Plastics Technology

MAY 2012

Plastics Technology - Dedicated to improving Plastics Processing.

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know-how injection molding john bozzelli All About 'Cushion' Many believe injection molding is more art than science. I don't have all the answers but have spent many years working to replace as much of the art as possible by using scientific principles. Perhaps one of the reasons the term "art" pops up is the confusion in our industry about terminology. To date, there is no accepted standard for the terms we use to describe process- ing and part problems. For example, what is the difference between a weld line, flow line, knit line, meld line, and … you get the picture. How can we get to the science when many of us are using different names for the same issue, and sometimes the same name for different issues? My goal for today is to pick one term: Cushion. I'll tell you what it means and why it's important. There are two definitions (see what I mean?) for cushion in molding: 1) the position of the screw when the hold or second- stage timer ends; 2) the most forward position the screw reaches during injection (first or second stage). Sometimes this is labeled "cushion minimum." While these definitions are not the same, the values they produce may or may not be, depending on the mold, machine, or process. Confused? To duplicate and document a process cor- rectly, the molder needs to know the difference and understand what the machine is reporting. Let's use an example to explain. Suppose that during first-stage injection, the screw reaches 6 mm from the forward zero position— the transfer point from first to second stage. Thus, 6 mm is the stroke transfer position (velocity to pressure control, or V to P). And supposed that there is enough hold pressure so there is no bounce- back of the screw and it continues forward to pack the part. At the end of the second stage (pack and hold), the screw is at 3 mm, which makes this both the position of the screw at the end of second stage and the screw's most forward position. In this scenario both defini- tions of cushion would result in the same 3-mm value. In another scenario, let's say the screw reaches the 6-mm stroke transfer position during the first stage, yet bounces back slightly due to a required low hold pressure to minimize over- packing at the gate. (For some parts screw bounceback is accept- able as long as the flow front does not hesitate in filling the cavity. Plastic is compressible.) Here, the position of the screw at 28 MAY 2012 PLASTICS TECHNOLOGY the end of hold is at 8.5 mm. Some machines will report a cush- ion of 6 mm, and the screw position at the end of hold as 8.5 mm. Another machine would simply report 8.5 mm as the cush- ion, and yet another might report cushion as 6 mm. My prefer- ence is to see both reported. This is not a case where one ma- chine is better than another, just that there are two definitions being used and you need to know what the numbers mean. Which definition is right or best? That's not my call. Better check out what your machines are reporting to you. Is your documentation correct? WHAT'S ACCEPTABLE? Now that the definitions are established, what should the cushion be and what is an acceptable range? My bet is that if we sampled 50 molders we would have lots of different responses. To start answering these questions, let's state the purpose of a cushion: to provide a pressure pathway to enable packing out the part. To properly pack out a part, there has to be plastic in front of the screw/non-return valve (check ring) to provide a means to transmit plastic pressure through the nozzle, sprue, runner, and gate(s) to the part. There are processes such as gas assist that do not require the injection unit to pressurize (pack out) the cavity, but for our discussion, we will stay with conventional injection molding. On a hydraulic machine you can program hold pressure: For

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