机械外文翻译--薄壁模具成功的秘密

中英文薄壁模具成功的秘密 要求生产一种小的轻的零件,就要我们寻找一个能够注出薄壁工件的注塑模具.现在,”薄壁”在微电子方面通常定义为少于1m壁厚.在大的自动化方面,”薄”可能意味是2mm左右.无论怎么样,越薄壁的地方,在生产过程中要求的变化就越多:更高的压力和速度,更短的冷却时间,和改注射的方法和工作排列的方式.这些过程的改变在模具,机构和零件设计中要引起一系列的思考机械方面的思考: 标准的注塑机都能够应用于大多数的薄壁注射.新标准的注塑机的容量远超过了十几年前的机器.先进的材料和技术,高超过的设计水平大大的增加了薄壁零件对标准注塑机的要求. 但是当薄壁不断的收缩,要求有更大的高速带来的特殊压力.例如微电子零件的壁厚少于1m,填充时间要少于0.5秒和注射压力大于30000psi是不罕见的.为薄壁注射而设计的水力机械通常储蓄的能量既用于注射又用于夹紧循环.纯电的和水电混合的机械的出现往往能够提供更高的速度和更大的压力.为了抵抗高压,在注射范围内,夹紧里应该是在5-7吨每平方英寸.另外,连接杆到压盘有助于减少弯曲,当墙壁厚度减少,注射压力上升.薄壁注射机的连接杆到压盘厚度的距离通常是2:1,或者是更低的比率.而且,随着壁厚的变薄,注射速度的闭环的控制,转移压力和其他的过程变量能在高速度和压力拥包的情况下帮助控制充满型腔.当它开始注射容量时，大量的塑料装入型腔太多了。我们建议注射40%70%的型腔容量到模的型腔里面。在薄壁注射的应用中经常能见到的大大地减少的总循环周期时间可以使把最小注射量降低到型腔容量的20%30%成为可能，但是 ，只有在彻底了解零件因材料变化而引起的其特性的变化的情况下才能实现。用户必须小心，小的注射量可能引起材料性能的降低，因此，意味着更长的只社时间。模子：本身的精度 速度是薄壁模能否做成功的关键的因素之一。更快的折射速度和更高的注射压力把溶解的热塑性的材料在一个足够的速度下注入狭窄的型腔以避免其凝固。如果标准零件注射时间在2sec内，如果它的厚度减少25%那么充型时间就能减少50%，即1sec钟就能充满型腔。 薄壁模具的好处之一是当壁厚减少时，需要冷却的材料也相应的减少。随着主要壁厚的减少，循环周期能减少50%，熔化状态下的系统的小心的管理能使分流道和主流道缩小循环周期的时间。热的分流道和主流道通常用于薄壁零件的注塑以利于把周期时间减少到最小。 模具的材料也应该被检查。P20钢在传统的应用中广大被使用，但是，由于薄壁注射的压力不断的增大，模具也必须做得更坚固。H-13钢和其它的坚韧的钢为薄壁的工具提供了额外的安全保证。（另外，如果可能，你也可以选用模具的材料这 可以使在高速度注入型腔的时候，不会加快模具的磨损。） 不过，比标准的零件来说精密的模具可能要多花费30%40%。可是，生产率成倍提高可以弥补这多花费的部分。实际上，薄壁的注塑的方法是经常用于省钱途径之一。100%的生产率的提高意味着要做的模具就更少因此在生产程序中节省更多的钱。这里是一些薄壁的工具设计上的技巧：1. 对于主要薄壁工作的应用，一般用硬度大于钢p20的材料，尤其是要求有大的磨损和腐蚀的时候。H-13和D-2钢就是最常用的两栖种材料着之一。2. 模具的锁定有时是弯曲的不对齐。3. 型腔孔的型心能有助于减少型心在转换时的破损。4. 在型腔和主流道下面用更重的支持板（通常是23英寸厚）和较重的导柱（一般是增加0.005英寸）5. 比传统的模具使用更大更多的推杆，以减少推杆的压力6. 考虑滑块和导套的放置。注射模具避免在复合材料上的缺陷 两钟或更多材料的注射模需要一个两个浇口浇铸方式或同时技术。不管使用程序如何，造模者在达到高质量塑件方面面对相同的挑战。任何多种材料成型过程的三个共同的问题是不足的聚合体的化学和机械结合，一个或更多成分的不完全填补，和一个更多的成分的“flash”。 这些情况能发生是否材料组合加强的和没被加强的，实心的和起泡的，刚硬的和软的，原料和再研磨，有色素和无色素，等等。 多种材料模和它的问题及问题的解决是复杂的题目，不能在简短的文章里彻底探讨清楚，接下来说明相关变量的范围，以及对一些比较重要的问题作简单的介绍。时间和温度引起材料之间结合不足的原因与材料注射时间和第二材料熔合时第一材料的温度有关。第一材料的过分冷却往往使熔合变弱。另外，第一次注射必须足够冷却才能不使第二次注射时不引起变形和错位。如果第一次材料仍然很软，而第二次注射来得太快，答二材料将在第一材料是形成缩孔和飞边。引起“流涎”现象。 在两个注射机上的流动材料（在一个注射机上第一次注射，接着把它插入到另一个注射机上）不易产生和旋转桌面的两个浇口的注射机上的流动材料一样好的结合。甚至当用相容材料时两次注射之间延长的时间相对要长，并且地一枪可能会太冷。一般认为一个比较高塑件温度有更好的化学/机械结合。如果当第一次注射转移到第二个模具上时吸附了一些灰尘，那么将会对结合有很大的影响。一些材料往往很自然比其它材料粘贴的更好。为了overmolding ,树脂供应者特别是TPES的制造者通过提高对其它聚合物的粘附范围努力地将某一等级最佳化。 添加剂和色素也会影响结合。在第一材料里面的玻璃纤维能提高与第二材料的结合质量。这些材料表面上的纤维能促进与第二注射材料的机械结合。 注意包含有像滑石或碳酸钙一样的填充物的材料应被足够烘干，因为这些填充物含有很多能是结合减弱的湿气。 质量影响元素 为了防止任一材料的没填充和装得太多（和飞边），机器的从注射到 注射的准确性明显的是一个关键的因素。一般建议注射量少于0.3%到0.5%。有注射速度闭环控制的注射机是最好的选择。 第二是选择一个有多种材料塑件成型经验的模具制造者。如果开始就有很好的模具设计，这样能省掉很多花费。例如，它有助于增加那些有通过用undercuts或相似设计获得 的机械结合的材料之间的热化结合。确保多孔模具平衡好，热流动的 maniflod也必须平衡好，而且下降的数字和大小一定对低压的填充物是充分的。模具的温度是另一个重要因素。当有核心lifter的移动模具的第二次注射时，温度准确控制是强制的。因为钢或钢合金有不同 的热膨胀，所以不正确的温度会引起lifter的契入和堵塞。 为了获得好的多种材料塑件成型，操作者必须有很好的训练。 当塑件制造结果不好时，错误的制造环境经常是罪魁祸首。因为它的 复杂性，所以如果当事情出错时，也只有懂得程序的人才被允许去纠正。 获得材料间好的结合也经常取决于当第二材料注射时第一材料的温度Secret of successful thin-wall moldingDemands to create smaller, lighter parts have made thin-wall molding one of the most sought after capabilities for an injection molder. These days ,”thin-wall” is generally defined by portable electronics parts having a wall thickness less than 1mm . for large automotive parts , “thin” may mean 2 mm . In any case, thinner wall sections bring changes in processing requirements: higher pressure and speeds, faster cooling times, and modification to part-ejection and gating arrangements .These process changes have in turn prompted new considerations in mold ,machinery ,and part design Machinery considerations Standard molding machinery can be used for many thin-wall applications. Capabilities built into newer standard machines go well beyond those of 10 years ago. Advances in materials, gating technology and design further expand the capabilities of a standard machine to fill thinner parts .But as wall thicknesses continue to shrink, a more specialized press with higher speed and pressure capabilities may be required. For example, with a portable electronics part less than 1 mm