Processible plastics in the rotational molding process.

     Generally different thermoplastic plastics can be processed in the roto mold process. The used raw material depends on the end use of the form part and considers next to the temperature resistant the chemical resistance and the mechanical characteristics.

 

Polyethylene - PE

The most used material in the rotational process id polyethylene. The polyethylene (PE) can be subdivided into different densities:

PE-LD    0,915     -              0,935 g/cm³
PE-HD   0,940     -              0,970 g/cm³
PE-LLD  0,870     -              0,940 g/cm³

Depending on the application case and your desired product materials with different densities and melting points used in the rotational molding process. For example materials with a rather high density are used for the production of greater tanks. For visible parts and lamp corpuses, where are highly attractive surfaces is wished for, materials with an especially high melting point are used, because there the mold qualities of the form part are especially high. Thereby labelings such as scales and logos can be reflected with a high grade of detail, which is rather difficult when materials with a low melting point are used.

Polyethylene offers the great advantage, that is has a wide manufacturing field and has really good resistances concerning aggressive chemicals. Furthermore the applicable temperature range for the produced parts is relatively great with a range from -20°C to +60°C.

Often occurred application areas are:
- lamp corpuses
- kayaks and boats
- containers for chemical use
-  containers for bulk goods

 

Cross-linked polyethylene (X-PE)

Through the admixture of different additives (most with a peroxide basis) a chemical process happens during the rotational process, which varies the mechanical characteristics of the material. Until the reaching of the activating temperature it is a polyethylene with a high melting point and therefore comparable with ‘bad’ mechanical characteristics. Due to the reaching of the activating temperature a chemical cross linking of the material happens, which thereby gets good impact and temperature resistance. The high melt-flow-index (MFI) of the material, at the beginning of the process, allows for the representation of complex contours. Due to the use of partly aggressive additives against steel we suggest solely the use of aluminium forms. The good static material characteristics allow for a reduction of the wall thicknesses of the form part without a reduction of the static characteristics, which makes the use of this material concerning weight critical construction parts preferable.

 

Polypropylene – PP

Next to polyethylene also polypropylene can be processed well in the rotational process. Thereby it is a thermoplastic, which has a better thermic resistance in comparison to polyethylene. The capacity of the form part lay up to 100°C in the upper part and -10°C in the lower part, in which the impact resistance drastically decreases below 0°C. The density lies between 0,895 g/cm³ and 0,92 g/cm³ and therefore shows a lower density as the above named polyethylene. The applications are relatable to those of polyethylene, although depending on the application mostly the focus on the higher temperature resistance is. However that causes that the manufacturing temperatures and cycles are higher/ longer in the rotational process, which increases the cost of the polypropylene construction part.

 

Polyvinylidene fluoride – PVDF

In the industry PVDF is valued because of its great thermic and chemical resistance and is often used in special applications where the normal thermoplastic plastics such as polyethylene and polypropylene reach their boundaries. The temperature resistance ranges up to 140°C and offers new possibilities to replace metals through plastics.  Additionally the material offers a great advantage of classification in the flammability class V0 (3mm UL94). The high melting temperatures increase the cycle times and process temperatures drastically and make high demand on the processors. Additionally the raw material costs for PVDF lay at a multiple of those of polyethylene and polypropylene. The material has a significant higher density than the before introduced materials with 1,71 – 1,78 g/cm³. The company ARICON Kunststoffwerk has the possibilities to process those high tech materials due to its great construction and processes.

 

Polyamide

Next to the materials introduced before there is also the possibility to process polyamide in the rotational process. Its great chemical resistance for organic solvents can for example be used to create a barrier for hydrocarbons. That is often used in the multilayer production of fuel tanks. The high stability and paint ability offers further advantages for the application in the visual area. Polyamide additionally offers the advantage that the produced form parts can be used at temperatures up to 140°C. Due to the production of a multilayer structure it is possible to combine the positive characteristics of polyethylene with those of polyamide. Because polyamide has the characteristic to absorb water (hygroscopic), it is necessary for the material to go through a drying process before the production. The consequences when this is not done are optical residual images in the material surface up to brittleness, which can destroy the form part. The density of polyamide varies according to crystallinity and execution (PA6 to PA12) between 1,01 g/cm³ and 1,235 g/cm³.

