In the bubble wrap molding process, precise control of bubble size and distribution density requires collaborative optimization across multiple dimensions, including raw material characteristics, equipment structure, process parameters, molding methods, environmental management, auxiliary processes, and quality inspection, to ensure stable product performance and compliance with application requirements.
Raw material selection is fundamental. Low-density polyethylene (LDPE) is the core raw material for bubble wrap, and its melt flow rate directly affects the bubble forming effect. To improve membrane flexibility, high-density polyethylene (HDPE) or ethylene-vinyl acetate copolymer (EVA) can be added for modification, balancing the membrane's strength and elasticity by adjusting the raw material ratio. Raw material uniformity is also crucial. Inconsistent particle size or excessive impurity content will lead to uneven bubble size; therefore, strict raw material screening and optimized mixing processes are necessary to ensure uniform mixing.
Equipment structure directly affects bubble forming accuracy. The molding die adopts a coat hanger-type T-shaped structure, dividing the molten material flow into two streams via a flow divider cone, which are extruded from two die openings to form two layers of film. The die opening width must be matched to the extruder screw diameter to ensure uniform film width. The vacuum forming roller is a key component. The diameter and depth of the uniformly distributed bubble-forming holes on its surface must be designed according to the target bubble size. The vacuum chamber inside the roller must be stably connected to a vacuum pump to ensure constant suction pressure and prevent bubble collapse or deformation due to pressure fluctuations.
Precise control of process parameters is crucial. The temperature of each section of the extruder needs to be set in a gradient: the feeding section has a lower temperature to prevent raw material agglomeration; the plasticizing section has a higher temperature to promote raw material melting; the homogenizing section has the highest temperature to ensure uniform material flow; and the forming die temperature should be slightly lower than that of the homogenizing section to stabilize the film morphology. The vacuum degree of the vacuum forming roller must be maintained within a stable range. If the vacuum degree is insufficient, the bubbles are prone to collapse; if it is too high, it may cause the film to rupture. The traction speed must be matched with the extrusion speed. If the traction is too fast, the bubbles are stretched and become thinner; if it is too slow, the bubbles are prone to merging and becoming larger.
The choice of forming method affects the bubble distribution. Flat film forming involves using vacuum forming rollers to draw air from the upper film into circular holes, forming bubbles. This process is suitable for producing single- or double-layer bubble wrap. For three-layer composite bubble wrap, an additional thin film is laminated onto the raised bubble surface. The bubble distribution density can be controlled by adjusting the thickness of each film layer and the lamination pressure. Blow molding, on the other hand, forms bubbles by blowing air into the bubble. This process is suitable for producing tubular bubble wrap, but bubble uniformity is more difficult to control, requiring optimization of the blowing pressure and bubble cooling rate.
Environmental management is equally crucial. The temperature and humidity in the production workshop must be stable. Excessive temperature can soften the raw materials, causing bubble deformation; excessive humidity can cause the film to absorb moisture, affecting lamination strength. The workshop must be kept clean to prevent dust from adhering to the film surface or entering the vacuum system, which could lead to surface defects or equipment malfunctions.
The use of auxiliary processes can further improve bubble quality. For example, adding nanoscale nucleating agents to raw materials can increase the number of bubble nucleation points, resulting in smaller and more uniformly distributed bubbles. Ultrasonic treatment of molten raw materials can break the entanglement of macromolecular chains, promoting uniform bubble nucleation. Furthermore, optimizing the cooling process, such as using segmented cooling—first rapidly reducing the membrane surface temperature with air cooling, then stabilizing the internal structure with water cooling—can reduce dimensional deviations caused by bubble shrinkage.
Quality inspection is crucial. Laser microscopy must be used to inspect bubble size and distribution density to ensure that bubble size deviations within the same batch of products are within acceptable limits and that distribution density meets standards. Tensile tests verify the buffering performance of the bubble wrap, ensuring it can withstand specified pressure without rupture. Additionally, weather resistance tests are necessary to simulate the performance changes of the bubble wrap under different environmental conditions, ensuring its stability and reliability in practical applications.
