Are there alternatives to internal mixers?

As suppliers to the automotive industry, rubber processors are very much familiar with the claims of their customers for highest quality at lowest price.

This means for equipment manufacturers offering automatic systems with efficient machines working at highest production and low operating costs getting continuously good quality.

A bottle-neck in rubber processing is today's mixing room installation. What has been done in the past to overcome this bottle-neck situation? Can the traditional mixing room equipment like internal mixers, mills and dump extruders be substituted by continuously working equipment? In other words, is there a chance to move from the discontinuous batch process to a continuous process like in plastics compounding? Do the available continuous mixing lines really give a better quality and is this really more economical? Or is it better to make out of today's mixing room installation a "Quasi"continuous system by combining the right internal mixer with the ideal peripheral equipment? This article tries to give an answer to these questions.

History

In our business, quality and pricing are concepts that beat nearly everything else. It didn't need a Mr. Lopez to understand that suppliers to the automotive industry can only be successful taking these standard claims into consideration.

About 70% of the world's rubber consumption goes into the automotive industry. Talking about mixing rooms and mixing room equipment means talking about the bottleneck for quality in every rubber processing plant. It should be the goal of everybody involved to overcome this bottleneck situation.

Historically during the first half of last century rubber started to be processed on roll mills. They didn't have today's modern design, but the principle of kneading and blending was already similar to what happens today.

At the end of the last century the roll mill job was partially taken over by internal mixers, at that time called a masticator.

It was about 1915 when one of these masticators delivered by Werner & Pfleiderer to a company in the U.S. gave problems by not pulling the rubber in between the rotors.

So Fernley Banbury, working at that time with Werner & Pfleiderer, was dispatched to this customer. He used a piece of wood to force the rubber in between the rotors and invented in that way a ram for the masticator.

As a result of the ram construction the masticator could not be discharged any longer by turning the mixing chamber to the front, so less than a year later, in 1917, he applied for a patent on the drop door. Still today rubber producers are using this traditional mixing equipment.

Today's mixing room

Looking at today's mixing rooms, the tire industry is working in two or more steps using for the production of masterbatch internal mixers with tangential rotor systems and dump extruders with roller-die and for the final batch two or three or more mills behind the intemal mixer.

In contrast, technical rubber manufacturers prefer internal mixers with intermeshing rotors making in many cases the final batch in one mixing step only. Behind the mixer are one or two mills, or one mill followed by a strainer.

Continuous mixing

Since the early 1960s raw material producers, machine manufacturers and rubber processors try to substitute the discontinuous batch process by a continuous rubber compound manufacturing process similar to plastics compounding.

Traditionally natural and synthetic rubber is packed in 30 kilo-bales, transported on pallets and delivered in this form to the mixing room. Here the desired batch-weight is obtained by manual or semi-automatic cuts on so-called hydraulic bale cutters.

In the 1970s, raw material manufacturers started to offer powdered or crumb rubber. To avoid sticking together, anti-adhesives and other substances were added.

But no major breakthrough was achieved. Manufacturing costs were too high. The large volumes receive i from the bales made packing and transport difficult and expensive. The number of recipes at the processors, plants was too large and the quantities per "just in time" product were too small.

So the problem was given back to the equipment suppliers who continued the experiments together with rubber processors.

Again, granulating of the polymer bales was the first step required. With today's knowledge, it is possible to granulate at a processor's plant most types of natural rubber, EPDMs and some other types of synthetic rubbers. Butyl rubber and other mainly oil-blended and therefore softer synthetics have a high sticking degree and give problems.

The bales are preheated to simplify granulation and the granules are transported automatically into a cyclone. Anti-adhesives like talcum powder prevent the granules from sticking together.

Behind the cyclone, a type of blending system might be installed consisting of a reversible conveyor belt for the stratification of the granulated rubber into rotary drums. From the rotary drums the granules are transported to the pre-mixer where all other substances are added. The dosing is based on parts per 100 parts of rubber by weight.

This is not to explain recipes. This is to remind that in every recipe there are minority substances which might have even less than one thousandth of the total weight. And within about 15 minutes one has to make sure that all these substances are well distributed within the pre-mix.

The pre-mix is conveyed into a feeding drum from where it finally enters the feed hopper of the extrusion process. Alternatively to this the pre-mix might consist of fillers and chemicals only and might be brought parallel to the rubber granules to the feed hopper of the extruder.

Known continuous rubber mixing equipment working in very specific applications with granules or powder or particulated rubber are shown in table 1.

Table 1 - continuous mixing
* FCM and MVX (Farrel)
* planetary gear extruder
* transfer mix extruder
* Ko-mixer (Buss)
  * EVK-extruder (W&P)
* ZSK twin screw extruder (W&P)
* other twin screw extruders

In all these cases the real continuous process starts at the and the largest problems are:

* the exact sufficient continuous dosing and premixing of polymers, fillers and chemicals;

* the complexity of the installation, which means beside looking at the investment costs the start-up of such a line takes time to reach a steady state of all relevant process parameters. Especially in larger machines (> 200 mm screw *) it might take hours to get there;

* the large quantities of different recipes for the various rubber applications.

If incompatible recipes have to be compounded one after the other, the clearing of a continuously working installation takes quite some time.

The future

But even if:

* raw-material suppliers delivered the material the way it is needed and at acceptable prices, and

* even if rubber processors reduced the member of recipes

* and even if you take fillers which are easy to disperse and which don't need high shear forces, the physical data achieved up to now permit the application of MVX or EVK or ZSK in very specific situations only.

But if the continuous process doesn't give the necessary and continuously needed high quality at lowest costs, what could be the solution? Our suggestion is to make out of your mixing room system a quasi-continuous process, and call it QCP.

What does this mean? QCP:

* starts with automatic storing, dosing and weighing of all fillers, chemicals and polymers;

* rubber should be simply articulated before entering the internal mixer

The internal mixer should have for example:

* a speed variable drive;

* a hydraulic ram for equal process conditions;

* the right rotor geometry for the individual application;

* sensitive reaction on cooling or heating, which means rotors, chamber sides, ram, drop door and side plates with cooling borings nearest to the compound;

* good access to all components for easy maintenance;

* the single screw dump extruder should have a simply designed double pusher or a conical twin screw in front of the roller-die;

The two-roll-mill should have:

* a hydraulic opening control;

* an individual variable drive for stepless friction control;

* peripherical drilled rolls (at least one);

* a stock blender;

* and on-line quality control similar to Rheomill.

Rheomill is a sensor system which enables to stipulate the floating behavior of rubber compounds when they are still worked on the mill.

During the milling process a wedge gib is put into the rubber. Sensor units measure viscosity, temperature and swelling behavior of the compound.

The whole mixing room should have an automatic electronic process control system. Automation starts with the PLC at machine level via process control level up to the host computer including weighing control, mixer control, production control and the control of additional devices.

The computer should not only control the overall system but also have control - on the mill or in front of the batch-off - an additional on-line quality control system like for example Relma.

Relma means remote laser microanalysis. The light of a pulsed UV-laser is focused onto the rubber surface producing a hot and bright plasma. The optical plasma emission is detected spectroscopically.

Thus you don,t check only if everything is included in the batch but you check also the distribution of the different ingredients in the polymer-matrix which means the rubber homogeneity can be characterized.

We feel that machine manufacturers and rubber processors should have QCP in mind when they think about the mixing room of the future.