thick, fill times of less than 0.5 sec and injection pressures greater than 30,000psi are not uncommon. Hydraulic machines designed for thin-wall molding frequently have accumulators driving both injection and clamping cycles. All-electric and hybrid electric/hydraulic models with high speed and pressure capabilities are starting to appear as well.To stand up to the high pressures involved, clamp force should be a minimumof 5-7tons/sq in. of projected area. In addition,extra-heavy platens help to reduce flexure as wall thicknesses drop and injection pressures rise. Thin-wall machines commonly have a 2:1 or lower ratio of tiebar distance to platen thickness. Also, with thinner walls, closed-loop control of injection speed, transfer pressure,and other process variables can help to control filling and packing at high speeds and pressures.When it comes to shot capacity, large barrels tend to be too large. We suggest you aim for a shot size of 40% to 70%of barrels capacity . The greatly reduces total cycle time seen in thin-wall applications may make it possible to reduce the minimum shot size to 20%-30% of barrel capacity, but only if the parts are thoroughly tested for property loss possible material degradation. Users must be careful, as small shot sizes can mean longer barrel residence times for the material ,resulting in property degradation .Molds: make em ruggedSpeed is one of the key attributes of successful thin-wall molding. Faster filling and higher are required to drive molten thermoplastic material into thinner cavities at a sufficient rate to prevent freeze off. If a standard part is filled in 2 sec, then a reduction in thickness of 25%potentially can require a drop in fill time of 50%to just 1 sec.One benefit of thin-wall molding is that as wall sections drop, there is less material to cool. Cycle times can drop by 50%with aggressive wall-thickness reduction. Careful management of the melt-delivery system can keep runners and sprues from diminishing that cycle-time advantage. Hot runners and heated sprue bushings are often used in thin-wall molding to help minimize cycle time.Mold material should be reviewed too. P20 steel is used extensively in conventional applications, but due to the higher pressures of thin-wall molding, molds must be built more robustly. H-13 and other tough steels add an extra degree of safety for thin-wall tools.If possible, you will also want to select a molding material that doesnt accelerate mold wear when injected into the cavity at high speeds.However, robust tools cost money-possibly even 30% to 40%more than a standard mold. Yet the cost is often offset by increased productivity. In fact, the thin-wall approach is frequently used to save money on tooling. A 100% increase in productivity can mean that fewer molds to be built, thereby saving money over the life of a program.Here are some more tips on tool design for thin walls:For aggressive thin-wall applications, use steel harder than P20,especially when high wear and erosion are expected. H-13 and D-2 steels have been successful in gate inserts.Mold interlocks sometimes can stave off flexing and misalignment.Cores that telescope into the cavity can help reduce core shifting and breakage.Use heavier support platesoften 2 to 3 in thickwith support pillars typically preloaded 0.005 inunder the cavities and sprue.Use more and large ejector pins than with conventional molds to reduce pin pushing.Consider strategic placement of sleeve and blade knockouts.Injection Molding Troubleshooter Avoid Pitfalls in Multi-Material MoldingInjection molding with two or more materials requires either a two-shot molding approach or a simultaneous coinjection technique. Regardless of the process used, molders face the same challenges in achieving high part quality. Three common problems with any multi-material process