IX Международная студенческая научная конференция Студенческий научный форум - 2017

There are basically two methods of mechanical assembly for plastic parts. The first uses fasteners such as screws or bolts; the second uses an interference fit such as a press-fit or snap-fit and is primarily used in thermoplastic applications. This latter method of fastening is also called design for assembly or self-fastening.

If possible, the designer should try to design the entire product as a one-part molding because it will eliminate the need for a secondary assembly operation. However, mechanical limitations often will make it necessary to join one part to another using a fastening device. Fortunately, there are a number of mechanical fasteners designed for metals that are also generally suitable with plastics. There are also many fasteners specifically designed for plastics. Typical of these are thread-forming screws, rivets, threaded inserts, and spring clips.

As in other fabrication and finishing operations, special considerations must be given to mechanical fastening because of the nature of the plastic material. Care must be taken to avoid overstressing the parts. Mechanical creep can result in loss of preload in poorly designed systems. Reliable mechanically fastened plastic joints require:

1) A firm, strong connection

2) Materials that are stable in the environment

3) Stable geometry

4) Appropriate stresses in the parts, including a correct clamping force.[2]

In addition to joint strength, mechanically fastened joints should prevent slip, separation, vibration, misalignment, and wear of parts. Well-designed joints provide all of these without being excessively large or heavy or without burdening assemblers with bulky tools. Designing plastic parts for mechanical fastening will depend primarily on the particular plastic being joined and the functional requirements of the application.

Mechanical fasteners and the design of parts to accommodate them are covered in detail in later sections of this chapter. The following section describes the use of press-fit and snap-fit designs that are integrated into the molded part to achieve assembly.

Design for self-fastening

It is often possible, and desirable, to incorporate fastening mechanisms in the design of the molded part itself. The two most common methods of doing this are by interference fit (including press-fit or shrink-fit) and by snap-fit. Whether these methods can be used will depend heavily on the nature of the plastic material and the freedom one has in part design.[1]

Mechanical Fasteners

A large variety of mechanical fasteners can be used for joining plastic parts to themselves and to other materials. These include machine screws, self-tapping screws, rivets, spring clips, and nuts. In general, when repeated disassembly of the product is anticipated, mechanical fasteners are used.

Metal fasteners of high strength can overstress plastic parts, so torque-controlled tightening or special design provisions are required.

Where torque cannot be controlled, various types of washers can be used to spread the compression force over larger areas.

Special consideration for composites

The efficiency of a composite structure is established, with very few exceptions, by its joints, not by its basic structure. The selection of a joining method for composites is as broad as with metals. Riveting, bolting, pinning, and bonding are all possible. Only welding and brazing cannot be applied to thermoset composites. However, thermoplastic and metallic matrix composites can be joined by welding or brazing. Composites can be mechanically fastened in a manner similar to metals. Parts are drilled, countersunk, and joined with a fastener.

Rivets, pins, two-piece bolts, and blind fasteners made of titanium, stainless steel, and aluminum are all used for composites. Several factors should be considered:

1. Differential expansion of the fastener in the composite

2. The effect of drilling on the structural integrity of the composite, as well as delamination caused by the fastener under load

3. Water intrusion between the fastener and composite

4. Electrical continuity of the composite and arching between fasteners

5. Possible galvanic corrosion at the composite joint

6. Weight of the fastening system

Adhesive Bonding

Adhesive bonding presents several distinct advantages over other methods of fastening plastic substrates. Solvent and heat welding may be considered, as a type of adhesive bonding process where the adhesive is actually part of the substrate itself. Bonding, as a method of assembly, is often preferred when different types of substrates (for example, metals to plastics) need to be joined, when high-volume production is necessary, or when the design of the finished part prohibits the use of mechanical fasteners.[2]

Although there are various ways of joining plastics to themselves or to other materials, adhesive bonding has often proved to be the most effective assembly method. In many applications the use of adhesives rather than metal fasteners reduces product cost and the weight of the assembly, and, in some cases, provides longer service life. Adhesive bonding can also be used very effectively in prototypes and with large or intricate assemblies that for economic or design reasons cannot be molded or processed as a single part.

However, the joining of plastics with adhesives can be made difficult because of their low surface energy, poor wetting, and presence of con-taminants such as mold release agents, low molecular weight internal components (for example, flexibilizers, UV inhibitors, and processing aids), and possible susceptibility to moisture and other environmental factors.

Adhesive bonding (also referred to as gluing or glue bonding) describes a water bonding technique with applying an intermediate layer to connect substrates of different materials. These produced connections can be soluble or insoluble. The commercially available adhesive can be organic or inorganic and is deposited on one or both substrate surfaces. Adhesives, especially the well-established SU-8, and benzocyclobutene (BCB), are specialized for MEMS or electronic component production.[4]

Adhesive bonding has the advantage of relatively low bonding temperature as well as the absence of electric voltage and current. Based on the fact that the wafers are not in direct contact, this procedure enables the use of different substrates, e.g. silicon, glass, metals and other semiconductor materials. A drawback is that small structures become wider during patterning which hampers the production of an accurate intermediate layer with tight dimension control. Further, the possibility of corrosion due to out-gassed products, thermal instability and penetration of moisture limits the reliability of the bonding process. Another disadvantage is the missing possibility of hermetically sealed encapsulation due to higher permeability of gas and water molecules while using organic adhesives.

References

1. Terry A. Richardson. Machining and Finishing // Modem Industrial Plastics. - College Audience, 2003. P. 13-22.

2. John L. Hull. Design and Processing of Plastic Parts. // Handbook of Plastics Elastomers and Composites. / Charles A. Harper, ed., McGraw-Hill –

New York, 2005. P. 247-269.

3. J. O. Trauemicht. Bonding and Joining, Weigh the Alternatives, Part 1 // Solvent Cements, Thermal Welding / Plastics Technology. – 1970. P. 125-128.

4. Charles A. Harper. Engineer’s Guide to Plastics // The Complete Guide to Properties and Performance / Materials Engineering - 1972. P. 101-118.

5. Edward A. Muccio. Finishing and Decorating Plastic Parts. // Plastic Part Technology. / ASM International - Materials Park. – Ohio. – 1991. P. 21-28.

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