Escape slot

Design of the vent (Fig. 10.22) is determined somewhat experimentally. The width of the opening and the taper angle must be chosen such that there is rapid freeze off, minimal polymer escape, and burning is eliminated. The escape slot should be located farthest from the vent. This allows the slot to be used as an indication for the completion of the mold filling. A piece... 

Have about 15 percent less capacity because, when vapor escapes from the slots on the bubble cap, it is moving in a horizontal direction. The vapor flow must turn 90°. This change of direction promotes entrainment and, hence, jet flooding. 

When a parison is blown, a large volume of air must be displaced from the mold cavity in a short time. Because blowing is carried out at relatively low pressure, it is essential to provide venting to allow this air to escape without resistance. Unless a gloss finish is required on the molding, it is common practice to sandblast the cavity to a fine matt finish. This helps air to escape as the expanding parison touches the cavity face but it is not sufficient in itself. Vent slots may be cut at appropriate points into the mold parting face to a depth of 0.05 to 0.15 mm. The appropriate point is where there is a possibility for air to collect as the hot plastic expands in the cavity. 

Consider a simple model of impaction. Air issues from a long slot of width W at a velocity u. A surface is placed normal to the discharging flow a distance S away. With this configuration, air leaving the slot must make a 90° turn before it escapes. Particles that fail to make this turn strike or impact on the surface and are assumed to be retained by that surface. 

In 1950, Bromley (B3) presented a complete derivation for film boiling outside a horizontal tube. The derivation is based on the fact that liquid is separated from the tube continuously. Important assumptions are involved (1) The vapor film is smooth, and vapor escapes from the top of the tube as if issuing from a narrow slot. (Figures 9 and 10 show that this is not truly the case.) (2) The rise of vapor due to buoyancy is a viscous flow. (3) The kinetic energy of the vapor is negligible. (4) The liquid-vapor interface is at the liquid boiling point. (5) The temperature drop across the vapor film is assumed to be constant. (6) Heat... 

The mold must be vented to allow for gas escape such vents must be placed near weld lines and also near the last areas to be filled. Typical vents are slots 6 to 13mm (0.236 to 0.512in) wide and 0.01 to 0.03mm (0.0004 to O.OOlin) deep such slots are located on the mating surface of one of the mold halves. If a negative pressure device is available it may be possible to vent the mold into the water channels. This can speed up mold filling, reduce component burning and, reduce the cycle times. 

When a stud is inserted into a blind hole, the escaping air may blow some of the still liquid adhesive out of the thread. This must be prevented to give complete thread filling in critical applications by machining a slot or flat on the side of the stud. 

By using extraction as close to the source of pollution as possible to minimise the escape of the pollutant into the atmosphere. The extraction devices can be either hoods, slots, enclosures or fume cupboards coupled to a system of ducts, fans and air cleaners. 

Types of trays (a) bubble-cap tray vapor rises through openings in the plate, reverses direction, and escapes through the slots of an inverted cup (b) sieve tray (c) valve tray tray openings are adjusted by means of floating disks that rise and fall with the vapor flow rate. 

A unique feature of toe cap/finish used in CSD bottles is the interrupted threads. The interrupted threads create a vent slot, which provides an escape route for the gas in the bottle on slight turning of toe cap. The internal pressure generated on a standard 28 mm diameter cap, at room temperature, is about 15 kgj and can more than double at temperatures reached in storage or while in transport. 

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