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Question Part of my company's business is the manufacture of blank plastic cards for financial and health care purposes. These cards are laminated under heat and pressure, ten books at a time. A book is formed as follows; A sheet of PVC core material (generally 0.026in thick) is placed between two PVC overlay sheets. This is then placed between two stainless steel transfer plates. Ten of these PVC and transfer plate sandwiches are placed between two .25 inch rubber sheets. Then two heavy gauge steel plates are placed around everything. This book is placed on the platens of the laminator, and cooked with nine other separate books for about a half-hour. Our static problem occurs with debooking. The stainless steel transfer plates will transfer quite a shock - I've been hit with an arc from a book over a foot away. Our operators use antistatic mats and gloves, but its still a bit of a shock. Essentially, they get hit with a low-level shock every time they touch a fresh stainless steel plate - nine times per book. From what little I know about ESD, I'm guessing that the static is triboelectric, generated by the PVC rubbing together under heat and pressure during lamination. The static charge is partially transferred from the plastic to the steel, which conducts the charge to the operator. I'm wondering if it is possible to actively remove the charge from the surface of the plastic sheets through the stainless steel transfer plates. While I have been unable to determine this, I am guessing that the charge on the plastic is negative. Could I create a temporary charge to the stainless steel transfer plates and draw the negative charge away from the surface of the plastic, eliminating, or at least reducing, the static? Thank you for your help. - Tim, Rocky Mount, VA
Answer Your problem sounds like it could be stemming from a combination of either a charge build-up from the thermo-electrochemical heating and compression process or triboelectric generation from the debooking as you stated. What you have described (the books) sounds like a HUGE capacitor. When you create more than one layer of alternating conductive and insulative (or even highly resistive) materials, you are in essence creating a capacitor. For an ESD "arc" to hit you 1 foot away, the electric field between the books must be huge, having a voltage well over 300,000 volts and probably approaching 600,000 volts at 25 degrees C at 760 mmHg. We would advise that you practice grounding. Ground all the steel plates to a common point, the same potential that your operators are grounded to. This will eliminate the huge potential differences between the operators and the steel plates of the books. You should further measure the electric fields, using a field meter, during the debooking process to determine if charges are being generated in lieu of grounding. If this is the case, you may need to further employ forced-air ionization over the books to neutralize these charges on the PVC.
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