Internal mixer is a standard rubber machinery for volume mixing in both tire industry and non-tire rubber industry.

When you use one, your most elementary requirement is to calculate the batch weight for your respective mixer model. Because when mixing rubber compounds, you should understand that different compounds based on the same polymer might require different batch weights. And different polymers will almost certainly require different batch weights.

Image Courtesy: Bainite Machines Pvt Ltd

Here’s 7 quick tips for you to fix the batch weight for your rubber mixing. (Updated on 23rd Dec 2015: Flip through this post in our digital edition and download here)

1) Theoretical Equation

The thumb rule is the theoretical equation

W= NV x SG x FF

where W – Batch Wt [kg]; NV – Net Mixer Volume [dm³]; SG – Specific Gravity (density) of the mixed batch [kg/dm³]; and FF= Fill Factor.

Generally, most mixer manufacturers share this calculation with you. But remember, what they give you is only a theoretical number. This is only a starting or reference point and you need to arrive at your own mixing batch weight for your compound recipes, following some of the other tips stated below.

2) Net Mixer Volume (NV)

Since Internal mixer has a fixed volume mixing chamber, knowledge of the net volume (in liters) is required. This can be obtained from the manufacturer directly or in some cases from their literature for their various models.

When the mixer is used regularly (or if you have procured a used-mixer) the effective volume increases due to wear on the rotors and mixing chamber. If not compensated for this inside wear, your batch volume will be effectively too small leading to insufficient ram pressure on the compound, poor dispersion and longer mixing times. Annual measurements of chamber are recommended to update your batch weight correctly.

Excessively worn out mixers will have to be rebuilt or reconditioned (Read our posts on mixer rebuilding – Top 25 Things You Should Know to Discuss with Mixer Rebuilder and 17 Essential Questions to Select the Right Rebuilder for your Internal Mixer)

3) Guesstimate the Fill Factor (FF)

If you have a Tangential Mixer (aka Banbury) , then your FF can range between 0.70 and 0.85. And for a Intermeshing Mixer (aka Intermix), your FF can range between 0.62 and 0.70.

Knowledge of the fill factor is necessary because an under-filled mixing chamber results in the ram bottoming out too soon. This reduces the pressure on the rubber stock and increases the mixing time. An over-filled chamber leads to unmixed ingredients staying in the mixer throat. This creates a mess under the mixer when the batch is dumped.

For example, NR-rich compounds in an intermeshing mixer has a fill factor of around 0.65 while for the same compound in a two-wing tangential mixer, it is about 0.75. This compound will have an increased FF of about 0.78 for a tangential mixer with four-wing rotors. Each polymer also has its ideal fill factor and that varies again with Mooney viscosity and filler system.

Fill factor of a mixer depends on the age of the machine, wear and tear of the rotors and chamber, the rotor type, rotor speed, rotor friction ratio, nature of elastomer, ratio of elastomers/ fillers, mixing sequence, kind of polymers, fillers and individual SG of the ingredients in your recipe, viscosity of ingredients, etc. Generally, the lower the compound viscosity, the fill factor is higher.

Hence, we initially guesstimate the FF before stabilizing on the figure later on through actual trials.

4) Estimate the Specific Gravity (SG) of your Compound

You can estimate the density of your compound by multiplying the quantity of each ingredient with its individual density (you can get this figure in any compounding handbook or ingredient supplier literature). Sum up your individual results and then divide this number by the total sum (usually phr). The result will give you the estimated density of the compound. (Mathematically, this is the weighted average calculation).

For example, lets consider a sample recipe (I got this recipe from a web search) as below:

Calculating, the SG of this Compound mix is arrived at 1.16 (=179.3/155).

5) Know your Internal Mixer

Knowing your internal mixer – its capabilities, design features like rotor (tangential or intermeshing), ram (pneumatic with dedicated air supply at the plant or hydraulic), variable speed capabilities of the motor, SCADA, PLC, automation and control features, etc.

Rotor speeds are critical because you can use higher speeds at the initial mix and then reduce the rotor speed to allow the batch to “knead” well.  This will allow you to get both your dispersion and distribution tasks of mixing right. Hence, when selecting a mixer explore variable speed drives since it give you advantage in your mixing process.

(If you are planning a new purchase, read and download our Questionnaire for Internal Mixer Selection)

Similarly, think of ram pressure.  If your ram pressure is too high you will cause excessive heat build up and poor flow of ingredients across the rotor tips. In intermeshing mixers, this will also cause internal pressure within the mixing chamber and might cause mixer failure. If ram pressure is too low, then you will not get the ingredients down into the rotors and this will result in poor mixing. (Read more about Hydraulic Ram here)

Image of HF Mixer

6) Watch the Ram Action

After the above reference calculations are done and mixing initiated; watch the ram action during the mix. The ram should start high, move up and down about an inch or two and bottom out when mixing is complete. Good mixing practice dictates that when the ram bottoms out about 30 – 45 seconds before the batch is dumped, you can be assured that the chamber is properly filled and mixed compounds will be of high quality.

You need to observe the position of the ram by watching the tell-tail rod attached to the top of the ram. Hence, this requires more of practice and experience than theoretical knowledge.

If you have a good mixing system with controls and feedback features, you can correlate the position of the ram with the current and rise in temperature – these are important to get an optimized batch size and high quality of mix.

7) Optimize Your Mix Batch Size (…Do Not Maximize)

The key to successful mixing is optimizing your mix batch size, and not maximizing. And good mixing is a form of art.

Most mixer users want to get the most out of their internal mixer (quite natural!) and they test its capabilities to the full. Finally, when they get poor mixing, they wonder if they have done the right investment! 

If you try to take your batch size to the upper limits of the mixer’s “capacity” as specified in the manufacturer’s manual (that is usually a peak magical figure) and you have raw material variations such as particle size or bulk density changes in your fillers, this can lead to poor mixing (dispersion and distribution of ingredients).

The right batch size will be smaller, but your internal mixer throughput is increased by shorter mixing time and thus more batches in the same period. Thus, optimizing your batch weight will allow you to get consistent batch quality and repeatability that are of paramount importance to your (or your customers’) downstream processes.

The key factors that will influence your mixing optimization are compound formulation, ram pressure, mix procedure, mixing speed and rotor design.

Each mixer is different and it would be very difficult to determine the optimized fill factor without actually conducting several mixing trials. Experience is a key to good mixing.

Summarizing, when mixing rubber compounds, different compounds require different batch weights. These 7 tips will help you calculate the optimized batch weight for your compounding recipes on an internal mixer quicker.

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