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Creating and Troubleshooting a Plastic Injection Molding Process - Pack & Hold Pressures


Plastic injection Molding Process - Pack & Hold Pressure and Time – Phrasing Reference – I will be referring to the “set point hydraulic pressure” when I am referring to PSI in this section.  Once we have properly established our 95 – 99% fill point of our injection molding process, we can set our pack and hold pressures and timers.  When making the very first shots in a new tool, you will probably want to set these parameters somewhere around 50 to 100 PSI to start with.  This will help protect the mold from over packing and flashed parting lines until you have established some basic process parameters, but also help avoid  “bouncing of the injection screw” that can occur when “zero” pressure is applied at the transfer point from first to second stage pressure.  Once again, if the full inertia of the screw was absorbed as it should be at this point, issues with the injection screw bouncing back after transfer should be minimal to begin with. 


If your press is so equipped, and you are confident that you will not flash out the tool, you can begin adding your pack pressure to your process.  I would work in 100 and not more than 200 PSI increments at this point.  The goal you will have at this point will be to as the term indicates, “pack out your part”.  You want to use this phase of the process to finish filling   the remaining 1 to 5% of the part from the 1st stage portion of your process.  If your machine is not equipped with a separate “pack” function, you can use the 1st step of your injection hold pressure settings to accomplish the same task.  In this phase of the injection molding process, you are attempting to bring the part to the peak cavity or plastic pressure you will need to produce a fully filled out, dimensionally correct, sink, & flash free part.  Depending on the type of resin used, this could be as little as a 100 PSI, or as much as 2500 PSI depending on your needs.  Pack time, will typically be somewhere between .5 and 2 seconds, but again will vary with the resin and part design used. 


Once the pack phase is complete, you will switch to hold pressure.  The purpose of the injection hold pressure phase of the process is to maintain the plastic pressure inside of the cavity until the gate has frozen off, which then acts as a barrier to any further pressure loss in the cavity.  The amount of hold pressure needs to be just high enough to maintain the cavity pressure during this time, without any loss of pressure or discharge of plastic resin back out of the gate.  Assuming the part has been designed correctly, this should give you a part of acceptable quality.  Hold “time” is established through use of a gate seal study.  What this is a simple test to determine when the gate is frozen, thus reducing any loss of time due to and unnecessarily long hold period.  The way to establish this point is done simply with a scale and can be hand plotted or loaded into a spread sheet to produce a graph if you desire. 


You will only need two columns for your study.  The first column, you will label as “time”.  The second column will be labeled as “weight”.  Start with a time where you know the gate is not yet frozen.  Let’s say for example we are estimating our hold time to be 20 seconds to the gate freeze point.  In your first column under time, write in the number 10; next line 11, and so on until you reach 25.  Now as you run your injection molding process, start with the 10 second hold time setting, and when the part is produced, weigh it and put the weight in the weight column on the line where you entered the 10.  Repeat this for every one second interval up to 20 (see example 1).  You should see the point where the weight stops increasing, and this will be the gate freeze point, and thus, there is no need to maintain hold  pressure beyond that point

Example 1

as it will just be wasted energy and cycle time , and cycle time equals “money”!  If  by the time you have reached the last point (20 seconds in our example) and the weight has not stopped increasing, continue until you get at least two consecutive weight readings, with little or no change.  You will now be able to begin your screw rotate and cooling phase at this point. 


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Written by: WM8C, August 17th, 2006.  Not for use without written permission




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