The Troubleshooter: Sensitive polycarbonate, aggressive chemicals, stressful times
It has been a few days since one of the big brown trucks has pulled up in front of my office. I suspect it is due to everybody getting busy and not having the time to send me parts to work on. It could also be that the molders laid some of their employees off when business slowed down and they just haven't had a chance to call them back yet.
November 2, 2011
I like to stay busy but a break now and then isn't all that bad. I know Jake likes it when we go for a walk in the woods 2 or 3 times a day instead of only once. This time of year his nose is picking up lots of smells that gets his blood flowing.
I am also running low on parts to talk about. If any of you want me to tell your story, please send parts and runners when you can.
This month's part comes to me from Canada and is an excellent example of how sensitive polycarbonate is to some chemicals.
The part is a cover and is center gated. The whole center section of the part has broken out and the molder wanted to know what caused it.
Polycarbonate is a tough material and has the advantage of being able to be molded as a clear part or it can be colored as a tint or even an opaque color. The only down side is that it is terrible with many chemicals. Even liquid soaps can cause polycarbonate to crack and craze in poorly designed sections during a washing operation.
The areas that crack and craze are those where we have sharp corners or sharp edges. Lack of radiuses in these sharp corners and edges is bad for most amorphous materials and a few crystalline materials as well, but polycarbonate is one of the worst.
The molder told me he was aware of the cracking and crazing issues but wasn't sure he knew how to protect against it in all cases. Of course I don't know what all the cases are that he was talking about but I told him I could offer a few suggestions.
First, radius all sharp corners and edges. I have been told that we lose 90% of the impact value in any 90° sharp corners. Why buy an expensive material like polycarbonate so we can have 14 - 18 foot pounds of impact strength in our molded part and then give up 90% of that value to poor design practices?
In the case of this part I can see the "football" shaped area that broke out of the part shows us the problem of unsuspected design problems. The cracking has occurred at the front area of 7 short ribs. The lack of a transition from rib to wall has caused the crack to begin at one rib and then run to the next, and the next, and so on.
Find the stress
I like to use two pieces of polarized film to look for areas of stress concentration that I try to eliminate with slower injection speeds, less hold pressure, higher material heats and often higher mold temperatures. I work through the design problems first and then work on the processing conditions second.
You can find the polarized lenses at most camera stores or scientific web sites on the internet. I probably use these polarized lenses for polycarbonate or acrylic more than any other material, but I also have used them with low-density polyethylene and polyurethanes from time to time. These lenses basically work with any clear or mostly clear material.
When you look at parts through these lenses you will see packing rings around the gate(s) and in areas of thick-to-thin transitions of the part. You will see two components to this viewing. You will see rings around the gate areas that will vary from being close together to being less congested. The color you will see seems to vary from a light blue to a black color; the black means the area is probably over packed and the blue means the part has about the right inject, pack and hold pressures.
So all this means we are looking for areas in our parts that have packing rings that are not so close together, and to stay away from the black colors.
The way I use the polarized lenses is to build a cube that is about 1 foot long on each side. Put a light source such as a fluorescent tube(s) on the bottom of the cube. Put a polarized lens above the light source, and then leave an open space so you can put your molded part in this area for viewing purposes. Then put a second polarized lens on the upper level of the box from where the part will be viewed.
The key to making this work is to be sure the second or upper lens is turned 90° to the lower lens. Put the part in between the two lenses and if you have turned the lenses correctly, you will see the packing rings and color shifts I have described to you in the paragraphs above.
You can also buy these polarized lenses already installed in various types of viewing boxes, but you will pay several hundred dollars for the experience. I have even used polarized fishing glasses and found them to be somewhat useful as a quick check for molded-in stresses.
I used the fishing glasses for a quick check on these parts and found the only stressed areas were those around the center gate and at each and every rib on the part. The outer ring of the part did not show any evidence of molded-in stress, but then again the ring was properly radiused. It always makes me wonder why a mold maker would radius one area of the mold and not the others. Maybe it is because many of the materials we work with are very forgiving in these areas and they have taken our focus off the ball.
Problem solved
I called the molder and passed along my comments and waited for his response. He came back with questions that involved my making a trip out his way to put on a seminar for his whole tool room. I reminded him that this type of education was not a quick fix program that will cure every single problem coming out of his shop. He needed to include the molding technicians, his management group and especially his customers in this type of activity. It takes me a full day to cover all the required bases, and then periodic follow-up sessions for the next 1 to 2 years.
He said he would work with his different departments, get their input and then get back to me with a schedule that will work for him. Then I have to see if I am free on each of those days. It is a lot of work but well worth it.
Jake just poked his head in the door as if to tell me I have been working long enough and needed to take a break so he can go for his afternoon walk. Great dog, perfect timing.
Abouth the author: Consultant Bob Hatch of Bob Hatch & Assoc. has more than 45 years of experience finding solutions to processing challenges. Reach him at [email protected].
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