Cooling holding pressure

Depressurization of the oxidation unit is achieved with the use of a high pressure control valve (Annin Wee Willie). This control valve can be set to operate in any one of three control actions proportional, reset, or derivative. Following the control valve, the hot products are directed to a water cooled high pressure condenser. Here they are cooled to ambient conditions before being sent to a holding vessel for GC andysis. 

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. 

The point on the pvT diagram at which the final isobaric cooling begins (4), controls the total part shrinkage. This point is influenced by the two main processing conditions —the melt temperature and the holding pressure as depicted in Fig. 3.43. 

The plasticating and injection unit. A plasticating and injection unit is shown in Fig. 3.45. The major tasks of the plasticating unit are to melt the polymer, to accumulate the melt in the screw chamber, to inject the melt into the cavity, and to maintain the holding pressure during cooling. 

The objective of this test method is to measure the cohesive stress and the time to failure of a crystalline polymer craze layer under rapid, uniform extension. The method is an impact variant of the Full Notch Creep test used by Fleissner [12], Duan and Williams [13], Pandya and Williams [14] and others. The specimen (Fig. 2), a square-section tensile bar, is injection moulded. At the mid-plane of the gauge length a sharp, deep circumferential notch reduces the cross-section to about one fifth of its original area. This notch plane is formed by a moulded-in, hardened steel washer. Specimens were injection moulded at 210°C into a warm (100°C) mould and air cooled to 40 C using a hold pressure of 45-50 bar. 

Pressures above 15 MPa help fill the mold before its components such as the mandrel and casings begin to cool. Above 25 MPa melt pressure, the mold part may crack at the gate. The equipment should allow control of transfer rate and transfer pressure. Once the filling is nearly complete (90%-95%), the transfer pressure should be lowered to half to two-thirds of its initial value to avoid stress concentration in the gate area. The pressure profile has to maximize hold pressure (Fig. 6.57) for packing without crack formation at the gate. 

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 status of mold and clamping unit during the holding pressure phase is identical with the one of the cooling phase. 

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). 

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

Warping Machine 1. Cooling time 4. Injection pressure 2. Holding time 5. Injection speed 3. Holding pressure... 

When the mould is hlled, the screw is held in the forward position or moves with a small displacement to maintain a holding pressure, during which time the material cools down and shrinks, allowing a little more material to enter the mold to compensate for volumetric shrinkage of the material. The packing phase is also called the holding phase. 

---