Snap Fits Enable Plastic Parts Assembly
With the proper design, snap-fit assembly ensures cost-effective, high-volume production of a wide range of parts.
To the casual observer, large piles of car parts are trash. But to an engineer looking to improve automotive design, they are a treasure. Such was the case at General Motors’ Competitive Assessment Center in Warren, MI, in the late 1980s.
“There were enough parts from disassembled cars on tables to fill three football fields,” recalls Paul Tres, principal and founder of ETS Inc., a plastics engineering consulting firm. “The place was nicknamed Mona Lisa because the parts were hung up like paintings for competitive analysis.”
At that time, Tres worked for DuPont as a development specialist, and GM used DuPont resins to make auto parts. He had already developed DuPont’s first snap-fit design software, which helped GM increase its use of the assembly method.
Tres was joined at Mona Lisa by many GM design engineers to find out as much as possible about how GM’s competitors built their cars.
All the engineers learned a great deal about snap-fit assembly there, because several European automakers were using the method quite extensively.
In 1990, the automaker formed a Design for Assembly committee that included Paul Bonenberger, a staff project engineer for GM who specialized in mechanical attachments. He retired from GM in 2006 and now provides consulting services as FasteningSmart Inc.“One of the committee’s objectives was to replace loose fasteners with snap fits in new designs whenever feasible,” notes Bonenberger.
Today, all automakers use snap-fit assembly extensively. Components assembled this way include door and instrument panels, dashboards, fuse boxes, and fascia for fenders and bumpers. Electronics, white goods, medical-device and aerospace manufacturers also are increasingly using snap fits.
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There are three main reasons for this. Snap-fit assembly is inexpensive, because the fastening system is molded into the part and no additional inserts, fasteners, preforms, solvents or adhesives are required. The method is well suited to high-volume production (up to 60 parts per minute), because assembly is easy and instantaneous. Finally, designers can use the method for parts of different shapes and sizes.
Hooks and Latches
There are four types of snap fit. The most common type features a cantilever beam with a hook that deflects and locks into a recess, or slides past a latch plate, in the mating part. Once the hook passes the recess edge, the beam returns to its original shape.
A second type, most common in food-storage containers and packaging, has fitments that snap on the top surface or perimeter of cylindrical, circular or nearly round parts. The two other types feature a ball-and-socket joint to transmit motion, or a torsion arm that works as a release or latch.
Cantilever snap fits can be permanent or reopened. In the former case, the hook locks into the opening at a 90-degree angle and prevents dislodging unless extreme force is used on the mated part. To evenly distribute stress along the length of the beam, it should be tapered from the tip to the base.
This type of snap fit can be designed to be reopened with ease or difficulty. Designs that allow easy opening often feature a U-shaped cantilever beam, with the hook edge resting on the outside of the part for better access. Difficult-to-open assemblies are intentionally designed to limit disassembly. They may feature a slot, rather than a deep recess, to hold the hook. The slot allows a person to access and release the hook from the outside with a screwdriver or similar tool.
“Designers often default to cantilever hooks whether or not they are appropriate,” claims Bonenberger, author of The First Snap-Fit Handbook. “Other lock styles exist and should be considered. Cantilever hooks are not robust enough for many applications because locks must be flexible [weak] for assembly but strong for retention. The cantilever-hook lock style is very poor at meeting these conflicting requirements.”
He recounts an incident where cantilever hooks failed to secure a plastic reflector to an interior panel. The reflector was originally attached with four screws. In a new design, four integral hooks replaced the screws. The hooks were strong enough but the reflectors sometimes fell off the panel.
Although they were well designed, the hooks failed due to other factors. A recess in the panel and handling difficulties led assemblers to install the reflector with a tipping motion rather than the required push, overstraining and weakening the hooks. Replacing the hooks with trap-style lock features solved the problem.
Unlike cantilever snap fits, snap-on fitments are never designed for permanent closure. However, they can be designed to open with great ease or difficulty. Tupperware is the most common application, but other examples include pen covers and childproof caps.
In general, snap-on fitments are stronger than cantilever beams and require greater force to achieve a good assembly. Regardless of application, they operate by elongation and recovery, typically of the female component. As a result, fitments require materials that have a relatively high elastic deflection limit, which is the point where material fails to fully recover from deformation. Maximum permissible strain ranges from about 50 percent for most reinforced plastics, to more than 70 percent for more-elastic polymers.
“Proper snap-fit design must always be material specific,” says David Sheridan, senior design engineer for Celanese Corp. “Often, the material chosen is not flexible enough, resulting in nonworking snap fits.”
Last fall, Celanese expanded its Hostaform S series of acetal copolymer (POM) resins to include the XT 20 and XT 90 grades for snap-fit and other applications requiring high-impact strength and flexibility. The POM resins are low-level impact modifiers with excellent mechanical, friction and wear properties. They also have low moisture absorption, high chemical resistance, and excellent thermal and dimensional stability.
For snap fits in electronics, the
company’s Celanex PBT (thermoplastic polyester) resin provides good insulation resistance and mechanical properties. This resin can be colored, and it complies with Restriction of Hazardous Substances (RoHS) and Waste Electrical and Electronic Equipment (WEEE) directives.
