There are many different kinds of rotomolding machine, and it can be classified according to mechanical mechanism and heating methods.
The mechanical structure.
As far as machinery is concerned, rotational molding machine can be classified according to mold installation way (depending on countershaft) and the operation mode.
The mold installation way: Center type and lateral type
1. The center type
This type of rotomolding machine’s mold set in the middle of the machine, the installation of the mold is located in the spindle line of the machine. The spindle of the rotational molding machine can be cantilever type, also two fulcrums, its fulcrums respectively in the two sides of the die-set, supporting the two short sleeves that link to the die-set. For its good stress, it is suitable for large and super large components.
2. The lateral dispersion type
Mold (or template) were respectively located on both sides of the spindle of the rotomolding machine .This structure is more convenient to load and unload. Generally speaking, the spindle of lateral rotational molding machine is cantilever and poor force. The equipment is adaptable to the production of plastics with small scale and large quantities.
The operation mode of rotomolding machine
There are batch type, semi automatic type, continuous type and jacketed type and so on.
1. The batch rotational molding machine. This kind of rotational molding machine also known as fixed arm roll presses the equipment with a fixed rotational arm in the oven. With main and auxiliary axis of rotational molding machine by manual or equipment pushed into the oven, the rotational arm will lock the spindle of frame and makes the frame drives die along the main and auxiliary shafts along each vertical direction to rotate according to the scheduled requirement, and at the same time to give heating. Afterfinishing the heating, torning off the spindle of frame and the fixed arm in the oven and pulled out from the oven rack and sent to the cooling chamber to cool to the required temperature.
A frame from the heating chamber to the cooling chamber can be realized through the roller or small car, in the process of moving, frame still can make the mold along main and auxiliary vertical rotate. Cooling is also carried out under rotation. At the time, the rotation depends on the rotational arm in fixed or mobile car which is arranged in the cooling chamber.
2. The semi automatically rotational molding machine.The main characteristic of such equipment is the rotational molding machine relies on the spindle automatically from the feeding and demoulding position moving to heating and cooling station. Semi automatic rotational molding machine has single arm type, double arm type and multi arm type.
A . Single arm type
It has only one arm that is spindle, there are two types: rocking and reciprocating. Their common characteristic is that they are at least provided with two stations, i.e. heating station and feeding, demoulding, cooling station. There are machines that separate feeding and demoulding and cooling station and set up three stations. The advantages of single arm type of rotomolding machine are simple in structure, low in price. But the heating oven is not fully utilized and the production efficiency is relatively low. This kind of equipment is more beneficial to the production of the trial mode, trial production and small batch production.
B.Double arm type
It has two arms, two spindle shafts, each equipped with an auxiliary axis. Usually this kind of rotational molding machine equipped with one oven and two cooling chambers, the oven for alternate use of arms. Double arm type’s rotational molding machine's main advantages is the oven can be fully utilized, and with only two arms, compared with the multi arm rotational molding machine it required less mold.
C. Multi arm type
This kind of rotational molding machine has three or more arms (spindles). The number of the arms usually adapts to the stations, each arm are equipped with a countershaft. Three arms rotational molding machine is usually equipped with feeding and demoulding, heating, cooling station; four arm rotational molding machine is usually equipped with feeding and demoulding, heating1, heating and cooling station. The multi arm rotomolding machine can make each station reduce the "empty" time as far as possible and reasonable used, giving full play to the potential of the equipment.
The rotation of each arm of the multi arm rotational molding machine can be controlled respectively. In the molding process, the exchange station of each arm can rotate along the horizontal direction as well as the perpendicular plane, the former is usually referred to as "merry- go- round", while the latter is called "truck". The rotating Trojan type covers a large area but has a low requirement of the plant; the waterwheel type is just the opposite: it has a higher requirement of plant and covers an area is relatively small.
3. The continuous rotational molding machine. This kind of equipment is equipped with single axis (a spindle and a countershaft) as well as multi axes (multiple spindle and the corresponding spiritual countershaft). The mold of this type machine is arranged in the rotating frame, the frame loading is carried out on a chain successively driven by a gear. The chain drives the mold to go through oven, cooling, demoulding and feeding station.
A feature is the spindle can be respectively according to the established procedures to operate. This kind of equipment is very high in automation, and that can prepare a variety of products simultaneously; another characteristic is high yield, high efficiency. Thus it is particularly suitable for the production of a variety of products and commodities.
4. The clamp sleeve type rotational molding machine. This kind of equipment is equipped with folder jacket rotomolding mold, the feeding and demoulding, heating, cooling were carried out in the same station. It covers the minimum area. The heating and cooling of this kind of mold rely on clamp sleeve to pump into the hot and cold oil. The high thermal efficiency and accurate temperature control is especially suitable for thick molding parts. The drawback is that the equipment and mold are more complex, with difficult processing, and price more expensive. The clamp sleeve type rotational molding machine can be used to forming large plastic parts.
The heating methods
At present, the main heat source of rotational molding machine are electric energy, combustible gas and fuel etc.. The main advantage of electric energy is that it’s simple to control and easy to implement as well as clean to the environment. The disadvantage is that the energy prices are high and the use of combustible gas or fuel combustion is difficult in technique, it is difficult to control. The advantage is the energy prices are relatively low.
The heating ways of rotational mold mainly have the air circulation, liquid medium circulation, infrared heating, direct flame heating and molten salt spray method, in which the hot air circulation method is the most common. The application of molten salt processing, due to the poor working environment, the high requirements of the mold and the lack of competitiveness in economy, is increasingly shrinking.
The cooling methods
The common cooling methods of rotational molding technique are water spray and air spray. Moreover the cooling of the clamp sleeve rotomolding machine by pumping into the cold liquid heat transfer medium (cold oil), after jacket type of circulation method.
Water cooling has high efficiency and also very convenient actually, but for some plastic parts of easy warpage (for example the rotational molding products of high density polyethylene), it’s not advisable to spray water to cool immediately after heating in order to avoid partial quench. It should be short air cooling or spray cooling first before water cooling .Spray cooling has the advantages of both high efficiency and uniform cooling, but the requirements of implementation is high; air cooling is easy to implement, but the cooling effect is relatively poor and the cooling cycle is relatively longer.
