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Keywords of this article:  injection molding 
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dryer 
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medical 
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extruder 
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injection mold 
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pelletizer 
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Layout for granulating PVC
Layout for granulating PVC
Photo: 123

In the last decades, the use of PVC has been more than successful all over the world. The versatile material is today one of the most important plastics internationally recognized and proven in the market. The good properties of PVC in functional, economic and ecological aspects will expect an increase of the per capita consumption in the next few years. The continuous innovations in fillers, additives, stabilizers and others combined with the various formulations allow PVC to be used in many areas with its individual properties.

Granulating

Certain applications and machinery require the usage of PVC in granular form, such as medical applications, cables, or bottles, to mention just a few. The main objective for using granules in these processes are the operational reliability, which is provided by the good flowability and dust free transportation of the granules. Only this form allows a constant feed intake of PVC and a stable homogenous plastification. The same applies for injection molding machines where the even feed intake has direct influence on the shot weight and quality.

A general overview beginning from the mixing, processing and granulating up to cooling or drying of the pellets will give an insight to the equipment needed for this task.

MIXING:

For the production of dry blend for granulating, the various components are transported into a high speed mixer. The high speed mixer is required to premix and distribute PVC, fillers, impact modifiers, reinforcing materials, plasticizer, stabilizer, lubrication, color, pigment and production aids. Due to the high temperature the plasticizer is absorbed into the PVC grain and volatiles like humidity can be evaporated by a vacuum nozzle.

Depending on the formulation the mixer temperatures of 100°C and beyond are common. The hot mixture is then emptied directly into the inlet hopper of the compounding machine or -depending on the compounding equipment - in a second step cooled down in a slowly rotating cooler mixer. Control of the dry blend temperature is important and is to be kept constant as it has an direct influence on the quality.

FEEDING:

There is one major benefit using a twin screw extruder in terms of the way of feeding the different raw materials. Instead of premixing all components prior to the compounding step also separate feeding of the raw material components is possible, e.g. color pigments or color masterbatches can be added separately without contaminating the upstream-located equipment. Color changes and formulation changes can be done in an easy and quick way. In addition the separate dosing of the components offers a directly controllable influence on the product quality; with just one setup a wide range of formulations can be processed and granulated.

COMPOUNDING:

Coperion’s Kombiplast KP is the ideal system for the processing of PVC. The two-stage Kombiplast processing system consists of a ZSK twin screw extruder and an ES-A single screw discharge. It boasts outstanding mixing behavior, excellent technical flexibility, and good compression and venting capabilities. The Kombiplast KP processes hot and cold dry blend as well as individually fed components.

The raw materials (either as premix or separately added) are transported to a twin screw side feeder (ZS-B). The feed intake is specially designed with high free volume of the screw and an optimized feed inlet, the variable screw speed of the ZS-B screws are determing the resulting throughput rate.

The mixture is conveyed to the first barrel of the ZSK twin screw extruder, a rear vent helps to remove air of the powder premix. The premix is conveyed to the plastification zone where it is gelled until a crumbly (rigid PVC) or more or less thoroughly plastified (flexible PVC) homogenous phase is reached.

DISCHARGE:

The mass is then transferred without creation of back pressure to the slowly rotating single discharge screw ES-A in a cascade design. Both, the ZSK twin screw extruder and an ES-A single screw discharge are electrically heated to have an exact temperature control, cooling of the twin screw extruder is by water, the single screw is cooled by air.

The single discharge screw is required to generate pressure to overcome the resistance of the die plate. When processing flexible PVC melt pumps are used for pressure build up. The critical aspect is the viscosity and shore hardness of the extruded product as well as the filler content. Once the viscosity has reached a certain level the lubrication of the bearings is not guaranteed. High filler content can lead to excessive wear in the bearings and cogs.

PELLETIZING:

The PVC “melt” is cut either in air w/o water film or completely under water into pellets and transported to a pellet cooler or dryer where the temperature is brought down to about 10°C to 20°C above room temperature. From there it is transported to storage / mixing silos or to bagging.

Hot die face, dry cut pelletizing has been historically used for years for the full range of PVC compounds, both rigid and flexible. Especially high throughputs are granulated with a full die plate of an eccentric granulating system. Coperion’s Eccentric Pelletizer EGR is ideal for the pelletizing of PVC. This system includes an offset, multi-hole pelletizer die plate, electric heated die ring, pelletizer arm assembly with 2 or 3 knife blades, pelletizer casing or hood with bottom discharge and a fluidized bed air-type pellet cooler. Coperion’s new type of knife rotor makes it possible to produce extremely low-dust PVC pellets. Rotating directly on the die plate of the EGR, and fitted with pelletizer knives that have also been optimized, the rotor (patent pending) permits particularly smooth and gentle cutting of temperature and shear-sensitive plastics.

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