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Slimmed down part design: thin is in

October 12, 1998

5 Min Read
Slimmed down part design:  thin is in

Reducing wall thickness has always been a prime goal for the consumer electronics industry. It makes sense--putting more chips into a thinner package expands the capabilities of items such as cell phones without increasing their size. In fact, industry watchers have noticed that the trend is migrating lately. More OEMs in a wide range of markets want to take out weight and raw material cost with slimmer-walled designs that maintain stiffness and other performance goals.

Keep in mind that stiffness in plastic parts is directly related to thickness. So how do you thin down without the trade off?

Figure 1. Meeting all the requirements for thin-wall parts often involves the right combination of materials, processing, design, and assembly techniques.

Two GE Plastics managers--Bob Balko and Joe Fouquart--believe they have some answers, and shared them with IMM. Balko, Lexan product manager, explains that when he initially attempted to help customers go thin wall, mold filling and finite-element analysis results didn't appear accurate. "Models used to generate analyses were based on a standard 1/8-inch (roughly 3-mm) thickness," he said, "and they just didn't work at 1 mm." GE collaborated with both C-Mold and Moldflow to develop software that would work on thin walls, then went back and modified material models internally.

The result: a thin-wall design guide that incorporated much of what Balko and Fouquart, who is a technical manager, learned during the project. Kurt Weiss, GE's thin-wall injection molding program leader, believes the knowledge base will help the company drive technology for thin-wall designs, stretching them to new application areas such as CPUs, network interface devices, automotive components, and small appliances.

Ericsson used thin-wall design techniques and a high-flow ABS (Cycoloy C1200HF) from GE Plastics to produce the housing for its newest cellular phone, a mere .04 inch (1.3 mm) thick.

All three experts agree that the term "thin-wall" is relative. For comparison, conventional molding technology refers to applications with wall thicknesses greater than 2 mm. Thin-wall designs are called "advanced" when thicknesses range from 1.2 to 2 mm, and "leading-edge" when the dimension is below 1.2 mm. Both of these latter types, according to the GE team, require high-flow materials and high-pressure, high-velocity processing.

According to Balko and Fouquart, there are always key design goals and limitations to address for portable electronic device housings, as shown in Figure 1. The main concerns, though, include impact, stiffness, and manufacturability. For both advanced and leading-edge designs, these areas are approached somewhat differently than in conventional scenarios. And while the guidelines were originally developed for electronics applications, they apply equally to thin-wall parts designed for other markets.

Impact Strategies
Designers can use one of two strategies to improve the impact strength of thin-wall parts--either use the unreinforced plastic housing to absorb the load or use filled thermoplastics to transfer it.

In the case of load absorption, the part needs to deform somewhat to absorb impact energy. Make sure that you have ample package space if you're using this strategy, because the housing needs to deflect without contacting internal components. Also, keep those same components secured tightly with snug fits and fasteners that still provide sufficient room for deflection during impact. Finally, avoid all sharp corners and stress concentrators to minimize the chance of brittle failure.

For load transfer, little deflection is required. Rather, the impact energy is transferred directly to internal structures and components. Package space can and should be minimal for this reason. When using this strategy, remember that the housing need only be stronger than the weakest internal component. Again, internals need to be fastened snugly, and stress concentrators and sharp notches avoided.

Figure 2. A variety of molded-in interlocks can add stiffness to thin-wall housing designs.

Optimum Stiffness
As wall thicknesses decrease, the need for designed-in stiffness rises exponentially. Larger wall sections and ribbing are not feasible options for thin-wall designs. Instead, say Balko and Fouquart, modify part geometry for greater stiffness, using curved surfaces rather than flat plates, for example. Also, selecting a reinforced plastic may be the answer for large unsupported spans such as those found on the back of LCD panels for a notebook computer. Here, ribs, bosses, and gussets would interfere with the slim packaging requirements.

Other approaches to stiffer thin-wall parts include using styling lines and curved surfaces to boost stiffness along with part aesthetics. Also, stiff internal components tied carefully to the housing can improve overall rigidity. Consider using molded-in features to interlock pieces as well (Figure 2).

A different set of rules applies to thin-wall parts that can handle ribs, bosses, and gussets. In traditional design practice, these features are normally limited to 60 percent of the nominal wall thickness. But in leading-edge thin-wall applications, all three can be made as thick as the wall section, providing that they are located close to the gate. Why? Because the high injection and packing pressures used for these types of parts minimizes shrinkage and thus sink marks and voids. Less shrinkage also means, however, that draft angles need to be increased for proper part ejection. When the features are located far from the gate, traditional design rules apply.

Figure 3. To help minimize anisotropic shrinkage in thin-wall parts, it is important to pack the part adequately while the core is still molten. Shown is a comparison between cores for a conventional, 3-mm-thick design and a thin-wall, 1-mm-thick wall section.

Manufacturability Issues
Although the higher injection velocities used for thin-wall parts minimize shrinkage, they also increase shear rate. As a result, orientation occurs more readily, causing both filled and unfilled materials to shrink anisotropically--more in the cross-flow than in the flow direction. To avoid this problem, make sure parts are adequately packed out (Figure 3). This can be tricky, because thin-wall parts freezeoff quickly. The key, say Balko and Fouquart, lies in extremely short injection times to allow packing to take place while the core is still molten. Using large gates, greater than the wall thickness, also ensures sufficient flow during packing.

While mold filling simulations can accurately represent filling, cooling, and pressure trends, they cannot predict material degradation during processing. The GE team cautions designers and molders to watch for excessive residence time, melt temperatures, or shear--all of which can cause material degradation.

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