There are many factors that influence the quality of a product, one of these is the choice of powders.

The rotational molding is a process frequently used for the formation of objects, using thermoplastic resins , that we need to be hollow.

The main feature of the process is that the mold rotates around two axes, or mutually perpendicular, moreover, with respect to injection molding traditional, the raw material, in the form of powder, is introduced into the mould, to then be heated and subsequently cooled.

What are the main differences with the injection molding process?

Perhaps the most obvious is that in rotational molding the raw material is used in the form of powder and not granules furthermore, the polymeric resin is found inside the closed mould, and not pressure injected into it.

Furthermore, the mould, in the rotational process, works on the basis of axial rotation unlike the static nature of injection moulding.

Finally, we can say that rotational process molds are cheaper as they do not have to consider the injection pressure.

Why choose rotational molding?

When objects with a hollow shape have to be produced, rotational molding is particularly suitable for its ease of adaptation to all required forms.

Furthermore, in the absence of great pressure inside the mould, the product tends to easily shrink and detach after its production, even if the objects are of large size.

Finally, we can say that through the rotational process, it is also possible to create very complex elements both from a structural and design point of view.

Main characteristics of rotational molding molds

We can say that the main materials that make up the molds are:

• Cast aluminium

• Electroformed nickel

• Stainless and non-stainless steel

When we are faced with the need for better uniformity in the heat exchange inside the mould, we will choose cast aluminium.

If we had to favor a faithful reproduction of the figures we could choose electroformed moulds, while in the presence of simple shapes and large formats, we can opt for the cheapest steel moulds.

If we talk about the thickness of the molds we can say that, normally, the cast aluminum molds have a thickness of 6-8 mm, while those in steel only 2- 3mm.

When designing the mold you should always keep in mind which raw material you will be using, as some polymers shrink sufficiently facilitating the extraction of the piece, others less, so as to make a slight draft angle necessary in the mold to facilitate the detachment of the product.

Rotational molding phases

As we said previously, rotational molding is nothing more than a heat exchange within a mold under conditions of movement.

The temperatures during the process can vary, within a certain range, continuously during the entire production cycle.

Despite these continuous variations in temperature, the quality of a product is established by calculating the exact permanence of the mold inside the oven. This time is called induction time.

We can therefore say that, in the first phase of the cycle, the induction time is that interval of heating of the mold in which the resin reaches melting temperature, which normally takes place through the blowing of hot air.

The induction time is characterized by the following variables:

• Oven temperature

• Speed of heat exchange

• Thickness of the mold

• Melting temperature of the resin

• Ratio between surface and volume of the mold

• Thermal transfer coefficient of the mold material

The second phase of the cycle, defined melting time , is the time required to completely melt the resin.

The melting time is characterized by the following variables:

• Thickness of the piece

• Temperature of the resin and heat of fusion

• Mold heating capacity

• Ratio between the surface of the mold and its volume

• Oven temperature

All of these variables have a significant impact on melt time and quality of the piece to be made.

However, the melting speed of the resin can, in some cases, be increased by raising the oven temperature, but it is important not to exceed in this operation because, if on the one hand it increases productivity, on the other an excessive permanence of the polymer in the mold, at very high temperatures, can lead to its degradation.

Choosing the powder to be used for rotational molding

As we have seen, the melting time of the resin is a crucial factor for the good performance of the mold and for the quality of the pieces to be produced.

So, we can say that even the size of the polymer particles that are used can influence the processo. In fact, a dimensionally larger resin increases the time needed to melt.

This occurs due to the decrease in contact surface between the particles and the hot parts of the mold, but this does not normally occur if a raw material size of less than 500 microns is used.

Beyond the important dimensional parameter of the polymer powders to be used, it can be said that a good raw material is one that flows quickly into sharp corners and in the recesses, adhering to the mold and melting without bubbles through the thermal contribution.

Moreover, from experience, the finest powders are used for resins with lower MFIs, in order to obtain a good surface reproduction, while the use of a polymer with high MFI may consider using larger particle sizes.

Cool cycle of the mould

The cooling of the mold and the product can take place through the use of both air and water.

Normally the air, pushed by the cooling fans, hits the outside of the mold, while the use of water jets is reserved for the inside.

The cooling time is very important as an acceleration of this phase, therefore a rapid cooling, could lead to a deformation of the piece with an increase in the percentage of the amorphous phase of crystalline polymers.

Machine translation. We apologize for any inaccuracies. Original article in Italian.