Solvay Specialty Polymers USA LLC has a broad portfolio of ultra-performance polymers that can be used to mold snap fits in electronics, including Kalix high-performance polyamide (HPPA) resins. These glass-filled compounds are designed to replace metal structural components in smart mobile devices.
HPPA resins provide high strength, rigidity and a high-quality surface finish for device housings, covers, chassis and frames. Different base resins are used to tailor performance and processing requirements. Bio-sourced and halogen-free flame retardant grades are available.
Solvay also offers AvaSpire, a versatile family of polyaryletherketone (PAEK) resins that offer strength and performance comparable to polyetheretherketone (PEEK) semicrystalline thermoplastic. PEEK is a preferred material choice when designing snap fits due to its flexibility. K.C. Desai, CAE technical marketing manager for Solvay Specialty Polymers, says the AvaSpire AV-600 series has a higher elongation at break than PEEK, while the AvaSpire AV-700 provides comparable strength, stiffness and chemical resistance but is more cost-effective.
The ball-and-socket-joint snap fit features a ball that engages a similar-shaped socket. Once engaged, the ball rotates in all directions within certain limits. This type of snap fit is often found on toys and inexpensive jewelry, as well as many consumer, commercial and industrial products.
Although the least used type, a torsion snap fit is great for parts that require frequent assembly and disassembly. It relies on, and imparts torsion force to, a shaft when opening the part. A typical application is a hinged lid on a box or container.
Helpful Hints
Although snap fits may reduce assembly costs, they need many design features to work correctly.
For example, there must be sufficient room for the beam or latch to flex and a lead-in ramp to deflect the snap. Shutoffs or side actions should be placed in molds to create an undercut for the hook or mating part. The parts may even need special features to keep them aligned while the snaps are being engaged.
Snap-fit assemblies also require more engineering calculations and analysis. Three key factors to review before finalizing a snap-fit design are stress concentration, creep and fatigue.
One of the most common causes of snap-fit failure is stress concentration at the sharp inside corner between the beam and wall to which it is attached. This stress can exceed the strength of rigid polymers, like glass-reinforced nylon, causing it to yield or break. More ductile materials, like unreinforced nylon, redistribute the peak stress over a broader region so they yield and deform rather than break.
“Moisturized nylons can yield up to 300 percent,” notes Tres. “Other plastics used in snap fits can yield as much as 900 percent or as little as 2 percent.”
One way to prevent stress concentration is to place a fillet radius along the tensile side, or both sides, of the beam where it meets the wall. The goal is to make the radius-to-wall-thickness ratio 50 percent. A higher percentage only marginally increases strength and may cause internal voids and sink marks.
Creep occurs gradually over time and reduces holding force between the two components. This joint relaxation can lessen seal pressure, resulting in fluid leakage, or allow excessive play (due to tolerance variations) that causes noise and vibration in the part.
Designing a low-stress beam or latch is one way to prevent creep. Another is incorporating a 90-degree return angle so the beam or latch relaxes in tension vs. bonding.
Fatigue should be addressed if a part is expected to be opened and closed hundreds or thousands of times. Stress from repeated opening and closing can cause various materials to fail–even high-strength materials that are formulated to withstand high stress levels.
The best way to select material for this type of snap fit is to compare the S-N (stress-failure) curves of several materials. This curve shows the expected number of cycles to failure at various stress levels and at different temperatures of exposure. Engineers should select the material with the lowest failure rate at the required stress level and temperature for the specific application.
When a snap-fit design deviates from theoretical assumptions, finite element analysis (FEA) is recommended to accurately determine deflection and stress. FEA is a computer-based tool that divides the design into small elements with defined deflection and stress characteristics. It then develops and analyzes a finite element model using several mathematical equations. FEA is also very helpful when designing complex parts because it incorporates many factors commonly ignored in classical calculations, such as shear deflection, deflection of the material around the base, irregular geometry and nonlinear material properties.
Additional computer-based design resources can be found on the ETS Web site at www.ets-corp.com/tools/development.htm. The resources include a snap-fit design calculator, various structural beam design software programs and a link to the Computer Aided Material Preselection by Uniform Standards (CAMPUS) database, which provides comparable data of different polymers from a variety of suppliers.
As the reflector example shows, calculations are important for designing lock features, but they are not enough. A better approach, Bonenberger contends, is to view snap-fit attachments (or interfaces) as a system. In the Snap-Fit Handbook, he calls this approach Attachment Level Construct (ALC).
“Most designers find themselves learning snap-fit through trial and error, which is an expensive and time-consuming process,” he says.
ALC focuses on three things: key requirements, elements and a logical development process. Key requirements are the common characteristics shared by all fundamentally sound snap fits: strength, constraint, compatibility and robustness. Elements are either physical features of a snap-fit attachment (interface) or attributes that describe the snap-fit application. Physical elements include constraint features (such as locks and locators) and enhancements. Bonenberger lists the attributes as function, basic shape, engage direction and assembly motion.
The development process begins with defining the application and identifying best practices using design rules captured in the ALC. Next, a few interface concepts are quickly generated and reviewed. The preferred (best) concept then proceeds through feature analysis, after which dimensions are added to the design. Step five is verifying the design (using prototypes if necessary) and end-use testing. If indicated, the design is fine-tuned in step six. The part, with its snap-fit interface, is now ready for production.
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