When the plastic melt fills the mold, it displaces the air. The displaced air must be removed quickly, or it may cause burn spot (due to the fast compression of trapped air pocket by the low-thermal-conductivity polymer melt), or it may restrict the flow of the melt into the mold cavity, resulting in incomplete filling (short-shot problem). Consider the injection-molding of a air bag housing. Notice that the part consists of a thin central region and a thick rim around it. A single gate is adopted in the original design. Most of the melt flow along the part side since. | A Air-Trap and Gate Location CoreTech System When the plastic melt fills the mold it displaces the air. The displaced air must be removed quickly or it may cause burn spot due to the fast compression of trapped air pocket by the low-thermal-conductivity polymer melt or it may restrict the flow of the melt into the mold cavity resulting in incomplete filling short-shot problem . Consider the injection-molding of a air bag housing. Notice that the part consists of a thin central region and a thick rim around it. A single gate is adopted in the original design. Most of the melt flow along the part side since the section is thicker and the flow resistance is lower than that in the central thinner region. That is the melt races away along the thick rim while the central region is filling at a slow rate. The filling along the rim is dominant and finally the melt backfills the central region and cause an entrapment of air there. In this case an air-trap problem is caused by the racetrack effect of melt flow. To avoid the buring or incomplete filling associated with the entrapment of air proper venting is required. Venting is provided by the clearance between knockout vent pins and their holes parting lines as well as additional venting slots in general to deep and 3mm to 6mm wide . Gate location is directly related to the consideration of venting location. In general the vent is located opposite the gate area near the end of filling or in the air-trap position. 31 Melt viscosity is reduced and flowability is improved by raising the melt temperature via viscous heating effect -Temperature raised 15 oC lower value for thermal sensitive material CoreTech System As the melt flows through the restricted gate the flow velocity is sufficient high and the melt is highly .
