Dazzling Plastics, Difficult EngineeringDazzling Plastics, Difficult Engineering
January 13, 2003
And it's not just products from the traditional design leaders that have adopted sleek styling and dazzling colors. "Even the volume-price players are now embracing aesthetics," says Rob Butterfield, director of industrial design firm Polymer Solutions Inc., a joint venture of Fitch and GE Plastics (Pittsfield, MA).
The reason: "In many industries, products no longer stand out on their functionality alone," says Butterfield. Aesthetics has become one of the only cost-effective ways companies can differentiate products from those of their competition.
Enter industrial designers and special-effect plastics.
The former's stock has risen as style's selling power has grown. Butterfield notes that ten years ago an industrial designer's big responsibility might be putting the corner radii on a box once it came out of engineering. Nowadays, he says, "design can increasingly tell engineering, 'Just make it this way.'"
"This way" often involves the use of special effects plastics. Also referred to as "visual effect" plastics, these materials contain specialty pigments that can impart a variety of eye-catching looks to plastics parts-including metallic, sparkling, pearlescent, and fluorescent appearances. Light-diffusing effects have also seen widespread use in the translucent plastic housings that now swaddle so many consumer products. And the use of color-shifting materials, whose appearance changes with the angle of viewing or with lighting conditions, has also started to spread into new products.
Though each of these special-effect materials differs from one another in terms of chemical make-up and appearance, they all tend to require some special attention when it comes to materials selection, part and tooling design, and the molding process. "Special effects are definitely not a drop-in replacement for conventional color technologies," says Ray Kosmider, a color research specialist for Dow Plastics (Midland, MI).
How do you cope and maintain control? Learn the differences and adjust your design steps accordingly.
To succeed with special effect plastics, forget for a minute their flashy appearance and focus instead on just how much these materials differ from their un-colored base materials. "Think of special-effect pigments as contaminants, so expect them to alter the mechanical and physical properties of the base plastic," says Stuart Swain, manager of color technology for RTP Co. (Winona, MN).
Recent advances in the base resins and software screening techniques have minimized most of these property differences. Tensile strength, stiffness, elongation, and heat deformation temperature, for example, don't tend to drop significantly in today's special-effect formulations, materials suppliers report.
But, some trouble spots do persist. "Impact is the big one," concedes Andrew Day, global aesthetics manager for GE Plastics. With materials containing metallic flake pigments, notched Izod impact can drop 20 to 50% compared to the base resin, according to material supplier estimates. Flame resistance can also suffer, and not all effects can pass the UL 94 requirements required by appliance and electronics makers. "It's not a trivial problem," Day says.
And then there's shrinkage. When heavily loaded with metal flakes, special-effect materials can shrink differently than traditional amorphous engineering resins: Their cross-flow and flow-direction shrinkage values diverge. "In this regard, they're a little like a glass-filled materials," explains Mark Matsco, manager of processing technology for Bayer Corp. (Pittsburgh, PA).
These special-effects materials also behave differently on the molding machine. Excess shear heat tends to break down delicate special-effect pigments, causing unpredictable color shifts or an intolerably dull appearance. These materials can also flow differently than their natural resin counterparts. Melt pressure distributions that would be acceptable for non-appearance parts can turn up as noticeable color shifts on the part surface. Additionally, some of the pigments "have sharp corners and are as hard as tungsten," so they wear out tools faster than natural resins, says Swain.
Special effects may also force you to rethink familiar plastics choices because not all effects and base resin combinations work equally well together. Swain notes that the pigments used for the color-shifting, or "angular-mesmerism," effect tends to work best in clear resins. So do edge glow, frosted, and pearlescent effects. "These can all work in a non-clear base, but they need a lot more pigment," he says, explaining that since opaque materials only show effects on the surface they require greater pigment loadings to achieve the desired effect.
Finally, special effect materials create a Catch 22: Parts made with these materials are vulnerable to surface defects, yet they go into applications with unusually high cosmetic quality standards. In part, it's the pigments themselves that produce these appearance problems.
Effects based on flakes or other pigments whose particles have a high aspect ratio tend to tumble and align themselves during the molding process, creating noticeable weld lines. This undesirable effect occurs wherever polymer flow fronts meet-around holes or other openings or projected features. An equivalent phenomenon can happen in areas of high flow, triggering visible flow lines. Making matters worse, these high-aspect-ratio pigments produce some of the most popular effects today-including metallic, mineral, pearlescent, and color-shifting ones. The need to control weld and flow lines drives many of the part design strategies used for special effects.
Discouraged yet? Don't be. "Early in the design process you can work around just about any problem," says Bayer Corp.'s Matsco.
Play by the (Design) Rules
With non-appearance parts, engineers can sometimes get away with ignoring design rules. Not so with special effects. Fortunately, the solution doesn't require a new set of design rules. "It's just a lot more important that engineers obey the existing rules," says Ken Kerouac, a plastics engineer and an injection molding specialist for Dow Plastics.
One of the more important of these rules is to design uniform wall thicknesses. "Designers still don't incorporate uniform nominal wall thickness as much as they should," Kerouac says. Bob Johnson, an engineer at GE Plastics Polymer Processing and Development Center (PPDC), agrees. "We always advise engineers to design parts with uniform wall sections," he says.
Uniform walls in visual-effect parts can minimize the tumbling and alignment of high-aspect-ratio pigments by promoting a smooth, or laminar, flow within the tool. "Laminar flow is a key to success with these materials," says Johnson. Also, the abrupt melt pressure changes that accompany wall-thickness transitions can cause noticeable color shift on parts, according to Matsco. He, too, is a member of the chorus that suggests keeping wall thickness uniform.
