Venting mold cavity

Most thermosetting resins are heat-activated, and their polymerization reaction ultimately cross-links the chains as the molecular weight of the material approaches infinity. Any polymerization via condensation reaction produces small amounts of gas or vapor (such as water) that dissolve under pressure within the hardening resin or must be vented out of the mold cavity. Venting ports may be provided within the... 

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. 

Venting The displacement of air from the mold cavity as the cavity is filled by polyurethane. Venting normally occurs through small holes or seams in the mold located at strategic positions around the mold to ensure that all air is vented. When the mold is completely air free and polyurethane filled, a small amount of polyurethane also vents, further ensuring a complete air-free filling of the mold. 

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. 

Venting can also be provided within the mold cavity by means of inserts equiped with vent slots, porous sintered plugs, or by holes with a diameter not greater than 0.2 mm. Such holes are machined only to a shallow depth and are relieved by a much larger bore machined from the back of the mold. 

The mold orientation should be such as to allow filling from the bottom of the mold cavity, allowing escape of air through a top flange at a hidden surface. This allows controlled venting, and positioning of vent pockets that can be trimmed from the part at a later time. 

Bad surface finish Incorrect gate type Injection pressure too low Inadequate venting Mold cavity soiled Mold temperature too low Flow length too great Excessive use of mold lubricant... 

The design of the gating and runner configuration (if any) is usually kept proprietary hy the molder. However, it appears that most SRIM parts are center-gated, with vents located along the periphery of the part. This configuration allows the displaced air in the mold cavity to he expelled uniformly. 

Polyethylene, polystyrene, and PP fall under the classification of nonhydroscopic resins. These types of polymer resins collect moisture on the surface of the pellet only. The moisture can originate from several potential sources. Such moisture in some cases can be removed very easily by moderate preheating immediately prior to feeding the material into the mold. In some cases it is sufficient to provide vents at the transition from the hopper to the mold cavity. In some situations the moisture can be removed by passing warm air over the material. The equipment utilized to heat air and dry resins is usually very simple, e.g., an inlet air filter, a blower, and a controlled electric heater, as shown in Figure 47.11. 

Efficient mold cooling also depends on accurate venting. Air trapped between the mold cavity and the plastic will significantly slow down cooling and also could cause problems with the part. Certain molds incorporate vacuum lines to ensure proper contact. 

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