How difficult is it to produce films with “no crystal defects”? Film manufacturers must read: Key challenges in crystal control and solutions.

Preface 

In the field of thin-film production, “ Crystal point ” It’s a quality defect that professionals go out of their way to avoid. These are tiny, hard particles visibly present on the surface of the film—particles that not only compromise the film’s optical and mechanical properties as well as its surface smoothness, but also adversely affect subsequent processing steps such as printing, lamination, and bag-making. As such, this defect is a critical barrier to entry for high-end film products. “ Obstacle ”。

From food packaging films to optical-grade protective films, the crystal point content is a key indicator for measuring film quality. However, achieving high-quality film production... “ Zero crystal point ” It requires overcoming multidimensional challenges related to raw materials, equipment, processes, and the environment. Starting with the root causes of crystal defects, this article dissects the core difficulties in achieving crystal-free production, providing valuable insights for membrane manufacturers seeking to enhance product quality. 

I. First, let’s get clear: What exactly are film crystal points?

The crystal defects in thin films are essentially... Particles that have not been fully plasticized during polymer melt extrusion, or impurity particles mixed into the production process. , according to their causes, they are mainly divided into two categories: 

1. Resin-type crystal points 

These crystal defects have the same material composition as the thin-film substrate and are caused either by improper raw materials or inadequate processing techniques. They may result from uneven heating of resin particles, leading to some particles remaining unfused, or from prolonged exposure of the melt to high temperatures, which can cause localized degradation and crosslinking, forming hard particles.

2. Impurity-type crystal points 

These crystal defects are foreign impurities that differ in composition from the matrix material. For example, they may include dust and metal debris entrained in the raw materials, iron filings generated by equipment wear, as well as fibers and dust particles present in the workshop environment. 

As long as the crystal dot size exceeds the customer’s requirements (typically... ≤50 μm High-end membrane ≤20 μm ), which constitutes a nonconforming product. 

II. Core Challenge: What makes it so difficult to produce thin films without crystal defects? 

(1) Difficulties in raw material control: A dual challenge posed by impurities at the source and resin characteristics. 

1. Resin raw material's “ Congenital defect ”

• The resin particles have an uneven particle size distribution: smaller particles tend to agglomerate, while larger particles are difficult to melt. This is partly due to the resin synthesis process. “ Fish-eye material ” It itself contains unpolymerized oligomers, which readily form crystal points. 
• Unstable resin flowability and melt index, along with significant batch-to-batch variations, can lead to uneven plasticization and increase the likelihood of crystal point formation. 

2. Risk of impurity intrusion during raw material pretreatment 

• Resins are prone to contamination by dust, paper scraps, and metallic impurities during transportation and storage. Even during drying processes, if the equipment’s filter mesh is damaged or the hopper’s seal is inadequate, impurities can still enter the system. 
• PET 、 PA If the resin is not sufficiently dried in terms of temperature or duration, residual moisture in the raw materials can lead to melt hydrolysis, producing degradation products and subsequently forming crystal points. 

(2) Difficulties in equipment precision: The entire process from the screw to the die. “ Temperature-controlled plasticization ” Challenge 

1. The challenge of achieving uniform plasticization in screw extrusion systems. 

• An unreasonable screw compression ratio and length-to-diameter ratio can lead to uneven resin shear, resulting in over- or under-plasticization in certain areas. 
• Caused by wear on the screw and barrel “ Dead zone ” Melt retention, degradation, and crosslinking can lead to the formation of crystal points. Metal debris generated by wear can also directly become impurity crystal points. 

2. The precision bottleneck of the die and filtration system 

• There is a trade-off between the mesh count of melt filters and production efficiency: A higher mesh count provides better filtration, but results in higher melt pressure, lower output, and a greater tendency to clog; a lower mesh count, on the other hand, fails to effectively trap fine impurities. 
• Uneven temperature distribution in the die head causes varying cooling rates of the melt at the die lip, leading to premature crystallization and the formation of crystal spots in localized areas. If material buildup in the die head is not promptly removed, it may undergo high-temperature carbonization and subsequently contaminate the melt, resulting in hard crystal spots. 

3. The difficulty of temperature control in the cooling and shaping stage. 

The uniformity of the cooling roller temperature and the surface finish of cast films are critical. Large temperature fluctuations can lead to inconsistent crystallization rates in the film, resulting in the formation of tiny crystalline regions. Scratches or contaminants on the cooling roller’s surface can also transfer onto the film, creating crystal-like defects. 

(3) Difficult process control: Dynamic balance of temperature, pressure, and speed. 

1. Precise control of the temperature curve is difficult. 

The extrusion temperature is a key parameter for plasticization. If the temperature is too low, the resin will not be fully plasticized, resulting in unmelted crystalline spots. Conversely, if the temperature is too high, the resin may degrade and crosslink, also forming crystalline spots. The temperatures at the front, middle, and rear sections of the barrel must be precisely matched; any loss of control in any one section can trigger crystalline spot issues. 

