With the deepening of global economic integration and the improvement of people's living standards, the market demand for various plastic products has grown rapidly. As a type of plastic products, rotational molding products are in increasing demand in the market due to their various forms and suitability for industrial production. With their excellent performance and wide range of applications, rotational molding products are expected to continue to Maintain the growth momentum and become an indispensable and important part of the market. In the lecture titled "The Future of Rotational Molding in Great America", Wen Yuan explained the molding process of rotational molding in detail, comprehensively and vividly explained the current development and application of rotational molding industry at home and abroad, and demonstrated the rotational molding through pictures. Products are used in different fields, including various types of boxes, casings, large pipes, decorations, etc.

Does the rotational molding process have low production efficiency per piece and high processing costs? Compared with blow molding, injection molding and other molding processes, what is the future development trend of the rotational molding industry? In response to a question, Wen Yuan said that the investment cost of the rotational molding industry is lower than that of other industries, and theoretically, there is almost no upper limit on the size of products formed by the rotational molding process.

It is reported that the "Talk about Plastics" lecture hall is a series of publicity activities organized by industry experts and enterprises by the China Plastics Processing Industry Association. Lecture halls and other methods are used to successively introduce the history, production, characteristics, application fields, usage methods and other knowledge of plastic products.

1. Definition of mold trial

2. Why do we need to conduct a mold trial?

（1）Improve the quality and efficiency of mold production

（2）Reduce the number of improvements

（3）Optimize mold design

3. Mold trial process

The mold trial process includes the following steps:

（1）Determine the test purpose and test standards

（2）Manufacturing trial mold parts

（3）Assemble trial mold

（4）Test and record results

（5）Analyze results and make improvements

In real life, plastic products have penetrated into all aspects of our lives. Because we are not professional in the material properties of plastic products, we choose some products made of harmful materials, which brings harm to our lives and health. Modern scientific evidence shows that some chemicals in waste plastic products are chronically toxic, and long-term exposure can cause harm to the human body. Mainly polyvinyl chloride monomer, the main component of plastic, will damage the human nervous system, liver and kidneys after being absorbed, so we need to know a simple way to identify plastic products.

1. Through observation, we learned that brand-new plastic products have bright colors, high transparency, and no variegation. As for waste plastic products, the color is darker, the color is darker, and the patterns are more serious.

2. Understand through the sense of smell that products made of new materials have no peculiar smell. Products made from recycled materials have a peculiar smell.

3. Compare the weight and understand that for plastic products of the same size, new plastic products are lighter, while waste plastic products are heavier.

4. Choose regular supermarkets, which are relatively large. The plastic products in regular supermarkets are relatively reliable. The prices at farmers' markets and roadside shops are slightly lower, but the quality is not up to standard.​

5. Look at the flexibility of plastic products. New material products have good toughness, are resistant to bending, will not deform, and will not break. The recycled material has poor toughness, is relatively brittle, and is easy to break.

Ingredients and Properties

‌PE material (polyethylene)‌: The main component of PE material is polyethylene, which is a thermoplastic resin that is odorless, non-toxic and feels like wax. It has good low temperature resistance, the lowest operating temperature can reach -100~-70°C, has good chemical stability, and can resist the erosion of most acids and alkalis, but is not resistant to acids with oxidizing properties.

‌PP material (polypropylene)‌: The main component of PP material is polypropylene, an odorless, tasteless, non-toxic milky white granular or powdery product. It has good mechanical properties, heat resistance and electrical insulation properties, good chemical stability, and does not interact with most chemicals.

Processing performance

‌PE material‌: It has good fluidity, low processing temperature, moderate viscosity and low decomposition temperature. It is a plastic with good processing performance. PE materials are easy to shape during processing and are suitable for blow molding, extrusion, injection molding and other methods. They are widely used in the manufacture of films, hollow products, fibers and daily sundries.

‌PP material‌: PP material has good fluidity at high temperatures and can maintain stable size and shape after cooling, so it has good formability and plasticity.

Application scope

‌PE material‌: PE material is widely used in packaging materials, containers, pipes, monofilaments, wires and cables and other fields because of its light and transparent, moisture-proof, oxygen-resistant, acid-resistant, alkali-resistant and other properties.

‌PP material‌: PP material is often used to make food turnover boxes, food bags, beverage packaging bottles, etc. due to its excellent mechanical properties and heat resistance.

1. Apply DSC to measure the thermal parameters of XLPE for rotational molding, and quantitatively analyze the non-isothermal crystallization kinetics of increases and decreases; a rotational rheometer was used to test the complex viscosity and other parameters of XLPE for rotational molding. By comparing with the complex viscosity of HDPE with only DCP added, it was shown that the starting cross-linking temperature of XLPE increased by about 10°C, broadening the processing Window; increasing the heating rate and cross-linking temperature can shorten the cross-linking reaction time and increase the cross-linking degree of the product. When the heating rate is greater than 7℃/min and the cross-linking temperature is above 190℃, products with a higher cross-linking degree can be obtained .

