April 2011 Archives

Although it is ideal that the surface of the molded product is a copy of the surface of the cavity and has a beautiful shape and gloss, some times the surface gloss may become clouded, or there may be variations in the gloss.

Particularly in the case of products with decorative surfaces in which the external appearance quality is very important, gloss defect can become a very serious defect.

The following methods can be thought of as the countermeasures for solving the problem of gloss defects.

(1) Countermeasures related to molds

1. There are no air vents or they are insufficient.
2. The gate is small.
3. The sprue and runners are thin.
4. The electroplating condition of the cavity surface is not good.
5. The polishing condition of the cavity surface is not good.
6. There are some deposits (soot) adhered to the cavity surface.

(2) Countermeasures related to the injection molding conditions

1. The charge quantity is insufficient.
2. The cushion position is too short
3. The holding pressure is too low.
4. The pressure holding time is too short.
5. The cavity surface temperature is low.
6. The preliminary drying of the molding material is insufficient.

Silver streaks are a phenomenon in which shining line shaped patterns appear on the surface of the molded product. These can be considered external appearance quality defects in exterior parts of consumer electrical products, automobiles, motorcycles, etc.

The cause of silver streaks is the air or gases contained in the molding material appearing on the surface of the molded product.

The following methods can be thought of as the countermeasures for solving the problem of silver streaks.

(1) Countermeasures related to molds

1. Enhancing the function of air vents.
2. Making gate size larger.
3. Making the cold slag well larger

(2) Countermeasures related to the injection molding conditions

1. Checking the preliminary drying conditions (temperature, drying time) of the molding material, and carrying out appropriate drying.
This is the most important countermeasure.
2. Making the injection speed lower and filling the cavity slowly.
3. Making the cylinder temperature lower.
4. Making the screw rotation speed lower.
5. Preventing stagnation of the plastic inside the cylinder.

(3) Countermeasures related to the molded product design

1. Making the wall thickness of the molded product as uniform as possible.

Burns are a phenomenon in which black burnt material appears on the surface of the molded product. At the time of packing molten plastic into the cavity, any air remaining inside the cavity gets compressed and generated heat thereby burning the plastic causing these burns.

Burns cause defects such as defects in the external appearance quality of the product, missing parts in the shape of the product, reduction in the physical characteristics, etc.

The following methods can be thought of as the countermeasures for solving the problem of burns.

(1) Countermeasures related to molds

1. Providing air vents.
2. Making air vents deep, providing sufficient air exhaust path.
3. Taking apart the mold and cleaning and maintaining the air vents.
4. Using a divided structure or nested structure for the mold and letting the air escape from the parting surface.
5. Using a vacuum suction apparatus.
6. Changing the gate position.

(2) Countermeasures related to the injection molding conditions

1. Making slower the injection speed.
2. Making the cylinder temperature lower.
3. Making the charge quantity smaller.
4. Preventing stagnation of the plastic inside the cylinder.

(3) Countermeasures related to the molded product design

1. Using a design that avoids thin walled parts in the product.
2. Changing the wall thickness of the product, and changing the flow pattern of the molten plastic.
3. Changing the shape in which air gets trapped.