<P>Overpacking and underpacking
An overpacked cavity pressure curve is shown in Figure 10b. It is characterized by a pressure peak in the packing phase. The pressure peak is </P>
<P>caused by the delay in switchover to holding pressure, so the high injection pressure is still applied after volumetric filling. The pressure </P>
<P>peak is relieved at the switchover to the lower holding pressure. Here lies an often overlooked cause of flashing which is easily detected if </P>
<P>one has cavity pressure sensing.</P>
<P>The momentary cavity pressure peak could produce a momentary Fc big enough to reduce the residual clamping force Fr to zero, causing the mould </P>
<P>to open and the part to flash. To remove the flashing, the straight forward thinking would be to increase the clamping force. Reaching the </P>
<P>limit of the machine rated clamping force, one would even move the mould to a bigger machine. Even if the increased clamping force overcomes </P>
<P>flashing, overpacking adds weight and stress to the part and makes the part more difficult to demould. An alternative is to reduce the </P>
<P>injection pressure. Too low an injection pressure causes defects such as sink marks. In actuality, the problem is easily and better solved by </P>
<P>switching over earlier to get back to the ideal cavity pressure curve. In precision injection moulding, overpacking creates a reject.</P>
<P>An underpacked curve is shown in Figure 10a. It is characterized by a pressure dip in the packing phase. The switchover occurs too early, </P>
<P>before the cavity is volumetrically filled. Part of the filling takes place at the lower holding pressure. Subsequently, the screw advance </P>
<P>increases the pressure. The part has reduced dimensions, is underweight, has sink marks and surface marks. It is again a reject.</P>
<P>A device that switches over at volumetric filling would avoid the problems of overpacking and underpacking and produces the ideal cavity </P>
<P>pressure curve. Switching is initiated at point 3 and completed in point 4 in Figure 9.</P>
<P>[img]NewUploadFile/2005-1/200512417328718.gif[/img]
Figure 10. Underpacked and overpacked cavity curves </P>
<P>翻译如下：</P>
<P>
过充填和充填不足
图10B显示的是过充填的模腔压力曲线，他的特征就是在充填阶段有个压力峰值。原因是转换成保压延迟了（没有及时转换成保压），因此在充填到设定（理想）体积之后高注</P>
<P>射压力仍在起作用。压力峰值在转换成保压之后会减小。这个地方在查找毛边产生的原因的时候很容易被忽略掉，如果安装了压力传感器，就很容易被探测到（通过显示器里面</P>
<P>的实际压力和设定压力曲线图）。</P>
<P>瞬间的压力峰值能够产生一个瞬间的Fc(一种力，当作冲击力吧)，这个力大的可以将缩紧力减为0，导致模具打开（一个缝隙，打开了就没有这么大的压力了），零件产生毛边</P>
<P>。要去掉毛边，最直接的想法就是增加锁模力。当（设定锁模力）达到机台的额定锁模力的最大值是，应该把模具移到大一点的注射机。即使增加的锁模力克服了毛边这一缺陷</P>
<P>，过充填会增加产品重量和内应力，并且让产品很难脱模。另外一个可以选用的办法是降低注射压力。太低的注射压力导致缩水等缺陷。实际上这个问题可以通过早点进行（速</P>
<P>度压力）转化，使实际曲线更接近理想曲线，而很简单很好的解决。在精密注射中，过充填将会产生不合格品。</P>
<P>图10a显示的是充填不足的曲线。他的特征就是在充填阶段有个压力谷值，在模腔还没有注射到设定体积就过早转化保压，发生在过低保压时。然后，螺杆前进增加压力。产品</P>
<P>的尺寸会偏小，重量不足，有缩水及表面（凹）痕。仍然是不良品。
设备在充填完成时的转换设定时，应该避免过充填和充填不足的问题，做出接近理想模腔曲线的压力曲线。转化在图9的第三点开始第四点完成。</P>
