Cooling Holding pressure, injection molding

Part sticking Machine 3. Cooling lime 1. Injection pressure 2. Holding pressure 4. Mold temperature 5. Holding time 6. Injection speed... 

In micromolding, both surface patterns were replicated to the ratio HR over 0.95 at standard molding condition. HR decreased with increasing distance from the gate in the case of 50 and 100 pm in depth. We could conclude that replication properties were affected by cooling rate and final polymer flow. Moreover, HR increased with increase of injection speed, holding pressure, injection temperature, and mold temperature. 

The sequence of events during the injection molding of a plastic part, as shown in Fig. 3.40, is called the injection molding cycle. The cycle begins when the mold closes, followed by the injection of the polymer into the mold cavity. Once the cavity is filled, a holding pressure is maintained to compensate for material shrinkage. In the next step, the screw turns, feeding the next shot to the front of the screw. This causes the screw to retract as the next shot is prepared. Once the part is sufficiently cool, the mold opens and the part is ejected. 

For the material within the mold cavity, the holding pressure phase is concluded as soon as the sprue solidifies-in other words, as soon as it is sealed. From this point on, no more material can enter the cavity. During the injection and holding pressure phases, the material in the cavity has already begun to cool against the relatively cold mold wall. The outer layers have soldified very quickly. The cooling time therefore starts as early as injection. 

The injection molding proeess ean be divided into the following phases injection, holding pressure, cooling, feed and. ... 

Standard dumbbell specimens (NFT 51-034-1981) are molded with an injection molding machine (DK codim NGH 50/100) in a temperature range between 115°C and 130°C, with an injection pressure and speed of 500 bars and 50 mm/s. Holding pressure and times are 500 bars and 12 s (PBAT) or 14s (biocomposites), respectively. Mold temperature is 30°C. The total cooling time is 22 s (neat PBAT) and 24 s. The injection-molded specimens are approximately 10-mm wide and 4-mm thick on the central part (NFT 51-0.34 1981). 

Depending on the design of the machine nozzle, with or without shut-off mechanism, it is time for the injection unit to move back either just after the end of holding pressure in case of a shut-off nozzle, so preventing the smallest amount of heat flow from the nozzle to the cooled mold (Figure 1.7) or after the end of the plasticizing process. 

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