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标题:成型诀窍:注射成型-平衡充填是保持尺寸精度的关键

1楼
上海北京顺德 发表于:2010/4/9 15:46:00

Know How: Injection Molding

<!-- InstanceEndEditable --><!-- InstanceBeginEditable name="ArticleTitle" -->Balanced Filling is Critical For Holding Molding Tolerances<!-- InstanceEndEditable -->
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By John Bozzelli

 

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Processors face a multitude of challenges whenever they approach an injection molding machine. One that has persisted for years (decades, actually) is the difficulty of getting multicavity tools to produce identical parts. While the financial benefit of multicavity production cannot be denied, rarely has the full financial benefit been realized. In many cases, the quality problems are so great that single-cavity production can actually be less expensive. So what is the problem with multi-cavity tools? Logic indicates that all one has to do is to geometrically balance the runners. Moldmakers do a pretty good job of this, but the problem persists nonetheless.

A brief thought experiment or study in polymer flow may shed light on root causes. As an example, let us follow the sequence of filling and packing in a two-cavity hot- or cold-runner mold. The runner is balanced with identical gate diameters and identical steel dimensions for each cavity. A short shot shows that the polymer fills the sprue and runner evenly. The next progressive short shot shows that even though the flow path is balanced, the polymer flow filling each cavity is not. Cavity “A” fills ahead of cavity “B.” There can be a number or reasons for this, such as a slight difference in gate land or venting. (And where more than two cavities are involved, even the most perfectly balanced molds are prone to uneven filling due to the physics of polymer flow, as described by Beaumont Technologies, beaumontinc.com.)

Plastics change=
Plastics change viscosity in response to shear. Multicavity molds are prone to numerous causes of shear variations in the tool.

The point is that in most multicavity molds, all cavities do not fill evenly—this is true the world over. The consequence is that parts are not truly identical. Couple this with increasing part complexity and tighter tolerances required for assembly or function, and we have projects running with high reject rates or blocked cavities. Frustration builds and the finger-pointing begins—maybe even a lawsuit. Is the part design, resin, mold, or processing at fault? In our imaginary example, mold-filling analysis does not not show the source of the problem—an all too real possibility.

For a partial explanation, let us take a look at the material, particularly the shear sensitivity of all resins. Figure 1 represents a typical curve plotting viscosity vs. shear rate on linear axes instead of the typical log-log scales. Fill times are included to provide information on injection rates. Short fill times mean high shear rates and long fill times mean low shear rates. Nearly all plastics show this relationship of changing viscosity in response to shear rate. The resin is significantly stiffer at long fill times with low shear rates and flows significantly easier at short fill times with high shear rates. The viscosity difference between the two is huge. What has this have to do with our non-uniform multicavity parts?

Let’s look at our two cavities during filling and packing, with this viscosity curve data to help us “see” from the plastic’s perspective. During filling of each cavity, the shear rate is high, viscosity is low (easy flowing), and all is fine. However, if we freeze-frame the situation at the instant of switchover from first to second stage (packing), we see cavity “A” is full and cavity “B” is short. (For the sake of this article, we’re ignoring the many possible reasons why.) What is the viscosity of the plastic in cavity “A”? There is little if any flow, shear rate is extremely low, and viscosity is off the top of the graph, perhaps something like taffy. Cavity “B” is short, so plastic is still moving, shear rate is high, and viscosity is low, perhaps like cold molasses.

cavity A
If cavity A fills before cavity B, its melt stops flowing before packing and thus its viscosity is much higher and resistant to packing.

Now we switch to the second stage. Does the hold or packing pressure get delivered to each of the two cavities equally? Think of the viscosity difference and which viscosity will transfer or deliver the packing pressure to which cavity better. Cavity “B” has the advantage of the lower viscosity and will pack better. Cavity “A,” with taffy-like high viscosity, will be sinky, too stiff to pack. Perhaps Fig. 2 helps to explain the issue. We add more packing pressure and Cavity “B” flashes while Cavity “A” remains sinky.

Now what do we do? Many people start jacking up the clamp pressure or looking for a parting-line problem or other tool rework. But all these are a waste of time; nothing helps. By now the importance of delivering uniform flow to both cavities during the entire filling stage becomes clear. What are the chances of processing around this situation? None! You are fighting Mother Nature, the inherent law of physics that pressure will take the path of least resistance.

If this discussion makes sense to you, apply the thought process to molds where you have one or more blocked cavities. Again you are trying to violate a law of plastic flow, and it’s not nice to buck Mother Nature. The only answer is to find and fix the reasons for uneven filling in the first place—a lesson for another day.

