Warning: Information in the blog is acted on at the sole risk of the reader. It is correct and accurate so far as we are able to determine, but is delivered without any warranty of any kind, including but not limited to direct or consequential damages, personal injury, or damage to product or property. Want to see something addressed in the blog? info@tributek.com 26 April 2008 -- A heat staking machine can be used for hot tool insertion as well as thermal staking or swaging. The most common inserts for hot tool insertion are brass female-threaded fasteners placed on molded holes to provide good purchase for bolts used to secure circuit boards or other components/subassemblies to a chassis or inside a case, or to secure the case halves. Brass inserts are preferred to steel for this operation because the copper content of the brass assures a relatively low thermal mass for the inserts and they will change temperature rapidly. The inserts are typically placed in the holes partially engaged. The press is actuated and the tips come into contact with the inserts. At this point, an insertion delay can be programmed to allow the inserts to warm up to a temperature that allows for melting of the thermoplastic material. After the delay, the press continues to press the inserts into the holes. Depth is usually controlled by a mechanical stop. After contacting the stop, the press can simply retract if the insertion is low precision, or the press can stop while compressed air is used to cool the inserts and tips to prevent the inserts floating back out of the hole on a cushion of expanding hot plastic. Getting the tip temperatures and delay times just right takes a bit of experimenting. Beware of the temptation to simply use press force to jam not-quite-hot-enough inserts into their holes, as holes almost always have suceptibility to cracking at the knit or weld lines when under hoop stress. The pull-out and torque strength of the set inserts can usually be improved by slowing the process down and allowing the heat to soak a little more deeply into the plastic during the process. As with thermal staking, inserts of various sizes can be set on multiple levels simultaneously. 10 April 2008 -- Should you call a thermal staking machine a heat staker? Most thermal staking presses actually have interchangeable tooling. Since they can be easily be converted from heat staking to thermal insertion, embossing, or degating, they should probably more properly be called thermal presses. It is possible to build a machine for the sole purpose of heat staking, in which case the term heat staker could be applied. 5 April 2008 -- Hot tool staking probably started with a screwdriver or some similar tool heated with a torch and then applied to a tab or post to capture another part. The process does not work much differently today, though the equipment and tooling has become much more sohpisticated. Most thermal staking machines today use electric coil heaters with embedded thermocoules and digital temperature controllers to maintain probe temperature. The tip, attached to the probe, is designed to have low thermal mass, that is to say it can change temperature easily. The tip is heated by the probe, which is mounted to a press that delivers the probe and tip assembly to the work and applies force to deflect the head(s) to be formed. When the tip contacts the work, it transfers heat to the work to melt the material and form the detail. If the press were simply retracted at this stage in the process, most materials would stick to the hot tip and either form strings or the head would actually be pulled off of the tab or post. To prevent this, pressurized (and sometimes cooled) air is delivered to the tip long enough to reduce the tip temperature enough to allow a clean release and leave a well-formed detail with sufficient strength to hold the assembly together. This action is usually called post-cooling. Following retraction, the tip is then reheated by the hot probe. If more than one probe assembly is installed on the machine, they can be individually controlled if there are a sufficient number of heater controllers on the machine, or they can be zoned together, with one thermocouple providing temperature feedback to the heater controller, and other heaters modulating in open-loop fashion. This works better then one might think when identical probe/tip assemblies doing the same work are on the same zone and heaters are closely matched. There is considerable freedom in tooling design for the heat staking process, and multiple size heads can be formed at multiple levels in the assembly. Probes cannot go into tight spaces in parts, though, as they may thermally deform nearby details. Likewise, not all plastics respond well to thermal staking, including matrials with sharply crystalline melting points such as polyamid (nylon), or narrow melt to degradation temperature ranges such as polyvinyl chloride. 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 a 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. 19 March 2008 -- One of the more basic questions about plastics assembly is how to know which process to specify for the assembly. Often, the size and configuration of the parts and the material inolved will rule out various options. Since very few people do thermoplastic assembly for the pure joy of it, the economics of the project will definitely enter into the discussion as well. Often, a designer will have a preference for or familiarity with a particular process and drive their design toward what they perceive as the right combination of factors to make that process feasible. Sometimes, the joint strength requirement will drive the decision. Sometimes, it's more a matter of which equipment is already owned. Whatever drives the decision, the cost of the headaches of using the wrong process will almost always be greater than the cost of doing it right in the first place. 18 March 2008 -- When considering whether to even attempt to blog about assembly of thermoplastic components, the issue of whether I was enough of an expert to be holding forth on the topic kept entering my mind. I have written a lot of magazine articles and technical papers over the years, but they have always been peer reviewed or at least reviewed by an editor of some kind. Blogging is riskier because of the possibility of a remark becoming part of the permanent record even after it has been removed from the site in favor of a better second thought. But then, there are a lot of people who blog on about many things of which they know little, and that does not stop them from doing it, or from being viewed as experts by others who almost mystically assign importance to anything they read on the internet. The truth is that anyone with an opinion and ten bucks a month can have a web site about whatever they wish, without having any qualifications whatsoever. Negative opinions and statements are generally presumed to be true while positive ones are presumed false. For example, if I said a certain restaurant was very good, it would be presumed false because my motive would be suspect; if I said a friend of a friend said they served dog meat, that would be enough to drive them out of business. The more pervasive the internet becomes, the greater this effect. Anyway, my desire is that by blogging the knowledge and experience resulting from my long career in the industry I may help someone to find the answer to a manufacturing puzzle he or she is trying to solve, and to shed light on the various tricks of the trade learned over the years that may or may not be well known or thought of at the right moment in time. Oh, and just to remove any question about the commercial intent of this endeavor, if you are reading this I hope you will buy our stuff. It's good stuff. What appears in the blog is true to the best of my knowledge and I will call it like I see it. So, enjoy the blog. I hope it turns out to be something of value for you. Tom Kirkland Warning: Information in the blog is acted on at the sole risk of the reader. It is correct and accurate so far as we are able to determine, but is delivered without any warranty of any kind, including but not limited to direct or consequential damages, personal injury, or damage to product or property. Could you do us a favor? If you want something and we don't have it on the site, like a replacement for a 502 or 802 converter or 200- or 400/800-style booster, please let us know by email. Thanks! All prices are FOB Elburn, Illinois, USA. Shipping to US addresses only at this time. Orders shipped to Illinois addresses will have Illinois sales tax added to taxable items. Paper warranty statement shipped with product supersedes any warranty statements made on web site. USA phone/fax 877-768-3892 www.tributek.biz - PO Box 8025 - Elburn IL 60119 USA All content is copyright (c) 2006-2008 www.tributek.biz division of www.tributek.com All rights reserved. Last updated 26 April 2008