This is part 2 of our series on assembly design. We have discussed two of the methods employed in assembly design in part 1 of this series, which can be found here .

Here is another method:

WELDING AND BONDING

Welding and bonding methods have a wide range of applications in assembly design and, in many cases, they are the only viable option for the engineer. Welding and bonding create permanent joints, so applications in which recycling and repairing are a possibility will not be very suitable for these methods. When it is determined that welding or bonding is viable, it is good practice to avoid mixing both techniques and to minimize the equipment used. We will be taking a look at the effects of welding and bonding on the design of assemblies. There are various types of welding, including spin, vibration, ultrasonic, hot plate, etc., all of which have their pros and cons regarding the assembly design specification.

Let’s take a look at some of these welding techniques.

Ultrasonic Welding

This is one of the most common welding techniques for plastics. The bonds resulting from this method are sound, permanent, and aesthetically appealing. Ultrasonic welding works through the transmission of mechanical vibrational energy at ultrasonic frequencies generated from an ultrasonic assembly unit. This ultrasonic vibration is transferred through a mating part to the area of the joint where the friction generated creates heat which, in turn, melts the plastic that then fuses to create a permanent weld.

Consider the following tips if you have selected the ultrasonic welding as a joining method:

• The thermoplastic must be suitably rigid to allow the transmission of the ultrasonic vibration to the joint location.
• Limitations arising from the equipment size and welding-horn design is a serious factor in determining the size and number of ultrasonic welds possible for every operation.
• Delicate components stand the risk of damage from stray welding energy and extra care and experience is required to minimize or avoid this possibility.

Vibration and Hot-Plate Welding

This method is used to create welds over a large seam between two parts. It is typically used when ultrasonic welding is not feasible, although the operating principle is not completely dissimilar. Vibration welding is also a friction dependent technique, requiring that the parts’ joint surfaces allow for sliding vibrations. The part geometry, in order to avoid weakening the vibration, must rigidly support the surfaces of the mating parts. The welding occurs with one part stationary and the other part vibrating along the joint plane and creating enough heat to melt the joint overlap. At that point, the parts are aligned and held together until the weld is set.

For permanent joints along a single plane, in which aesthetics is not a major consideration, hot-plate welding is a relatively cost effective option. This technique involves contact between a heated platen and two plastic parts until the joint interface is slightly melted. The platen is then withdrawn and the softened plastic parts are clamped until the weld sets.

Both hot-plate welding and vibration welding can cause flash along the interface of the joint when used for simple butt-weld configurations, so you should incorporate flash traps in your joint designs for applications where flash cannot be tolerated.

Spin Welding

Spin welding is used almost exclusively for welding continuous circular joints. As with the other methods of welding, spin welding employs the generation of heat to soften and weld the parts together. The only difference is the motion involved in generating this heat – in spin welding, one part is spinning against a part that remains stationary and the generated heat melts their interface. Then the spinning stops and the parts are held together until the weld is set. Parts that are joined together using spin welding also use flash traps to minimize flash.

Solvent and Adhesive Bonding

Solvent and adhesive bonding is arguably the least restrictive joining method for plastic parts in terms of design and part geometry. Bonds created using this method are permanent and involve the parts being joined by using a strong solvent to soften the joint interface before the parts are pressed together. This chemical reaction takes the place of using friction to partially melt the mating parts as in the welding techniques. Adhesives simply join two mating parts by adhering to both surfaces and then curing to form a rigid permanent bond.

Solvent bonding, despite allowing a lot of freedom in part design, has limitations in that the choices of plastic materials usable for this technique have to be compatible with the solvent and vice versa. When joining two different materials, the solvent must be compatible on both of them. Adhesive bonding, on the other hand, has a great advantage in that it allows the joining of not only dissimilar plastics but also with products that aren’t plastic – glass, metals, fabrics, etc.

A few things to keep in mind while using adhesive bonding:

• If you will be using polycarbonate resin, allow for vapor dispersion as the bond cures. Trapped vapor resulting from solvents can be corrosive to the resin.
• Consider the total time it takes the adhesive to cure and the total cost that can be incurred from the special requirements of using the adhesive.
• Adhesives that are UV-cured work best when you’re using a transparent or translucent plastic material. The design should allow access of the joint to a UV source.

In part three of this series, we will be looking at some other factors to keep in mind during the injection molding process in regards to how they influence the ease of assembly design.

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