1. Introduction
As an essential material in everyday life and industrial production, the manufacturing process of rubber goods has long been a hot focus of industry study. Rubber compound is the primary raw material for rubber goods, hence the preparation procedure is vital. This article will go over the entire rubber compound preparation process and explain what further has to be done when it is released from the internal mixer.
1.1 Preparation process of rubber compound
Rubber compound preparation consists mostly on the following steps: raw material weighing, internal mixing, and unloading. First, weigh the raw components, such as natural rubber, synthetic rubber, filler, and additives, using the formula ratio. The raw components are then placed in a double-drum internal mixer for thorough mixing, which takes around 5-15 minutes in most cases. When the rubber compound has met the specified performance characteristics, it is removed from the internal mixer. This concludes the preparation of the rubber compound.
1.2 Further treatment of rubber compound
The rubber compound output from the internal mixer cannot be directly used in the subsequent manufacturing process; further treatment is necessary. The key procedures are chilling, pelletizing or slicing, and storing. First, the high-temperature rubber compound must be cooled down. Once chilled, the rubber mix is chopped into particles or slices for storage and transportation. Finally, the processed rubber mix is stored in a dry, light-proof, temperature-appropriate location to maintain its quality.
2. Characteristics of the rubber mix after being discharged from the internal mixer
2.1 Physical state
When the rubber mix is released from the internal mixer, its temperature is typically between 120 and 180°C. Mechanical churning increases the viscosity of the rubber mix, making it harder to flow. Furthermore, because the various raw ingredients are not thoroughly mixed together during the internal mixing process, the discharged rubber mix is likewise non-uniform. These physical properties pose challenges to the following processing procedure.
2.2 Chemical properties
Chemically, the rubber molecules’ chain length and cross-linking degree altered dramatically during the internal mixing process. On the one hand, mechanical shearing breaks the rubber molecular chain, resulting in a broader molecular weight dispersion. Under high temperature circumstances, however, cross-linking reactions occur, increasing the rubber’s cross-linking degree. This complicated molecular structure significantly decreases the rheological characteristics of the rubber mixture.
2.3 Existing problems
Combining the foregoing physical and chemical features, it is clear that the rubber compound recently expelled from the internal mixer is unsuitable for direct molding. Its high temperature, viscosity, non-uniformity, and complicated molecular structure will provide significant challenges to following molding operations like as molding and extrusion. To fulfill molding requirements, the rubber compound must be further treated.
3. The necessity of thin-through pouring
As previously stated, the rubber compound emitted from the internal mixer exhibits a number of physical and chemical properties that make direct molding challenging. To solve these issues, the rubber sector often uses thin-through pouring on the rubber compound. This treatment approach can significantly increase the performance of the rubber compound and establish ideal circumstances for future molding. We’ll go over why thin-through pouring is necessary in detail below.
3.1 Reduce the viscosity and fluidity of the rubber compound
The rubber compound emitted from the internal mixer is highly viscous, making it difficult to flow and shape. Thin-through pouring can lower viscosity and enhance fluidity by mechanical action. This is due to shear stress applied to the rubber mix during the thin-through mixing process, causing the molecular chain to break and reducing the molecular weight distribution. At the same time, repetitive plastic deformation can aid in the equal distribution of raw materials. These modifications serve to lower the viscosity of the rubber mix while increasing its processability.
3.2 Improving the uniformity and processability of the rubber mix
As previously stated, the rubber mix emitted from the internal mixer is not completely consistent. Through repeated shearing and extrusion, thin-through mixing may improve the uniform distribution of raw material components in rubber. At the same time, this mechanical action might remove some microstructures in the rubber mix, such as agglomerates, therefore increasing its processability. As a result, thin-through mixing is an excellent way to increase the homogeneity and processability of the rubber mixture.
3.3 Inhibit excessive cross-linking reaction and maintain good mechanical properties
During the internal mixing process, rubber molecules undergo a degree of cross-linking reaction, increasing the cross-linking degree of the rubber mix. If the cross-linking reaction continues, the rubber mix’s mechanical characteristics will decrease. Thin-through pouring can prevent excessive cross-linking by decreasing the temperature and minimizing the residence period, allowing the compound rubber to retain acceptable mechanical qualities.
4. Process principle of thin-through pouring
Thin-through pouring is a commonly used method in the rubber industry. It may significantly increase the performance of compound rubber while also creating suitable circumstances for further molding processing. So, how does thin-through pouring perform these functions? We’ll look at the thin-through pouring technique idea in detail below.
4.1 Reducing molecular weight through mechanical shearing
During the thin-through pouring operation, the compound rubber will experience significant mechanical shear stress. This shear force can break rubber molecular chains, lowering the compound rubber’s average molecular weight. Breaking molecular chains widens the molecular weight distribution, which improves the compound rubber’s flow qualities. As a result, mechanical shearing is the primary process used in thin-through pouring to lower molecular weight.
4.2 Improving molecular weight distribution and improving flowability
In addition to lowering the average molecular weight, thin-through pouring can improve the molecular weight dispersion of compound rubber. Long-chain molecules are more prone to break under extreme shearing, resulting in a more concentrated and uniform molecular weight distribution. This change in molecular weight distribution helps to improve the fluidity and processability of the rubber mix.
4.3 Promoting the rearrangement of crosslinking groups
Furthermore, mechanical activity during the thin-through mixing process might cause the rearrangement of crosslinking groups in the rubber mix. The high temperature during the internal mixing step will cause certain crosslinking reactions. However, the crosslinking may not be homogeneous, affecting the performance of the final product. Thin-through mixing can rearrange these crosslinking groups by adding shear stress to the rubber mixture, resulting in a more homogeneous crosslinking structure. This adjustment helps to retain the rubber mix’s strong mechanical qualities.
5. Process flow of thin-through mixing
5.1 Discharge of rubber mix from internal mixer
First, the rubber mix is ejected from the closed internal mixer. During the internal mixing process, the rubber, and other compounding materials were well combined, and a certain degree of crosslinking reaction occurred, resulting in a mixed substance with high viscosity and low flow.
5.2 Entering the double-roll open rubber mixer
The discharged rubber mixture is then fed into the double-roll open rubber mixer. This rubber mixer consists of two horizontally placed rollers with a space between them to provide an open feeding area.
5.3 Repeated extrusion between rollers
In the rubber mixer, the rubber mix will repeatedly penetrate the space between the rollers and be subjected to intense shear and compression forces. This mechanical motion can greatly lower the viscosity of the rubber mix while increasing its fluidity and processability. At the same time, repetitive plastic deformation can help to ensure homogeneous dispersion of the compounding ingredient.
5.4 Obtaining a rubber mix with excellent performance for molding
The thin-pass refining technique optimizes the molecular weight distribution of the rubber mix, improving fluidity and homogeneity significantly. Such a rubber mix has good molding and processing performance, allowing for the easy execution of following molding operations such as extrusion, injection, and calendering, resulting in rubber products of great performance.