2. Use polarizing microscope and DSC to study the crystallization behavior of XLPE and gel.

3. Using the air temperature inside the mold as an indicator, XLPE rotational molding is divided into stages such as material preheating, material melting, bubble removal and compaction, cross-linking, cooling crystallization and solidification; the heating temperature is 270℃~290℃, and the heating time is 28min~ 32min is the optimal XLPE rotational molding process. Within a certain range of heating temperature and heating time, there is a time-temperature equivalent relationship between a 10°C increase in heating temperature and a 1-min extension in heating time; the distribution of cross-linking degree, crystallinity and crystal particle size of XLPE rotational molded products along the thickness direction was quantified.

4. The relationship between the mechanical properties of XLPE products and the degree of cross-linking, crystallinity and crystal particle size was analyzed.

Rotational foam molding is a new rotational molding process that has developed rapidly in recent years. It is based on the usual rotational molding process and combined with foaming molding technology to simultaneously complete rotational molding and rotational molding during the rotational molding process. The process of foam molding is a unique molding process to produce integrated rotational molded foam products. Rotational foam molding can be completed by the "one-step method" of inputting multiple raw materials at one time, or by two or multiple inputs. The outer surface of the obtained rotomolded foam product is dense and smooth, and there is a foam core inside. A dense inner layer can also be added to the foam core layer as needed to form a "sandwich" structure. It is also possible to mold solid foam products that are fully filled with foam layers.

Rotational foam molding is more complicated than the usual rotational molding process. If the thickness of the foam layer is controlled and the input of foam material is too small, the thickness of the foam layer will be insufficient, which will affect the function of the product, such as insufficient thermal insulation effect or insufficient density. , and also affects the strength of the product. However, if there is too much foam, it will swell and break the product or even the mold, causing greater damage.

Rotational molded foam products have many unique advantages: because they have multiple product layers, they have high mechanical properties, can be insulated, and can reduce the cost of the product. Rotational foam molding is currently mainly used to produce rotational molded products that require thermal insulation, high strength requirements, or to reduce product production costs, such as insulated boxes, boats, pontoons, etc.

In recent years, a very popular molding method in blister molding is rotational molding. Rotational molding, also known as rotational molding, rotational molding, rotary molding, etc., is a hollow molding method for thermoplastic plastics.

Rotational molding originated in the United Kingdom in the 1940s. The first rotational molding patent was issued in 1887, and the first commercial rotational molding machine was manufactured in 1934. At first, it was only used as a supplement to injection molding, blow molding, and extrusion. With the maturity of polyethylene powdering technology, it has increasingly become a very competitive molding method in the plastic molding process.

In recent years, in developed countries in North America and Europe, the rotational molding industry has developed rapidly, and its development speed is higher than the average development speed of the entire plastics industry. And the development of the entire rotational molding industry is relatively complete. In addition to rotational molding machine manufacturers and rotational molding product manufacturers, there are also a considerable number of other professional auxiliary factories, such as rotational mold manufacturers, release agent manufacturers, rotational molding special Pigment production plants, grinding machine manufacturers, insert manufacturers, and mixing equipment manufacturers suitable for rotational molding, as well as numerous rotational molding-specific raw material supply factories, etc.

PE bucket manufactured using rotational molding process



My country's rotational molding market



According to statistics, there are more than 1,500 rotational molding product companies in my country; in the early 1970s, there were more than 500 companies engaged in the production of rotational molding products in the United States, with more than 500 rotational molding machines.

For the problem of uneven wall thickness of rotational molded products, specific problems must be analyzed in detail. When there is a large wall thickness difference between one part of the product and other parts, if it is caused by structural problems between the rotational molded product and the mold, the structure of the mold must be changed, and the mold corners designed with too small an angle should be replaced with larger smooth arcs. to transition.

For uneven temperatures in the heating furnace, it is necessary to improve and adjust the air volume, angle and circulation path of the hot air to ensure that the mold can be heated evenly in all directions. For parts of the mold that are relatively deep and difficult for heat to reach, you can use an airflow amplifier to add heat. For problems caused by differences in heat absorption in different parts of the mold, such as the flange parts, you can preheat or reduce the heat insulation factor first. In areas where the thickness of the product is expected to be reduced, an additional layer of thin iron sheet can be welded to the corresponding part of the mold to delay the transfer of heat into the mold and reduce the molten material adhering to the mold cavity, thereby reducing the thickness of this area.

