Plastics Technology: May 2009 Archives
One of our more advanced customers brought to us this observation...
Jim M.
It is interesting that this test does not utilize the pack & hold portion of the cycle; just the fill portion. Experience shows, especially with high 2nd stage pressures, that the screw will drift, and some reflection of that would be a good indication of check ring holding. Your thoughts...?
Our Response
Every check ring will leak and drift. What is most important in maintaining a consistent fill is the repeatability of this leak. The Dynamic Check Ring Repeatability Test is intended to measure this variability in this leak during fill. In most cases, if the drift is variable during 2nd stage, it will also be variable during fill. Although each process is different... if the screw does not bottom out, and cushion varies less than 5% of the overall shot size from shot to shot, usually the amount of drift is not a major factor.
Additional Thoughts
It is always great to hear someone asking such highly technical questions because it means they are focusing on the finer points of processing rather than subjects of general knowledge.
-Andy
A good friend of mine just asked the following question...
Ron Z.
Is there an industry standard for how much torque a set up person will use to clamp a mold? I know there are specs for the bolt used but I think they're too high. We use multiple clamps and multiple bolts per mold so the load is distributed all around. Our smallest machine is 55 tons and our biggest one is 500 tons. If you know of any standard, please let me know. Thanks in advance.
Our Response
For your molds, we typically recommend around 50-60 ft-lbs... If the diameter of the platen bolts is larger on your biggest machines, you may be able to go a little higher. The two main factors would be the weight of the mold, and the condition of the platen... many industry people would recommend generic values around 80-120 but they usually assume ideal situations.
Additional Thoughts
Keep in mind, the clamp functions by stretching a bolt to apply a bending force to the clamp which is held back by the platen threads. As a result, the more torque you use, the quicker one of these will fail (this failure is almost always found in the platen threads). This why we recommend a conservative torque value with additional clamps used when necessary.
It is always a good idea to have a specific torque value to be used at your facility. Much time can be lost when a platen bolt hole becomes stripped and needs repair.
-Andy
