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模塑后的尺寸稳定性(第一部分)  发帖心情 Post By:2013/2/6 20:31:00 [只看该作者]


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Dimensional Stability After Molding

Materials Know How

By Michael Sepe from Michael P. Sepe LLC

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From: Plastics Technology
Issue: January 2013

The degree to which molded parts shrink as they cool is largely dependent upon the composition of the material being processed.

Click Image to Enlarge

The graph for this part shows that reaching a stable dimension takes 2 hr. This is due to the fact that PBT is a semi-crystalline polymer, this particular grade contains no fillers or reinforcements that can limit the shrinkage of the polymer, and the nominal wall of this part is 0.250 in., so the cooling process is relatively slow.

Everyone involved in plastics processing knows that molded parts shrink as they cool. The degree to which this occurs is largely dependent upon the composition of the material being processed. Semi-crystalline materials shrink more than amorphous ones, and fillers will reduce the shrinkage rate of any particular polymer to a degree that depends upon the type and amount of filler that is added.

Part geometry is also a contributing factor—thin-walled parts shrink less than products with thicker walls simply because they cool to an equilibrium state more quickly. Processing conditions also have an influence on this behavior. Higher packing pressures will reduce the level of shrinkage, and lower mold temperatures also have this effect, although in the case of mold temperature the benefits may be only temporary.

One of the most difficult aspects of constructing a mold is determining the compound effects of all these factors on the relationship between the tool dimensions and the size of the related part features.

While everyone accepts shrinkage as a fact of life, there is somewhat less agreement regarding the time frame required for a part to become dimensionally stable. This is of practical importance since dimensional checks are part of first-article approval procedures, process-capability studies, and ongoing quality checks during production runs.

Measurements made too early will provide bad data, but waiting longer than necessary creates the risk that corrective actions made to a process that is out of specification will come too late. Efforts to produce parts to print often make use of tools known as hot gauges. These are go/no-go devices that can be used to check a part that is still in the process of cooling and shrinking but has reached a point where the final dimension can be accurately predicted based on an established relationship between the dimension of the hot part and the final dimension of the stable product.

Studying the process of shrinkage in any given part produces a relationship between size and time that follows a function known as exponential decay. When plotted on a linear scale this produces a graph such as the one on this page. The dimensions drop rapidly in the first few minutes and the rate of change becomes slower as the part approaches equilibrium. Eventually, no further changes are noted and the part is considered to be stable. A comparison of the controlling tool dimension and the final size of the part will produce an actual value for the mold shrinkage.

It is an interesting exercise to compare this actual value with the published values for mold shrinkage on the material data sheet. In the case of the part in the accompanying graph, the material was an unfilled PBT with a published mold shrinkage value of 0.017-0.023 in./in. The critical dimension on the part print is given as 4.941 + 0.008 in., and the actual mold shrinkage value for a part molded to the nominal dimension is 0.02197 in./in.

The critical parameter in making a good prediction of the final part size is the time required for the curve to flatten out. The graph for this part shows that the process of reaching a stable dimension takes 2 hr. This is due to the fact that PBT is a semi-crystalline polymer, this particular grade contains no fillers or reinforcements that can limit the shrinkage of the polymer, and the nominal wall of this part is 0.250 in., so the cooling process is relatively slow. The cycle time for this part is 60 sec, so if the part is running out of specification, waiting 2 hr before making a correcting adjustment will cause the rejection of 120 parts. The ability to correlate the size of the part at 15 to 20 min, when the curve has “turned the corner,” to that of the final part can save hundreds or even thousands of rejected parts over the course of a year.

But is the time frame to complete shrinkage 2 hr for all parts and material types? As it turns out, it is not. The actual time required to reach dimensional stability will depend upon all of the factors mentioned above. For thin-walled parts molded in amorphous materials, the wait time may be as little as 15 min and even parts with wall thickness of 0.125 to 0.140 in. will be stable in half an hour. Most molded parts are stable within an hour. But there are maddening exceptions.

First, part size is important. Mold shrinkage is a percentage of the as-molded dimension. A mold shrinkage of 0.010 in./in. is equal to a 1% dimensional change. Often our judgment of stability is indexed to what we can measure. If we are using tools that have a resolution down to 0.001 in., then a part with a critical dimension of 1 in. will appear to have reached a stable condition before a part with a critical dimension of 20 in. simply because a last 0.001 in./in. change will barely be detected in the smaller part and will likely be only a small percentage of the total print tolerance, while in the larger part this same percentage change may equal the entire tolerance range.

