As the use of plastic parts increases in various segments of the appliance and electronics industries, specifying the optimal method for joining molded plastic parts has become more challenging and important than ever. This is due to the expanding range of polymer formulations available, along with the increasing number of additive options for reinforcing or modifying the material.

The ease of re-forming thermoplastics creates numerous options for joining plastic parts. Two of the more commonly used methods are ultrasonic welding and heat staking. Where greater joint strength is desired, a threaded metal insert can be embedded into a plastic part, allowing it to be assembled with a threaded fastener.

The most important point to understand up front is that there is no one-size-fits-all approach that is best for everybody. Choosing the proper process depends on many tangible (and intangible) factors that make every case different. Typically, a feasibility study must be performed for a specific application to determine the best method.

Enlarge this picture Improper insertion can cause hoop stress in surrounding material, as illustrated on the left. Hoop stress can lead to cracking. A proper, stress-free insertion is illustrated at right.

Naturally, each process has some inherent pros and cons, but design engineers often find that the increasing use of thin-walled part designs and space constraints generally point to one of two thermal processes as optimal, either heat staking or heat-based installation of inserts. Depending on the complexity of the assembly, using both methods may be appropriate in some cases.

When joining two plastic parts together that are molded from the same material, there is no other process that can match the benefits of ultrasonic welding. It is fast, clean and repeatable. On the other hand, ultrasonic welding can also be loud, create particulate matter, mark or scuff the plastic surface, and can cause stress failures in a part. Part size, material selection (crystalline vs. amorphous), and the field of welding are also important criteria that can limit the applicability of ultrasonic welding.

With respect to joint designs, staking post configurations, and insertion-hole size specifications, design engineers should realize that while most manufacturers of ultrasonic welding, heat staking and insertion equipment distribute general design configurations, these guidelines are targeted for preliminary concepts and layouts, and not the final production product where application-specific needs may differ. Design engineers should always consult with the experts to discuss expected performance results and equipment investment

Insertions

Custom-machined, 12-point heat-staking tip tooling with independent Z-axis adjustability in small area (1.5 sq. in.).

Ultrasonic processes can turn out to be expensive when chosen incorrectly, so it is critical for a design engineer to provide sufficient information to a vendor to receive appropriate advice. That includes all information to them about materials, the fragility of internal components, desired bond strength, sealing criteria, aesthetic needs, cycle-time requirements, and the desired overall size and shape of the finished heat-stake head.

The limitation of ultrasonic welders is most evident in applications where it is necessary to stake posts and/or bosses or install threaded inserts in large quantity or in varying planes. The limitation in tooling design — horns specifically — for these types of operations featuring large parts with multiple planes requires the purchase of multiple welding heads where it is desired to accomplish the assembly in one cycle.

The limitation of ultrasonic welding does not apply to situations such as installing a small insert into a PC/ABS part one at a time. That, in fact, is a great application for ultrasonic welding, provided the noise is an acceptable level and the plastic boss has a radius at the base.

However, heat technology would always be the preferred choice for situations such as installing one large insert over 1/2-in. in diameter into a 33 percent glass-filled nylon part with 10 percent carbon filler, or installing 20 inserts into a housing that has bosses located on various planes and different directions.

Studies of insert installation have demonstrated that in at least 80 percent of applications where insertion was performed ultrasonically, the welder was actually cold-pressing the insert by pneumatic pressure only. In these cases, after insertion, the welder produces a loud squeal as a result of ultrasonic energy making contact with the metal insert, and this noise typically gives the equipment operator a false impression that the insert was actually melted into the plastic. When one cuts a cross-section of a part where this is occurring, one finds that the plastic has not been consistently melted; the plastic has not flowed around all of the angular knurls and horizontal undercuts that give the insert its true holding properties such as resistance to rotational torque, and tensile pull out.

More often than not, ultrasonic welding tends to induce a lot of residual or hoop stress on the material surrounding the insert. Such stress can lead to cracking or splitting of the plastic material. Importantly, such cracking does not always become immediately visible. Often, the cracks don’t appear until the product is already out in the field. This is more commonly the case where certain types of semi-crystalline and/or glass-filled resins were used. Ultrasonic insertion also creates a fair amount of metal particulates and flakes that can be damaging to sensitive electronic components. Heat-insertion tooling induces minimal stress to the surrounding plastic and is able to successfully install inserts into parts with extremely thin walls.

By contrast, a properly set up thermal insertion system flows the plastic sufficiently to provide complete filling of the knurls and undercuts. When the insert is molded to the mating hold in this fashion, the installation delivers up to 15 percent to 20 percent more holding retention on rotational torque out and tensile pull out. In addition, heat systems can be designed so that there will not be enough pneumatic pressure to install the insert without melting, so that installation will not begin until the insert has reached the proper temperature to melt its way into the mating hole. Thermal systems from Sonitek also offer a pre-heat feature for larger inserts that allow the heated installation tip to rest on the insert for an adjustable period of time to improve thermal transfer into the insert prior to installation into its mating hole.

