Frontier of technology: Rotomolding process

       01-Introduction to the rotational molding process

There are many processing and molding processes of plastics and their composite materials, in addition to our common extrusion molding, injection molding, blow molding and other processes, rotational molding is also a processing method for plastic products. Rotomolding, also known as rotational molding or rotational casting, is a plastic molding process.

Rotational molding is also known as rotational molding, rotational molding, rotational

molding, rotational molding, etc., which is expressed in English as RotationalMoulding.

The rotomolding process is to first add the plastic raw materials to the mold, and then

the mold continuously rotates along the two vertical axes and heats them, and the

plastic raw materials in the mold are gradually and evenly coated and melted and

adhered to the entire surface of the mold cavity under the action of gravity and heat

energy, formed into the required shape, and then cooled and shaped into products.

At present, the foreign rotational molding industry is developing rapidly, while the

domestic rotational molding industry is developing faster than other plastic molding

processing industries due to various factors, such as slow processing cycles and

limitations of materials used.

It should be noted that the production efficiency of rotational molding products is

very low, and generally a product needs to be produced in 30-60 minutes, including

forming, cooling, disassembling and assembling molds. Therefore, the quantity of the

product is determined by the number of molds, and multiple molds can be rotated

on the machine at the same time.

-02-Characteristics of the rotational molding process

1. Low cost of rotational molding: for products of the same specification and size, the

cost of rotational molding mold is about 1/3 to 1/4 of the cost of blow molding and

injection molding;

2. The edge strength of the rotational molding product is good, and the thickness of

the edge of the product can be achieved by more than 5 mm, which completely solves

the problem of thin edge of hollow products

3. Rotomolding can place various inlays

4. The shape of the rotational molded product can be very complex, and the thickness

is more than 5 mm

5. Rotational molding can produce fully enclosed products

6. Rotomolding products can be filled with foam materials to achieve thermal insulation

7. There is no need to adjust the mold, and the wall thickness of the rotomolded

product can be adjusted freely

8. The product is hollow and seamless, the wall thickness is uniform, and the wall

thickness can be adjusted to make the product have high strength

9. It can provide a variety of surface pattern treatment, produce various color products,

can be used for anti-ultraviolet and anti-aging treatment, and make a variety of stone

and wood simulation effects

-03-Application of rotational molding technology

At present, rotational molded products can be used in transportation, traffic safety

facilities, entertainment industry, river channel dredging, construction industry, water

treatment, medicine and food, electronics, chemical industry, aquaculture, textile

printing and dyeing and other industries.

1. Container rotational molding parts, such plastic parts are widely used in water

storage tanks, various liquid chemical storage tanks (such as acids, alkalis, salts,

chemical fertilizers, pesticides, etc.), gasoline containers (gasoline storage tanks and

fuel tanks for automobiles and aircraft), battery shells, etc.

2. Rotational molding parts for automobiles, mainly using polyethylene and polyvinyl

chloride resin, rotomolding various pipe fittings, such as air conditioning bends,

backrests, handrails, etc.

3. Sports equipment and various substitutes. There are mainly various parts of PVC

rotomolding, such as water balloons, floats, bicycle seat cushions, small boats, and

buffer shock absorbers between boats and docks. Surfboards, boats, etc. are also

rotationally molded parts.

4. Toys, models, crafts, etc. Since rotary molds can be manufactured by precision

casting, electroforming and other processes; The surface of the rotational molding

part has a good effect on the fine structure of the mold cavity surface, so the rotational

molding method can make the product quite delicate and beautiful, so it is often used

to make products with greater ornamental value, especially toys, models, handicrafts,

etc.

In addition to the above, rotational molding products are more commonly used in

various boxes, shells, large pipes and other parts, such as turnover boxes, garbage

bins, machine shells, protective covers, lampshades, bathrooms, toilets, telephone

rooms, yachts, etc. Rotomolding products have been widely used in the storage and

transportation of liquid chemicals, chemical enterprises, industrial coating, washing

tanks in rare earth production, reaction tanks, etc., as well as river and sea buoys,

domestic water tanks and other fields.

The main advantages of rotational molding

Key benefits of the rotational molding process:

（1) It is suitable for molding large and extra-large parts

The vast majority of plastic molding processing processes, such as injection molding,

compression molding compounds, extrusion, blow molding, etc., in the molding proce, plastics and molds are under quite high pressure (pressure), so when applying these

molding processes to produce large plastic parts, not only must the mold that can

withstand a lot of pressure must be used, so that the mold becomes bulky and complex,

but also the plastic molding equipment must also be designed and manufactured very

firmly, and the processing and manufacturing difficulty of the machine mold will

increase accordingly, and the cost will increase.

On the contrary, because the rotomolding process only requires the strength of the

frame to support the weight of the material, the mold and the frame itself, as well as

the closing force to prevent material leakage, even if the rotational molding of large

and extra-large plastic parts does not need to use very bulky equipment and molds,

the processing and manufacturing of the machine mold is very convenient, the

manufacturing cycle is short, and the cost is low.

Theoretically, there is almost no upper limit on the size of parts formed by the

rotational molding process. For example, the rotomolding process is used to prepare

a polyethylene cylindrical rotational molding container with a diameter of 1.2m and a

height of 1.2m, and the mold is only welded by cold work with a 3mm thick book steel

plate, and the rotomolding frame is welded with No. 8 channel steel.