Parts with very thick walls present some significant challenges simply because the time required for the entire part to cool is extended. This is particularly problematic for unfilled semi-crystalline materials, where shrinkage is related to the process of crystallization. Crystals are well-ordered regions and consequently occupy less space than the amorphous regions. With increasing degrees of crystallinity come increased levels of shrinkage.

If you want to get a feel for the magnitude of this relationship, refer to a data sheet for PEEK. This polymer is slow to crystallize and therefore can be produced in either an amorphous or a semi-crystalline structure depending upon the cooling rate. The density for amorphous PEEK is 1.26 g/cc while for the semi-crystalline variety it is 1.30 g/cc. This is a 3.2% difference and can translate to as much as a 0.015 in./in. difference in shrinkage in all directions.

But even with a typical wall thickness there are semi-crystalline materials that do not seem to follow the rules. Parts molded in these materials appear to keep changing for an extended period of time—days, not hours. And while most of these changes involve the part becoming smaller, there are instances where part size increases. The reasons for this seemingly anomalous behavior, and how to best cope with the associated challenges, will be the subject of Part 2.



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  发帖心情 Post By:2013/2/6 20:43:00 [只看该作者]

 
此主题相关图片如下:pbt的模后收缩与时间关系.jpg
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The graph for this part shows that reaching a stable dimension takes 2 hr. This is due to the fact that PBT is a semi-crystalline polymer, this particular grade contains no fillers or reinforcements that can limit the shrinkage of the polymer, and the nominal wall of this part is 0.250 in., so the cooling process is relatively slow.

该部件的图像显示,达到稳定尺寸需要花2个小时。这是由于PBT是一个半结晶聚合物,这个特定的牌号不含有限定聚合物收缩的填料或增强剂,这个部件的设计壁厚0.25in,所以冷却过程相对慢。

Everyone involved in plastics processing knows that molded parts shrink as they cool. The degree to which this occurs is largely dependent upon the composition of the material being processed. Semi-crystalline materials shrink more than amorphous ones, and fillers will reduce the shrinkage rate of any particular polymer to a degree that depends upon the type and amount of filler that is added.

从事塑料加工的人都知道,模塑件随着冷却而收缩。模塑部件随冷却而收缩的程度主要取决于所加工材料的组成。

半结晶材料收缩大于无定形材料,填料将减少任何聚合物的收缩,其程度取决于所添加填料的类型和填充量。

[此贴子已经被作者于2013-3-11 11:49:11编辑过]

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  发帖心情 Post By:2013/2/6 21:31:00 [只看该作者]

 

Part geometry is also a contributing factor—thin-walled parts shrink less than products with thicker walls simply because they cool to an equilibrium state more quickly. Processing conditions also have an influence on this behavior. Higher packing pressures will reduce the level of shrinkage, and lower mold temperatures also have this effect, although in the case of mold temperature the benefits may be only temporary.

部件几何也对收缩率有影响薄壁部件收缩小于厚壁部件,只是因为薄壁部件可能更快冷却到平衡状态。加工条件也对收缩性能有影响。较高的保压压力将减少收缩率水平,较低的模温也有这种效果,尽管模温的影响只是暂时的。

One of the most difficult aspects of constructing a mold is determining the compound effects of all these factors on the relationship between the tool dimensions and the size of the related part features.

建造一个模具的困难之一就是确定这些因素对模具尺寸与相应部件尺寸之间关系的综合影响。

While everyone accepts shrinkage as a fact of life, there is somewhat less agreement regarding the time frame required for a part to become dimensionally stable. This is of practical importance since dimensional checks are part of first-article approval procedures, process-capability studies, and ongoing quality checks during production runs.

尽管大家公认收缩是一个事实,但是在一个部件达到尺寸稳定的时间框架上,很少达成一致意见。这个问题有其实际上的重要意义,这是由于尺寸稳定检查是首件产品批准程序、加产能研究和未来生产流程中的质量检验的一部分。

[此贴子已经被作者于2013-3-11 11:51:57编辑过]

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  发帖心情 Post By:2013/2/6 23:14:00 [只看该作者]

 

Measurements made too early will provide bad data, but waiting longer than necessary creates the risk that corrective actions made to a process that is out of specification will come too late. Efforts to produce parts to print often make use of tools known as hot gauges. These are go/no-go devices that can be used to check a part that is still in the process of cooling and shrinking but has reached a point where the final dimension can be accurately predicted based on an established relationship between the dimension of the hot part and the final dimension of the stable product.

