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intern summary	printable PDF

Examining Root Causes at X-Cel Optical

Mechanical Fixture Eliminates Need for Solvent Cleaning

X-Cel Optical manufactures bi-focal and tri-focal eye glass lenses by fusing a multi-focal glass button to a larger glass lens and then grinding the assembly to a curvature and lens power in the range of 0 to 16 diopters.

Buttons are made by fusing together two or three glass segments of different refractive indices. The segments' surfaces must be ground flat to fine tolerances before joining. For this grinding, up to 400 segments of similar dimensions were placed on a flat plate and molten wax was poured over them. The wax solidified when cooled, affixing the segments to each other and to the plate. The waxed plate assembly was inverted and pressed down on the cast iron lap which was dusted with abrasive emery powder for grinding. Once ground, the wax was melted to release the segments. The day's lenses were cleaned in a trichloroethylene (TCE) vapor degreaser, using a three-hour cleaning cycle to remove wax residue before the segments were fused. In 1995, X-Cel used over 10,000 pounds of TCE for producing multi-focal buttons.

Incentives for Change
To comply with the vapor degreaser National Emission Standard for Hazardous Air Pollutants (NESHAP) rule by late 1997, X-Cel would have had to upgrade or replace the degreaser, spending nearly $50,000. The NESHAP was intended to reduce toxin exposure for populations near industries.

The market for glass lenses was shrinking, pushing the need for increased production flexibility with smaller batch sizes. But, X-Cel's degreaser and grinding operations worked best with large batch sizes.

Mechanical Blocking Replaces Need for TCE
Instead of looking at alternative cleaners to reduce its use of TCE, X-Cel looked at the root of its problem—the fixture wax. Fixture wax is very difficult to remove. The company decided to explore mechanical blocking to eliminate the wax.

Previous attempts to use a vise-like mechanical fixture to hold lens segments had been promising. But, loading and unloading the fixtures was slow and costly; their capacity was limited; and chipping and poor tolerance control for the ground surfaces caused high product loss. Segments also had to be cleaned individually. Adding separators between each segment nearly eliminated chipping, but the fixtures were limited to holding only 10 lens segments.

A MnTAP intern took the latest design prototype and addressed the remaining drawbacks. To reduce chipping, Teflon® dividers were placed between segments as cushions. A steel support bar was placed in the middle, allowing up to nine segments to be grouped together. Both elements reduced bridging­—the tendency for columns of lens segments to buckle up in the center when pressure is applied at the column ends.

To reduce cycle time, a larger fixture was built so more segments could be processed at a time. A middle support bar doubled fixture capacity. Teflon® separators permanently attached to the fixture eliminated much of the extra handling required by the previous separator designs. Clamping bars running the length of the fixture required just three clamping screws on each side to apply pressure, rather than one screw for each of the five columns of segments. The fixture was able to hold 90 lens segments.

The intern also improved loading procedures. A comb-like device was designed to separate the Teflon dividers so that five segments could be placed in a row at one time rather than handling each segment separately. Cordless screwdrivers are used to speed loading and unloading, and to tighten the loaded fixture consistently without over-tightening. Later, X-Cel staff increased the fixture's rigidity by adding stiffening bars under the guide rods.

X-Cel staff replaced the three-hour vapor degreasing cycle with a new 10-minute cleaning cycle. The new cycle consists of an ultrasonic dip using tap water, a de-ionized (DI) dip rinse, a DI water spray rinse cascaded to reduce water and hot air dry.

X-Cel Optical’s operations required two fixture sizes, one for holding most button segments and a shallower fixture to handle trifocal segments.

Capital Costs
The total capital cost was $0.40 per 100 pair of lenses made, based on a 10-year expected service life. Grinding machinery, 10 fixtures, and the washer and drier plus their installation each constituted about one third of the change-over cost. All of the equipment was either constructed by X-Cel staff or purchased used and significantly modified.

Operating Costs
Button grinding using the mechanical fixture requires 50 percent more labor than wax blocking due to smaller batch sizes and increased handling—from loading the fixture through grinding, cleaning and unloading. Although labor is higher for using the mechanical fixture, wax blocking has higher material cost. In total, mechanical blocking reduces operating costs by $0.12 per 100 pair.

Costs (per 100 pair)
	Wax	Mechanical
Button grinding labor	2.54	3.95
Emery	0.44	0
Rosin	0.32	0
Wax	0.12	0
TCE	0.47	0
Respirator cartridges	0.13	0
Total materials*	1.48	0.00
TCE	0.08	0
Wax	0.01	0
Total disposal	0.09	0.00
Gas (wax burners)	0.01	0
DI water	0	0.05
Utility total	0.01	0.05
Total operating costs	$4.12	$4.00
* Iron oxide costs were unchanged and are not included. The diamond lap tool has roughly the same costs and life as the previously used cast iron laps.

Other Benefits
The market for glass lenses has declined and continues to decline. Mechanical blocking is one cost reduction option that helped X-Cel maintain this product offering.

Smaller batch sizes can be run efficiently with the mechanical fixture. In 1995, a single batch took three days to move through the grinding department. The mechanical fixture allows it to be done in two hours. Special orders can be completed in days rather than weeks. The grinding department has less work-in-process waiting to be batched and processed.

Using the mechanical fixture, X-Cel Optical was able to convert their emery grinding process to diamond grinding. Diamond grinding is faster, cleaner and produces less sludge. When grinding with emery, the emery powder makes the area dirty and enters the sludge along with the glass fines. Diamond lapping was not possible with wax blocking because the wax would gum up the abrasive.

The MnTAP intern carried out parallel work at the St. Cloud Vision-Ease Lens plant. Production of multi-focal lens segments was switched over to a mechanical blocking system for the same reasons. The fixture held only 60 segments due to constraints of the grinding process. It also used two moveable, steel support bars rather than a single stationary support bar and a single fixture height. Vision-Ease accommodated tri-focal segments by placing a thin plate under the center of the fixture to raise the tri-focal segments. Costs and other details were not looked at in depth because the glass, multi-focal button production operations at the St. Cloud plant were shut down due to decreased demand shortly after the conversion was accomplished.

Root Cause Solutions
X-Cel Optical and Vision-Ease Lens sought solutions to the root cause of the problem with the vapor degreasers. The companies did not look narrowly at alternative cleaners and cleaning processes as a way to eliminate the use of the toxic chemical TCE. Both companies looked at the soil on the parts and questioned why it was there. Both decided that changing the soil to make cleaning easier was simpler than trying to find a different, aggressive cleaning method to remove the wax.

More Information
MnTAP has a variety of technical assistance services available to help Minnesota businesses implement industry-tailored solutions that maximize resource efficiency, prevent pollution, increase energy efficiency, and reduce costs.Our information resources are available online. Or, call MnTAP at 612.624.1300 or 800.247.0015 from greater Minnesota for personal assistance or more information about MnTAP’s Intern Program.

This project was conducted in 1995-1996 by MnTAP intern, Linda Gerard a physical science junior at the College of St. Benedict.

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