Introduction
In the injection molding industry, trapped air is one of the injection molding defects frequently encountered. Trapped gas can directly cause injection molding defects such as material shortage, charring, air streaks, and surface fogging, and indirectly cause injection molding defects such as shrinkage marks and exposed glass fibers. Trapped air can be classified into three types based on its location: poor exhaust at the parting surface, trapped air at the end of the material flow in deep rib positions, and encapsulated air. The most fundamental solution to trapped gas is to improve the exhaust structure of the mold. However, only by truly understanding the root cause of trapped gas can one know how to solve it from the mold. Otherwise, it will increase the number of mold trials and verifications, causing unnecessary waste. In fact, not only will trapped gas not be solved, but injection defects such as flash may also occur.
Case Analysis
Package air (package wind)
When the two material flows merge and the gas mixed in the middle cannot be discharged, it will cause injection molding defects such as gas shortage, gas charring and gas streaks. The following pictures respectively show gas shortage, gas charring and gas streaks.
Solution measures: 1) Add ejector pins, breathable steel, exhaust inserts, etc. at positions not requiring the appearance surface of the mold to increase exhaust. 2) By adjusting the injection molding speed, the wall thickness of the product, or the gate position, the position of the gas bag can be moved to a position that is easy to exhaust. 3) Adjust the material flow by reversing the flow channel to avoid air entrapment; 4) For some products, the charring phenomenon can be optimized by reducing the injection speed.
Trapped air at the end of the deep-ribbed material flow
When the parting surface of the mold is sealed by the rubber compound at the end of the deep rib position and the material flow is still flowing in a direction perpendicular to the parting surface, the gas cannot be discharged from the parting surface, resulting in injection molding defects such as gas shortage and gas burning.
Solution measures: 1) Add exhaust inserts, inserts or breathable steel at the end of the deep-ribbed material flow; 2) If it is difficult to open exhaust at the end, exhaust can be added as much as possible at other positions where it is easy to add exhaust. When adjusting the process, low-pressure and low-speed injection should be adopted at the end of the glue filling to meet the customer's minimum material shortage requirements. 3) Sometimes, product design engineers will deliberately create an inner arc at the position where the air is trapped in the deep ribs to achieve the purpose of product aesthetics.
Summary
Trapped air is impossible to completely avoid in the injection molding industry. Therefore, during the new product development stage, the phenomenon of trapped air should be avoided through mold flow analysis. Emphasis should be placed on the overall exhaust system design of the mold during the mold design stage. During the process design stage, it is necessary to avoid material decomposition, set reasonable clamping force and injection speed, and propose an optimization plan for mold exhaust as early as possible. Special attention should be paid to the daily maintenance and preventive upkeep of molds during the mass production stage.