Preliminary Study on the Production Technology of PET Foam

According to the different density, PET foam can be divided into low-density PET foam products and high-density PET foam products. Generally, foam with a density below 100kg/m³ can be considered low-density foam. The current minimum density can be around 60kg/m³, and the density of high-density foam is around 300kg/m³.

1. The technical status of China domestic PET foam industrialization

In terms of the application of PET foam, the survey found that, whether it was a few years ago or at present, most of the PET foam products are mostly used in the field of wind power, that is, as a filling and reinforcing material for wind turbine blades. The application consumption in other fields still accounts for a relatively low proportion, but under the joint promotion of the supply and demand industries, it is expected that the application range of PET foam will become wider and wider in the future.

Due to the good expectations of all parties facing the prospects of the PET foam market, equipment localization and product localization have become the pursuit goals of relevant scientific research institutions, equipment manufacturers and production companies. Especially in the period from 2019 to 2020, driven by relevant domestic policies in China, a wave of wind power equipment has been launched, and based on the short supply of other traditional filling materials (balsa wood and PVC foam) for wind turbine blades, Or the quality is not stable or the price is high, etc., which directly led to a greater demand for PET foam.

2. Production process of PET foam

When it comes to the production process of PET foam, personal understanding should be a collective concept, which should include at least the following aspects: first, the foaming method, which is only involved in the extrusion foaming method; second, the different dehumidification methods of materials are divided into pre-dehumidification and online dehumidification; third, the configuration of the extruder; fourth, the type of mold, especially the structure and specific design of the die plate; fifth, the specific design setting, adjustment and optimization of technological parameters in the production process.

The production process is generally based on the basic physical and chemical properties and processing characteristics of PET resin, while considering production efficiency and effects. First, an important characteristic of PET resin is that the viscosity decreases sharply after melting, that is, the melt strength is low. It is difficult to make an ideal foam with pure PET raw materials, and generally requires viscosity enhancement modification; secondly, PET is easier to absorb water, and it is easy to hydrolyze in the molten state of processing (of course, it also includes other forms of processing degradation such as thermal degradation), thereby further reducing its viscosity, which in turn affects the foam molding, cell state and mechanical strength after molding; the third is PET processing. The high temperature makes it difficult to plasticize and homogenize the formula materials.

In view of the characteristics of melt hydrolysis, dehumidifying all formula materials is an indispensable process before or during processing of all PET products, especially foamed products.

The specific process parameters in the production process mainly include the output (material addition) setting, the screw speed, the temperature of each section of the extruder barrel, the mold temperature, and the control melt pressure. In theory, there can be infinite combinations of these parameters. In fact, which combination is the best is unknowable. We don't have to find the so-called "best combination", and we just need to explore a certain combination. It is sufficient to obtain the products with the preset requirements under certain kinds or certain combinations.

3. Comparison of advantages and disadvantages of different material dehumidification processes

The dehumidification process is one of the core technologies, and it is inconvenient to openly discuss in detail, and it is inconvenient to openly comment on tendencies.

4. PET foam production equipment

The production equipment of PET foam is a relatively large system if it is to be comprehensively described. This article gives a brief overview.

4.1  Material dehumidification methods and equipment

4.2  Extrusion system

The extruder system is one of the most critical systems in the PET foam production line system, and therefore one of the most technologically-content systems.

The nominal capacity of the extruder is closely related to the design of the screw structure. Under the same screw diameter configuration, the capacity is related to the screw's pushing ability, and the screw's pushing ability is related to its length-to-diameter ratio and the screw thread structure (threaded element selection and arrangement), and the actual production capacity is also highly related to the design of the motor power of the extruder, the reduction ratio (torque) of the gearbox, and the pressure (melt pressure) capacity of the system. Therefore, the design of the machine system should consider the adaptation and balance of various factors. This is a problem that the designers of our equipment system need to consider comprehensively. Obviously, it is difficult to achieve a very ideal state through a single design configuration. It is necessary to optimize and become more ideal after observing the equipment performance, product state and performance in the test machine or a long-term production practice process.

As an extruder for foaming, its capacity is not only determined by the pushing capacity of the extruder, but also related to the requirements of the foaming process. The good or bad condition of the foamed product is not only related to the formulation and process parameter settings, but also highly related to the dispersing (homogenizing) ability and plasticizing ability of the extruder. Therefore, the designed output of the extruder is only a theoretical value or a nominal value, and the actual output that can be achieved depends on whether the foam product produced can meet the preset related requirements.

4.3 Thoughts on the design of molds and mold plates

The design of the mold and the die plate is very important, and it is also the place that best reflects the core technology. The author has a deep understanding of this.

4.4 Auxiliary system

The auxiliary system of PET foam production equipment mainly includes the front traction cooling roller group, the transition roller group, the secondary traction unit, and the enhanced cooling unit.

Although the auxiliary machinery system does not have a major impact on the main quality of the foam product, it is of great importance to the continuous and stable operation of the entire equipment.

5. Common problems and solutions in production

Common problems in the production of PET foam generally include the following categories. The first category is equipment issues, the second category is formulation issues, the third category is process issues, and the fourth category is product status issues. The fifth category is the mechanical properties of the product. Among these types of problems, the latter two types of problems are caused by the first three types of problems.

In summary, product problems mainly reflect the following aspects: (1) Thick cells or poor uniformity; (2) Partial or large-scale cell collapse; (3) Local or large-scale “pinholes” are too many; (4) The product is brittle and tough; (5) The thickness uniformity of the product is poor; (6) The mechanical strength is insufficient.

All problems are presented for reasons. We can analyze the causes. However, when analyzing the causes, it is generally not only a single factor analysis, but a comprehensive analysis of multiple factors. In particular, product-related issues must be analyzed systematically.

This article only briefly enumerates the common problems in production. As for the cause analysis and problem solving methods, although the author has accumulated a lot of practical experience, it is not intended to be presented and discussed in this article because the performance of the problem may vary greatly depending on the difference, and only specific problems can be analyzed in detail.