Heat staking is the process of melting and reforming a thermoplastic stud to lock a dissimilar material in place mechanically.
Heat Staking Design Tips
Typical heat staking applications are found in a variety of industries, including the automotive, consumer appliances, consumer electronics, telecommunications, and medical industries.
Creating plastic products using direct-contact heat staking can be a repeatable, economical, and safe method of assembly. The number of posts staked at one time is limited only by the size and configuration of the parts to be assembled. The following method is designed to precisely control heat and pressure to provide clean, odorless, and safe high-quality upset staking of modern plastics without the high cost and noise of other staking processes.
Heat Staking Direct Contact Method
The direct-contact method uses heated probes with pressure, in contact with plastic posts, to compress and form a stud joining two pieces of plastic or plastic and other materials. Excellent results are possible when post design and material selection are initially thought of as heat-staking applications. Precise heat and pressure can re-form posts made from a variety of plastic materials. Some of the most commonly used plastics are acrylonitrile butadiene styrene (ABS), polystyrene (PS), polypropylene (PP), polycarbonate (PC) and glass-reinforced nylon (GFN).
Heat Staking Design Considerations
Very simple design guidelines are required: post diameter, height, and geometry are the essential factors in obtaining good heat-staking results. Working height above the mating piece should be approximately 1.5 to 2.0 times the post diameter, as shown in Figure 1. That height will result in stud-head diameters having approximately a three-to-one head-to-post diameter ratio. Typical post diameter will be determined by the selected material and its moldable wall thickness. As an example, ABS and PC post diameters typically can be 0.04 to 0.09 inches; structural foam diameters typically are 0.09 to 0.25 inches.
Post geometries and stud-head geometries can vary as the application dictates. For through-hole fastening, flat-tip probes for staking flat heads shown in Figure 2a are the easiest to tool and maintain. Dimpled stud heads are visually more attractive but require a machined recess in the probe tip, as shown in Figure 2b. For high-strength retention, a cross-shaped post, Figure 2c, or hollow boss, Figure 2d, should be used. Hollow or cross shapes are often used to minimize sink marks in front surfaces, a problem common to injection molding of large posts or thick wall sections.
Rosette-shaped probes, shown in Figure 2d, will flair out a hollow boss, leaving the hole available for other attachments that use self-tapping hardware. Designs requiring the retention of either glass lenses or other components that need side swaging can be successfully implemented using angled probes contoured to form the plastic posts to the side, capturing the component by its edge, as shown in Figure 3.
Spring plungers can be designed into the staking head to compress the assembly and ensure proper contact between parts to be joined. Joint gaps common to some other types of plastic assembly caused by part warpage, poorly fitting parts, or operator error can be virtually eliminated using spring plungers in the staking head.
Heat Staking Dis-assembly
Dis-assembly for recycling or reworking of heat-staked assemblies also can be accomplished using precise heat and pressure. High-heat piercing with a 60° pointed probe will reflow the stud-head material, allowing a quick, clean removal of the stud head. Reassembly can be performed on reworked parts using alternate holes and self-tapping hardware, or a second set of posts for staking assembly can be performed on reworked parts using alternate holes and self-tapping hardware, or a second set of posts for staking.
Our Thermal Press Heat Staking design engineers will be glad to go over your particular product assemblies requirements.
Contact Thermal Press International for all of your heat staking needs.