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Rubber compounding, including mastication, mixing, hot mixing, re‑mixing, and other processes, is primarily carried out using open mills and internal mixers.

NR and CR are typically processed on an open mill using a narrow roll gap and thin passes, repeated as needed to achieve the required plasticity (within a specified range), thereby meeting the downstream process requirements for compound flowability and the final product’s physical‑mechanical properties. For NR, “accelerator M” or a plasticizing agent is introduced; for CR, a self‑prepared dextrin‑water turbid solution is sprayed in—both methods can accelerate the plasticizing process. (Note: CR plasticization involves a “structural transformation” phase; proper control of this phase facilitates optimal plasticizing results.)

Compounding is widely regarded as the most critical processing step in rubber product manufacturing. Textbooks and specialized monographs have already provided extensive discussions on the factors that influence compounding quality in open mills and internal mixers, as well as on the numerous parameters used to evaluate compounding processability. However, when it comes to the pivotal yet complex and highly variable “feed order,” such standard references often offer only simplified, generalized overviews, while the literature tends to be relatively vague, frequently reflecting a tendency to merely echo others’ views or adopt a “copy‑and‑paste” approach. From another perspective, the feed order may, in fact, sometimes constitute a proprietary, confidential element.
In practice, many formulations do not require an especially strict “order of addition”; as the saying goes, “the absence of a fixed method can sometimes be more effective,” and even a somewhat haphazard mixing process can yield a “qualified product.” However, for other formulations, variations in the mixing procedure can lead to significant differences in performance. If the “order of addition” is neglected, it may prove impossible to produce a compounded rubber that meets specifications, jeopardizing subsequent processing steps and resulting in non‑conforming products—often with an exceptionally high scrap rate. Therefore, it is essential to carefully analyze, deliberate, experiment, and compare options to identify the optimal “order of addition,” taking into account factors such as the timing and duration of each addition, the quantity added, and the batch sequence. To explore, develop, and validate an appropriate addition sequence, one must not only understand the intrinsic properties of the rubber and the compounding ingredients but also thoroughly investigate the interactions—whether additive, synergistic, or antagonistic—among different rubbers (especially when used in combination), between rubber and compounding agents, and among various categories and grades of compounding agents. This includes assessing their compatibility, as well as the nature and extent of physical and chemical reactions involved.
People should not only strengthen their theoretical grounding in these areas; more importantly, they need to engage in ample practice, comparison, critical reflection, and verification. Here, we offer a few examples drawn from open‑mill mixing as illustrative evidence and reference. As for the “causes” or “rationales,” we neither wish nor deem it appropriate to dwell on them at length—let that be left to readers to “experience and ponder.” Begin by “using what is,” setting aside for now the question of “why”; when conditions permit, strive gradually to “understand the why,” thereby approaching problems and resolving them with greater rationality and moving toward the state where “the subtlety of application resides in the heart.” If one insists on “knowing the why,” one may well have to “wait a very long time—for history itself!”
Example 1: Direct bonding of “NR/brass valve stem.”
Method A: First, perform plastication on the NR until it reaches the required plasticity level A; then proceed with mixing, achieving a plasticity level AA in the compounded rubber. After bonding, the product meets specifications, with an adhesion rate exceeding 97%–100%.
Method B: NR is first subjected to plasticizing until it reaches plasticity grade B (B<><>
 
Example 2: Direct bonding of CR to brass (using the imported adhesive SW).
When the CR coating is properly applied, the non-uniform peel adhesion strength is no less than 6 MPa (and can even reach 10 MPa), with 100% adhesive coverage; if the coating is insufficient, the strength is only around 4 MPa.
 
Example 3: NR hollow tire (small, air‑filled tire for model aircraft) — a semi‑finished product in which the two halves have been vulcanized to a certain degree (e.g., around TC70) with the aid of an adhesive layer, and then bonded into a complete tire while containing a blowing agent.
If NR is fed into the mixing process without achieving an appropriate plasticity, it is difficult to produce a finished product with uniform thickness and a smooth surface free of bubbles and scars; moreover, during assembly, poor adhesion may occur, leading to scrap. For “adhesive-grade rubber,” the plasticizing and mixing stages demand even more precise rubber‑processing techniques. With a well‑formulated recipe and proper plasticization and mixing, even when the two halves are vulcanized far beyond the tc100 point, they will bond exceptionally well.
NBR, SBR, BR, EPDM, and IIR typically offer grades with varying Mooney viscosities. It is difficult to achieve or increase plasticity through thin‑rolling alone; however, performing a brief thin‑roll prior to mixing, or conducting a period of calendering and kneading, can enhance uniformity, reduce agglomeration, and improve both the quality of the compounded rubber and the final product.

In short, the “feed‑order” in the mixing process is endlessly variable, and its complexity defies full articulation. The only way forward is through extensive hands‑on experience and critical reflection, enabling one to synthesize and distill relatively sound practices—while firmly avoiding mere blind conformity or mindless replication.