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There are a number of barrier screw designs available which offer some benefits not provided by general purpose screws. Barrier screws provide more efficient mixing without increased backpressure and, in some cases, recovery times may be decreased. These advantages are offset by the increased risk of black speck formation. The deep flights in a barrier screw may have stagnant areas in which there is a reduction in the flow of the material. The molten plastic tends to stay in these areas and degrade, ultimately causing black specks in the parts as the degraded material flakes off the screw. When purchasing barrier screws, it is recommended that the molder work closely with the screw designer to ensure that stagnant areas are avoided and that the screw is properly designed for the material being used.
Plasticating screws for thermoplastics generally have interchangeable tips.The two most commonly used tips in the injection molding of polyolefins are sliding check ring and ball-check non-return valves. In the molding cycle, as the screw moves forward to inject material into the mold, the non-return valves close to prevent material from flowing back over the flights of the screw. Typical sliding check ring and ball check valves are shown in Figures 19 and 20.
Because of their tendency to wear, it is critical to periodically inspect the condition of sliding ring shut-off tips. Excessive wear will result in inconsistencies in shot size and melt temperature.
Figure 18. Screw type configurations used in injection molding machines
The typical length-to-diameter (L/D) ratio for polyolefin reciprocating screws is about 20-30:1, with a compression ratio of 2-3:1. Longer screw lengths are generally preferred as they provide better homogeneity of temperature and melt quality.
Nozzles
The injection-unit nozzle is connected to the barrel and directs the flow of the melt into the mold. The nozzle extends into the fixed platen and mates to an indentation in the front of the mold called the sprue bushing.
The nozzle may have a positive shut-off device or it may be open and rely on the freezing-off of the melt in the gate areas of the mold to keep the resin from flowing back into the injection unit. Some nozzles may be connected to a temperature control device to control the melt temperature.
Clamp mechanisms
There are three basic types of injection-molding-machine clamps: mechanical, also called toggle units, hydraulic and a combination of these called hydromechanical clamps.
Toggle clamps, which are less expensive to build, are most widely used on small tonnage machines (typically, less than 500 tons). The toggle action can best be understood by looking at your arm when it is bent at the elbow and then when it is fully extended. In the toggle clamp, a hydraulic cylinder moves the unit’s crosshead forward, extending the toggle links and pushing the platen forward. The mechanical advantage is low as the clamp opens or closes, which permits rapid clamp movement. This action slows and the mechanical advantage increases as the platen reaches the mold-close position. The slow speed is important for mold protection.
Figure 19. Typical sliding check ring showing injection stage (top) and retraction stage (bottom)
Figure 20. Typical ball check assemly showing injection stage (top) and front discharge-retraction stage (bottom)
Full clamp pressure is reached when the linkage is fully extended. To adjust the toggle clamp to different mold heights, the entire toggle mechanism and moving platen assembly are moved along tie rods.The position of the toggle mechanism depends on where the mold closes when the toggle is at full extension. The toggle opens when hydraulic pressure is applied to the opposite side of the clamp cylinder. See Figure 21.
Figure 21. Toggle clamping system
Hydraulic clamps generally are used on injection molding machines in the 150-ton to 1,000+-ton clamp tonnage range. In this type of clamp, hydraulic oil is used to move the platen through the full closing and opening strokes. The fluid is metered into a reservoir behind the main ram. At first quite rapid, the oil flow is slowed as the ram reaches the mold-close position in order to protect the mold. An oil fill valve closes when the mold is closed. The area behind the ram is then pressurized to build full clamp tonnage. To open the mold, the oil valve is first partially opened to smoothly open the mold. Once the mold halves are separated, the clamp accelerates to a fast open speed (Figure 22).