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Custom Plastic Injection Molding Process - Basic Defect Identification and Troubleshooting
Some basic plastic injection molding surface defect identification causes and troubleshooting| Types of defects in injection molding - Let's begin by listing some of the common defects, their possible causes, and some things we might be able to do about them. One thing to remember is that as with all injection molding processes and machines, this can vary greatly from part to part, and machine to machine. Now that we have the proverbial disclaimer out of the way, let's begin, starting with the visual types of defects.
Defect - Blush - a dull spot or blemish, most commonly found near a gate or other extreme stress or high sheer areas of the part. It also will often have a "grainy appearance" on a smooth surface part..
Potential causes - melt temperature, barrel temperatures, hot runner temperature, back pressure, mold temperature, fill speeds, valve gate firing position, decompression settings, polish of mold surface, resin build up on mold surface, and a plugged vent.
Potential corrections - Fill speed faster or slower in proper segment, barrel temperatures up or down, hot runner temperatures up or down, back pressure increase or decrease, decompression increase of decrease, if applicable, valve gate firing positions advance or delay, correct mold polish if dull, clean the mold surface if resin or volatiles are present, and open or close the vents as needed.
Simple isn't it?
Defect - Splay or silver streaks, blisters, and bubbles. Found in various locations of any part.
Potential causes - wet material, melt temperature, barrel temperatures, hot runner temperature, throat cooling temperature, back pressure, mold temperature, fill speeds, valve gate firing position, decompression settings, plugged vent, screw RPM's, residence time in barrel, polish of mold surface, contaminated resin, damaged screw, barrel or tip assembly, blocked nozzle tip.
Potential corrections - Fill speed faster or slower in proper segment, barrel temperatures up or down, hot runner temperatures up or down, back pressure increase or decrease, decompression increase of decrease, if applicable, valve gate firing positions advance or delay, correct mold polish if dull, clean the mold surface if resin or volatiles are present, and open or close the vents as needed, check resin for contaminants, check injection unit components for wear, check for blockages of nozzle or tip as valve gate orifices.
Still simple isn't it? You can be a professional molder in no time with these guides I am supplying to you…or can you? Of course you can't. If you look closely at the possible causes and corrections for both defects even though they are very different from each other, you will see many of the same things listed in both areas for both cause and correction. This is where we start separating the knob turners from the rest of the process technicians and troubleshooters.
A good troubleshooter knows that the basic process of injection molding is a sum of it's total parameters and that any one process parameter can affect many of the others. So knowing that, how do we solve an injection molding problem when there are so many options that cause it as well as to fix it?
Part of troubleshooting an injection mold process and a necessary beginning is the learning process, be it college classes, injection molding machine manufacturer training and seminars, on the job training, etc. Without this basic knowledge, it won't matter how many troubleshooting "quick charts" you are given, your chances of being successful are minimal. Take college training if you can, but if that is not an option for you, get training wherever you can find it. There is no better tool in your arsenal than this for deciding where to start. The second tool is experience and time performing the job of troubleshooting, preferably working with an experienced process engineer. Even with formal training, this is extremely important. I have wasted a lot of time on young college graduates that thought they had all the answers who quickly learned differently. Finally yet importantly, and in my mind sometimes the most important, is good old-fashioned "common sense". Without this you can get caught up in the textbook answers and miss the obvious, and that is exactly what many times will happen.
One last suggestion I have is this...if you have the opportunity, spend some time in your maintenance and tooling department learning what makes the injection mold and molding machine tick. I had the opportunity early in my career to spend three years as the plant engineer for a medium sized injection molding company and the knowledge I gleaned on
injection molding machines and auxiliary equipment operation was some of the most valuable learning I ever did. I was also able to spend about 2 years working with the mold makers as a tooling engineer at that same company and it was extremely beneficial to my career. It is so much easier to work with the injection molding process when you have a thorough understanding of all the major factors affecting the process and not just what you learned in books. In the next section, I will present a potential molding problem and we will go through a scenario used to try and solve it.
Written by: WM8C, August 25th, 2006. Not for use without written permission
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