About the Author
John Bozzelli is the founder of Injection Molding Solutions (Scientific Molding) in Midland, Mich., a provider of training and consulting services to injection molders, including LIMS, and other specialties. E-mail john@scientificmolding.com or visit scientificmolding.com.

<!-- InstanceEndEditable -->
Know How: Injection Molding

<!-- InstanceEndEditable --><!-- InstanceBeginEditable name="ArticleTitle" -->Balanced Filling is Critical For Holding Molding Tolerances<!-- InstanceEndEditable -->
<!-- InstanceBeginEditable name="Synopsis" --><!-- InstanceEndEditable -->
<!-- InstanceBeginEditable name="Author" -->

By John Bozzelli

 

<!-- InstanceEndEditable -->
2楼
上海北京顺德 发表于:2010/4/9 15:46:00
Know How: Injection Molding

<!-- InstanceEndEditable --><!-- InstanceBeginEditable name="ArticleTitle" -->Balanced Filling is Critical For Holding Molding Tolerances<!-- InstanceEndEditable -->
<!-- InstanceBeginEditable name="Synopsis" --><!-- InstanceEndEditable -->
<!-- InstanceBeginEditable name="Author" -->

By John Bozzelli

<!-- InstanceEndEditable -->
3楼
上海北京顺德 发表于:2010/4/9 16:29:00

Processors face a multitude of challenges whenever they approach an injection molding machine. One that has persisted for years (decades, actually) is the difficulty of getting multicavity tools to produce identical parts. While the financial benefit of multicavity production cannot be denied, rarely has the full financial benefit been realized. In many cases, the quality problems are so great that single-cavity production can actually be less expensive. So what is the problem with multi-cavity tools? Logic indicates that all one has to do is to geometrically balance the runners. Moldmakers do a pretty good job of this, but the problem persists nonetheless.

 

每当加工者要去摆弄注塑机时,他们都会面临许多挑战。持续几十年的挑战之一是难于找到多型腔模具来生产一模一样的部件。尽管多模腔生产的经济性不可否认,但是完美的经济性却罕能实现。在许多情况下,还存在如此大的质量问题,以至于单模腔生产实际上还便宜一些。所以,多模腔的问题是什么?逻辑表明,所有要做的事情就是在几何上平衡流道。模具商做了很好的工作,但问题依然存在。

[此贴子已经被作者于2010-4-9 16:40:22编辑过]
4楼
上海北京顺德 发表于:2010/4/9 17:09:00

A brief thought experiment or study in polymer flow may shed light on root causes. As an example, let us follow the sequence of filling and packing in a two-cavity hot- or cold-runner mold. The runner is balanced with identical gate diameters and identical steel dimensions for each cavity. A short shot shows that the polymer fills the sprue and runner evenly. The next progressive short shot shows that even though the flow path is balanced, the polymer flow filling each cavity is not. Cavity “A” fills ahead of cavity “B.” There can be a number or reasons for this, such as a slight difference in gate land or venting. (And where more than two cavities are involved, even the most perfectly balanced molds are prone to uneven filling due to the physics of polymer flow, as described by Beaumont Technologies, beaumontinc.com.)

 

在聚合物流动方面的一个思路简洁的实验或研究就能找到根源。举一个例子,我们在一个热获冷的流道模具上按照充填-进胶的顺序进行实验。流道用同样的浇口尺寸进行平衡,每个模腔采用同样的钢板尺寸。采用欠注的方法可以显示,聚合物可以均衡地在主流道(sprue)和流道中充填。接着一个欠注实验表明,即使流动路径平衡了,聚合物料流充填也不会平衡。模腔A先于模腔B先充填。这可能有很多原因,例如浇口厚度(gate land)或排气有轻微的差异。(当涉及两个以上的模腔时,即使最完美平衡的模具也会出现不平衡进料,这是由聚合物流动的物理特征决定的。

5楼
上海北京顺德 发表于:2010/4/9 17:32:00

The point is that in most multicavity molds, all cavities do not fill evenly—this is true the world over. The consequence is that parts are not truly identical. Couple this with increasing part complexity and tighter tolerances required for assembly or function, and we have projects running with high reject rates or blocked cavities. Frustration builds and the finger-pointing begins—maybe even a lawsuit. Is the part design, resin, mold, or processing at fault? In our imaginary example, mold-filling analysis does not not show the source of the problem—an all too real possibility.

 

上述实验的意义是,在许多多模腔模具中,所有模腔都不能均衡充填--在全世界都是这样。结果是,部件不完全相同。此外,制件复杂性不断增加以及组装或功能要求的公差也更加苛刻。还有,我们使用高的顶出速度赶进度或者制件卡住模腔。挫折感积累起来了,相互指责开始了-甚至成了官司。部件设计、树脂、模具或加工有过错吗?在一个设想的例子中,模具充填分析没有显示出问题的根源--问题的现实可能性。

6楼
上海北京顺德 发表于:2010/4/9 17:52:00
Plastics change=
Plastics change viscosity in response to shear. Multicavity molds are prone to numerous causes of shear variations in the tool.

