Dalian hollow molding (also known as blow molding) is a widely used processing technology in the production of plastic products, mainly used to manufacture various hollow containers, bottles, oil tanks and other products. During this process, the uniformity of wall thickness is directly related to the mechanical strength, appearance quality, and performance of the product, and is one of the key indicators for measuring product quality.
2、 Overview of Hollow Forming Process
The basic principle of hollow molding is to extrude or inject thermoplastic into a tubular billet, then place the billet in a mold, inflate it tightly against the inner wall of the mold with compressed air, cool and shape it to obtain the desired hollow product. According to different molding methods, it can be divided into several main types such as extrusion blow molding, injection blow molding, and stretch blow molding.
3、 The main factors affecting the uniformity of wall thickness
1. Billet forming stage
Billet is the foundation of blow molded products, and its thickness distribution directly affects the wall thickness uniformity of the final product. The factors that affect the uniformity of billet thickness include:
Design and rotation speed of extruder screw
Uniformity of gap between machine head and mouth mold
Melt temperature distribution
The phenomenon of billet sagging
2. Blow molding stage
The key factors affecting the uniformity of wall thickness during blow molding process are:
Blowing pressure and distribution
Uniformity of mold temperature
Precision of mold closure
Inflation ratio control
3. Material characteristics
Different plastic materials have different melt strengths, viscoelasticity, and cooling shrinkage rates, which can affect the formation and blowing behavior of the preform, thereby affecting the wall thickness distribution.
4、 Key technologies for controlling wall thickness uniformity
1. Billet thickness control system
(1) Axial wall thickness control
By controlling the position of the core rod of the machine head through a program, the gap between the die and the mouth can be adjusted in real time during the extrusion process of the billet, achieving thickness adjustment at different axial positions. Modern equipment usually adopts a multi-point control system driven by servo motors, which can preset dozens of control points.
(2) Radial wall thickness control
By using a rotating head or a specially designed die structure, the billet can achieve a more uniform thickness distribution in the circumferential direction. For asymmetric products, the problem of uneven inflation can be compensated for through eccentric mouth mold design.
2. Optimization of blow molding process parameters
(1) Blowing pressure control
Adopting segmented pressure control technology, lower pressure is used in the early stage of blow molding to prevent local excessive stretching, and pressure is increased in the later stage to ensure complete molding. The pressure is usually adjusted within the range of 0.2-1.0MPa.
(2) Blowing time control
Accurately control the timing of pre blowing and main blowing to avoid premature inflation causing the top to be too thin or late inflation causing the bottom to be too thick.
(3) Mold temperature control
The temperature in different areas of the mold should be kept uniform, and the temperature difference is generally controlled within ± 3 ℃. For complex shaped products, the local cooling rate can be adjusted through a zone temperature control system.
3. Application of advanced control technology
(1) Wall thickness prediction and simulation
Using CAE software to simulate the formation and blowing process of billets, predict wall thickness distribution, optimize process parameters and mold design. Common software includes Moldflow, Polyflow, etc.
(2) Closed-loop control system
Real time monitoring of wall thickness through online thickness measuring devices (such as ultrasonic and laser thickness gauges), feedback adjustment of the billet control system, and dynamic compensation.
(3) Servo electric control system
Replacing traditional hydraulic systems, improving the response speed and positioning accuracy of billet control, the position control accuracy can reach ± 0.01mm.
5、 Wall thickness control strategy for special shaped products
1. Asymmetric products
For asymmetric products such as square containers and shaped bottles, the following measures should be taken:
Design non-circular cross-section billets
Adjust the blowing direction or use multi-directional blowing
Design of mold cavity compensation
2. Large volume products
Key points for controlling the wall thickness of large containers (such as IBC drums and car fuel tanks):
Increase the number of control points for billet thickness
Adopting a storage cylinder head to ensure sufficient material supply
Optimize the structure and position of the blowing rod
3. Multi layer co extruded products
The wall thickness control of multi-layer composite products needs to consider:
Matching of flow characteristics of materials in each layer
Interface stability control
Adjustment of layer thickness ratio
6、 Common problems and solutions
1. Uneven vertical wall thickness
The significant difference in thickness between the upper and lower parts of the product can be resolved by optimizing the molding process and adjusting the blowing timing.
2. Uneven circumferential wall thickness
Manifested as thickness fluctuations in the circumferential direction, the roundness of the mouth mold, the rotation system of the billet, and the concentricity of the mold should be checked.
3. Localized thinness
Usually appearing in angular or concave areas, it can be improved through mold chamfering design, local cooling adjustment, or air replenishment technology.
7、 Quality Control and Testing Methods
1. Online detection technology
Infrared thermal imager monitors temperature distribution
Real time measurement of ultrasonic thickness gauge
Machine vision system detects appearance defects
2. Offline detection method
Slice thickness measurement
CT scan analysis
Destructive cross-section measurement
3. Statistical Process Control (SPC)
Establish a control chart for wall thickness data, monitor process stability, and promptly detect abnormal fluctuations.
The control of wall thickness uniformity in the hollow forming process is a systematic engineering that requires comprehensive consideration from multiple aspects such as materials, equipment, molds, and processes. With the advancement of control technology and the development of detection methods, modern hollow forming can achieve wall thickness uniformity within ± 5%, meeting the requirements of most application scenarios. In the future, with the deepening application of intelligent manufacturing technology, wall thickness control will become more precise and efficient, further expanding the application fields of hollow products.
