Analysis of Bubble Defects in Injection Molding

Introduction

Bubbles, also referred to as air pockets, air marks, or air holes, can be divided into two categories: water bubbles and vacuum bubbles. Vacuum bubbles occur in some thick-walled products. The surface of these products cools more rapidly, while the center cools at a slower rate. This leads to uneven volume shrinkage, resulting in the formation of cavities in the thick-walled sections. Water bubbles are formed because the moisture and gas present in the plastic cannot be released during the product's cooling process, thereby creating bubbles inside the product.

Reasons for Formation

I. Materials

1. Poor fluidity.

2. Inadequate drying of the plastic, which contains moisture.

II. Molds

1. Poor venting of the mold.

2. Abrupt changes in the wall thickness of the product design. Due to varying cooling rates in different parts of the product, bubble formation is likely. When there is a significant difference in wall thickness, the melt flow in the thin-walled area is slow, while the melt in the thick-walled area advances quickly. This can trap gas in the cavity, forming air pockets.

III. Processes

1. Excessively fast plasticizing process.

2. Insufficient injection pressure.

3. Excessively high injection speed.

4. Excessively high melt temperature.

5. Insufficient mold temperature.

Solutions

1. Thoroughly dry the materials to eliminate moisture.

2. Improve the venting of both the injection machine and the mold.

3. To prevent thermal decomposition of the materials, reduce the melt temperature. Meanwhile, apply back pressure to stop air from entering the materials.

4. Extend the holding pressure time and increase the mold temperature.

5. For molded products with significant thickness variations, lower the injection speed and raise the injection pressure.

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