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Staking & Insertion - Why Heat is the Better Choice
by Michael J. Brunetti
Back in the mid to late 1960s, most molders and product designers thought that heat insertion and even heat staking were non-repeatable, unreliable, or inconsistent methods of installing metal inserts into plastic or staking over of plastic posts. This was due to a few factors. First, ultrasonic technology was sweeping the country – billed as a ‘fix all’ for all types of welding situations, instead of just for welding plastic to plastic – the situation for which it is best used. Second, heat technology, temperature controllers, and the use of thermal dynamics were not as advanced and readily available as they are today.
At that time, ultrasonic welders were being used widely to install one insert (or in some cases, several at a time) and everyone thought this was the best thing since sliced bread! I can assure you that after over 22 years of first-hand experience in this industry and starting out as a post-molded insert designer, I have learned that ultrasonic insertion, in many cases, was not being properly used and was definitely not the best method for the majority of insertion and staking applications.
Just walk through any molding facility today and you will find an old (or new) ultrasonic welder installing just one insert at a time. In at least 80 percent of these situations, the welder is actually ‘cold pressing’ the insert by utilizing pneumatic pressure only, and then loudly “squealing” with ultrasonic energy/contact with the metal insert, thereby giving the operator a false sense of confidence that the insert truly has been melted into the part. Cross section these inserts and you will find that the plastic has been inconsistently melted and flowed around all of the angular knurls and horizontal undercuts, which give the insert its true holding properties, such as rotational torque and tensile pull out.
When properly set up, on the other hand, a heat insertion system will give you a complete ‘filling’ and match to all of the knurls and undercuts to mold itself into the mating hole, resulting in up to 15-20 percent more holding retention on rotational torque out and tensile pull out. Heat systems can be set up so that there is not enough pneumatic pressure to install the insert and will not begin to install until it has reached the proper temperature to melt its way into the mating hole. For larger inserts, a ‘pre-heat’ feature is available that allows the heated installation tip to rest on the insert for an adjustable period of time in order to improve thermal transfer into the insert prior to installation down into its mating hole.
Ultrasonics, more often than not, tends to create quite a bit of ‘residual or hoop stress’ on the material surrounding the insert, which leads to cracking or splitting. In most cases, if this is not immediately visible, you will find that the cracks show up later in the field - especially in a lot of semi-crystalline and/or glass-filled resins. Ultrasonic insertion also will create a fair amount of metal particulate and flakes that can be damaging to sensitive electronic components. Heat insertion tooling applies very little stress to the surrounding plastic and is able to install inserts into parts with extremely thin walls, such as cell phones and PDAs.
Heat tooling has no limit on the quantity of inserts to be installed or the varying levels or planes of a product. Sonitek literally has built systems that have installed over one hundred inserts or have performed staking on over one hundred locations on multiple planes in a single cycle. Heat tooling has no real design constraints. It can get down deep into tight restricted areas at very small diameters or work over contours and protrusions. Additionally, heat tooling can have many staking or insertion points that are in close proximity and can allow independent adjustability from point to point in an x,y, and z axes.
When staking electronic components (such as PCBs or other sensitive components), ultrasonic vibrations can damage fragile parts during the staking process upon contact with the plastic. Ultrasonic staking also has a strong tendency to crack the post at the base of the plastic stud that is being staked if there is not a generous enough radius. Heat, by its very nature, imitates the molding process by re-forming and then cooling the plastic while it is held under pressure in the tip cavity, just like a molding machine. Post cooling features destroy the plastic memory, thereby achieving an extremely tight stake, preventing adhesion or stringing on the tip cavity, and producing minimal tool wear - even with highly glass-filled materials. Sonitek has recently introduced the ServoStaker, which incorporates servo motors for the z axis travel, to provide significant advantages on applications utilizing very soft materials, such as thermoplastic elastomers and thermoplastic rubbers, as well as staking over extremely thin-walled bosses.
While this article sets out the advantages of utilizing heat for staking and insertion over ultrasonic welding, it is important to note that both have their merits and that specific applications exist for each. For example, if only one or two inserts or stakes are required to be installed in a high-speed application, in some instances, ultrasonics may lend an advantage when properly set-up due to reduced cycle times. Heat, on the other hand, can overcome speed by easily increasing the number of parts being done per cycle for less cost than ultrasonics. In the end, as a general rule for staking and insertion, heat is a much better choice.
Michael J. Brunetti is vice president of sales and marketing for Sonitek Corporation of Milford, Conn. As an industry expert for over 22 years, Brunetti is credited with many patents on ‘symmetrical’ inserts. He has worked with several major heat equipment companies and Fortune 500 companies as a consultant, applications engineer, and threaded insert designer. Sonitek Corporation is a manufacturer of heat staking and ultrasonic welding equipment for plastic assembly. For more information, call (203) 878-9321 or visit www.sonitek.com.