2. The coordination of extrusion pressure and speed is difficult. 

• Unstable extrusion pressure can lead to uneven melt flow rates within the die: if the flow rate is too high locally, cooling may be insufficient; or if the flow rate is too low, the melt may stagnate and degrade, both of which can result in crystal points. 
• The matching degree between the traction speed and the extrusion speed is highly demanding. If the speed difference between the two is too large, it will cause uneven stretching of the film, increase the regularity of molecular chain alignment in localized areas, and lead to the formation of tiny crystalline regions. 

3. It is difficult to maintain the stability of process parameters. 

Film production is a continuous process, and the start-up, shut-down, and material-change phases are characterized by crystal defects. “ Peak incidence period ” During startup, the melt temperature has not yet stabilized; during shutdown, melt residue remains; and when changing materials, the new and old materials are mixed unevenly. These process fluctuations can all give rise to crystal points. 

(4) Environmental and Management Challenges: Often Overlooked “ Hidden pollution sources ”

1. It's difficult to control cleanliness in the production environment. 

Cleanrooms are standard equipment for high-end film production, but they come with high maintenance costs. Dust, fibers, hair from operators, and debris from clothing can all fall into the melt or onto the film surface, creating crystal spots. Additionally, static electricity generated during the film cooling and winding stages can attract dust, further exacerbating contamination. 

2. It's difficult to meticulously control the details of production management. 

• If equipment is not cleaned regularly enough, accumulated material and carbonized residues inside the screw, barrel, and die head can get mixed into the melt. If the filter screen is not replaced on time and becomes clogged, it will cause a sudden increase in melt pressure, leading to uneven plasticization. 
• When switching between different grades and types of resin, if cleaning is insufficient, residual resin and the new material may form crystal spots due to poor compatibility. 

III. Breakthrough Approach: Core Control Strategies for Reducing Film Crystallization Points 

(1) Source Control: Refined Preprocessing of Raw Materials 

1. Select high-quality resin raw materials, giving priority to products with uniform particle size distribution and low fish-eye content. Suppliers are required to provide raw material testing reports. 

2. Improve the raw material pretreatment process: Perform resin screening and magnetic separation to remove impurities; set drying temperature and duration according to resin characteristics to ensure that the moisture content meets the specified standards; keep the hopper and conveying pipelines sealed, and install dust removal equipment. 

(2) Equipment Upgrade: Enhancing Plasticization and Filtration Precision 

1. Employ high-performance screws such as barrier-type and pin-type screws to enhance shear mixing efficiency and improve plasticization uniformity. Regularly inspect the screw and barrel for wear, and promptly replace worn components. 

2. Optimize the filtration system and adopt coarse filtration. + The multi-stage filtration system features fine filtration, with the mesh count of the filter screen selected according to product requirements. It is equipped with an automatic filter screen switching device to prevent filter clogging and associated process fluctuations. 

3. Reinforce the insulation of the die head to ensure uniform temperatures across all zones; regularly clean out accumulated material and carbonized deposits from the die head to keep the die lip smooth and pristine. 

(3) Process Optimization: Dynamic Adjustment of Plasticizing and Cooling Parameters 

1. Determine the optimal plasticizing temperature range for different resins through small-scale trials, and establish a scientific temperature curve to avoid temperatures that are either too high or too low. 

2. A closed-loop control system is employed to monitor and adjust the extrusion pressure and traction speed in real time, ensuring stable melt flow rate and film draw ratio. 

3. Optimize the startup, shutdown, and material-change procedures: Thoroughly preheat the equipment before startup, promptly clean the melt before shutdown, and thoroughly clean the screw and die head when changing materials. 

(4) Environmental Control: Creating a Clean Production Space 

1. Construct cleanrooms that meet standards, equip them with high-efficiency air filters, and control the dust levels in the workshop. Operators should wear antistatic, dust-free suits, gloves, and masks to prevent human-induced contamination. 

2. Install an electrostatic elimination device during the film cooling and winding stages to reduce the likelihood of static electricity on the film surface attracting dust. 

IV. Industry Trends: Technical Upgrade Directions for the Production of Crystal-Free Films

1. Application of Intelligent Monitoring Technology 

An online crystal defect detection system has been introduced to monitor in real time the number and size of crystal defects on the film surface. When the defect levels exceed the specified thresholds, an immediate alarm is triggered, enabling intelligent control throughout the production process. 

2. Research and Development of New Materials and New Processes 

Add compatibilizers and nucleating agents to improve the resin’s plasticizing performance; adopt a reactive extrusion process to reduce the risk of resin degradation during processing. 

3. The popularization of green and clean production 

Promote solvent-free, low-energy film production processes to reduce pollutant emissions and minimize the likelihood of impurity crystal formation at the source. 

Conclusion 

Thin-film production “ No crystal points ” not just control over a single link, but rather... A systems engineering approach integrating raw materials, equipment, processes, and the environment. It tests the fine management and technological R&D capabilities of membrane enterprises, and also represents an essential step for thin-film products to move from the low-end to the high-end. 

As downstream markets place increasingly higher demands on film quality, overcoming the challenge of crystal defects and achieving stable, defect-free production will become the key to enhancing membrane manufacturers’ core competitiveness. In the future, breakthroughs in intelligent technologies and new materials may lower the barrier to producing crystal-free films; however, meticulous attention to detail will always remain the essence of film production.