Joining and fastening

When two surfaces made from the same thermoplastic have to be joined, welding is the preferred method. There are several ways in which welding can be accomplished, but they all depend upon melting the surfaces to be joined, and bringing them together under pressure. During this operation, melted material flows outwards from the weld, forming a bead on the side of the component, as illustrated in Fig. 8.8. 

In spin or vibrational welding, heating is achieved by rubbing the two surfaces together at a sufficiently high speed to melt the polymer by friction. Spin welding is suitable for circular objects, whereas vibrational welding can be achieved by either circular or linear motion. Total cycle times are of the order 

Press-fitting is used to join plastic parts to each other or to metals, especially to fit wheels on to shafts. The interference J, defined as the difference between the internal diameter d of the hub and the outer diameter of the shaft is given by 

8 (a) Section through a hot-plate welded joint in glass-reinforced polypropylene, showing how the melt flows outwards to form a bead, (b) In some designs, the weld bead is hidden. 

9 (a) An ultrasonic welding machine and (b) a section through a moulded part intetxJed for ultrasonic welding, showing the energy director.  

10 Use of the ultrasonic machine for staking, in which the top of a plastic peg is melted to form a retaining head, in a similar manner to riveting. 

11 Features of snap-fitting using cylindrical and cantilevered lug joints. 

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Table 5-1 presents a general guide for selecting joining and fastening methods for GRTP. 

Different methods are used for joining or fastening and assembling plastic products and plastic to other materials. It is im-... 

Mechanical Fastening, Joining, and Assembly, James A. Speck... 

They reduced the number of parts to be joined, and rejected, to the extent possible, the use of mechanical fasteners to eliminate stress concentrator sources of RPs. The design for the lOm/32.5 ft high vertical stabifizer and the two 7m/22.75 ft long horizontal stabilizer torsion boxes essentially involved tape-wound, hollow rectangular spar... 

The hole diameter should be not less than the thickness of the thinnest laminate being joined and be not more than one and a half times the thickness (1.0 h/t<1.5). The clearance of the hole should allow the fastener to be inserted easily, even when all the other fasteners are in place and linger tight, but should not be more than 5% of the fastener diameter. Great care is required, therefore, in forming the holes. 

P(2) The stiffness properties (elasticity and geometry) of the joined members and fasteners shall, directly or indirectly, be taken into account when determining the fastener load distribution and far field load distribution. 

Consider two plates with uniform thickness (Figure 5.9) which are Joined together by a certain number (n) of fasteners and subjected to a uniaxial tensile or compressive load Fx. The cross-sectional area of the fasteners is denoted by Aj. As the load is applied, the plates and fasteners deform and the load is distributed among the fasteners. 

Fastening, Joining, and Assembling Reference Issue, Machine Design, November 17, 1988. 

The principal means of assembling polyurethane RIM parts are fasteners and adhesives. For self-tapping screws, the pilot holes can be molded-in, providing dense skin surface for a better grip. The thermoplastic materials can be joined by fasteners and adhesives as well as all the welding techniques. 

Amino molding compounds can be fabricated by economical molding methods. Because of their poorer flow characteristics, the urea resins are usually compression-molded, but transfer molding is common with the melamines. Amino resins exhibit relatively high mold shrinkage. Product applications for amino resins include dinnerware, buttons, toilet seats, knobs, handles, ashtrays, food utensils, mixing bowls, and military equipment. Parts are typically joined with adhesives and fasteners. 

Thermal welding is the joining and sealing of thermoplastic garment materials without the use of adhesives or other means (sewing, staples, other fasteners). There is... 

Where permanent types are used, the parts themselves can be joined and narrow gaps filled with weld or brazing metal. Where separation is wider, a strip of metal can be welded or brazed at both ends across the gap. Bonds connecting one vibrating part to another part that may or may not vibrate should be of a flexible material that will not fail under vibration. Corrosion because of the joining of dissimilar metals may cause the electrical resistance across the bond to increase. This is especially noticeable in humid or corrosive atmospheres. The types of metal for the bond and its fastenings must therefore be selected with care. Grounds and bonds should... 

Fastening, Joining and Assembly Components. Machine Design. Reference issue, Nov. 

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