Snap Fit, Press Fit, Fasteners, and Adhesives

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3 April 2008 -- Press fits work on a somewhat different principle than snap fits. In a snap fit, the goal is to temporarily deflect a detail and have it return to rest after capturing a matching detail in another part. Press fits work by permanently deflecting a detail and using the surface friction and surface affinity of the plastic material to hold the assembly together. The classic press fit is a pin engaging a slightly smaller hole. The technique is not limited to round pins in round holes; a common variant is a round pin in a hexagonal hole. The interference fit between pin and hole and the wall thickness of the boss are very important in making this type of assembly work. Sloppy molding practice that embrittles the bosses or leaves a weak weld line in the boss can sabotage the best press fit design. The optimal interference dimensions and length of engagement also depend on material properties including stiffness and lubricity, and creep can be an issue as assemblies age. Some years ago a technology was introduced that executed the press fit at high velocity and claimed to actually weld the pin in the hole through heat produced by surface friction. As with all other techniques, success depends on good design and careful testing to get the details right. Press fit assemblies generally cannot be disassembled, which may be either an advantage or a disadvantage compared to snap fits. At one manufacturer, a war broke out between engineers who favored press fits and engineers who favored ultrasonic welding. Recollection is that good points were made on both sides, and it is possible that the conflict my still be unresolved more than a decade later.

26 March 2008 -- The words snap fit indicate exactly what is going on with this type of assembly. Some detail of one part is deflected from its rest position by some detail of the other part and it snaps back to rest position to lock the assembly together. A post with a hook detail going into a slot is the most easily imagined form. There are in reality a vast array of snap fit possibilities. It is impossible to relate more than general information in this format, but the imagination of designers has not yet been exhausted when it comes to the endless variety of configurations. What remains constant, however, is the need to deflect the material enough to get sufficient engagement to do the job without stressing the material to the point of weakening it. This is why it is generally advantageous to have many smaller details rather than one larger one. Another key consideration with snap fits is the need to incorporate the necessary undercuts without creating unnecessarily complex and expensive mold features. Often, snap fits can be designed so the part will snap out of the mold without requiring side action, other times, the part wall can be relieved so the undercut can be formed with a straight-draw detail. Snap fits can be designed so the part can be disassembled, but quite often a snap fit is a one-way street. Given the creep issues with plastic parts as they age, careful thought and attention needs to be paid to the dimensions of the details so that assemblies do not loosen up over time. Also, if the assembly requires a seal, the seal will have to be provided for mechanically and separately from the snap detail.

24 March 2008 -- Fasteners have been used to assemble thermoplastic components almost as long as there have been thermoplastic components. Of course, some of the earliest plastic parts were things like combs that did had no assembly requirement, but it wasn't long before someone drilled a hole in a part, put a wire loop in it, and hung it on a chain around his/her neck. Thus the realm of fastener use on plastic parts was born. Thermoplastics parts have been bolted, screwed, riveted, clipped, stapled, and held together with just about every fastener imaginable, but there are a few key points to consider when using fasteners on plastic parts. First, few plastics exhibit a really high degree of crystallinity, so are therefore amorphous to some degree or another when "solidified." Fully amorpous materials, which includes a great many thermoplastic materials, can never be said to be really "solid." They just flow so slowly at normal temperatures that we humans experience them as solids. What this means is that almost all thermoplastic materials exhibit more or less creep; creep being the tendency of a thermoplastic part to change dimension over time. Since most thermoplastic parts have some degree of internal tension, the parts generally get smaller over time. This is why the vinyl trim in an older car will pull away from openings and open up gaps. Creep is exacerbated by stress on the material in the form of temperature extremes, vibration, and such. Creep can be a major problem for any thermoplastic assembly but is particularly troublesome where screws may loosen as material shrinks, or where a lessening wall thickness will cause a rivet to become loose and start "working" or moving around in a hole. In some cases, where a fastener needs to be set to a certain torque to remain tight, like in an under-hood automotive application, it becomes necessary to insert metal sleeves into bolt holes to prevent crushing of the plastic material and looseneing of the bolt as the plastic shrinks. Where bolts need to be threaded into a thermoplastic component, especially when disassembly is a possible need, threaded inserts are strongly recommended. There are several great books on fastener use in thermoplastic components, so again we'll not belabor a point that is near the edge of our expertise here. Just be sure to have done your homework before incorporating fasteners in a new design to avoid unpleasant surprises later in product life.

21 March 2008 -- It's hard to say which thermoplastic joining process came first, hot plate welding, hot tool staking, solvent bonding, adhesives, snap or press fits, or fasteners. We know that ultrasonic welding came in the early 1960s, vibration welding in the late 1970s, and laser welding in the 1990s, but the others are really anybody's guess. I believe some of the earliest plastic materials were cellulosics, so it's hard to imagine anyone having much early success with heated tools, though it is possible. Another early material was polyamid (nylon), so again, hard to imagine a lot of success with heated tools. My guess is adhesives came first. But it's just a guess. Adhesives have been used in plastics assembly for a long, long time. Pure adhesives work totally on the basis of surface affinity; in the simplest terms, the materials stay together because chemically they simply want to. Some adhesives use a combination of surface affinity and solvent action and therefore are possibly more correctly called cements (I am open to correction on this). Solvent bonding is the action of chemically breaking down the surface of the joint by dissolving the materials, allowing flow, and then resolidification through the evaporation or diffusion of the solvent. Solvent bonding is not technically an adhesive process, but it is usually lumped into that category by those of us outside of that realm. The materials to be joined will greatly influence the choice of adhesives or solvents. Solvent action depends on participation of the molecules of the material, and solvents that will bond one plastic will quite often have no effect whatsoever on another. Surface affinity bonds are somewhat more generic, with the most basic rule that low surface tension plastics work best with high surface tension adhesives and vice-versa. Anyway, we're coming dangerously close to exhausting my knowledge on the subject (and we may already have crossed the line) so this discussion will end with the following admonition: If considering adhesive or solvent bonding, work very closely with the suppliers of the bonding agents, as reliable and durable bonds depend on getting the chemistry right.

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Last updated 14 March 2018.