Periodically opening the lab press during the initial heating phase is a critical technique often referred to as "breathing" the mold. This action creates a momentary escape path for excess water vapor generated as the thermoplastic starch mixture rises in temperature. By allowing this steam to discharge, you prevent it from becoming trapped within the material, which is the primary cause of structural defects.
The accumulation of trapped steam is the leading cause of internal voids and surface imperfections in thermoplastic starch. Venting the press allows this pressure to equalize, ensuring the final sheet achieves high density and a uniform structure.
The Physics of Moisture and Molding
Vapor Generation Under Heat
Thermoplastic starch mixtures inherently contain moisture. As the platens heat the material, this water converts into high-pressure steam.
If the press remains tightly closed throughout the entire cycle, this vapor has nowhere to go. It remains suspended within the viscous matrix of the melting starch.
Preventing Internal Voids
When steam is trapped, it forms pockets of gas within the material. Upon cooling, these pockets become permanent internal voids.
By momentarily opening the press, you allow the gas to expand and escape into the atmosphere. This enables the starch to consolidate into a continuous, solid mass without porous defects.
Improving Surface Quality
Gas bubbles do not only affect the internal structure; they often migrate to the interface between the material and the mold.
This results in a pitted or rough surface texture. Effective venting ensures the material presses flush against the mold surface, resulting in a smooth, high-quality finish.
Critical Process Considerations
The Importance of Timing
This venting process must occur during the early stages of hot pressing.
If you wait too long, the material may begin to cross-link or solidify around the gas bubbles. At that point, opening the press will not effectively remove the voids and may disrupt the sheet's dimensional stability.
Managing Phase Transitions
While venting addresses gas entrapment, it does not manage the material's thermal contraction.
To prevent shrinkage or deformation after the hot cycle, the material requires rapid, controlled cooling. As noted in standard protocols, transferring the sheet to a cold plate press (typically around 16 °C) immediately after hot molding locks the microstructure and ensures physical stability.
Optimizing Your Pressing Protocol
To achieve the best results with thermoplastic starch, align your process with your specific quality goals:
- If your primary focus is structural density: Ensure you vent the press multiple times early in the heating cycle to eliminate all internal micro-voids.
- If your primary focus is surface aesthetics: Prioritize venting to prevent gas from interfering with the mold-material contact interface.
- If your primary focus is dimensional accuracy: Combine effective hot-press venting with an immediate transfer to a cold press to minimize shrinkage.
Mastering the "breathing" cycle is the single most effective way to transform a porous, defective sample into a dense, engineering-grade sheet.
Summary Table:
| Stage | Action | Primary Benefit |
|---|---|---|
| Early Heating | Periodic Press Opening ("Breathing") | Discharges trapped steam and prevents internal voids |
| Mid-Heating | Main Pressure Application | Consolidates material into a dense, continuous mass |
| Post-Heating | Cold Press Transfer (approx. 16°C) | Locks microstructure and prevents thermal shrinkage |
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References
- Jorge Luis López Terán, M. Beltrán. Development of Antibacterial Thermoplastic Starch with Natural Oils and Extracts: Structural, Mechanical and Thermal Properties. DOI: 10.3390/polym16020180
This article is also based on technical information from Kintek Press Knowledge Base .
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