Causes and solutions for silver streaks in medical injection molding products

Silver streaks refer to splash marks formed on the surface of plastic parts, usually caused by the volatilization of air and moisture in the plastic, or the decomposition and burning of foreign plastics after mixing. These marks usually radiate outward from the gate in a fan-shaped manner. Many plastics absorb moisture during storage. If they are not fully dried before processing, the residual moisture will be converted into water vapor during the injection molding process, resulting in splash marks on the surface of the plastic parts. In addition, plastics will also enclose a certain amount of air during the plasticization stage. If this air cannot be discharged smoothly during the injection molding process, it will also cause silver streaks. Some cracked or burnt plastics will also produce silver streaks on the surface of the product.

Analysis of the causes of silver streaks

1. Volatile substances are contained in the melt:

Silver streaks often appear as needle-shaped silver-white streaks formed by elongated flat bubbles, mainly including degradation silver streaks and water vapor silver streaks. Silver streaks are usually caused by volatile substances precipitated from the front end of the flow material. For example, degradation silver streaks are caused by the partial degradation of thermoplastics after heating, and small bubbles formed by gas decomposition are distributed on the surface of the plastic part. The traces left by these small bubbles are generally “V” shaped, with the tip facing away from the center of the gate. Water vapor silver streaks are mainly caused by excessive moisture content in the raw materials. The bubbles generated when the moisture evaporates cause silver streaks to form on the surface of the plastic part, especially in highly absorbent resins such as polyamide and impact polystyrene. If the moisture in the melt cannot be completely discharged, it will cause water vapor silver streaks.

Measures to solve the silver streak problem of PPSU medical injection molding products:

– Raw material processing: For degradation silver streaks, try to choose resins with uniform particle size, screen out powder in the raw materials, reduce the use of recycled materials, and remove residual foreign materials in the barrel; for water vapor silver streaks, the raw materials must be fully dried according to the drying requirements of the resin.
– Process operation: For degradation silver streaks, reduce the temperature of the barrel and nozzle, shorten the residence time of the melt in the barrel, and prevent local overheating of the melt; reduce the screw speed and forward speed, and shorten the boost time. For water vapor silver streaks, increase the back pressure, increase the screw compression ratio, reduce the screw speed or use a venting screw.
– Mold design: For degradation silver streaks, increase the cross-section of the gate, main channel and runner, expand the cold material hole, and improve the exhaust conditions of the mold; for water vapor silver streaks, increase the mold exhaust holes or use a vacuum exhaust device to exhaust the gas retained in the molten material as much as possible, check whether the mold cooling water channel is leaking, and prevent the mold surface from being overcooled and frosted, which will cause silver streaks on the surface of the plastic part. If there is moisture on the mold surface, white silver streaks will appear on the surface of the plastic part. In addition, the release agent used in the injection process may also produce a small amount of volatile gas, and the amount should be minimized to reduce the demolding resistance by improving the surface finish of the mold cavity.

2. Poor plasticization of the molten material:

If the molten material is not heated enough in the barrel, resulting in poor plasticization, the incompletely melted particles will be exposed on the surface of the plastic part, forming streaks. These streaks are shaped like mica flakes, with an area close to the size of a grain. When observed through light, the transparency of the streaks is poor, and sometimes the protruding particles of raw materials that have not been melted through can be clearly distinguished.

Solution:

– Increase the barrel temperature and extend the molding cycle. Try to use an internal heating injection port or increase the cold material hole and flow channel. If conditions permit, an injection molding machine with a larger barrel length-diameter ratio can be used to enhance the plasticization ability of the molten material.

Analysis and elimination methods of silver streak defects

1. The moisture and volatile content in the molding raw materials are too high: the raw materials should be pre-dried.

2. Poor mold exhaust: exhaust holes should be added to improve the exhaust performance of the mold.

3. Poor contact between the nozzle and the mold: the position and geometric dimensions of the two should be adjusted.

4. Silver streaks appear in specific parts: check whether the corresponding mold cavity surface has surface scars. If so, the surface scars of the mold cavity should be removed by machining.

5. Mixing of different types of resins: mixing of different types of resins should be prevented.

By identifying the causes of silver streaks and taking appropriate measures to solve them, the quality of plastic molded products can be effectively improved.