The System of Plastic Injection Mold
The cycle begins with the mold closing fast to just before the leader pins where mold safety is set, the closing speed and pressure is set low to prevent accidental mold damage. When both halves of the mold touch high pressure is built to create clamp tonnage.
Plastic injection molding uses thermoplastic materials (plastics that soften and fluidize when heated.) The plastics used in the injection molding process are liquefied at temperatures between 220° C and 270° C. The thermoplastic material used in the plastic injection molding process is supplied in the form of small pellets. These pellets are loaded into the hopper, then gravity-fed into the barrel and the screw assembly.
The reciprocating screw compresses and melts the plastic material. It also moves the material toward the mold. The reciprocating screw is divided into three zones: the feeding zone, the transition zone, and the metering zone.
· Feeding zone – where the plastic first enters the barrel and screw, it conveys the plastic forward
· Transition zone – the plastic starts to melt by shear heat, the flight depth of the screw continue to get decrease
· Metering zone – the final section, with shallowest of the flight depth, it improves the melt and color quality
The outside diameter of the screw remains constant throughout these zones; but the depth of the screws' flights decreases from the feeding zone to the metering zone. The flights of the screw compress the material against the inside of the barrel. This motion creates viscous (shear) heat. Although there are heater bands on the barrel, this shear heat is the primary heat that melts the plastic. (The main function of the heater bands is to maintain the temperature of the molten material.) Plastic is melted by 80% of the screw and 20% by heater bands.
The nozzle forms a sealed connection between the barrel and the sprue bushing of the mold. The temperature of the nozzle is usually set near the plastic’s melt temperature. The mold’s sprue bushing has a concave radius. The nozzle fits into this radius with the aid of a locating ring. The melted plastic travels to the mold via a sprue bushing, then a runner system and finally the gate. This process is called plastication.
When the part is about 95% full, called the 1st stage of injection is complete the part begins to take shape in the mold cavity. This begins the pack and hold phase of the process. The final 5% of the part is controlled by velocity and pressure. The pack and hold phase continues until the gate is sealed to stabilize the part dimensions.
Cooling time is dependent on wall thickness, plastic actual melt temperature, mold temperature and shrinkage. The better designed part more efficient the cooling the faster the cycle time lower the cost. Water is channeled through the mold to improve the cooling. During the cooling time plastication takes place.
Plastication is the process of melting plastic and preparing the next shot. The plastic is dried to a manufacturer specification as specified on a material data sheet. Once dried the plastic is conveyed from a hopper to JIT loader and enters the feed throat. The feed throat has water lines through it to prevent bridging of plastic at the feed throat.
Attached to the front of the screw is a non-return valve. The purpose of the non-return valve is to act as a plunger and extruder. The non-return assembly allows material to flow in front of the screw creating the next shot. During injection, the non-return assembly prevents the shot from flowing back into the screw sections.
The mold will open when the next shot is created and the cooling time has timed out. The mold will be set to open slowly to release the vacuum that is caused by the injection molding process and prevent the part staying on the stationary side of the mold. Then pick up speed and then slow down at end of travel. The end of travel will be determined by the design of the part to minimize the cycle time. Once the mold is open, the part is ejected usually by the ejector pins. When the ejector pins are retracted and all criteria of the molding cycle are met the next cycle begins.