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Creating and Troubleshooting a Plastic Injection Molding Process - Screw Rotate Time, Back Pressure & Cooling Time


Custom Plastic Injection Molding - Screw Rotate, Back Pressure & Cooling Time – Now that we have a basic understanding on how to setup the injection, pack, and hold profiles, it is time to set up our injection molding process, screw recovery time and pressures.  There are two factors controlling or affecting this screw rotate time, the first of which is the injection screw speed control, which controls the actual RPM’s of the injection screw. The second part of this control is the back pressure set point, which is pressure that is applied as “counter pressure or force” to the back on the injection screw itself, during the entire screw rotate cycle. 


The manufacturer of your machine will, once again determine the actual RPM’s that a given machine is capable of.  Typically, the smaller machines are going to have a higher RPM rate capability than their larger counterparts.  Some of the reason for the faster screw speeds on smaller machines are the faster cycle time requirements, that are very common on smaller tonnage injection molding machines.  Another is just the sheer amount of torque required to turn an 8 oz. injection screw is considerably less than the torque required to turn the screw on a 300 oz. injection screw.  Screw speed settings on most injection molding machines are usually a percentage set point between 0 and 99%.  The set point for this should be determined by the speed required to get the screw back somewhere in the neighborhood of 2 – 4 seconds before the “cooling or mold closed” portion of the cycle ends.  This prevents excessive soaking and idle heat exposure time for the plastic resin, which reduces the risk of having burnt resin developing in your melt stream.  It also helps increase the life of the injection molding screw and barrel surfaces, as the slower the RPM’s used are, the slower the wear is that results from performing this function.


Back pressure is a tool to help provide a consistent and homogenized melt stream.  If you applied zero back pressure to your screw during the process, the movement of plastic down the screw flights during rotation would create an uncontrolled and inconsistent shot density during each cycle.  This would contribute to shot-to-shot variations in the entire molding process, because of the direct effect it has on shot size consistency.  Back pressure also adds a frictional component to the melting process, taking some amount of the load off the barrel heaters as well.  These settings will often be a low as 50 PSI and as high as 500 PSI, with 100 to 200 PSI being somewhere near the “common middle” of the range.  The use of back pressure is vital to our melt consistency, and also to the removal or prevention of cold pellets in the melt stream.  It improves the consistency and homogenization of the melt, and insures that shot after shot it will be the same.  If you are mixing colors into your plastic, it can also aide in the mixing process.


Cooling time is a little more difficult to establish.  We could write an entire article on the calculations for determining what the expected cooling time will be for a given injection molded part will be.  Plastic wall stock thickness, part geometries, dimensional requirements, the thermal conductivity of your mold, mold cooling temperatures, and more, all play a role in the determination of this time.  In order to maintain the simplicity of the processing information I have been providing, we will look at this a simpler way.  Cooling time is the amount of time it takes to bring the plastic back from a molten state, to a solid state.  A little trial and error will give you some good starting points for this.  If your mold has sprues and runners, they could determine this time more so than the parts, as the material must be solid enough to be removed or ejected from the mold.  I can’t give you any guidelines for this as it will vary greatly form part to part, depending on the factors I have already mentioned. 


Use common sense.  If you are poking holes through your part with ejector pins, the plastic is probably still too soft.  If you start on the high side and work downward in time, you can usually tell when you are getting too short on time as the part will stick, start warping, have severe dimensional issues, develop excessive sinks over thick areas, and other similar defects.  The goal is still to reduce this time to as little as possible and still produce an “acceptable quality product”.  Faster cycle times mean more money in the till at the end of the production day!


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Written by: WM8C, August 18th, 2006.  Not for use without written permission


More to come...stay tuned


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