 

Polycarbonate – PC

Polycarbonates have a special place in the roto area, because the applications refer only to the design and special constructions. The process is due to a low or not occurring shrinkage in the rotational process not easy and is only offered by few companies. The material has a low impact and chemical resistance. Therefore polycarbonate has a glasslike light transmission and is transparent and especially interesting for design applications. Through an admixture of pigment a variety of noble effects for lighting corpuses or design objects can be produced. Polycarbonate also reaches the classification V0 according to the UL94. Form parts made of polycarbonate can be used at temperatures over 120°C, which increases the demands on the production process.
Polycarbonates are often used for the production of CDs, DVDs, Blue-rays glasses and helmet sights.

 

Foaming material

Through the usage of modified polyethylene, which was transferred with faming agent, the plastic can be brought to a foaming producing process. Through the rotational process the material will be heated and the foaming agent activated. That triggers the foaming of the polyethylene and it expands by 4-6 times. The thereby produced structures include many air pockets and offers the advantage while it has a light weight to also have a great volume and thereby a higher stability. The thereby produced pimples go through the whole material and are not completely closed and open porous. Because of this negative characteristic it is often used in combination with conventional materials (see the multilayer).
The application areas are hulls for an increased stability while a light weight, chilling boxes for a reduced warmth transfer through the isolation layer and design objects with sound absorbing characteristics. The density of the material lays between 0,15 g/cm³ and 0,31 g/cm³.

 

Electric conductible material

The admixture of soot reduces the surface resistance of the plastics and makes this to a certain grade conductible, up to 2,5 x 10^5 Ω/m². After the soot is added to the raw material the material will be extruded again and made to powder. This guarantees the firm and sustainable integration of the soot pigments in the raw material, whereby this can also be used in the food industry. The high conductivity allows container and special formed parts to be used in EX restricted areas. Due to the soot the plastics are coloured in black and not translucent. Many application are in EX protected areas in which the potential discharge can lead to a light arch and an explosion. Food applications with for example flammable mediums such as alcohol but also bulk good containers in which plastic granulate is mixed or poured in, make the use of electric conductible material necessary. Due to a constant friction of the plastic it can lead to a potential charge, which can for example be discharge through a person.

 

Flame-retarding material

Polyethylene and polypropylene can be also varied to a flame-retarding material according to the UL94 through different additives.
Through an admixture of additives the chemical and impact resistance decreases. In exchange the materials reach the classification V0 or V2 according to the UL94 and can be used for example for the production of design products such as lamp corpuses, furniture or garbage cans. By means of this material it is possible to also fulfill the formalities for the establishments of products in public building. The material is in natural transparent or in every other wished colour available. The density of the material lays at 1,029 g/cm³ (PE) or 1,1 g/cm³ (PP).

 

Multilayer production

As already above mentioned it is possible to execute a layer structure in the rotational mold process.
This can have the following causes:

• Design: Due to the manufacturing of different colours the outside contour of the form part (for example a vase) can be produced in white and the inside in a deep red. This has the advantage that an after colouring is unnecessary and by a throughout coloured material also light scratched can be coated on the upper side.

• Improvement of the chemical characteristics: The usage of a material mixture can lead to an improvement of the chemical characteristics. As already mentioned in the paragraph about polyamide, an outside layer can be produced of polyamide, which can be supported by an inside layer of polyethylene.  Thereby the form part has stability of a part made of polyethylene and the positive chemical characteristics concerning hydrocarbon for the application as a motorcycle tank.

• Improvement of the stability: By means of the foaming material hollow container can be produced in the rotational molding process, which show a high stability without the weight a full material construction part. The outside layer of the corpus is rotated of PE or PP and offers an attractive closed surface. Then a layer made of the foamed PE, which is in comparison thicker but also lighter, follows. This increases the stability of the form part, without the drastic increased weight. Due to an open porous surface at the inside it cannot be used for every application. Therefore often a triple-layer is produced, which as an additional full material layer in the inside. The result is a robust but also light for part as for example kayaks or for corpuses with isolating characteristics, such as chilling boxes.