First of all, from the overall scope of plastic processing, rotational molding is only a smaller branch, and its output value and scale only account for a very small part of the entire plastic processing industry. Rotational molding, a non-mainstream processing method, is inherently determined to not be the focus of the plastics processing industry.

Secondly, the rotational molding industry lacks large and influential companies, and rotational molding products have not been widely popularized and applied in our country, which makes people's awareness of the rotational molding industry low.

Furthermore, the lack of industry information, the scarcity of research talents, and the lack of publicity are also one of the reasons why rotational molding has not attracted enough attention.

However, compared with other mainstream plastic processing methods, rotational molding has its unique advantages, especially in agriculture (such as drone plant protection machines) and intelligent fields (such as smart robots, smart lifebuoys, etc.), and is increasingly favored by brands. It is favored by finished product manufacturers because rotational molding has great cost advantages under the demand of small batch manufacturing and can leave customers with several times the money.

We will see that more and more rotomolded products have replaced or are replacing products previously made of metal, wood, glass, ceramics, fiberglass and other materials. Originally, they were mostly made of wood, iron and other materials, but boxes of these materials are easy to store. It has shortcomings such as corrosion, easy damage or rust, heavy weight, and inconvenient loading, unloading and transportation. Therefore, after the rotational molding box is produced, it is quickly accepted by users and used more and more.

1. Main materials

Rotational molding toner: usually made of polyethylene (PE), polypropylene (PP) and other materials.

Other process toners (such as injection toners): may be made of PVC, ABS and other plastics.

2. Production process

Rotational molding toner: Its production usually involves mixing the pigment and resin through physical friction, high-speed rotation, etc., so that the pigment is evenly dispersed in the resin.

Other process toners (such as injection molding toners): usually made by dissolving the pigment in a solvent, mixing it evenly with the resin and then drying it to form a powder.

3. Performance characteristics

Rotational molding toner: It has good weather resistance, gloss and waterproofness. These characteristics make rotational molding toner widely used in the manufacturing of outdoor products and daily necessities.

Other process toners (such as injection molding toners): Their performance characteristics may be more focused on internal product applications, such as electronics, electrical appliances, toys and other industries.

4. Scope of use

Rotational molding toner: Due to its good weather resistance and waterproof properties, it is suitable for the manufacture of outdoor products such as furniture, trash cans, water tanks, etc.

Other process toners (such as injection molding toners): are more suitable for coloring internal products.

5. Price

Rotational molding toner: Since the raw materials are relatively cheap and the production cost is low, the price is more favorable than other process toners (such as injection molding toner).

6. How to use

Rotational molding toner: usually painted or mixed on the surface of plastic products for color mixing.

Other process toners (such as injection molding toners) are directly injected through an injection molding machine to color the inside of plastic products.

The perfect result of rotational molding is to produce high-quality rotational molded products. However, due to factors such as raw materials, equipment, molds, processes, and product structures, various problems often occur during or after the processing of rotational molded products. Most of these problems are problems with the products, some are equipment and mold problems, and some are process problems.

To sum up, the more common problems in rotational molding processing are:

The product has bubbles and pinholes;

The surface and inner wall of the product are uneven and frizzy;

The wall thickness of the product is uneven, with some parts thick and some parts thin;

The corners of the product have chipping and bridging phenomena;

Products are prone to shrinkage, warping, and deformation after being demoulded;

Product color solution;

The product has low strength and poor rigidity;

The product cannot be released from the mold;

The bonding fastness between inlays and products is low;

Molding heating time is too short or too long;

Flash appears on the product;

Blockage of ventilation tube;

The mold deforms or even explodes;

…

These problems that occur during the rotational molding process not only affect the appearance of the product, but also damage the mechanical properties of the product, affect the use efficiency of the product, and even lead to the scrapping of the product. Therefore, it is necessary to fully understand the importance of solving the above problems and find out the causes of the problems, thereby improving the efficiency of rotational molding processing and the quality of products.

1. In terms of raw materials and additives, choose raw materials with better toughness and impact resistance. Raw materials with low density or low melt flow rate generally have better toughness than raw materials with high density or high melt flow rate; try to use special rotomolding materials; Increase the amount of raw materials; the raw materials should be kept dry and not expired; it should be clear whether the colorants or additives used are suitable for the rotational molding process and whether they can withstand the long-term high temperature of rotational molding without decomposing. Do not use them blindly. Add Just the right amount, not too much.

