Smoke and Deposits in High-Temperature Molding Are Not Simply Due to "Insufficient Viscosity": IOTA Outlines Silicone Oil Selection Criteria, Featuring IOTA-2437, 255, 255A, and BJ400


During continuous high-temperature molding, the appearance of smoke, a transparent oily film, or yellowish-brown deposits on the mold surface often prompts an immediate, intuitive response: "increase the viscosity of the silicone oil." In recent technical documentation, Anhui IOTA Silicone Oil Co., Ltd. (IOTA) refutes this assumption, noting that the causes of smoke and deposits are distinct. While increasing viscosity might reduce the volatilization of low-molecular-weight components, it can also impair the oil's ability to spread and transfer effectively, potentially exacerbating residue buildup on the mold. The fundamental question that must be addressed first is: where exactly are the smoke and residues originating?


IOTA categorizes the sources of smoke and deposits in high-temperature molding into seven areas: the actual surface temperature of the mold exceeding the material's rated operating temperature; the volatilization of low-molecular-weight components or diluent carriers at high temperatures; the thermal oxidation of silicone oil exposed to heat in the presence of air; excessive application of the release agent; the migration of plasticizers, oligomers, or filler-treatment agents from the molded material; incompatibility with existing release-agent layers or cleaning agents; and localized over-application due to unstable spray-atomization parameters. This diagnostic framework makes one point clear: if the smoke stems primarily from diluents or substances exuding from the molded material, simply adjusting the viscosity of the base oil will not solve the problem.


To help distinguish between these phenomena, IOTA provides specific troubleshooting guidelines: white mist appearing immediately after spraying is typically linked to low-boiling-point carriers or low-molecular-weight components; a transparent oily film is more likely the result of over-application or the accumulation of high-viscosity materials; yellowish-brown residues observed after continuous operation point to prolonged thermal oxidation or the exudation of components from the molded material; and localized black, charred deposits suggest the presence of "hot spots" on the mold or the repeated thermal degradation of existing residues. Since color alone cannot definitively indicate chemical composition, it is advisable—when necessary—to collect samples of the fresh release agent, mold residues, and exuded substances for comparative analysis. Dimethyl Silicone Oil vs. Methyl Phenyl Silicone Oil: The Difference Lies in Thermo-oxidative Stability, Not Merely a "Grade Upgrade"


In terms of material classification, dimethyl silicone oil offers a broad viscosity range and is well-established for spreading and mold-release applications; however, it remains susceptible to volatilization and oxidation during continuous high-temperature use. Methyl phenyl silicone oil, by contrast, typically exhibits superior thermo-oxidative stability due to the incorporation of phenyl groups. Technical literature indicates that high-molar-mass methyl phenyl silicone oils can remain stable in air at 250°C for several hours, withstand even higher temperatures for short durations, and support long-term operation at 250°C. Nevertheless, this does not guarantee that switching to a methyl phenyl silicone oil will entirely eliminate residue buildup. As noted in the comparison table provided by IOTA, one cannot judge mold-release performance or potential residue buildup solely based on the presence of phenyl groups; factors such as phenyl content, viscosity, volatile composition, and compatibility with the existing system must all be verified under continuous operating conditions.


Regarding specific product grades, IOTA’s portfolio offers options tailored to various temperature ranges and application requirements. **IOTA-2437** is a low-viscosity methyl phenyl silicone oil (15–22 cSt at 25°C; flash point: 170°C) designed primarily as an **internal mold-release agent** for thermoplastic and thermosetting resin systems. Application data shows that when pre-mixed with resin pellets at a dosage of 0.01%–0.5%, it enables 500 to 2,000 molding cycles—for engineering plastics such as PC, PA, PBT, and PPO—without the need for additional external mold-release agents, while keeping the mold cavity clean and free of residue. For external spray-on applications, **IOTA-255** methyl phenyl silicone oil offers long-term heat resistance up to 250°C (with short-term tolerance up to 300°C), utilizing phenyl modification to enhance oxidation resistance. Meanwhile, **IOTA-255A** features a higher phenyl content, extending its operating temperature range to between -70°C and 320°C, with an open-cup flash point of ≥305°C. Furthermore, **IOTA BJ400** methyl-phenyl polysiloxane exhibits thermal stability within a temperature range of -30°C to +280°C, making it suitable for use as a mold-release agent or lubricant. While IOTA’s product documentation mentions that it can "withstand temperatures above 300°C, endure mold temperatures without producing smoke or carbonizing, and leave no residue on the mold," this claim pertains to the performance targets verified for a specific grade under defined operating conditions; it should not be generalized as a universal promise applicable to all types of methyl-phenyl silicone oils.


**Defining Operating Conditions: A Prerequisite to Selecting Viscosity**


IOTA provides a checklist of operating conditions covering eight key dimensions: the actual surface temperature of the mold (including localized peak temperatures); the production mode (continuous vs. intermittent) and cumulative operating duration; the degree of air exposure (e.g., in open heating environments or areas with localized exhaust); the type of molding material and its potential outgassing or deposits; the form of the release agent (neat silicone oil, emulsion, solvent-based, or aerosol); application parameters (dosage, dilution ratio, nozzle distance, and re-application frequency); the specific nature of any performance failure (such as smoke generation, transparent oil films, yellowish-brown residues, carbonization, or difficulty in demolding); and compatibility requirements for downstream processes like painting, printing, bonding, or electroplating. Specifying a particular silicone oil grade or viscosity without complete data lacks engineering reliability.


Regarding validation methodology, IOTA recommends a comparative study involving three groups: the material currently in use, a high-viscosity candidate, and a methyl-phenyl silicone oil candidate. This test should keep the mold, molding material batch, temperature, cycle time, application dosage, and spraying parameters constant. Key metrics to record include the cycle at which smoke, oil films, or significant buildup first appear, as well as the quality of the demolding process, the appearance of the finished parts, and the number of continuous production cycles achieved before mold cleaning becomes necessary. For products requiring secondary processing, the adhesion of subsequent coatings, prints, or bonded joints should also be included in the validation scope. The significance of this framework lies in the fact that product grades such as IOTA-2437, 255, 255A, and BJ400 serve as "potential options" rather than "plug-and-play" solutions that bypass the need for thermal-oxidative diagnostics and operational condition matching. When addressing smoke and deposit formation in high-temperature molding processes, the essential first step is to identify the source of the issue before considering factors like viscosity and phenyl content.