Cooling ejector pins

A further source of stress may arise from incorrect mould design. For example, if the ejector pins are designed in such a way to cause distortion of the mouldings, internal stresses may develop. This will happen if the mould is distorted while the centre is still molten, but cooling, since some molecules will freeze in the distorted position. On recovery by the moulding of its natural shape these molecules will be under stress. 

Once the polymer has cooled to its solid state, the molding is ejected. This is accomplished with the aid of ejector pins that protrude from the mold walls as it opens. Small items typically drop directly into a catch pan or onto a conveyor belt below the mold. Larger items are removed manually. 

If there are other methods of ejection planned, such as the use of strippers or aif ejection, the ejector pins and their mechanism can add considerably to the mold cost. Also, the effect on cooling (as outlined before) will increase, tbe v molding cycle and thereby reduce the productivity of the mold. 

At the end of the molding cycle, the mold opens and cooled parts are ejected from the injection mold. This requires the ejection system shown in Fig. 5.64.1 When the mold opens, the plastic part typically stays with the B side of the mold. Once the mold is opened, the hydraulic ejection cylinder extends, forcing the ejection platen forward (in all-electric machines, the cylinders are replaced by servomotors). Since the platen is usually tied to the mold s ejector plate by ejector rods, the ejector plate also moves forward. This forces the ejector pins (in the mold) forward... 

The point at which the cap is cool enough to eject, yet warm enough to strip off the core, will vary according to the means of ejection employed. Ejector pins provide very localized forces at the base of the cap. An ejector plate creates an ejection force which is distributed uniformly across the base of the cap. Therefore the cap can be ejected in a softer condition with the use of a stripper plate. That results in a cycle... 

Part release Once the resin is cured and the part is solidified, the part is removed from the mold with the aid of ejector pins. Then, the part is cooled down outside the mold, while the mold surfaces are cleaned, and an external mold release agent is applied to the mold surfaces for the next molding cycle. 

The injection molding process consists essentially of three consecutive steps, starting with supply and softening of the thermoplastic material in a heated cylinder and its subsequent injection, under high pressure, within a relatively cold mold, where the material gets hard and takes the final shape. Finally, the molded article is cooled and expelled from the mold by ejector pins, compressed air or other auxiliary equipment. ... 

Part ejection is accomplished with ejecting devices similar to injection molding ejector pins. Care should be taken to assure that the part stays in the mold half which has the ejector pins. It is important that the ejector pin be flush with the mold to prevent an increase in the cooling cycle and/or formation of sinkmarks. 

A diagram of the injection molding process is shown in Figure 6.22. The extruder is similar to other fabrication techniques where the granular resin enters a hopper, where the material is transferred to a heated barrel and a reciprocating screw moves the molten plastic to the molding cavity. The molding cavity is unique to this process. The molten polyethylene is forced xmder pressure into a closed mold that is continually cooled. After the molded part cools, the mold opens and the fabricated part is ejected from the mold. An injection molded article can usually be identified by the ejector pin marks that are usually present on the molded part. 

Figure 7.9 Sequence of mold operations. (a) Plastics material is injected into the closed mold. The mold remains closed while the molding cools, b) The mold opens, leaving the molding attached by shrinkage to the core, (c) The ejector plate is moved forward, causing ejector pins and stripper bars to push the molding off the core.

Tooling. Tooling is relatively expensive for injection molds. Production molds are normally machined and burned (electro-discharge machined) from tool steels (P20) and must be sfructured to withstand high pressures. Precision is required to incorporate cooling lines, ejector pins, and cavity pressure transducers or temperature monitoring sensors. The use of hot runner manifolds adds further complexity and cost, but may be justified because cold runners and their associated costs are eliminated in manufacturing. 

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