1、 Analysis of the Causes of Bubble Problems

The occurrence of bubbles during the extrusion molding process in Dalian is a common but serious problem that affects product quality. The causes of its formation are complex and diverse, mainly including the following aspects:

Raw material factors: The presence of moisture or volatile substances in raw materials is the main cause of bubble formation. When plastic particles are heated and melted in an extruder, these substances vaporize to form bubbles. In addition, impurities and low molecular weight substances in the raw materials can also decompose and produce gases at high temperatures.

Improper process parameters: Setting the temperature too high can lead to material decomposition and gas production; If the temperature is too low, it may lead to high viscosity of the melt and difficulty in releasing gas. Excessive rotation speed of the screw will result in insufficient residence time of the melt in the barrel, preventing the gas from being expelled in time.

Equipment issues: Unreasonable screw design, insufficient compression ratio; Uneven plasticization caused by barrel wear; Poor design of mold flow channels leads to unstable melt flow; Equipment factors such as vacuum exhaust system failure can lead to the generation of bubbles.

Improper operation: uneven feeding, insufficient exhaust during restart after shutdown, and other operational issues can also cause bubbles.

2、 Raw material processing and selection strategy

Thoroughly dry raw materials: For materials with strong moisture absorption such as PET, PC, PA, etc., strict pre drying treatment must be carried out. It is generally recommended to dry at 80-120 ℃ for 4-8 hours to reduce the moisture content to below 0.02%. Using a dehumidifier dryer yields better results than a regular hot air dryer.

Select high-quality raw materials: Avoid using materials that are damp, expired, or have a high proportion of recycled materials. Recycled materials should undergo strict screening and reprocessing, and a certain proportion of defoamers should be added if necessary.

Adding additives: Properly adding defoamers, deaerators, and other additives can help eliminate bubbles. However, attention should be paid to the amount added, as excessive amounts may affect product performance.

Raw material pretreatment: For easily decomposable materials, vacuum degassing treatment can be considered, or heat stabilizers can be added to the formula to prevent high-temperature decomposition.

3、 Optimization methods for process parameters

Temperature control: Establish a reasonable temperature gradient, gradually increasing the temperature from the feeding section to the machine head to avoid local overheating. General suggestion:

Feeding section: 20-30 ℃ lower than the melting point of the material

Compression section: close to the melting point of the material

Measurement section: 10-20 ℃ higher than melting point

Head temperature: slightly lower than the measuring section

Screw speed adjustment: Select the appropriate speed according to the material characteristics to ensure that the melt has sufficient residence time for exhaust. The general principle is to use low speed for high viscosity materials, and increase the speed appropriately for low viscosity materials.

Back pressure control: Increasing back pressure appropriately can improve the quality of melt plasticization and help eliminate bubbles. However, excessive back pressure can lead to a decrease in production and an increase in energy consumption, and it is generally advisable to control it within 3-10MPa.

Vacuum exhaust: For multi-stage screws, a reasonable vacuum exhaust section should be set up, and the vacuum degree is generally maintained at -0.08 to -0.095MPa. The exhaust port should be located in the area where the melt has been largely plasticized but not completely melted.

4、 Key points for equipment improvement and maintenance

Optimization of screw design: Select screws with mixing and exhaust sections, and the compression ratio should be appropriate (generally 2-3:1). For materials that are prone to generating bubbles, barrier screws or pin screws can be used.

Mold channel design: The channel should have a smooth transition to avoid sharp turns and sudden changes in cross-section. Add exhaust grooves in areas where gas accumulation may occur, with a depth generally ranging from 0.02-0.05mm.

Regular equipment maintenance:

Check the wear of the screw and barrel, and replace them in a timely manner

Clean the filter screen to prevent clogging and pressure fluctuations

Calibrate the temperature control system to ensure accurate temperature

Check the sealing of the vacuum system and maintain good exhaust effect

Auxiliary equipment configuration: Consider installing a melt pump to stabilize pressure, or using an online degassing device to further remove gas from the melt.

5、 Production operation specifications

Standardize the startup procedure: Run at low speed before starting up, and gradually increase the speed after the molten material is evenly discharged. After replacing materials or long-term shutdown, sufficient exhaust should be carried out.

Stable feeding: Maintain uniform feeding to avoid fluctuations in melt pressure caused by fluctuations in quantity. The use of a forced feeding device can improve feeding stability.

Process monitoring: Regularly inspect product quality and adjust process parameters in a timely manner if bubbles are found. Establish a system for recording key parameters to facilitate problem tracing.

Environmental control: Keep the production environment dry and better control the relative humidity below 50%. The raw material storage area should be moisture-proof, and unused raw materials after opening should be sealed and stored.

6、 Problem diagnosis and solution methods

When there is a bubble problem, the following steps can be followed for troubleshooting:

Observation of Bubble Characteristics: Uniformly distributed small bubbles are usually caused by the presence of water or volatile components in the raw material; Local large bubbles may be due to poor exhaust of the mold; Irregular bubbles may be caused by uneven plasticization.

Targeted solutions:

Raw material issue: Strengthen drying and replace raw material batches

Process issue: Adjust parameters such as temperature, speed, back pressure, etc

Equipment issue: Check screws, molds, and vacuum systems

Operational issues: Standardize operational procedures and strengthen training

Validation effect: Adjust only one variable at a time, observe the effect of the change, and avoid complicating the problem by changing multiple parameters simultaneously.

By systematically analyzing the causes of bubble formation, starting from various aspects such as raw materials, processes, equipment, and operations, adopting a prevention oriented and comprehensive management strategy, it is possible to effectively avoid bubble problems in extrusion molding, improve product quality, and production efficiency.