to the front screw.This causes the screw to retract as the next shot is prepared. Once the part is sufficiently cool, the mould opens and the part is ejected. FOAMING PROCESS: This involves the blowing of a volatile organic liquid, which is entrapped into a polymer network resulting in the formation of foamed plastics. Foamed polystyrenes are produced in this process. COMPONENTS OF GEAR TYPE INJECTION MOULDING MACHINE: GEAR BOX: A gearbox is a mechanical method of transferring energy from one device to another and is used to increase torque while reducing speed. Torque is the power generated through the bending or twisting of a solid material. This term is often used interchangeably with . Located at the junction point of a power shaft, the gearbox is often used to create a right angle change in direction, as is seen in a rotary mower or a helicopter. Each unit is made with a specific purpose in mind, and the used is designed to provide the level of force required. This ratio is fixed and cannot be changed once the box is constructed. The only possible modification after the fact is an adjustment that allows the shaft speed to increase, along with a corresponding reduction in torque. In a situation where multiple speeds are needed, a transmission with multiple gears can be used to increase torque while slowing down the output speed. This design is commonly found in automobile transmissions. The same principle can be used to create an overdrive gear that increases output speed while decreasing torque.
Distinct shrink rates for general plastic gears A good method to determine shrink requires to two-step approach. Shrink factors are estimated for the gear in question. After the tool is made and the first gears are molded, they are then profile inspected for exact involute geometry. The individual shrink rates are then determined, a new cavity is made to the measured shrink and the final gear geometry is properly sized. Only profile inspection will be able to accurately determine involute shrinkage. Gear roll testing may give some idea of shrinkage anomalies, but it can also give misleading indications. Sometimes heavily glass filled material is selected for gears due to its low shrink rate. Shrinkage then becomes less of an issue in mold design. This approach can also cause its own problems. Unfilled engineering resins such as nylon and acetal mold into very precise shapes, albeit with shrinkage. Glass filled materials will have knit lines where injection flow fronts merge. These knit lines can cause distortion at the tooth surface as well as localized weak spots on the gear.
as the controlling geometry, this feature will be a variable in the actual part. It is necessary to set realistic tolerances for these truly variable features. The Involute Shrinkage of a Molded Gear Typical errors in a Molded Part The only way to be certain that a plastic molded gear is within tolerance is by scanning the involute profile and determining the actual physical geometry of the gear. The molded part can be completely out of specification and still give acceptable roll test results. Below is a profile inspection of such a gear. The involute base circle was very far off the defined value. The gear had 64 teeth and a master used to measure the gear had 64 teeth. With such a large number of meshing teeth in roll testing, there was almost no tooth-to-tooth error. The gear simply appeared large, even though the base circle was small. The molder thinned the teeth, brought the gear into