Rubber Compounding and Mixing: Crafting quality starts with the recipe; extends to mixing

By Cody David

The genesis of rubber product manufacturing is the compounding and mixing of numerous raw ingredients. This transformative process initiates the creation of versatile materials that ultimately become functional rubber products.

The process goes beyond the mere mechanical amalgamation of these ingredients; it’s a precise science, amplified with artistry, that determines the intrinsic properties of a rubber compound. Physical characteristics, performance attributes and ultimately, the quality of finished rubber products are directly influenced by the mixing process. Understanding the nuances of rubber mixing is essential for delivering products that consistently meet, and in WARCO’s case, exceed both industry standards and customer expectations.

The Goal: Ingredient Dispersion

Understanding dispersion and shear in rubber mixing is crucial for achieving consistent and high-quality results. Dispersion refers to the uniform distribution of all ingredients, which ensures that each component is thoroughly blended. The ultimate goal of rubber mixing is to achieve a high level of dispersion as poor ingredient distribution can lead to failed physical properties and a lack of desired characteristics. Lumps, pinholes or a bumpy, uneven textured surface on finished parts can be a key indicator of poor dispersion. This can result in sub-standard wear and propagate fissures and tears.

Shear describes the force applied to the rubber compound to drive dispersion, with higher shear rates leading to better dispersion. By controlling shear forces, rubber manufacturers can optimize the mixing process to develop the desired properties in their products.

There are four key measurement parameters play a significant role in achieving effective dispersion:

  • RPM or revolutions per minute indicates the speed of mixing equipment rotation
  • Amps measure the electrical current drawn by the mixer and correlates to the workload of the mixer
  • Temperature is monitored to precisely blend ingredients and prevent scorching or premature curing
  • Time is fundamental to control the duration of mixing required to reach homogeneity

2) The 2 Methods of Rubber Compound Mixing

There are two primary methods for mixing rubber compounds: mixing with an open-mill or utilizing an internal mixer. While open-mill mixing offers simplicity and provides for small batch sizes, internal mixer mixing provides enhanced control and efficiency gains through speed and larger batch sizes.

Open-Mill Mixing

Open mill mixing occurs on a two-roll mill, a foundational method facilitating efficient blending and dispersion. The shear rate is primarily controlled by adjusting the speed of and the nip or space between the mill rolls. However, achieving uniform dispersion may be challenging due to the limited control over mixing parameters compared to internal mixer mixing.

Open-mill mixing’s strengths lie in its simplicity, low equipment cost and ability to mix small batches. However, it comes with limitations regarding precision and control over mixing parameters. It typically allows for the measurement of only two of the four key mixing parameters: RPMs and time. Moreover, it is a slower process with each batch requiring 30-45 minutes to blend properly. Achieving nuanced formulations, particularly those with specialized requirements, may present challenges.

Internal Mixer Mixing

Internal mixers are specialized machinery, offering heightened control over mixing parameters which provides more consistent results. This method ensures a precise blending process that enhances the overall quality of the rubber compound.

Each of the four key mixing parameters are precisely optimized as well as a host of additional controls and monitoring functions. Operators can perfect the shear rate of the mixer rotors against the walls of the mixing chamber to ensure thorough dispersion of ingredients, leading to a uniform and consistent rubber compound.

Internal mixer mixing is a complex process that offers improved control and efficiency along with a higher investment in machinery. The benefits include a precise blending process, significant speed improvement of a mere 5-7 minute cycle time per batch, and command over mixing parameters. This level of control is integral for high performance compounds designed for applications requiring critical performance and specific requirements.

Steps in the Rubber Mixing Process

1. Introduction of the Polymer

The rubber mixing process commences with the introduction of the polymer, the fundamental building block of rubber compounding. The choice of polymer establishes the foundation for the material’s performance and dictates its inherent characteristics. See to learn about each polymer’s properties.

2. Sequenced Addition of Ingredients

While the polymer typically has the largest impact on the characteristics, additional ingredients are included to determine features such as curing and processing performance, cost consideration, hue designation, anti-degradation, and much more. This includes fillers, plasticizers, curing agents, accelerators, etc. See our previous article, for a general breakdown of each ingredient type’s effect.

3. Achieving Homogeneity in the Compound

The ultimate goal is achieving thorough homogeneity. A seamless blend of all ingredients creates a uniform and consistent rubber compound. A homogeneous rubber compound ensures uniform characteristics throughout production processing, curing and the final application of finished rubber goods. This is achieved by carefully moderating the four key mixing parameters according to the compound requirements while carefully considering chemical interactions of ingredients. Each rubber compound performs and reacts differently to the process and therefore requires an experienced operator execute on each compound’s detailed and exact mixing procedure to create the ideal mix.

The WARCO Advantage

The key to manufacturing quality rubber products begins with precise control over compound mixing parameters coupled with a deep understanding of the materials and their interaction with each other through the process. Our technical managers leverage their expertise in chemistry, mixing parameters, compound ingredient reactions, form production process and cure methods, and more to establish the ideal procedure of temperature, time, ingredient sequences, and shear rate.

An effective rubber manufacturer must be well equipped with testing equipment to measure the unique parameters of a compound prior to and after the form/cure production process. This feedback loop allows for a higher standard of quality and consistent, repeatable, and reliable results.

Unlike many other rubber manufacturers that depend upon external laboratories and mixing facilities, WARCO develops proprietary formulations, mixes compounds, and produces your sheet, molded and extruded rubber products in a single facility. Our expertise spans beyond our 114-year history providing quality rubber products to American industry and the global supply chain. Our wealth of experience, unique flexibility and integration, and tight control over each step of production are among the key contributors to how WARCO became America’s Choice for Quality Rubber.

This is the second article of our new series, Stretching the Imagination: Ideas to Innovation, where we break down the major steps of rubber production. Stay informed of the latest articles by signing up for our newsletter at

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