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Creating and Troubleshooting a Plastic Injection Mold Process - Shot Size,  Injection Speeds and Pressures

Plastic injection Molding Process Injection Controls


Once we have all of our temperatures set and coming up to their set points, we can begin setting our various injection controls and their starting points.  We are again approaching this as if we have never shot this injection mold before and are developing the process from scratch.  If you have an established process you would of course make all of these settings according to your injection process or setup sheet which ever you use for this.


Shot size - One of the first things we are going to set in our injection molding controller, is the shot size.  This is one of the most important settings you will make with any “new” injection molding process.  The main reason that this setting is so important is because it has a huge bearing on whether or not you will create a large “blob of flash” or a nearly fully formed plastic part on your first shot.  The reason I say this is that I have seen countless new molding process technicians and engineers make this mistake, and it is often due to the fact that they don’t “understand” this part of the process, and they “guess” at this setting.  Eight out of ten times, the final result is a flashed injection mold and prematurely damaged parting lines.  As you gain experience, you will be able to make better determination of the shot size needed to insure that the first shot is a short shot, but until then, use what ever information you can, to aide you in making the correct decision. 


Take the time to calculate this out if you have the information available for you to do it with, and you’ll avoid having to explain to someone why their brand new $400,000 mold has to return to the tool shop to have the parting lines corrected due to flash problems you have now created. If you have a injection molding machine with a 75 ounce injection unit on it and you are molding a 20 ounce part, you can see the huge potential for a problem.  It’s always better to err on the extremely short side, that to over fill and flash the part.  The risk to an extreme short shot in some molds, is that the part might stick in the cover when it opens, requiring you to remove it, which under certain conditions can be very tedious and risky to the mold.  Always use brass tools when working around mold surfaces for removing stuck parts as the softer brass will not usually damage the tool steel.


Fill speed & pressure – Fill pressure should always be set to the injection molding machines maximum pressure (usually 2000 to 3500 hydraulic PSI or 20000 to 35000 PSI cavity pressure).  In the “old days” as I now fondly refer to them, we use to restrict and try to control the first stage or fill portion of the process with both the pressure and speed controls.  Over the years, with the advent of decoupled molding theories, we all got smarter and learned not to control the pressure but allow the machine to always have it’s maximum injection pressure available to try and avoid “pressure limiting” our process, and simply control the fill speed using the profile or fill speed controls and a proper transfer point.  The trick to being successful with this is to absorb the injection screws inertia after transfer before applying pack or hold pressure to the process.  This is also why the 95 – 99% full rule is so important in this process. 


So to recap, we are going to fill our part with the injection molding machines maximum pressure being applied, and control the rate of fill which we want to be as “fast a we possibly can”, transfer to second stage pressure early enough that the screw will stop on it’s own before bottoming out (inertia absorption), and providing us with a part that is 95 – 99% filled.  Sometimes “as fast a possible” is misconstrued for the “fastest speed the machine is capable of”, but it is more

factor of what the mold runner, gate, and part design is capable of.  “As fast as possible” could be five inches per second, or .5 inches per second depending on those factors just mentioned.  The term “pressure limited” applies when the injection plastic into the mold requires more pressure than the machine is capable of giving.  This needs to be avoided at all costs so as not to limit your process window of control.  Make the proper runner, gating, or wall stock changes as needed to correct this condition or you will have process stability problems until you do.


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Written by: WM8C, August 14th, 2006.  Not for use without written permission




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