测量过早将产生坏的数据,但是等得太久就会产生这样的风险:对不合格工艺采取纠正措施来得太晚。按照图纸生产部件的工作常常是利用称之为热规的工具,.这种工具是一种通止规,它们用于阻止还在冷却和收缩过程中但是还没有达到预期的最终尺寸的部件的变化,这里所谓的预期尺寸是通过建立热部件尺寸与稳定产品的最终尺寸之间的关系获得的。

Studying the process of shrinkage in any given part produces a relationship between size and time that follows a function known as exponential decay. When plotted on a linear scale this produces a graph such as the one on this page. The dimensions drop rapidly in the first few minutes and the rate of change becomes slower as the part approaches equilibrium. Eventually, no further changes are noted and the part is considered to be stable. A comparison of the controlling tool dimension and the final size of the part will produce an actual value for the mold shrinkage.

研究任何给定部件的收缩过程都将产生尺寸和时间之间的一个关系,它遵守指数衰减函数规律。当按照线性标尺绘图时,我们就获得上述所示的图像。尺寸在头几分钟下降很快,随着部件达到平衡,变化速率变慢。最终,没有发生进一步的明显变化,部件被认为达到了稳定。比较用于控制的热规尺寸和制件的最终尺寸将会获得模塑收缩率的实际值。

[此贴子已经被作者于2013-3-11 11:35:00编辑过]

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It is an interesting exercise to compare this actual value with the published values for mold shrinkage on the material data sheet. In the case of the part in the accompanying graph, the material was an unfilled PBT with a published mold shrinkage value of 0.017-0.023 in./in. The critical dimension on the part print is given as 4.941 + 0.008 in., and the actual mold shrinkage value for a part molded to the nominal dimension is 0.02197 in./in.

一个有趣的练习就是用材料数据表给出的模塑收缩率的文献值与这种实际值进行比较。就上图所示部件而言,材料是未填充PBT,文献模塑收缩率为0.017-0.023 in./in。部件图纸给出的临界尺寸为4.941 + 0.008 in,按照标称尺寸,该部件的实际模塑收缩率为0.0219in/in

The critical parameter in making a good prediction of the final part size is the time required for the curve to flatten out. The graph for this part shows that the process of reaching a stable dimension takes 2 hr. This is due to the fact that PBT is a semi-crystalline polymer, this particular grade contains no fillers or reinforcements that can limit the shrinkage of the polymer, and the nominal wall of this part is 0.250 in., so the cooling process is relatively slow. The cycle time for this part is 60 sec, so if the part is running out of specification, waiting 2 hr before making a correcting adjustment will cause the rejection of 120 parts. The ability to correlate the size of the part at 15 to 20 min, when the curve has “turned the corner,” to that of the final part can save hundreds or even thousands of rejected parts over the course of a year.

要对最终部件尺寸做出好的预测,关键参数是曲线变扁平时的时间。这个部件的图像显示,达到稳定尺寸的过程需要花费2个小时。这是由于PBT是一个半结晶聚合物,这个具体的牌号不含限制聚合物收缩的填料和增强剂,而且该部件的标称厚度达到0.250英寸,所以,能却过程相对较慢。这个部件的循环时间是60秒,所以假如这个部件不合格,要等2小时才能做出正确调整,这将产生120个废品部件。如能将曲线在15-20分钟“拐弯”时的尺寸与最终部件尺寸关联起来,一年将会节约数百乃至数千个丢弃的部件。

But is the time frame to complete shrinkage 2 hr for all parts and material types? As it turns out, it is not. The actual time required to reach dimensional stability will depend upon all of the factors mentioned above. For thin-walled parts molded in amorphous materials, the wait time may be as little as 15 min and even parts with wall thickness of 0.125 to 0.140 in. will be stable in half an hour. Most molded parts are stable within an hour. But there are maddening exceptions.

First, part size is important. Mold shrinkage is a percentage of the as-molded dimension. A mold shrinkage of 0.010 in./in. is equal to a 1% dimensional change. Often our judgment of stability is indexed to what we can measure. If we are using tools that have a resolution down to 0.001 in., then a part with a critical dimension of 1 in. will appear to have reached a stable condition before a part with a critical dimension of 20 in. simply because a last 0.001 in./in. change will barely be detected in the smaller part and will likely be only a small percentage of the total print tolerance, while in the larger part this same percentage change may equal the entire tolerance range.