are insufficient chemical or mechanical bonding of the polymers, incomplete filling of one or more components, and flashing of one or more components. These conditions can occur whether the materials combinations is reinforced and unreinforced ,solid and foamed, rigid and soft, virgin and regrind, pigmented and unpigmented , etc.Multi-material molding and its problems and solutions is a complex subject that cannot be explored thoroughly in a short article . The accompanying table indicates the range of variables involved. A few of the more important factors bear a brief discussion.Time and temperature One cause of insufficient bonding between materials relates to the timing of the injection of the materials and temperature of the first material when it is joined with the second . Too much cooling of the first material tends to weaken bonding. On the other hand, the first shot must be cooled enough not to be deformed or displace when you shoot the second one. If the second shot comes too soon, while the first material is still soft, the second material can compress and flash over the first one ,causing ”splash marks”.When running parts on two injection machines(molding the first shot on machine one and inserting it into the mold of the second machine ),bonding is not apt to be as good ad on a two-shot machine with rotating table. Even when using compatible materials, the delay time between the two shots is relatively long and the first shot is likely to be too cold . A higher part temperature is recommended for better chemical /mechanical bonding. Also, if the first shot picks up dust while being transferred to the second mold , bonding will also be negatively affected.Apart from process conditions, material choice can greatly affect bonding . Some materials naturally tend to adhere better than others, and resin suppliers-particularly makers of TPEShave been working hard to optimize certain gradesfor overmolding by increasing their range of adhesion to other polymers.Additives and pigments can affect bonding. Glass fibers in one materials can enhance bonding with the second . Fibers on the surface of the material promote a mechanical bond with the second shot . Note that materials containing fillers like talc or calcium carbonate should be dried adequately . These fillers hold a hot of moisture, which can detract from bonding.Elements of qualityTo prevent underfilling or overfilling (and flashing)of either material, the shot-to -shot accuracy of the machine is obviously a critical factor. Shot variability of less than 0.3%to 0.5%is recommended. A machine with closed-loop injection-speed control is the best choice. Next, pick a mold maker with experience in multi-material parts. You can save a lot of money if you have the mold designed well from the start . For example, it can be helpful to supplement the thermal/chemical bonding between two materials with a mechanical joint achieved by using undercuts or similar designs.Make sure multi-cavity molds are well balanced . Hot-runner manifolds must be balanced too, and the number and size of drops must be sufficient for low-pressure filling.Mold temperature is another important factor. Accurate control of the temperature is mandatory when running molds with core lifters for the second shot . Incorrect mold temperature can cause a lifter to wedge or jam, because of differential thermal expansion of the steel or steel/brass combination.Operators must be well trained for successful multi-material molding . Wrong machine settings are often the culprits when parts dont turn out right . Because of its complexities ,only people who understand the process should be allowed to attempt corrections if something goes wrong . Achieving a good bond between materials is often dependent on the temperature of the first material when the second is injected.