Design in Plastics - PART 5

Plastics Assembly Methods

Previous articles examined design considerations for injection-molded parts (August 2000), physical and mechanical properties (November 2000), thermal, electrical and chemical properties (February 2001) and injection molding and design with plastic (May 2001).

By Frank Jaarsma, Ticona Corporation, Summit, NJ

The best way to deal with parts assembly is to avoid it entirely, instead molding multiple parts as one unit. The next best option is to minimize handling through proper design and by choosing the best joining method. This article reviews the most common ways for assembling plastic parts, which range from molded-in features like snap fits to chemical bonding, thermal welding and mechanical fasteners.

Molded-in assembly features are fast, inexpensive and minimize errors. Many popular CAD programs make these elements easy to create during the design process.

Figure 1
Basic Snap-Fit Designs

The best examples are snap-fits, which contain an element that flexes past an interference and rapidly returns to its unflexed position (Fig. 1). Snap-fits are useful for high-volume production for either one-time assembly or for release and reassembly over hundreds of cycles, such as battery compartment covers in calculators. In designing snap fits, avoid sharp corners or structural discontinuities that increase stress. A tapered finger provides more uniform stress distribution. Snap-fits may require complex tooling, such as molds with side action that generally need undercuts. Snap-ons or snap-ins are variations of this design for round part applications. They involve small deflections of large portions of a part or even the entire part (Fig. 2).

Figure 2
Typical Snap-On and Snap-In Configurations

Another type of molded-in feature that allows rapid assembly contains male and female threads for containers, caps and other parts. Internal threads need more complex tooling (e.g., unscrewing or collapsing mechanisms) than external threads, which can often be molded by splitting the mold cavity across the parting line. It usually is not practical to mold threads finer than 28 pitch.

Press-fits involve inserting a pin into a hole in a boss and usually need relatively simple tooling. They should be designed to prevent excessive hoop stress in the boss that can cause creep, stress relaxation, fractures or crazing.

Chemical bonding uses solvents and adhesives to join similar and dissimilar materials. It is fast, economical, requires little or no part preparation or special equipment, creates excellent liquid and gas seals, and causes no assembly stresses.

In solvent bonding, a liquid applied just before assembly dissolves the joint surfaces. This is enough for a weld to remain after the solvent evaporates. This method is limited to compatible materials that dissolve in the same solvent or solvents. The chemical resistance of many plastics, especially crystalline resins, limits this method.

In adhesive bonding, a third substance bonds a plastic to another plastic or to metal, rubber, ceramic, glass, wood, etc. Adhesives frequently used with thermoplastics include epoxy, acrylic, polyurethane, phenolic, rubber, polyester and vinyl. Cyanoacrylate adhesives are popular because they work rapidly. Many adhesives contain solvents that partially dissolve the plastic surface, which improves adhesion. Surface preparation is also critical for successful adhesion. Many materials must be roughened or etched to eliminate overly smooth surfaces. They also may need thorough cleaning because grease, mold release compound, and other contaminants can spoil a bond.

Joints for this type of bonding can take many forms (Fig. 3) and should be tested in the actual end use. Testing should also make sure the chemicals used do not deteriorate the plastics. This effect is generally slow, often requiring long clamp times and sometimes special ovens or curing conditions. In addition, the chemicals used may be toxic, so worker protection, ventilation and solvent recovery can be issues.

Figure 3
Joint Designs for Solvent and Adhesive Bonding

Thermal welding involves melting the bond line between two parts to form a weld. This method is a fast, economical, and safe way to weld compatible plastics having similar melt temperatures.

Ultrasonic welding is the most common thermal method for joining small and medium-sized parts of amorphous and crystalline plastic. The process normally lasts less than 2 seconds and forms a continuous, leak-proof joint that often is as strong as the base material, using equipment that applies high-frequency energy (20 to 40 KHz) directly to the interface between parts. Hygroscopic resins should be welded as soon as possible after molding, since moisture can weaken the bond.

In vibration welding, parts are rubbed together to create frictional heat. Rubbing usually involves amplitudes of 0.1- to 0.2-in. and frequencies of 120 and 240 Hz. It creates strong joints and works best with large parts that have irregular joint interfaces.

Spin welding joins parts with circular joint surfaces using relatively simple equipment, sometimes just a drill press. It involves holding one part firmly and pressing a rotating part against it at a steady pressure. The weld usually forms in less than 3 seconds.

Radio frequency (RF) welding, or heat sealing, is widely applied to flexible films and sheets of such materials as vinyl and polyurethane. It is also used to join films and injection-molded parts. Welding occurs as heat from a strong RF field is generated by a metal die in the shape of the joint. The die also applies clamping pressure to complete the weld.

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