The basic concept of injection molding revolves around the ability of a thermoplastic material to be softened by heat and to harden when cooled. In most operations, granular material (usually through a feeding device known as a hopper), heated, and softened (plasticized or plasticated), forced out the other end of the cylinder, while it is still in the form of a melt, through a nozzle into a relatively cool mold held closed under pressure. Here, the melt cools and hardens until fully set-up. The mold is then open, the piece ejected, and the sequence repeated.

Thus, the significant elements of an injection molding machine become: 1) the way in which the melt is plasticized (softened) and forced into the mold (called the injection unit); 2) the system for opening the mold and closing it under pressure (called the clamping unit); 3) the type of mold used; 4) the machine controls.

Methods of melting and injecting the plastic differ from one machine to another and are constantly being improved. Conventional machines use a cylinder and pistom to do both jobs. This method simplifies machine construction but makes control of injection temperatures and pressures an inherently difficult problem. Other machines use a plasticating extruder to melt the plastic and piston to inject it while some have been designed to use a screw for both jobs. Nowadays, sixty percent of the machines use a reciprocating screw, 35% a plunger (concentrated in the smaller machine sizes), and 5% a screw pot.

Many of the problems connected with injection molding arise because the densities of polymers change so markedly with temperature and pressure. At high temperatures, the density of a polymer is considerably7 lower than at room temperature, provided the pressure is the same. Therefore, if molds wire filled at atmospheric pressure, “shrinkage” would make the molding deviate from the shape of the mold.

To compensate for this poor effect, molds are filled at high pressures. The pressure compresses the polymer and allows more materials to flow into the mold. Shrinkage is reduced and better quality moldings are produced.

Vocabulary

thermoplastic adj. 热塑性的

granular adj. 粒状的

resin n. 树脂

cylinder n. 圆筒

hopper n. 贮(chu)斗,装料斗

plasticize vt. 使塑化

plasticate vt. 使增塑

nozzle n. 喷嘴,喷头

eject vt. Throw out with force 推出,弹出

sequence n. the following of one thing after another in another

clamp v. 紧固

conventional adj. 常规的,普通的

piston n. 活塞

inherently adv. in itself or oneself

plunger n. 柱塞

shrinkage n. loss in size; becoming smaller 收缩

deviate vi. [from] be different

compensate vi. [for] provide (something) for some loss

Phrases and Expressions

revolve around 围绕着

in the from of 以…的形式

be connected with 与…有关

provided (that) 假如