Blow molding venting

When a parison or preform is inflated, it displaces the air around it within the mold. If no provision is made to vent the mold, compression of the air around the parison or preform can raise its temperature to such an extent that it can scorch the surface of the product. To avoid this problem, we equip blow molds with vents. These can consist of slit vents at the parting line between mold halves, porous plugs of sintered metal, or small holes drilled into the cavity walls. 

Level Control Filler. Plastic bottles can be manufactured clear thus for consumer appeal, they are filled to a level. This means the volume may vary but will appear to be equal due to the visually perceived common level in the containers. Flow is by gravity through a nozzle into a sealed container, whereas air equilibrates via a vent tube (Figure 8). When the rising liquid reaches the air vent port, flow stops. There is no overflow of product as in a pressure or pure vacuum filler. Aeration is at a minimum, perceived fill level is extremely accurate, and the filler is relatively simple and easy to maintain. Size consistency of the blow-molded container is critical to control, because this dictates the amount of product being sold. 

FIGURE 2.21 A two-stage vented extruder with a valved bypass. (After Fisher, E. G. 1971. Blow Molding of Plastics. Iliffe, London.)... 

Generally, the blow mold is a cavity representing the outside of a blow-molded part The basic structure of a blow mold consists of a cast or machined block with a cavity, cooling system, venting system, pinchoffs, flash pockets, and mounting plate. The selection of material for the construction of a blow mold is based on the consideration of such factors as thermal conductivity, durabrhty, cost of the material, the resin being processed, and the desired quality of the finished parts. Commonly used mold materials are berylHum, copper, aluminum, ampcoloy, A-2 steel, and 17-4 and 420 stainless steels. 

Poor detail definition Blow air pressure too low Poor mold venting Cold mold Increase blow air pressure and blow time Improve venting Increase mold temperature... 

Vents are placed along the mold-parting surface to allow the escape of trapped air between the expanding parison and the cavity. If these are too deep, an objectionable mark is left on the bottle. Because an air pressure of 1 MPa (145 psi) is used in injection blow molding, these vents are kept less than 0.05-mm deep. 

A roughened cavity surface is very helpful in blow-molding PE to assist in the movement of trapped air to the mold vents and to improve heat transfer rates. 

All blow molds have to be vented. The air that first occupied the product area must escape more rapidly than the rate at which the hot plastic is blown to fill the product area within the closed blow mold. This is known as venting. 

Increase material temperature Increase mold temperature Improve mold venting Increase air pressure Increase air blowing rate Decrease material temperature Decrease mold temperature Decrease part wall thickness Improve mold coring Increase material density Increase air pressure Increase melt index (with decreased stock temperature)... 

The blow molds for injection blow molding and for dip blow molding can be made from aluminum because there are no cutting edges required (for venting see Section 1.4.2.1). 

On the basis of volume, by far the majority of PVC is extruded. Rigid applications include pressure and drain/waste/vent pipe, siding, window profile, fence deck, and rail. Packaging applications include bottles (by extrusion blow molding) and rigid sheet. Rexible applications include medical tubing, hose, wire insulation (by extrusion coating), and cove base. 

Low gloss 1. Increase material temperature 2. Increase mold temperature 3. Improve mold venting 4. Increase air pressure 5. Increase air blowing rate... 

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