Plastics joining joint design

Figure 8.24 Inside/outside fitment (a) small shaft-large hub (6) large shaft-small hub and (c) wedged joint. (Source Jordan I. Rotheiser, Joining of Plastics Handbook for Designers and Engineers, Hanser Publishers, Munich-Hanser I Gardner Publications, Inc., Cincinnati, 1999.)...

From the viewpoint of the adhesive technologist, plastics are an anomaly bonded joints can be made in which the plastic fails before the adhesive, but certain plastics are very difficult to bond. Although plastics often present difficulties in adhesive bonding (Table 4.2), all can be joined successfully with the correct choice of adhesive, joint design, and surface preparation [28] ... 

Understanding the principles of plastics joining, the capabilities of each method and proper implementation of the appropriate joint design, can provide the designer with a clear direction at the outset and a successful an repeatable assembly result for the part. 

Bolted, bon and bonded-bolted joints have barely been introduced. Further befTavioral and design information is available in the book Joining Fibre-Reinforced Plastics edited by F. L. Mathews [7-11], which has chapters written by various authors including two chapters on design by L. J. Hart-Smith. 

L. J. Hart-Smith, Design of Adhesively Bonded Joints, in Joining Fibre-Reinforced Plastics, F. L. Mathews (Editor), Elsevier Applied Science, London and New York, 1987, pp. 271-311. 

McCarthy MA, Lawlor VP, Stanley WF. An experimental study of bolt-hole clearance effects in single-lap, multi-bolt composite joints. J Compos Mater 2005 39(9) 799—825. Hart-Smith LJ. Design and empirical analysis of bolted or riveted joints. In Joining fibre reinforced plastics. Elsevier Science 1987. pp. 227—69. 

Hart-Smith, L.J., Design and analysis of bolted and riveted joints in fibrous composite structures, Douglas paper 7739, presented to International Symposium on Joining and Repair of Fibre-reinforced Plastics, Imperial College, London, 1986. 

PVC pipes can be solvent welded but mostly they are joined by mechanical compression or push-fit connectors. Various designs exist and the most successful are capable of connecting any type of pipe within specified diameter range. Some mechanical fittings for softer materials such as PE use compression of the pipe wall into grooves as a means of sealing the joint. This technique is unsuited to harder plastics such as PVC. The multiple application connectors rely upon compression of a separate elastomeric seal. 

Semidovetail joint. The sernidovetail joint is probably the most common joint used for position and contour control in plastic parts. The semidovetail joint is used in place of a full dovetail joint because the latter is not required since location in the inside direction is controlled by the joint on the other side of the part. More importantly, the semidovetail joint requires two-thirds the wall thickness of a fiill dovetail joint. A semidovetail joint around the entire perimeter of the part provides location, tends to mask minor warpage and debris from joining devices (surplus adhesive, solvent, welding flash, etc.), and is reasonably tolerant of dimensional variations. In addition, if designed steel-safe, it can be readily adjusted if the molded parts turn out to have too... 

Equipment costs for each method vary considerably, as do the amount of labor involved and e speed of the operation. Most techniques have limitations regarding the design of the joint and the types of plastic materials that can be joined. 

When two pieces of plastic must be joined together in service but must also be suited to simple disassembly, threaded joints are possible. Such joints may be plastic to plastic, as in toothpaste tubes and caps, where male and female threads are molded. If metal screws are to be used, female threads can be molded in a plastic part to accept the metal screw. If such a threaded application does not have to withstand much tension in the screw, a selftapping screw may be turned into a molded hole of appropriate diameter. Self-tapping screws designed for plastics applications are better suited than those for metal appUca-tions. Metal female-threaded inserts may be molded into plastic parts to accept metal screws in a subsequent assembly operation. Metal female-threaded inserts may also be staked into a molded hole of appropriate size, sometimes mechanically, often with heat ( heat staking ) to soften the thermoplastics at the interface so as to achieve more positive bonding (Figs. 9.16 and Fig. 9.17). 

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