The Molding Cycle

We can explain the injection molding process with reference to the molding cycle illustrated in Fig. 13.2. The cycle consists of four stages injection, packing, cooling, and ejection. 

The injection stage starts at the twelve o clock position in Fig. 13.2 the mold is closed at this point and a reservoir of molten polymer is accumulated ahead of the screw, as shown 

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. 

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Between 250 and 450°F (121 and 232°C), plastics used include glass or mineral-filled phenolics, melamines, alkyds, silicones, nylons, polyphenylene oxides, polysulfones, polycarbonates, methylpentenes, fluorocarbons, polypropylenes, and diallyl phthalates. The addition of glass fillers to the thermoplastics can raise the useful temperature range as much as 100°F and at the same time shortens the molding cycle. 

Are secondary operations required except to remove sprue No No Yes, e.g., removing material where a window is required (often done within the molding cycle)... 

An injection mold performs several functions during the molding cycle. In addition to giving the polymer the desired shape, it distributes the molten polymer, cools it, and ejects the product. 

The technique for monitoring the dielectric loss factor is relatively simple. Two metal electrodes are placed opposite each other at critical locations on opposite sides of the mold. When the sheet molding compound (SMC), is placed between the electrodes, a capacitor is formed. The dielectric power loss is monitored continually throughout the molding cycle, as outlined in Section 6.1.2.2. 

When the mold is opened, the part should be easily removable. Cavities are made with a slight taper to reduce frictional drag of the part on the mold. The half of the mold attached to the movable platen is equipped with ejector pins, which push the part out of the cavity while the mold is being opened. When the mold is closed, the pins are flush with the cavity surface. Release agents or lubricants facilitate ejection and shorten the molding cycle. Some complex parts require that the mold open in several directions in addition to the direction of the platen movement. For a threaded part, eg, a bottle cap, part of the mold must be rotated to remove the article from the mold. 

The work of Sterman and Marsden and Plueddeman was carried out with glasscloth polymer laminates (10, II, 14,15). In this way the effect of the silane could be optimized as to time and temperature of the molding cycle. Additionally, the glass was being used in its maximum strength form in that it was continuous. An additional factor is the absence of any shear forces. As an example, for a polystyrene laminate, the use of an epoxyfunctional silane increased the flexural strength 90% over the bare... 

Microprocessors that control processing improve the precision of the product (Chapter 3). It optimizes the process by controlling the machine cycle based on temperature and time, rather than time alone by using an infrared thermometer to monitor the mold s outer temperature. The molding cycle data for the oven and cooling chambers can be stored for different products and recalled when needed. Cycle time, oven temperature, major and minor axis speeds, fan and water spray times are typical of functions under complete computer control. 

The molding cycle takes anywhere from a few minutes to an hour depending upon the type of plastic used and the size of the charge. The cycle steps are... 

Preheating is often used to reduce the molding cycle. It can aid in providing even heat through the material and can cause a more rapid rise in heat than occurs in the mold cavity. A warm surface plate, infrared lamps, hot-air oven, or screw/barrel preheater can accomplish preheating. The best and quickest method is high-frequency (dielectric) heating. 

The elastomeric polyamides are relatively expensive (for Pebax about 10.00/kg for low-quality grades and about 17.00 for high-quality grades for Grilamid ELY the prices are almost double), but because in finished articles the density is lower, the wall thickness much lower, and the molding cycles shortened, the final cost may be lower than that of corresponding goods in rubber. 

Blends were produced in a small Banbury mixer. About 3 lb of dry-blended material was added to the Banbury. After the flux point, blends were run for two more minutes and dumped. Conditions were speed No. 2, 30 psi on the ram, and a dump temperature of 310°F. Test specimens were molded on a 3-oz Van Dorn injection-molding machine front, middle, and rear zones were 500, 485, and 470°F, respectively. The mold temperature was 160°F the molding cycle was typical for ABS. Test methods were ASTM Standard Procedures. 

Increased efficiency starts with the molding cycle. It must be reduced if gains are to be made in production optimization. A shorter molding cycle is the place where optimization starts. What automatically follows as a consequence of this is, increased... 

At the end of the molding cycle, the mold is opened and the part is ejected from the core. The part is allowed to cure under ambient temperature, during which shrinkage may occur because of the residual stresses developed owing to differential cooling at variable sections of the part. 

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