但是,对于所有的部件和材料类型,完成收缩的时间框架都是2小时吗?事实证明,不是这样。要求达到尺寸稳定的实际时间将取决于上面提到的所有因素。对于用无定形材料模塑的薄壁制件,等待时间会小于15分钟,而具有0.125-0.140英寸壁厚的部件甚至在一个半小时后达到稳定。许多模塑部件在一个小时内达到稳定。但是,也有令人郁闷的例外。首先,部件尺寸是重要的。模具收缩率是模塑尺寸的百分比。0.001in/in的模具收缩率等于1%的尺寸变化。我们常常用我们能测量的东西作为稳定性判断的指标。假如我们使用一个精度达到0.001in的工具测量,那么一个具有临界尺寸1in的部件似乎将比一个临界尺寸为20in的部件先一步达到稳定状态,这只是因为0.001in的变化不能在一个较小的部件上被探测出来,而且可能只是总的图纸公差的一个小比例;而在较大部件上,这个同样的百分比变化可能等于整个容差范围。


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  发帖心情 Post By:2013/3/11 11:36:00 [只看该作者]

 

Parts with very thick walls present some significant challenges simply because the time required for the entire part to cool is extended. This is particularly problematic for unfilled semi-crystalline materials, where shrinkage is related to the process of crystallization. Crystals are well-ordered regions and consequently occupy less space than the amorphous regions. With increasing degrees of crystallinity come increased levels of shrinkage.

具有很大壁厚的部件表现出很大的挑战,只是因为整个部件冷下来的时间要延长了。对于未填充半结晶材料,这尤其成为问题,这里的收缩率与结晶过程相关联。结晶是有序度高的区域,因此比无定形区占据更少的空间。随着结晶度的增加,收缩率就增加了。

If you want to get a feel for the magnitude of this relationship, refer to a data sheet for PEEK. This polymer is slow to crystallize and therefore can be produced in either an amorphous or a semi-crystalline structure depending upon the cooling rate. The density for amorphous PEEK is 1.26 g/cc while for the semi-crystalline variety it is 1.30 g/cc. This is a 3.2% difference and can translate to as much as a 0.015 in./in. difference in shrinkage in all directions.

如果你想对于结晶度对于收缩率的影响程度找到一种感觉,就请参考PEEK的数据表。这个聚合物结晶慢,因此可以通过控制结晶速度来获得无定形或者半结晶结构。无定形结构PEEK的密度是1.26 g/cc,半结晶PEEK的密度为1.30 g/cc。存在一个3.2%的密度差,它将可能转化成在所有方向上的高达0.015 in./in.的收缩率差

But even with a typical wall thickness there are semi-crystalline materials that do not seem to follow the rules. Parts molded in these materials appear to keep changing for an extended period of time—days, not hours. And while most of these changes involve the part becoming smaller, there are instances where part size increases. The reasons for this seemingly anomalous behavior, and how to best cope with the associated challenges, will be the subject of Part 2.

即使对于典型的壁厚,有些半结晶材料似乎也不遵守这些规则。用这些材料模塑的部件似乎一直在变化,时间延长到数天,而不是数小时。尽管这其中的许多变化是使部件变小,但是也有部件尺寸增加的例子。你想知道这些表面看来类似行为的原因以及处理这类挑战的方法吗?且听下回分解。

About the Author

Michael Sepe is an independent materials and processing consultant based in Sedona, Ariz. with clients throughout North America, Europe, and Asia. He has more than 35 years of experience in the plastics industry and assists clients with material selection, designing for manufacturability, process optimization, troubleshooting, and failure analysis. Contact: (928) 203-0408 ?
mike@thematerialanalyst.com.

[此贴子已经被作者于2013-3-11 11:38:44编辑过]

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  发帖心情 Post By:2013/3/11 13:23:00 [只看该作者]

 

Dimensional Stability After MoldingPart 2

模塑后尺寸稳定性:第二部分

 

Materials Know How

By Michael Sepe from Michael P. Sepe LLC

From: Plastics Technology
Issue: February 2013

After molding, acetal parts can continue to shrink at room temperature and even in the cold.

模塑成型后,POM部件能够在室温乃至冷的温度下收缩。

 

 


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  发帖心情 Post By:2013/3/11 13:24:00 [只看该作者]

 

My first exposure to the prolonged shrinkage process exhibited by some materials came when I was molding large parts in acetal  homopolymer. Most of my experience with molding acetal parts to close tolerances had come from producing small gears with diameters of no more than half an inch.

当我模塑一个大的POM部件时,我第一次遇到的延长的收缩过程出现了。我模塑接近容差的POM部件的经验来自于生产小齿轮,其直径不超过半英寸。

However, our company landed a project that involved housings and quadrant gears with critical dimensions in the range of 3.5 to 4 in. and print tolerances of ±0.010 in. on parts with nominal wall thicknesses of 0.110 in. One specific dimension governing the spacing of alignment holes called for a specification of 4.046 ±0.010 in.

