Ejection, molded part

In blow molding, a tube of molten plastic material, the parison, is extruded over an apparatus called the blow pipe and is then encased in a split mold. Air is injected into this hot section of extruded stock through the blow pipe. The stock is then blown outward, where it foUows the contour of the mold. The part is then cooled, the mold opened, and the molded part ejected. In very heavy sections, carbon dioxide or Uquid nitrogen may be used to hasten the cooling. This process is widely used in molding high- and low-density polyethylene, nylon, polyvinyl chloride (PVC), polypropylene, polystyrene, and polycarbonates. 

This method, like extrusion, requires a molten polymer. The equipment is schematically illustrated in Fig. 3.5a. In this method, polymer granules are fed into a hopper and a screw transports the polymer through a barrel where the polymer is heated to make it flow, and the polymer melt is forced through a nozzle into a closed mold. The mold is cooled after it is filled, and the molded part ejected, and the equipment readied for the next shot. Injection molding is thus a semi-continuous batch process and is usually used for large-scale production, where high throughput and reproducibility are important. The process is economical. The method is commonly used for thermoplastics. E.g. PLA, PGA and copolymers. Some... 

Because of the high wall adhesion of the molded parts, ejection can be influenced by different mold surfaces. Polished surfaces tend toward strong adhesion due to the silicone roughened surfaces tend to allow for easier demolding. 

The injection mol ding process eliminates the restriction of straight-sided components required when parts are ejected from a die, and offers opportunities for external undercuts and threads. A wide variety of alloys can be processed, including alloy steels and stainless steels. Material properties of injection molded parts are available (32). 

Injection molding involves two distinct processes. The first is melt generation, mixing, and pressurization and flow, which is carried out in the injection unit of the molding machine. The second is product shaping, which takes place in the mold cavity. Injection molders, therefore, have two distinct parts the injection unit and the mold/clamping unit. The function of the injection unit is to melt the polymer and inject it into the mold the function of the clamping unit is to hold the mold, open and close the mold, and eject the finished product. 

Compression molding involves the use of preforms (bulk molding compounds, sheets of glass fibers impregnated with the thermoset precursors, etc.) that are placed in a heated mold. When the mold is closed, the plastified preform flows to fill the mold and cure takes place in about 1-2 min. Then, the mold is opened and the final part ejected. 

Crystallization temperature and the heat of crystallization at various cooling rates (or the ejection temperature of the molded part) ... 

The clamping debars (rods) can be used to support the fixed and movable platens on which the mold is attached. They serve as equally loaded tension support members of the clamp when the mold is closed. The open distance between tie rods through which the mold must fit and eject molded parts sets up the maximum outside dimensions of the mold that can be used. Different designs are used to meet different... 

The body mold closes on the parison that is blown normally by a neck calibrating blow pin. Immediately, with the mold still closed, the liquid contents are injected through the pin. The pin is then withdrawn and the neck is formed and sealed under vacuum by the neck-forming members. Both mold parts then open to eject a filled and sealed container. Small containers may be formed entirely by vacuum rather than blowing. 

Internal mold release agents are added to facilitate part ejection from the mold. The most common types of agents include metal stearates, fatty acids, fatty acid amides, and hydrocarbon waxes. Zinc stearate and calcium stearate are the widely used mold release agents in SMC and BMC. 

Warpage results from residual stresses within a molded part. The ejected part is not constrained by the mold and, if its residual stresses are higher than the modulus of the material, material distortion occurs. Some fillers can help to decrease warpage. 

The main processing technique in the automobile industry is injection molding in which pressure is applied to make the polymer flow into a preferred form [3]. Plastics must be cooled below the Tm (melt transition temperature) before removal from the mold [3], The molding process follows a basic injection cycle consisting of mold close, injection carriage forward, injection of material, metering, retraction of carriage, mold open, and part ejection. This cycle has been in place basically since 1946,... 

The blends of PC with PET, PBT and PETG were opaque whereas the blends with PCT and PCTG were transparent. The blends of an unidentified copolycarbonate, (coPC) with PET, PBT, PETG and PCTG were transparent. The opacity of the blends was attributed to their crystallinity. Mason [1995] found the irradiated (30 kGy, gamma irradiation) DB to show more acceptable yellowness index than irradiated PC (3 for DB as compared to 15 for PC), good injection molding characteristics (spiral flow similar to that of PC), similar part ejection force for PC and DB, and reasonable chemical resistance. 

The injection mold is a series of steel plates, which when assembled produces the cavity that deflnes the shape of the molded part. Conventional molds consist of the mold frame, components, runners, cooling channels, and ejector system. The mold frame is a collection of steel plates that contain mold components and runners, cooling and ejection systems. Components are parts inserted into either bored holes or cutout pockets in the mold frame. The polymer melt enters into the mold cavity or cavities through the runners, which are passages cut into the mold frame. The hot polymer material in the mold... 

Solution In compression molding of thermoset, once the resin is cured the molded part can be ejected while still hot and allowed to cool outside the mold. Qn the other hand, compression molding of thermoplastics involves heating followed by cooling to solidify the molded part. Therefore, in compression molding of thermoplastics the relatively longer heat-and-chill cycle times involved are imeconomical for large volume production. 

Under transfer molding, the mold is first closed. The plastic material is then conveyed into the mold cavity under pressure from an auxiliary chamber. The molding compound is placed in the hot auxiliary chamber and subsequently forced in a plastic state through an orifice into the mold cavities by pressure. The molded part and the residue (cuU) are ejected upon opening the mold after the part has hardened. Under transfer molding, there is no flash to trim only the runner needs to be removed. 

The mold is cooled, usually by water, to solidify the plastic. When the plastic object has been cooled sufficiently to maintain its shape, the mold is opened, along the mold parting line, and the object is ejected. The cycle time in injection molding depends on the size of the molded part, as well as on the molding conditions and on the thermal and mechanical characteristics of the plastic being molded. In some case, the cycle time can be as short as two seconds. 

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