For computer monitors, thin is in

DN Staff

July 17, 2000

6 Min Read
For computer monitors, thin is in

Seoul, Korea -Big plastic parts. Thin walls. That may sound like a recipe for disaster, but increasingly it's a prescription for cost reduction. Thinwall injection molding, already well-established as a way to reduce the weight and real-estate requirements of small molded parts, has spilled over into large parts as engineers strive to take the cost out of computer, automotive, and appliance components. "Thin is in" because thinner walls use less material. And in the manufacturing equivalent of a double play, less beefy walls also mold faster, cutting piece-part costs even further.

The latest, and perhaps greatest, example of thinwall technology's translation from small to large parts comes in the form of computer monitor housings. Given their size and the price pressures they face on the crowded shelves of computer stores, monitors are an ideal candidate for cost reduction, according to Sang Woo Kim, who leads R & D efforts for Samsung Electronics. "The overall industry trend is to thinner and thinner walls as long as there is no mechanical failure," Kim reports. Samsung recently embraced the trend when it concluded a six-month project to reduce monitor wall thicknesses from its usual 3.2 mm to a comparatively svelte 2.3 mm. Even with the thinner walls, the new housings still attain the required flame rating and fulfill all the company's mechanical performance requirements, Kim says.

Physical properties of plastics for monitor housings

Tensile modulus

Tensile strength @ yield

% Elongation @ break

Flexural strength @ yield

Flexural modulus

Izod impact, notched 73F

HDT 66 psi, 0.125" (F)

HDT 264 psi, 0.125" (F)

Specific gravity

He won't divulge exactly how much Samsung will save per monitor cabinet. But the material and cycle time savings can add up fast, according to GE Plastics Application Engineer Brian Nourse, who has helped Samsung and other GE customers develop new thinwall monitor designs. In a case study provided by Nourse, another monitor company stands to reduce its material costs by more than 5% by dropping wall thickness from 0.125 to 0.08 inches. Add in the processing advantages and the company's overall part cost for this developmental monitor cabinet could drop almost 10%.

Reaping the manufacturing advantages of thinwall monitors doesn't come easy. Thin walls or not, the monitor cabinets have to possess enough mechanical strength to survive impact tests that simulate the monitor's rough-and-tumble journey from factory to desktop. Nourse notes that these UL 1950 impact tests typically involve dropping a 500g ball onto the housing surfaces from a distance of 1.3m. For another, monitor housings must also achieve a UL 94 flame rating of 5 VB-a burn test made all the more difficult with less material in the walls.

Samsung's push to thinner monitor walls called for a rethinking of the housing design and materials. On the design side, the thinwall monitors did require some rib-and-boss re-engineering as well as a new design for the vents on top of the monitor. "We had expected a more remarkable warpage and shrinkage on top of low mechanical performance of the finished part," Kim recalls. None of that downside came to pass, however, once the size and location of these internal structural elements had been optimized with the help of CAE and physical tests performed in conjunction with GE.

Monitor case study

Material

Wall thickness

Weight

Material cost/lb

Material cost/part

Processing

Cycle time

Number of cavities

Machine cost/hour

Processing/part

Tooling

Parts/year

Number of tools req'd

Cost/tool

Total cost of tools

Tool amort. 1 year

Total cost of cabinet

Savings/part

Savings/year

For a thinwall material, the company moved from a flame-retardant ABS to a brand new PC/ABS formulation-Cycoloy CU6800 from GE. According to Kim, the material features non-halogenated flame retardant for worldwide Eco-label compliance and balances two important but conflicting attributes: a long flow length to fill these large parts and impact resistance to meet performance requirements. Monitors may not have the thinnest walls in absolute terms, but they push the thinwall molding envelope from a flow-length standpoint. To put things in perspective, Nourse notes that monitors have a 250:1 flow-length-to-wall-thickness ratio-far thinner than the 120:1 normally considered to define thinwall parts. As for mechanical properties, Nourse adds that PC/ABS's five-fold elongation advantage over ABS allows the thinner monitor walls to withstand greater sidewall deflection without failure.

That Samsung achieved the 2.3-mm walls on the same molding machines it employs for its conventional housings represents another of the project's "big technical breakthroughs," Kim adds. While specialized high-speed, high-pressure molding machines or advanced processing methods-such as sequential valve gating-can make thinwall molding easier and even enable monitor housings to get down to 2.0 mm, they can also can add expense. "Our main purpose was to get to a 2.3-mm wall thickness with the best economy," explains Kim. To that end, the company's only hardware concessions to thin walls were some tooling modifications-mostly to the ejector system. Kim characterizes the additional tooling costs as "small potatoes compared to the economic advantages of thinwall molding."

The first of Samsung's thinwall monitor housings was slated for commercial production in April. Kim says the company will then apply the concept to other monitor models. "We now know we can produce thinwall products commercially with zero risk," Kim says.


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