但是,我们公司启动了一个项目,是做外壳和扇形齿轮,临界尺寸在3.5-4英寸部件图纸容差±0.010 英寸,标称壁厚0.110英寸。确定定位孔间隔的一个尺寸要求规格在4.046 ±0.010 in.

During initial sampling we produced parts with measurements that ranged from 4.038 to 4.042-in. in a 30-piece capability study. Statistical process control (SPC) was a relatively new concept in U.S. manufacturing at that point, so the fact that we were operating at one end of the tolerance range did not particularly concern us. We were quite satisfied that the dimensional range was tight and all the parts were to print.

在起初的制样过程中,我们进行了一个30件产能的研究,我们生产的样品部件尺寸测量范围从4.0384,.042那时,在美国制造业,统计过程控制(SPC)是一个较新的概念,所以,尽管我们运行在容差范围的一端,但是这并没有使我们特别担心什么。我们相当满意,尺寸范围精确,所有的部件符合图纸要求。

[此贴子已经被作者于2013-3-12 11:02:58编辑过]

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  发帖心情 Post By:2013/3/11 14:07:00 [只看该作者]

 

But the parts had been measured about 90 minutes to two hours after they were produced. The parts were room temperature to the touch and based on our experience with other semi-crystalline materials we were satisfied that everything was fine. The next day the quality assurance people pulled us into the lab to show us that half of the parts we had produced the day before were too small to meet the print. A review of all 30 parts showed that they had all continued to shrink and were now 0.004 in. smaller than the previous afternoon. In another day they moved an additional 0.001 in. and then things seemed to settle down. Later I observed similar problems with large parts produced in polypropylene, even in filled PP.

部件生产后,我们一直测量90分钟到两小时。部件摸上去达到了室温,基于我们对半结晶材料的经验,我们很满意,一切都很好。第二天,质保人员把我们拉进实验室,让我们看到,昨天生产的一半部件太小而不能符合图纸要求。一眼就能看到,30个部件都继续收缩了,比昨天下午的尺寸小了0.004英寸。再过一天,这些部件又缩了0.001英寸,然后才尘埃落定。后来,我又观察到类似问题,这些出现问题的部件是用PP生产的大部件,甚至是用填充PP生产的大部件。

In order to understand what was happening it is important to appreciate the relationship between mold shrinkage and crystallization in semi-crystalline materials. The more the material crystallizes the more it shrinks. Optimal levels of crystallinity are desirable. Semi-crystalline materials offer improved levels of fatigue and wear resistance compared with amorphous polymers and they generally provide improved creep resistance at elevated temperatures. But if the material is molded in a manner that prevents the development of crystallinity, these properties are not realized at the intended levels.

为了理解所发生的问题,重要的是领会模塑收缩率与与半结晶材料的结晶关系。材料结晶越多,它就收缩越多。最高的结晶度当然是我们所需要的。与无定形材料相比,半结晶材料为我们提供了改进的耐疲劳性和耐磨性,它们通常提供了改进的高温耐蠕变性。但是假如材料模塑时以某种方式阻止了结晶度的提高,这些性能就无法按照理想的水平实现。


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上海北京顺德
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  发帖心情 Post By:2013/3/11 14:08:00 [只看该作者]

 

The opportunity for crystallization exists in a temperature window below the melting point of the polymer and above the glass-transition temperature (Tg). Processes like injection molding involve rapid cooling of the polymer as it enters the mold. Even when running a material such as PEEK, where the mold temperature may be 375 F (190 C), this represents a thermal shock to the flowing polymer that enters the mold at 700 F (371 C). This rapid reduction in temperature is needed to solidify the material so that the part can assume its intended shape. But as long as the polymer remains at a temperature above its glass transition, about 295 F (146 C) for PEEK, there will be sufficient mobility at a molecular level to allow the crystal structure to develop. Once the temperature of the material drops below this point no more crystals can form.

结晶的机会存在于熔点以下玻璃化温度(Tg)以上的温度窗口。在注塑过程中,当聚合物进入模具时,存在聚合物的快速冷却。即使在摆弄PEEK这样的材料时,也会存在对流动聚合物的热震(廖注:温度突然变化),虽然模温高达190,但是聚合物向模具的进料温度更是高达700 F (371 C)。快速减少温度是为了快速固化材料以便部件呈现出想要的形状。但是,只要聚合物仍然在玻璃化温度以上,PEEK大约是295 F (146 C),那么聚合物将在分子水平仍然具有足够的活动性,从而使结晶结构能够进一步发展。一旦材料的温度掉到这个温度以下,就不能形成结晶结构了。


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