Molded screw thread

Articles of circular cross section may be made in iron paste molds. To keep the inner surface of the paste mold moist, it is coated with shellac or varnish and a mixture of charcoal and linseed oil is baked on. Hot iron molds ate used for ware of any shape, particularly for screw threading, multiple decoration, or raised lettering. 

The Hartford-Empire 28 is a press-and-blow machine used to make articles such as drinking glasses (tumblers). It uses paste molds and the ware is rotated to avoid the mold seams jars with screw threads cannot be produced. The product leaves the machine as an almost closed, hoUow object and is finished by severing and fire-polishing with a bum-off machine. 

Internal plastic thread The strength of plastic threads is limited, and when molded in a product involving either an unscrewing device or a rounded shape of thread similar to bottle-cap threads, they can be stripped from the core. Screw threads, when needed, should be of the coarse type and have the outside of the thread rounded so as not to present a sharp V to the plastic that can produce a notch effect. 

The relatively high thermal expansion of plastics poses a problem in the use of molded metal inserts which are sometimes required for electrical contacts, screw thread mountings, or increased strength. To minimize stresses due to inserts, manufacturers usually use plastics with low coefficient of expansion. 

In this design, as with the shape of screw threads, etc., the shape of the bead must be shaped so tibat it will slide out of its groove. Also, the relation betwe the height of the prHove k, the diameto- d, and the molding material is as ma-portant as that shown for striping screw threads. 

Machine Screws, Bolts. Parts molded of thermoplastic resin are sometimes assembled with machine screws or with bolts, nuts, and washers (Fig. 7.6), especially if it is a very strong plastic. The threads used to engage the mechanical fastener may either be molded into the part (molded-in threads or molded-in inserts) or attached after the molding operation (post-molded inserts). 

A process in which melted plastic is injected into a mold cavity, where it cools and takes the shape of the cavity. Bosses, screw threads, ribs, and other details can be integrated, which allows the molding operation to be accomplished in one step. The finished part usually does not require additional work before assembling. Any IPN in which the individual polymers are thermoplastic. The polymers may contain physical cross-links as in ionomers where ionic clusters join two or more chains together. Nowadays, phase-separated polymeric systems, e.g., block and graft copolymers or thermoplastic polyurethanes, are frequently considered thermoplastic IPNs. 

The high modulus and creep resistance of reinforced PPS compounds make them more suitable than many other plastic materials for fastening with bolts, screws, or rivets. For one-time bolted assemblies, tapped or molded-in threads with bolts screwed in three to four diameters may be used. However, with bolted assemblies where repeated assembly and disassembly of parts are expected, or if torque retention at elevated temperature is a concern, ultrasonic or molded-in metal inserts may be used. Molded-in metal inserts should be preheated to the mold temperature before being placed in the tool for molding. For assembly with self-tapping screws, BF-type or BT-type steel thread cutting screws work well, using hole sizes as recommended for phenolformaldehyde thermosets by the... 

Automatic ejection (or demolding). The molds are equipped with mechanical or electric driven screw-thread cores for moving parts of the mold. 

For rigid plastics and large undercuts, use must be made of movable or rotating side cores, which then obviously influence mold constructions. Screw threads are an example of an undercut frequently met. To eliminate undercuts, consider tapering a wall so that a sliding shutoff can be used (see Fig. 11-97). 

The choice of method, or of a combination of these mediods, is not only governed by the shape of the article and the properties of the polymer (its flexibility, rigidity, shrinkage, etc.) but also by the standards of quality to be met by flie article. For articles with an external screw thread, for instance, either the first method or the third can be used. However, if the first is used, the mold parting line shows, which may be undesirable for aesthetic or design reasons. The method used should depend entirely on the circumstances of the article. 

When covalent bonds form between carbon chains, a cross-linked polymer results and the physical properties change sharply (Fig 21.34[c]). Cross-linked polyethylene is used for plastic screw caps on soda bottles. This plastic is rigid enough to mold as a solid and has enough mechanical strength to hold the screw thread needed to tighten the cap on the bottle. 

The necessary mounting points in a rotomolded plastic product can be tightly fixed inside the mold as threaded inserts. Around the inserts, the plastic melts and forms a constant bonding. Threaded inserts can be held inside the mold with a screw, which has to be screwed in and out for each cycle. In industry, snap inserts (fixed inserts using spring loaded balls) or magnetic holders have established themselves. 

Along with the possibility to mold-in threaded inserts, threads can be shaped directly inside the plastic material. Specific thread formers made from brass, steel, or aluminum are used to shape either internal or external threads. These have to be manually screwed off to allow the demolding of the plastic products. 

Injection stretch blow molding processes use hollow preforms produced by injection molding. The preform is short and thick-walled relative to the finished blown article. The neck profile complete with screw thread is entirely formed by injection molding and is not modified by the blowing process. The other end of the preform is closed and typically dome shaped. 

Mechanical fastener connections are used when there is the need to disassemble the components. The most common reason for disassembly is for repair of the internal components of a device, which can be electronic, mechanical, or both. There are two main types of mechanical connections for assembling polymer components that will be considered. One type is the threaded inserts and bolts. The other common situation is when screws are threaded directly into the unthreaded polymer for mechanical fastening of another component. Threads can also be molded into the polymer component. Molding in threads increases the cost of the molding operation. 

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