Staking

A 24-point, heat-staking platen with post-cooling ports.

The staking process involves the re-forming of a thermoplastic stud, tab, wall, or other protrusion over a mating component to retain or mechanically lock the part in place. Generally, the mating component is a dissimilar component such as a printed circuit board, metal bracket, metal stamping, bezel, light pipe, RF shield, or a part made from an incompatible polymer.

One drawback to using ultrasonic energy for staking is that the ultrasonic vibrations can often damage fragile components. This is particularly true in the case of printed circuit boards that many contain sensitive parts. Ultrasonic staking also has a strong tendency to crack the post at the base of the plastic stud being staked if there is not a generous enough radius.

Heat staking by its very nature imitates the molding process by re-forming and then cooling the plastic stud while it is held under pressure in the tip cavity. Heat staking equipment from Sonitek has a post-cooling feature that destroys the plastic memory. This feature helps to achieve an extremely tight stake, prevents adhesion or stringing on the tip cavity, and significantly reduces tool wear, even with highly glass filled materials. Physically, and visually, the heat-staked assembly appears to be an extension of the molding or fabricating process.

Sonitek has recently developed the ServoStaker System, a piece of equipment that incorporates servo motors for the Z-axis travel to deliver significant advantages on applications using very soft materials, like thermoplastic elastomers (TPEs), thermoplastic rubbers (TPRs), and thermoplastic polyolefins (TPOs). One of the primary advantages of using a servo-controlled system is the ability to apply near zero force onto the staking stud, thereby allowing the material to melt and form properly and uniformly into the cavity, creating a very good stake. Pneumatics would tend to apply premature force that can bend the post and result in less than acceptable results. The low force technique also provides an advantage when staking over extremely thin-walled bosses.

Heat-based tooling has virtually no limit on the quantity of inserts or staking points to be deployed and can work on varying levels or planes of a product. Sonitek has built systems that have performed work (inserts and/or stakes) on more than 100 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, and can work over contours, and protrusions. Heat tooling can also have many staking or insertion points in close proximity and permit independent adjustability from point to point in the X,Y, and Z axes.

While the previous examples focused on the advantages of utilizing heat for insertion and staking over ultrasonic welding, it is important to note that both have their merits and that applications exist for each. 

For more information  email: info@sonitek.com

SIDEBAR | Stake Styles |

Enlarge this picture

There are several different types of staking styles, each of which has a different set of advantages. The different properties can relate to part design, joint strength, and aesthetics of the final assembly, so designers should take care in selecting the style appropriate for their application. Below is a brief description of the more commonly used staking styles.

Hollow Stake

• Works well with large-diameter studs (no smaller than 0.080 in. O.D.).
• Produces a large, strong head.
• Does not have to melt a large amount of material (less time, less force).
• Avoids sink marks on the opposite side of molded component.
• Enables parts to be re-assembled with self-tapping screws when repair or disassembly is necessary.
• Aesthetically pleasing (can be made to look like it was “molded” on).

Knurled Stake

• Alignment is not an important consideration from an application standpoint.
• Ideally suited for high-volume production.
• Three styles available: fine knurl, medium knurl, and coarse knurl.
• Generally the pitch/texture of the knurl is related to diameter of stud to be staked.
• Can knurl a large tool and hit many stakes without alignment worries.
• Good use on heated platens where thermal expansion is generally a challenge.
• Works well when the mating component has a countersink.
• Greatly reduces cycle time.

Rosette Flared

• Recommended for large-diameter posts.
• Flares out the material, giving 360 degrees of even holding strength.
• Stakes/moves more volume easily.
• Lower staking forces required.
• Slightly less cycle time, as opposed to a dome stake on large studs.
• Alignment is critical.
• Requires very accurate positioning so that center point of tip contacts center of stud.
• Not generally recommended for use on heated platens (best on probes) because of thermal expansion.
• Not generally recommended on small-diameter studs.
• Aesthetically pleasing (looks like a rivet).

Flush Stake

• Used for applications requiring a flush surface.
• Requires that the mating component has sufficient thickness for a countersink, counterbore, or a combination of the two.
• Volume of the boss is critical to fill the countersink properly.

Dome/Conical

• Generally used with bosses with an O.D. of 0.250 in. or less.
• Aesthetically pleasing.
• Produces a tight stake.
• Recommended for crystalline material with sharp melting points such as 33 percent glass-filled nylon, highly defined melting temperatures, (post cooling a must).
• Good for glass-filled materials, or materials with abrasive fillers.
• Good for materials that degrade easily (post cooling).
• Dome stakes come in two profiles: high and low.
• High-profile stake is typically 0.750-in. high or more.
• Low-profile stake type is typically 0.375-in. or less.
• Works well into counter bored holes.