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Creating and Troubleshooting a Plastic Injection Molding Process - The "Art of Injection Molding vs. Scientific Molding" - Part 2

Custom Plastic Injection Molding - Scientific or Decoupled Molding - Scientific or Decoupled molding employs some simple facts that in my earlier days escaped many of us.  The main purpose as with all other methods of plastic injection molding processing was to provide top notch quality products to your customer every time, with a process that was very consistent and repeatable.  This needed to be the case even with older injection molding machines or moving between presses of different sizes and capabilities.  What made this possible was taking the injection molding machine out of the injection process picture and replacing that with the idea of controlling the “variables of the process” with little regard as to the actual injection molding machine settings.  Now, maybe you can’t appreciate the revelation that this was to us at the time, but I can tell you it was very hard to swallow in the beginning that we might have been going about it all wrong all that time.  The concept of decoupled or scientific molding fairly easy to grasp once you let go of the old ways you were taught, and if your new to the industry you’ll have no problems what so ever. 

 

In it’s simplest form, it is the process of controlling the process variables instead of the machine.  Since we are after a few key variables to control instead of sometimes hundreds of machine settings, we already made our task much simpler.  There are a few things we need to know about our injection molding machine(s) to make this possible.  One of the first is the intensification ratio of our molding machine to determine proper plastic pressure in our process.  This is calculated by knowing the intensification ratio of the screw which is the surface area of the injection piston divided by the surface area of the screw.  This data is often supplied by the manufacturer and is sometimes noted on a plate on the injection unit or in your injection molding machine operation manual.  If you have a machine with a maximum hydraulic pressure of 2500 PSI and an intensification ratio of 11:1, this tells you that you have a peak plastic melt pressure of approximately of 27,500 PSI.  Or if your machine has maximum hydraulic pressure of 2000 PSI and an intensification ratio of 7.7, your maximum plastic melt pressure is approximately 15,400 PSI.  Where this comes into play is in the scenario of the latter calculation we just made and we have a part that requires 18,000 plastic melt PSI, we could very likely have an issue with molding this part without tooling modifications.  We would be in essence “pressure limited” which is a condition we always want to avoid in the “scientific molding realm of things.

 

So how do we manage a “scientific process” as compared to the “black art of molding” processes we used to?  What we are going to do is establish some key variables and monitor them instead of a bunch of injection molding machine control or knob settings.  This will be after the point of establishing our injection molding process using the techniques discussed here to fore, and having qualified our process.  A very simple starting point might be the following items: fill time, cycle time, fill weight, and part weight.  The fill time is a good measure of the plastic and injection unit performance against your established standards.  Cycle time is important for maintaining the cost performance of your job.  The fill weight is a great measure of your shot size consistency and will tell you that you are maintaining your 95 – 99% fill point.  Your fill weight is easily verified or checked by turning off your hold and pack pressures for a shot, and then measuring that weight against a previously established standard fill weight.  You should be within approximately 1% total weight of each shot taken.  The part weight is a measure of your pack and hold pressure and time set up.

 

As you can see, just by measuring these 4 part variables, we have a basic understanding of how our injection molding machines major functions are performing.  The fill time tells us that our velocity and the injection pressure profile are working as they should, the fill weight will tell us that the transfer position and back pressure profiles are remaining consistent, and as mentioned in the previous paragraph, the part weight lets us see that the pack and hold phase is also

performing as requested.  The cycle time check is a good look at the entire process including any clamp and ejector speeds and pressures, part removal times, etc.  You can add as many variables as your desire for your given process but keep the KISS (keep it simple stupid) principle in mind as you do this so that you are concentrating on the important aspects of your injection molding process.

 

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

 

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