The transitions around ribs, bosses, snap-fits, and other projecting features require extra attention too. "Wherever there's mechanical stress in the part, you have to be careful," Matsco says. He points out that even a small drop in mechanical properties from special-effect pigments makes the materials more susceptible to stress concentrators. The consensus among the plastic engineering experts surveyed here: Radii near these features should be more generous than for equivalent non-appearance parts. They recommend radii of at least 0.015 and 0.030 inches. And Matsco argues that you should stick to the high end of this range with "bending features" like snap fits. "We've seen a lot of failed parts with inadequate radii," he says. At the same time, he warns, don't beef up radii too much or sink marks could result.
Rib sizing and location also touches on Johnson's points about flow. He recommends that you size ribs to avoid unnecessary flow disturbances by changing rib thickness-to-wall thickness ratios. Parts that can normally get away with ratios between 60 and 70% should go to something more like 50% when visual-effect materials are used. And Dow's Kerouac suggests that you orient ribs so that the melt flows along them rather than across them-in order to minimize the chance that surface will pick up the rib pattern as witness lines.
These simple part design strategies go a long way to improving the appearance of special effect parts, but they may not completely eliminate persistent weld line problems on parts with prominent holes or other surface features. In these cases, a couple of drastic solutions exist: Stamping out holes after molding, normally a needlessly expensive secondary operation, can do away with weld line predicaments. And in-mold-decorating technology can increasingly impart some of the same visual effects as molded-in effects. GE Plastics, for instance, recently introduced films containing visual effects. "The effects reside in the film, and films don't get weld lines," says Les Goff, applications technology manager.
Like good part design practice, smart tooling designs can also help or hinder efforts. One easy fix involves gate sizing. Materials suppliers recommend opening up the gates somewhat to reduce shear and any pressure spikes at the gate. Exactly how much depends on the application, but Johnson cites a rule of thumb: "The gate size should be three times the size of the largest pigment particle." The large-gate approach will treat your visual effects gently, but your molder might not like it. The molder will lobby for smaller gates because they freeze off faster and reduce cycle times. "Tell them that you want generous flow and low shear, so it's necessary to sacrifice some cycle time," Kerouac says
Gating choices also play a crucial role in weld line formation. The more gates needed to fill a part, the greater the chance of increasing the number of converging flow fronts. So if filling doesn't present a problem, reducing the number of gates often helps reduce weld lines.
Sometimes, however, adding gates can cut weld lines too-by moving flow fronts to non-visible areas of the part. Kerouac notes, however, that this strategy may necessitate going from a cold runner system to advanced hot runners whose gates can be opened and closed in a sequence. These sequential-valve gating systems usually see use in large, hard-to-fill parts-such as automotive bumpers. To fight weld lines, though, you might consider it for smaller parts too, in Kerouac's view.
There's a definite hierarchy of evil when it comes to the number of gates. According to Matsco, flows from opposing gates usually produce the least dramatic weld lines, while weld lines close to gates offer the next best appearance. "Flow around features is the next level of weld line," he continues. And the worst ones come at the end of flow, especially in dead-end part features. "We don't fully understand why," he says, but he speculates that fountain flow and low pressure in these regions results in orientation of flake and accumulation of excess pigment.
Gating style, meanwhile, plays a crucial role too. Special-effect materials work with a wide variety of gate types, but suppliers recommend you opt for those styles that don't impinge on the part's cosmetic surface. "You never want a gate right on the visible surface if you can help it," Matsco says. For this reason, edge gates often get the nod over tunnel gates-even though the latter style costs less to build and breaks away cleanly from the molded part. "If I were building a new mold that could switch between special effect and conventional materials, I would build an insert at the gate to switch back and forth to tunnel gating as needed," Matsco says.
Looking beyond gating, visual effects also have a couple of other secondary tooling factors. Given the inherent appearance quality standards for these parts, their tools tend to be textured and polished to the highest standards. The expense of the special-effect materials-and their diminished tolerance for regrind-also pushes many jobs into hot runner tools. And sometimes tooling for special effects requires harder tool steels to fight the wear from abrasive pigments. The bottom line: Expect tooling for special effect parts to cost from 5 to 10% more than a comparable tool for conventionally colored part, according to supplier estimates. That cost differential would shoot up substantially-to 20% or more-should the special effects trigger the addition of a hot runner system.
Mind Your Molding
Once you've designed the special-effect part and tool just so and picked the right material, beautiful parts will just fall from the mold, right? Well, don't count on it. With special-effect colors, bad molding practice can ruin all your hard work. "Injection molding expertise is critical to success with these materials," Kerouac emphasizes. All sorts of molding transgressions can work against special effect-materials. "Nine times out of ten if there's a flow problem in the tool, a molder will try cranking up the heat," he says. He's also seen molders concerned about their machine utilization try to run jobs on machines that are too big, increasing the residence time of the sensitive material in the molding machine's barrel. Both of these practices, common at even decent molding shops, can thermally degrade the special-effect materials. And the machines that run special-effect materials need to use a carefully selected screw design to ensure the pigment dispersion. "Or else you'll get particles of pigment and unmelted resin in the part, which later act as stress concentrators and reduce mechanical performance," Kerouac says. And make sure you pick a molder with experience in quality and molding machine control. "The tighter processing window of these materials can make it difficult for your average melt-and-squirt molder to run them with acceptable yields," Matsco says.
Most design engineers cannot watch their molder every minute, but there is an easy way around the molding pitfalls. "Find qualified molders and bring them into the project up front," Kerouac says. And don't stop with the molder. Special effects materials need nothing so much as a systems approach, so also consult with materials suppliers and toolmakers as soon the project gets off the ground. "Early involvement of everyone who can influence the part's final appearance is the best route to success," Kerouac says.
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