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A Basic Custom Plastic Injection Molding Machine Process


Barrel, Screw, & Barrel Heaters


The Screw and Barrel

The reciprocating screw in injection molding or just “screw” resides inside the barrel.  It is made up of two components which are the injection molding screw and screw tip assembly.  It serves two purposes, the first being to convey material forward in the barrel to the front of the injection unit for each new shot, and two, inject the plastic under high pressure into the mold.  It is very important that these components are inspected for wear at least yearly as they are very high tolerance components and very critical to the injection molding process.  One way you measure your screw and barrel performance is to record your recovery time and screw RPM’s, when the screw and barrel are both new, and also note which resin that measurement was performed with.  Then at 6 month intervals, check your recovery times, and make sure that they remaining relatively the same.  If you start noticing a longer recovery time (20% or more), there is a chance that you are starting to get some wear.  The only way to know that for sure is to remove the screw from the barrel, inspect them both and measure them with micrometers and compare the size to the print.  The manufacturer can give you the recommended maximum where for your particular screw and barrel but usually it’s around .006 - .010 of an inch.  There are three main zones to most standard injection molding screws and these are the feed, transition, and metering zones. You can also get specialty injection molding screws with mixing sections or other sections built to your own specifications as needed.  Mixing or barrier wave injection molding screws would be an example of a different flight options for a screw.  The screw, during the rotation phase, can also provide heat to the melting process through friction, improving the quality of your melt.  The injection pressure applied by the screw is the heart of your process.


Barrel Heaters

The purpose of the barrel heaters is somewhat self-explanatory, and that is to “heat the barrel”.  The reason we need to do this is provide an environment in which to begin the plastic processing portion of our cycle.  It is imperative that your barrel temps are up to your “set temperature” for the type of resin you are using, before turning the screw.  You will also want to add some “soak time” to this to be sure that all of the solidified resin in the barrel and on the screw has fully softened or melted.  The result of not doing this is usually a broken screw tip assembly or in some extreme cases, even a snapped of screw.  You'll notice I don’t mention how much time your soak time will need to be, and that is because it will vary by the material you are using and the efficiency of your barrel heaters.  Normally, at least a half hour of “soak time” is recommended for most plastics and this is commonly a standard feature on most machines made in the last 10 -15

years.  If you have the ability on your machine to “bypass” this part of the process, my recommendation is don’t and that is from experience with seeing the results of people who have tried that and failed.  At upwards of $1500 for a screw tip assembly and upwards of $15,000 or more depending on the size of your screw, you don’t want to have to explain to your manager how that happened to you.


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Written by: WM8C, July 28th, 2006.  Not for use without written permission






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