As for the uneven wall thickness caused by improper rotation speed ratio or speed control of the mold, the rotation speed of the main and secondary shafts should be appropriately adjusted to maintain a balanced proportion and uniform speed to obtain uniform coverage thickness and appropriate powder trajectory. A constant-torque or constant-speed motor that can be automatically controlled can be used to ensure that the main and secondary shafts rotate at a constant speed. When biaxial rotational molding is performed, the speed ratio of the two axes should not be an integer multiple, so that the spiral material flow traces of each rotation do not repeat each other.

1. The position of the mold closing line should be conducive to production operations such as mold opening and closing.

2. The position of the mold closing line should take into account the aesthetic needs of the product.

3. The setting of the mold closing line can make the mold structure simpler.

4. Sometimes due to some special reasons such as the complex shape of the product, the mold closing line can also be designed to be more complicated. For example, some parts are higher or lower than the main parting line, etc.

The first step in rotational molding is grinding, so what are the requirements for grinding?

The quality of powder is related to the pinholes on the surface of the product, the bubbles in the product wall, and of course production efficiency.

The fine powder is on the outside and the coarse powder is on the inside. When heated in this way, the fine powder close to the mold melts first to form the outer skin. If the powder is very rough, there will be gaps between the powder and air inside, which will lead to pinholes after demoulding. In addition, rough powder will trap air in it. Too much air cannot be completely melted during the heating process, and the final manifestation is bubbles in the wall.

When experts encounter problems such as bubbles and pinholes, the first thing they look at is the quality of the powder. If the observed powder particles have a lot of tails, or there are a lot of large particles, then it is necessary to adjust the settings of the grinder.

So, how do we describe the quality of powder? There are three main attributes:

Dry Flow: Describes the flow rate of powder. Generally speaking, the faster, the better;

Bulk Density: Describes the accumulation of powder. Generally speaking, the bigger the better;

Particle Distribution: Describes the size distribution of powder. Generally, the finer the better.

For the results, the dry flow rate is generally between 25 seconds and 35 seconds. The higher the particle fullness, the better the fluidity; the more tails, the worse the fluidity.

Okay, so how do we control the quality of powder? Needless to say, it is a better grinder! The number of teeth of the grinding mill disc, the gap, and the working temperature of the grinding disc will all affect the above three indicators.

Regarding grinding, there are newer cutting-edge developments.

The purpose of roasting the surface of rotomolded products is to improve the surface gloss, reduce surface pores, and make the scratches and burrs after trimming the mold closing seam round. ‌ After roasting, the surface quality of rotomolded products is significantly improved, as shown in the following:

1‌.Improve surface gloss‌: Baking can make the surface of rotational molded products smoother, increase the gloss, and make the appearance more beautiful‌.

2‌.Reducing surface pores‌: By roasting, the pores on the surface of the product can be eliminated or reduced, improving the compactness and overall quality of the product‌.

3‌.Smooth the scratches and burrs after trimming the mold closing seam‌: Fire treatment can round the scratches and burrs after trimming the mold closing seam, reduce sharp parts, and improve the safety and aesthetics of the product‌.

Technical details and precautions for roasting:

In the rotational molding process, baking is also a link that requires technical control. Since local defects on the inner surface of the mold may cause the polyethylene melt to flow in and form bonds, it is very important to coat the inner surface of the mold with a layer of thermally stable material to prevent adhesion‌2. In addition, attention needs to be paid to controlling the temperature during the heating stage to avoid scorching of materials and ensure product quality.

Good wear resistance: Specially treated iron molds, such as quenching, tempering and other heat treatment processes, can significantly improve their hardness and wear resistance, making them resistant to wear during use and extending their service life.

Excellent thermal conductivity: Iron material has good thermal conductivity, which can maintain stable temperature distribution and heat transfer during the use of the mold, helping to control product quality.

Higher strength and toughness: Iron molds perform well in terms of strength and toughness and can withstand large impact and pressure, making them suitable for some application scenarios that require higher mold strength.

Relatively low cost: Compared with some advanced alloy materials, the production cost of iron materials is low, making it convenient for small and medium-sized enterprises to manufacture molds.

Shortcoming

Higher weight: Iron material is heavier than other lightweight materials (such as aluminum alloy), which increases the difficulty of handling and installing the mold, and also limits its use in some application scenarios that require lightweight design.

Relatively poor thermal conductivity: Although iron has good thermal conductivity, its thermal conductivity still needs to be improved compared to some materials with better thermal conductivity (such as copper, aluminum, etc.). When manufacturing high-temperature products, iron molds may easily generate thermal stress, causing mold deformation and cracking.

Difficulty in processing: Iron material has high hardness and toughness, which increases its processing difficulty and cost. In the mold manufacturing process, more complex processing techniques and equipment are required to ensure the accuracy and surface quality of the mold.