，被制造成鉻鉬釩合金工具鋼。硬度在54～56HRC，它能夠工作7700次而毫物損傷，Premium H13的硬度在50～52HRC，在6000次的工作之后，塑件開始發(fā)生變形。
2． 熱導合金鋼
銅合金的使用已經(jīng)有很多年了，一直以來都在努力縮短銅合金重新成型的周期，通過利用特殊核心技術(shù)，塑料的成型次數(shù)最高。銅鈹和經(jīng)在硬度范圍30～40HRC范圍使用。另外銅鎳合金硬度能達30HRC，在P20的范圍之內(nèi)。此級別的合金的優(yōu)勢是熱傳導性和硬度范圍超過其他銅合金，也超過鋁合金的性能。此級別合金穩(wěn)定性、抗腐蝕性腔（參見例2）。
例3． 型腔磨削加工，優(yōu)質(zhì)H13，硬度45HRC
3． 不繡鋼的設計
我們必須重新審視不銹鋼的作用，很多新型的不繡鋼優(yōu)點突出，高的拋光能力，高的硬度等級可達H13，硬度HRC50～52。事實上這些合金保持著它們優(yōu)良的抗腐蝕性和最小的表面磨損，使的模具經(jīng)久耐用，這些優(yōu)點是值得我們認真考慮的。
例4． 鉆孔加工冷卻水道；優(yōu)質(zhì)H13，45HRC。
這個區(qū)域會影響不繡鋼模具的壽命，過去通過異敬管插頭來降低影響。熱處理的限制螺紋的加工存在問題，由于扭矩螺紋終止線存在壓力，死區(qū)表面在腐蝕環(huán)境中產(chǎn)生蝕斑，這些區(qū)域可能導致裂紋。新型的不銹鋼合金材料能最小優(yōu)化此不足，通過接頭的設計控制水封機構(gòu)，而不需要在模具中加工螺紋。
4． 預硬模具材料
回望鋼的制造技術(shù)和價格的關系，我們現(xiàn)在看到加工技術(shù)的實際應用，這使我們可以在更大的范圍應用更硬的材料，預硬鋼。它可能加工硬度等級H13，44～46HRC的材料，甚至更硬的材料。這種材料的優(yōu)點在于可以先使之變硬，因為被賦予了這樣優(yōu)良的特性，所以它的冷卻速度很快，不會產(chǎn)生裂紋。粗加工的時候，鋼的完整性很重要，從而減少了很多修補和后處理的過程。這種節(jié)省時間和成本的方法，在熱處理的過程種是很重要的，時間的利用率高了。利用節(jié)省的時間可以優(yōu)化鋼的特性?；鼗鸬男Ч紱]這么好，冷卻率的降低，使原料量降低，因為模具制造者很少會花時間處理額外的原料，通過快速的淬火使原料變成產(chǎn)品。
例5．完成的型腔加工; 優(yōu)質(zhì)H13, 45 HRC 。
一些例子怎樣用機器制造被硬化的H13 給出在例3, 4 和5 。
結(jié)論
有許多方法降低生產(chǎn)成本，而不僅僅是與每磅模具材料費用有關的方法。我們要提供給顧客的是最終的產(chǎn)品，按照顧客要求的完整的、穩(wěn)定的符合設計合理性的制造過程和產(chǎn)品要求的模具給顧客。新材料和方法的使用是的模具的制造比以前更出色，并且使模具的整體的費用降為最低。
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