CD Production has come a long way since then. Advances in injection molding machines have shortened the cycle to 3 to 4 seconds, and a manufacturing system was developed to increase production to greater than 90 percent. Yet, while many molders have kept up with molding machine technology, some have overlooked another important element in CD production: the dryer.

• Initial thinking was that a clean production room was necessary, similar to the
production of microchips and processors.
 
• One central sputter (for cutting the CDs with aluminum), historically the most
expensive equipment for the manufacture of CDs, was located in a separate clean
area, requiring that racks of raw CDs be moved from the presses to the sputtering
area. Transporting the CDs increased the potential for contamination, contributing
to higher reject rates.
 
• Multiple presses were also serviced by a single central dryer and conveying
system. Contamination issues prevented the installation of a dehumidified air dryer
in the cleanroon.

With today's production lines, a full cleanroom operation is no longer necessary, and robots move the molded CDs to sputter systems press-side within a contained clean area. Add to this quality improvements in polycarbonate, and you have significantly reduced cycles times, says Rajkovich.

However, as production speed increased, the hopper dryer that previously was perfectly sized to site on the press's feedthroat was no longer large enough to contain the volume of resin needed. Dryers were moved press-side, requiring a system to convey the material to the feedthroat.

Drying Optical PC and PMMA
 
To keep up with increased CD production, dryer manufacturers have taken steps to ensure their equipment can accommodate high yields. Rajkovich says that to achieve the highest yields, processors should take note of the following considerations when drying optical PC and PMMA:
 
• Avoid contamination from dust particles in the environment by properly handling
large bags of resin and maintaining filters regularly. Large bags should be emptied
in a near-dust-free area, preferably in an air-conditioned room. Air filters should be
cleaned frequently and replaced every six to twelve months.
 
• Avoid contamination from particles generated from drying units (black spots from
electrical heaters, dust from the desiccant agent).
 
• Avoid contamination from PC or PMMA dust generated during the resin's transport.
Keep conveying distances short to reduce contamination from friction-generated
dust. Dust generated from the friction of conveyed resin can result in burning in the
injection unit and produce black spots on the disks. 

• Long conveying distances also present the risk of the material remoisturizing or
cooling down before processing. Final moisture content of the granule should be
less than .005 percent. Moisture problems can also be the result of postdrying
activity such as a leak in the conveying system (usually at the inlet of the press), in
the cooling system of the feedthroat, or in the mold system. 

• For optimum yield conditions, the dryer should be located next to the injection
molding machine in an air conditioned room. - Amiee Chitwood

PC shipments: Trucks or bags?
 
Although there may not be a large price advantage to purchasing polycarbonate via a truck instead of bags, Rajkovich says the handling savings for the molder can be substantial. Benefits include costs saved in forklift use and spillage from damaged packaging, no packaging material to recycle, and less required storage room. To store resin delivered by a truck, stainless steel silos with proper ventilation should be used. This ventilation system requires filters to avoid contamination. Rajkovich recommends supplying dried air to the silo to avoid high initial moisture of the material and exercising caution when extending the flange connection during the filling and emptying of the silo.

November 2001 issue of Injection Molding 
Reprinted with permission from Injection Molding Magazine; 
Copyright © 2001 by Canon Communications.