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More Frequently Asked Questions:
Source: February, March, and May 2003 issues of Injection Molding Magazine in the "Networking, Questions and Answers" section.
- Hot Shots
- Corroded Feedscrew
- Black Speck Problem
Hot Shots
Q:
We put hot shots on the floor when purging barrels during startup. This can be dangerous if someone steps on them. When the hot shots cool, we are left with cold shots on the floor, making it slick.
Either way, the shots are a nuisance to deal with, but we don't know a good way to handle them in a safe and easy manner. Does anyone have suggestions on how to handle them?
Reprinted from the February 2003 issue of Injection Molding Magazine in the "Networking, Questions and Answers" section.
A:
I suggest running the material into a metal pan, thus eliminating the possibility of somebody stepping on it or the floor becoming slick.
Another option is to actually run the hot shots into a pail of water, which will instantly cool the material and reduce odor.
Finally, I would also encourage all plastics processors to use a commercial purging compound.
Tim Cutler
Dyna-Purge Business Manager
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Corroded Feedscrew
Q:
Recently we came across a feedscrew with internal threads that had completely corroded where the screw tip is installed. What could be the reason?
The materials processed were nylon, Noryl, Arnite, ABS, and PC. What practices could prevent this kind of corrosion on the feedscrew threads?
Reprinted from the March 2003 issue of Injection Molding Magazine in the "Networking, Questions and Answers" section.
A:
In some cases dissimilar metals can be incompatible and subject to reactive corrosion. Is the screw tip the same metal composition as the feedscrew, or at least a nonreactive metal to the feedscrew?
With respect to the polymers mentioned, they are not intrinsically corrosive (unless they are FR grades), but could break down under excessive heat or shear.
As such, they should be purged on a regular basis to minimize their contact with the screw, thus reducing the chance of corrosion.
If a commercial purging compound is currently being used, make sure it is not a chemical compound since some contain potentially corrosive chemical agents.
Tim Cutler
Dyna-Purge Business Manager
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Black Speck Problem
Q:
We are troubleshooting a problem with black specks that appear on an automotive headlight lens molded in polycarbonate.
The specks seem to originate from an accumulation on the screw. What are the requirements to prevent or reduce this oxidation?
What roles do screw design and melt adhesion to screw surface play? Would it help to change the metal selection or use surface treatment?
Reprinted from the May 2003 issue of Injection Molding Magazine in the "Networking, Questions and Answers" section.
A:
Black specking is one of the most aggravating and troublesome problems for processors to deal with.
A common cause is overheating, which can lead to degradation of the material. Resins, when processed, will accumulate in cavities, gaps, and other negative flow areas, such as around the flights on screws.
As the material is exposed to excessive temperatures, including shear heat, it carbonizes and eventually gets pushed out by other processing resins (or a purging compound).
Polycarbonate is typically processed around 520 to 600F. This issue of clarity must also be considered since you can't mask the black specking problem in a clear part.
While reviewing the role of the screw as a possible culprit, I would strongly encourage you to consider the molding process and your preventive maintenance program.
First, I would suggest running the PC at the lowest possible temperature. This minimizes the chance of resin degradation, the source of black specking.
Futhermore, you should institute a comprehensive purging procedure. For example, purging before shutdown and startup can save you thousands of dollars in lost production time and parts.
Tim Cutler
Dyna-Purge Business Manager
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