Threads molding

The core then unscrews the threaded, molded part. 

Fine Powder Resins. Fine powder PTFE resins are extremely sensitive to shear. They must be handled gendy to avoid shear, which prevents processing. However, fine powder is suitable for the manufacture of tubing and wire insulation for which compression molding is not suitable. A paste-extmsion process may be appHed to the fabrication of tubes with diameters from fractions of a millimeter to about a meter, walls from thicknesses of 100—400 )J.m, thin rods with up to 50-mm diameters, and cable sheathing. Calendering unsintered extmded soHd rods produces thread-sealant tape and gaskets. 

Screw Assembly. Self-tappiag screws are used for joining ETEE parts. Eor maximum hoi ding power, the boss diameter should be about double the screw diameter, and the engagement length about 2.5 times the screw diameter lubricants should be avoided. Threaded inserts can be molded ia place, pressed ia, or driven ia ultrasonicaHy. 

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. 

The injection mol ding process eliminates the restriction of straight-sided components required when parts are ejected from a die, and offers opportunities for external undercuts and threads. A wide variety of alloys can be processed, including alloy steels and stainless steels. Material properties of injection molded parts are available (32). 

When the mold is opened, the part should be easily removable. Cavities are made with a slight taper to reduce frictional drag of the part on the mold. The half of the mold attached to the movable platen is equipped with ejector pins, which push the part out of the cavity while the mold is being opened. When the mold is closed, the pins are flush with the cavity surface. Release agents or lubricants facilitate ejection and shorten the mol ding cycle. Some complex parts require that the mold open in several directions in addition to the direction of the platen movement. For a threaded part, eg, a bottle cap, part of the mold must be rotated to remove the article from the mold. 

Threads can be molded or tapped into a plastic. Molded internal threads usually require some type of unscrewing or collaps-... 

Control temperature of mold surfaces, to chill plastic to rigid state From side holes, slots, undercuts, threaded sections... 

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. 

If a self-threading screw can be substituted, it will not only appreciably decrease mold maintenance and mold cost but most likely, with proper type selection, also give better holding power. A screw that has a thin thread with relatively deep flights can give high holding power. If the screw or plastic is preheated... 

Bottle caps made from different plastics are extensively used. Some closures are of the simple cork snap type design, but most are of the screw type. Strong, accurate threads can be molded, which represent undercuts. Simple designs should be used when permitted, such as wide-pitch threads. The thread should be designed to start about 1 /32 in. (0.08 cm) from the end of the face perpendicular to the axis of the thread. It is usually practical to mold up to 32 threads per in. more than this number can give certain molders trouble. 

Moldable holes and inserts can complicate molding and may require extra postmolding operations. Thus, the most economical designs are those that minimize the number of holes. With many plastics, both external and internal threads can be molded, but sharp V threads should be avoided because they can cause the plastic to bridge, resulting in incomplete thread fill. Rounded or modified buttress threads will allow improved thread fill. 

Fig. 8-47 Simple complex EBM shape that includes a threaded core using a 3-part mold.

Nylons belong to the class of polymers known as engineering polymers that is, they are strong, tough, and heat resistant. We can readily extrude and mold nylons to form a wide variety of useful objects, such as tubing, furniture casters, and automotive air intake ducts. Nylons are commonly spun into filaments or fibers. These can be used directly, or braided, or twisted to form threads, yarns, cords, and ropes, which may be further woven to make fabrics. In their fibrous forms, nylons are used in carpets, backpacks, and hosiery. 

The choice of clamping system is very much dependent on the rate of production. Conventional threaded bolts are the simplest choice, but they may also be the most cost efficient for low volume production with cycle times in the range of hours. On the other hand, for high-volume production mold closing as well as mold opening must be achieved much quicker, and the choice may be a hydraulically driven mold carrier. 

---