2.In terms of molding process, appropriately adjust the heating temperature and heating time, neither under-burning nor over-burning, to prevent decomposition and oxidation of raw materials or insufficient melting, which will lead to a significant decrease in the strength of the product; keep the vents open to balance the pressure inside and outside the mold. Avoid pinholes and pores in the product and improve the density of the rotomolded product; maintain an appropriate rotational speed ratio to make the wall thickness of the molded product uniform; appropriately increase the cooling intensity and cooling rate.

③In terms of rotomolding product design, rationally design the structure to avoid acute angles and planes where stress is concentrated, increase the arc transition angle of corners and grooves; increase the wall thickness of the product; add design reinforcement structures in areas where the product is subject to greater stress, etc.

The rotational molding process can produce many different types of hollow plastic products, but how to make the products more suitable for the rotational molding process requires designing rotational molding products that meet the requirements of the rotational molding process, so that the rotational molding products can give full play to their best qualities. As for the effect, the following is the experience accumulated by Dongguan Keshan Rotational Molding Equipment in the rotational molding industry for many years. I hope it will be helpful to the rotational molding colleagues.

How to solve the problem of poor coating of modified engineering plastics in rotational molded products?

1. Eliminate the factors that are unfavorable to the heat conduction of the insert, so that the embedded metal product has an excellent heat transfer structure.

2. Under the premise of considering the rotational molding conditions and the strength requirements of the insert, the insert should be as small as possible relative to the surface height and volume of the product.

3. The depth and width of the anti-rotation and anti-pull grooves on the insert must comply with the rotational molding requirements.

4. When rotomolding, preheat the inserts according to actual conditions, so as to achieve good results, which is more suitable for rotomolded products with large and medium-sized inserts.

At present, most orders for rotational molding products from overseas directly indicate the use of linear density low-pressure polyethylene (LLDPE) rotational molding profiles for rotational molding. The products are mostly different products with higher requirements for rotational molding technology. Specifications of cleaning equipment casings, lawn mower casings, shipping boxes, storage boxes, various mechanical automated packaging wooden boxes, two-layer insulated boxes with polyurethane foam layers inside, automobile parts, etc. my country's rotational molding technology and rotational mold manufacturing technology are rapidly improving, but there are still differences compared with European and American countries in the rotational molding product design stage.

The rotational molding process plays a very important role in product quality. After the product is designed and the stamping mold is manufactured, the mold design that is likely to cause product defects is generally not easy to change casually. It is more important to find ways to solve the defects of rotational molded products from the forming process.

1. Material issues: If the materials used contain a large amount of volatile substances or moisture, bubbles may easily occur during the heating process. Especially when manufacturing plastic products, toxic substances in the materials may also produce harmful gases. In addition, a melt index that is too low may also lead to the formation of bubbles.

2. Temperature problem: During the manufacturing process, if the heating temperature is insufficient or the heating time is not enough, the material may not be fully heated and it will be difficult to fully fit the mold, resulting in bubbles.

3. Pressure problem: Insufficient or uneven pressure during manufacturing can easily lead to the generation of bubbles. These bubbles may be caused by gas being indented into the material, or they may be caused by insufficient fit between the material and the mold.

4. Mold design: The design of the mold may also affect the generation of bubbles. For example, the diameter of the mold ventilation pipe is too small or blocked, the pressure inside and outside the mold is not balanced enough, the internal pressure increases, and the resistance increases when gas escapes, resulting in residual gas in the raw material, resulting in bubbles in the product. In addition, if the parting surface of the mold is not closed tightly or there are blisters and pores on the cavity surface of the mold, bubbles may appear on the rotomolded products during molding.

5. Gas in the raw material: The gas in the raw material cannot be completely eliminated. During the heating process, it gradually melts, flows and adheres to the mold cavity. During this process, the gas trapped in the raw material is gradually squeezed to the surface, but due to the melting process Due to the existence of body surface tension, if the force of gas pushing outward is insufficient, it will be unable to break away from the melt surface and can only nest in the raw material.

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.