For a partial explanation, let us take a look at the material, particularly the shear sensitivity of all resins. Figure 1 represents a typical curve plotting viscosity vs. shear rate on linear axes instead of the typical log-log scales. Fill times are included to provide information on injection rates. Short fill times mean high shear rates and long fill times mean low shear rates. Nearly all plastics show this relationship of changing viscosity in response to shear rate. The resin is significantly stiffer at long fill times with low shear rates and flows significantly easier at short fill times with high shear rates. The viscosity difference between the two is huge. What has this have to do with our non-uniform multicavity parts?

 

为了部分说明这个问题,让我们看一眼材料,特别是树脂的剪切敏感性。图1显示了一个典型的在线性坐标系中的粘度-剪切速率曲线。考虑充填时间以提供注射速率的信息。几乎所有的塑料都显示出剪切粘度随剪切速率变化的关系。在较长的充填时间中和在高的剪切速率下,树脂更“倔强”一些,而在较短的充填时间下和较高的剪切速率下,树脂流动更容易。两者之间的粘度差别是巨大的。这与非均衡多模腔部件有关吗?

[此贴子已经被作者于2010-4-9 18:02:37编辑过]
7楼
上海北京顺德 发表于:2010/4/9 18:25:00

Let’s look at our two cavities during filling and packing, with this viscosity curve data to help us “see” from the plastic’s perspective. During filling of each cavity, the shear rate is high, viscosity is low (easy flowing), and all is fine. However, if we freeze-frame the situation at the instant of switchover from first to second stage (packing), we see cavity “A” is full and cavity “B” is short. (For the sake of this article, we’re ignoring the many possible reasons why.) What is the viscosity of the plastic in cavity “A”? There is little if any flow, shear rate is extremely low, and viscosity is off the top of the graph, perhaps something like taffy. Cavity “B” is short, so plastic is still moving, shear rate is high, and viscosity is low, perhaps like cold molasses.

 

让我们看一下两模腔模具的充填情况,粘度曲线可以帮助我们看到塑料的真实图景。在每个模腔的进料过程中,剪切速率升高,粘度变低(易于流动),这些都是有利的因素。但是,假如我们立即从充填的第一阶段切换到第二阶段(充填)并将这个过程定格,我们就能看到模腔A充满,而模腔B欠注。模腔A的粘度是什么?如果任何流动速度和剪切速率都很低(廖注:似乎为“高”),那么粘度就就很小,粘度就从曲线的顶部掉下来,或许像太妃糖什么的。模腔B欠注,所以塑料还在流动,剪切速率太高(廖注:似乎为“低”),或许像冷的磨拉石。

8楼
上海北京顺德 发表于:2010/4/9 18:28:00

Now we switch to the second stage. Does the hold or packing pressure get delivered to each of the two cavities equally? Think of the viscosity difference and which viscosity will transfer or deliver the packing pressure to which cavity better. Cavity “B” has the advantage of the lower viscosity and will pack better. Cavity “A,” with taffy-like high viscosity, will be sinky, too stiff to pack. Perhaps Fig. 2 helps to explain the issue. We add more packing pressure and Cavity “B” flashes while Cavity “A” remains sinky.

 

 

9楼
上海北京顺德 发表于:2010/4/9 18:29:00

Now what do we do? Many people start jacking up the clamp pressure or looking for a parting-line problem or other tool rework. But all these are a waste of time; nothing helps. By now the importance of delivering uniform flow to both cavities during the entire filling stage becomes clear. What are the chances of processing around this situation? None! You are fighting Mother Nature, the inherent law of physics that pressure will take the path of least resistance.

 

If this discussion makes sense to you, apply the thought process to molds where you have one or more blocked cavities. Again you are trying to violate a law of plastic flow, and it’s not nice to buck Mother Nature. The only answer is to find and fix the reasons for uneven filling in the first place—a lesson for another day.

 

cavity A
If cavity A fills before cavity B, its melt stops flowing before packing and thus its viscosity is much higher and resistant to packing.

About the Author
John Bozzelli is the founder of Injection Molding Solutions (Scientific Molding) in Midland, Mich., a provider of training and consulting services to injection molders, including LIMS, and other specialties. E-mail john@scientificmolding.com or visit scientificmolding.com.

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[此贴子已经被作者于2010-4-9 18:30:39编辑过]
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