(2) It is suitable for the production of multi-variety and small-batch plastic products

Because the mold for rotational molding is not affected by external forces, the mold is

simple, the price is low, and the manufacturing is convenient, so it is very convenient

to exchange products. In addition, the rotational molding equipment also has great

mobility, a rotational molding machine, can not only install a large mold, but also

arrange multiple small molds; It can not only mold parts of different sizes at the same

time, but also can also mold products of very different sizes and shapes at the same

time, as long as the raw materials used in the rotational molding products are the same,

the thickness of the products is the same, and they can be rotationally molded at the

same time, so the rotational molding process has greater mobility than other molding

methods.

(3) Rotational molding is very easy to change the color of the product

Rotomolding adds the material directly to the mold every time, which makes all the

materials enter the product, and after the product is taken out of the mold, the material

required for the next molding is added, so when we need to change the color of the

product, it will not waste a bit of raw materials, and there is no need to spend time

cleaning the machine and mold. When we use multiple molds to rotationally mold the

same plastic product, we can also add different colors of materials to different molds

and rotationally mold different colors of plastic products at the same time.

(4) It is suitable for forming hollow parts of various complex shapes

In the process of rotational molding, the material is gradually coated and deposited on

the inner surface of the mold, and the product has a strong ability to replicate the fine

structure such as the pattern on the mold cavity; At the same time, because the mold is

not subject to external pressure during the molding process, it can directly use precision

casting and other methods to prepare molds with fine structures and complex shapes,

such as toys and animal molds.

(5) Save raw materials

The wall thickness of rotomolded products is relatively uniform and the chamfer is

slightly thicker, so it can give full play to the efficiency of the material and is conducive

to saving raw materials. In addition, in the process of rotational molding, there is no

waste material such as runners and gates, and once debugged, there is almost no return

material in the production process, so the utilization rate of this process for materials is

extremely high.

(6) It is convenient to produce plastic products with multiple layers

Using the rotational molding process, only a reasonable matching material with different

melting temperatures needs to be loaded into the mold for rotational molding, and the

plastic with a lower melting temperature is first melted by heat and adheres to the mold

to form the outer layer of the part, and then the material with a higher melting

temperature is melted on it to form the inner layer of the part. Or first put the outer

plastic into the mold and rotomold the outer layer, then add the inner layer material,

and then rotationally mold to make multi-layer rotational molded products. Either

method can be achieved without complex equipment, and if multi-layer plastic products

are made using blow molding or injection molding, special multi-layer molding

machines and complex molds are required.

Limitations of the rotational molding process

(1) The rotational molding process is usually only suitable for the production of hollow

parts or shell parts (the latter is often obtained by dissecting hollow parts). This is

because rotational molding relies on the material loaded into the mold to gradually melt

and adhere to the cavity surface of the mold. The apparent density of plastics (especially

powdered plastics) before molding is usually smaller than after molding, so except for

foam products, the rotomolding method cannot be used to make solid parts.

(2) The rotational molding process cannot prepare products with very different wall

thicknesses and mutations in wall thickness. This feature is also directly related to the

fact that rotational molding relies on the gradual melting of the material and adhering

to the surface of the mold cavity for forming. To adjust the wall thickness of the part,

we can change the method of heating (strengthening or reducing) the parts of the mold

to make the wall thickness of the part moderately adjusted, but because the thermal

conductivity of the metal mold is very good, this adjustment effect is quite limited.

(3) The rotational molding process is difficult to make flat side parts. When the mold

rotates, the material is not easy to stay on the flat part, which can easily cause the wall

thickness of the flat part to be too thin, thus reducing the use effect of the part.

The main disadvantages of the rotational molding process

Large energy consumption

In each rotational molding cycle, the mold and mold base are repeatedly subjected to

alternating changes between high and low temperatures, so the rotational molding

process usually consumes more energy than other plastic molding processes. In order

to reduce the energy loss of the mold repeatedly heated and cooled, a jacketed

rotational molding machine is developed, which uses a pump to pump the cold and hot

medium into the jacket of the rotational mold through a special circulation system, and

directly heats and cools the mold.

The special case that is diametrically opposed to the large energy consumption of the

rotational molding process is the use of nylon monomer caprolactam, rotational molding

of nylon parts, such as the use of caprolactam to directly rotationally mold nylon 6 parts,

rotational molding and polymerization process are carried out at the same time,

rotational molding is carried out at a temperature lower than the melting point of nylon

6, and the product can be taken out at a higher temperature, so the temperature of the

rotational mold does not need to be repeatedly heated and cooled down in a large

temperature difference. It is said that the use of caprolactam rotomolding nylon 6

products has much lower energy consumption than using nylon 6 blow molding or

injection molding nylon 6 parts.

The molding cycle is long

In the process of rotational molding, the material does not withstand the strong action

of external force, the material does not have a violent turbulent state of movement, only

relying on it in contact with the surface of the mold cavity gradually melted by heat and

attached to the surface of the mold cavity, after the surface of the mold cavity is coated

with molten plastic, the heat required for the heating and melting of the inner surface

of the plastic also needs to be conducted through the molten plastic layer, the thermal

conductivity of the plastic is generally poor, so the heating time of rotational molding is

quite long, usually it takes more than 10min, Sometimes it even takes more than 20

minutes, so the entire rotational molding cycle is relatively long.

The labor intensity is high

In the process of rotational molding, the processes such as charging and demolding are

not easy to mechanize and automate, and are usually operated manually, so their labor

intensity is greater than that of blow molding, injection molding and other molding

processes. The dimensional accuracy of the product is poor The size of the rotational

molding product, in addition to the influence of the plastic variety, is also restricted by

various factors such as cooling speed, the type and amount of release agent (total release

effect), so the dimensional accuracy of the rotational molding product is difficult to

control, so the rotational molding is only suitable for plastic products with no special

requirements for dimensional accuracy, such as containers, toys and other products.

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