Uneven wall thickness is a common issue in blow molding, mainly caused by differences in the blow-up ratio of various areas of the parison. The following methods are widely used to improve wall thickness uniformity in blow-molded parts.
1.Profiled Die
Profiled die technology refers to locally machining grooves or modified contours on the die or mandrel of the extrusion head. By increasing the die gap at specific locations—typically at corners or areas with higher blow-up ratios—the local wall thickness of the parison can be increased.
This produces a profiled parison that compensates for radial wall thickness variations caused by uneven blow-up ratios, resulting in a more uniform final product.
By applying targeted die profiling and continuously optimizing the design through multiple trial runs, a properly designed profiled die can significantly improve wall thickness distributio
2. Axial Wall Thickness Control Technology
Axial wall thickness control enables the extruded parison to achieve different thicknesses along the axial direction according to the varying blow-up ratios of the final product. This ensures a more uniform wall thickness after blow molding.
This method works by axially moving the mandrel or die according to preset positions, thereby changing the die opening and adjusting the parison thickness.
At present, accumulator heads used in blow molding machines are generally equipped with axial parison control functions, with the number of control points ranging from 30 to 256, depending on the system design.
3.Radial Wall Thickness Control Technology
Although axial wall thickness control improves thickness distribution along the height of the product, the horizontal cross-section of the parison remains uniformly circular. For products requiring significantly different blow-up ratios at specific radial positions, axial control alone is insufficient.
To address this, radial wall thickness control technology was developed. This method allows the parison to change from a circular to a non-circular cross-section within specified zones.
Currently, two typical radial control designs are widely used:
(1) Flexible Ring Technology
Flexible ring systems use electro-hydraulic servo control to deform a thin flexible ring in one direction or in two symmetrical directions, thereby adjusting the parison thickness.
A key advantage of this technology is that radial control remains effective regardless of product shape, as long as the die diameter remains unchanged.
Recent breakthroughs in the development of radial parison thickness control systems have been achieved in Jiangsu, China. This technology enables accurate multi-point radial thickness control, typically allowing 2 to 16 control points, with the potential for even more. Industrial-scale applications are currently under development.
(2) Die Lip Profiling Technology
Die lip profiling achieves parison thickness adjustment through the vertical movement of a profiled die ring. Compared with flexible ring structures, its main advantages are longer service life and lower manufacturing complexity.
In some designs, the profiled section of the die ring is made as a replaceable insert, allowing quick replacement and reduced maintenance costs. Further research is still needed to lower costs and accelerate widespread adoption of this technology.
Radial wall thickness control is particularly effective for improving the quality of large hollow products and can significantly reduce product weight.
For example, in 200-liter plastic containers, material savings of 5% to 10% can be achieved. Although the additional cost of installing a radial control system is currently relatively high, continued technological development is expected to promote its wider application in medium and large extrusion blow molding machines.
4.Combined Axial and Radial Wall Thickness Control
The combined application of axial and radial wall thickness control technologies produces optimal parison geometry and more ideal wall thickness distribution in finished products.
At present, several domestic blow molding machine manufacturers in China offer radial wall thickness control systems as optional equipment for large-scale blow molding machines.
5. Parison Temperature Differential Method
The deformation resistance of a parison is closely related to its viscosity, which is directly influenced by temperature.
Higher temperatures result in lower viscosity and lower deformation resistance, making the parison easier to stretch during blow molding. Conversely, lower temperatures increase viscosity and deformation resistance, reducing deformation.
During extrusion, forced cooling can be applied to areas with higher blow-up ratios, creating a controlled temperature gradient along the parison.
In the free-blowing stage, cooler areas with higher viscosity deform less, while warmer areas deform more. During the constrained blowing stage, areas with initially lower deformation continue to stretch until the final shape is formed. This process improves overall wall thickness uniformity.
6. Combination of Vacuum Forming and Extrusion Blow Molding
Differences in local blow-up ratios are a major cause of wall thickness variation. The blow molding process can be divided into free blowing and constrained blowing stages.
Free blowing stage: From the introduction of compressed air until the parison first contacts the mold cavity. During this stage, deformation is unconstrained and relatively uniform in all directions.
Constrained blowing stage: From initial contact with the mold cavity until full contact is achieved. Areas touching the mold cool rapidly, increasing viscosity and limiting deformation, resulting in thicker walls. Areas not yet in contact remain hotter, deform more easily, and become thinner.
If the free blowing stage could end simultaneously across all regions, a perfectly uniform wall thickness would be achieved.
By applying vacuum suction first to pull the parison toward areas with higher blow-up ratios, followed by compressed air blowing, the blow-up ratio differences can be reduced. This combined process effectively improves wall thickness uniformity in the final product.
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