Due to their unique structure, square-bottom plastic bags require targeted optimization of their bottom load-bearing capacity during the blown film process. This involves the coordinated efforts of multiple aspects, including raw material selection, process parameter adjustment, equipment improvement, and structural design.
Raw material selection is fundamental to improving the load-bearing capacity of square-bottom plastic bags. High-strength, high-toughness polymer materials, such as high-density polyethylene (HDPE) or polypropylene (PP), should be prioritized. These materials have a dense molecular chain structure, enabling them to withstand greater tensile forces without easily breaking. If environmental friendliness is also a concern, biodegradable materials can be added, but modification techniques are needed to balance their degradation rate and mechanical properties. For example, adding nano-cellulose reinforcement to PLA (polylactic acid) can improve the bottom's puncture resistance while maintaining its biodegradability. Furthermore, appropriate amounts of plasticizers and anti-aging agents should be added to the raw materials. The former reduces material brittleness, preventing cracking due to stress concentration under load, while the latter slows down performance degradation over long-term use, ensuring load-bearing stability.
Precise control of process parameters is crucial for optimizing the load-bearing capacity of square-bottom plastic bags. The blow-up ratio and draw ratio need to be adjusted according to the characteristics of the bottom structure: an excessively high blow-up ratio can lead to an overly thin bottom film, reducing load-bearing capacity; an excessively low ratio can cause material buildup at the bottom folds, leading to stress concentration. The optimal blow-up ratio range is usually determined experimentally to ensure uniform bottom film thickness that meets load-bearing requirements. The draw ratio also needs strict control, as it directly affects the longitudinal tensile strength of the film. If the draw speed is too fast, the bottom film may be overstretched, causing molecular chain breakage; if it is too slow, the film thickness will be too thick, wasting material and affecting production efficiency. Temperature control is also crucial. The extruder temperature needs to be set according to the raw material characteristics to ensure sufficient plasticization and prevent decomposition; the die temperature needs to be uniform to avoid uneven bottom film thickness due to temperature differences.
Equipment improvement is the technical support for enhancing the load-bearing performance of square bottom plastic bags. For square bottom structures, the traditional blown film machine die needs to be modified, adopting a non-circular die design to form a thicker film layer in the bottom area of the extruded preform, enhancing the bottom load-bearing capacity. Meanwhile, during the cooling process, cooling devices, such as localized air vents or water cooling systems, need to be added to the bottom area to accelerate the cooling and shaping of the bottom film, preventing deformation or uneven thickness due to insufficient cooling. Furthermore, the traction device also needs optimization, employing a multi-roller traction system to ensure uniform stress on the bottom film during traction, avoiding damage caused by excessive localized stress.
Structural design optimization is a direct means of improving the load-bearing capacity of square-bottom plastic bags. Stress can be distributed by increasing the number of bottom pleats or adjusting the pleat shape. For example, using a "W"-shaped pleat design, compared to the traditional "V"-shaped pleats, can more evenly distribute stress during bottom load-bearing, improving overall load-bearing capacity. Additionally, adding reinforcing ribs or thickening layers at the bottom folds can further enhance the bottom's tear resistance. Moreover, rounded corners at the bottom edges can reduce stress concentration and prevent damage during use due to sharp edges.
Online testing and quality traceability are crucial for ensuring the stable load-bearing performance of square-bottom plastic bags. An online thickness gauge needs to be installed on the production line to monitor the thickness of the bottom film in real time. If the thickness deviates from the set range, the system will automatically adjust the process parameters to ensure uniform bottom thickness. At the same time, a quality traceability system should be established to record information such as raw material batch, process parameters, and production time for each batch of products. Once a product with unqualified load-bearing performance is found, the root cause of the problem can be quickly traced and the production process can be adjusted in a timely manner.
