What Is The Necessity Of Cyclic Pressure Release In Pla Compression Molding? Ensure Structural Integrity & Density

The cyclic pressure release operation is a fundamental requirement for structural integrity. In the context of compression molding for Polylactic Acid (PLA) and starch biocomposites, this operation—often referred to as a "degassing cycle"—is strictly necessary to expel air and volatiles trapped within the material matrix. Without this repeated release and re-application of pressure, the material cannot achieve the density and uniformity required for functional use or testing.

By repeatedly releasing and re-applying pressure, you actively force out gas pockets that cause structural weaknesses. This process is non-negotiable for producing low-porosity composite plates that yield reliable, reproducible mechanical test results.

The Mechanics of the Degassing Cycle

Exhausting Trapped Volatiles

When biocomposites are subjected to heat and pressure in a lab press, air and volatile substances often become trapped between the material layers or particles.

The cyclic pressure release creates a physical escape path for these gases. By momentarily releasing the clamp force, you allow the trapped air to escape from the mold cavity before the polymer fully consolidates.

Reducing Porosity

If pressure is applied continuously without a release cycle, gases are sealed inside the composite, creating microscopic voids.

The degassing cycle combats this by allowing the material to settle more compactly after the gases are vented. This significantly lowers the porosity of the finished composite plate, resulting in a denser, more solid material.

Impact on Material Performance

Eliminating Internal Stresses

Trapped gases are not just empty space; under heat, they become pressurized pockets that exert force on the surrounding material.

This creates internal stress within the composite plate. The degassing cycle relieves this pressure, allowing the polymer chains to relax and preventing the formation of residual stresses that could warp or weaken the part.

Preventing Structural Defects

The presence of gas pockets acts as a barrier to adhesion between the PLA matrix and the starch reinforcement.

If these pockets remain, the material is prone to delamination (layer separation) and other internal defects. The pressure release cycle ensures better bonding between phases, preventing these critical failure modes.

Common Pitfalls to Avoid

The Risk of "One-Shot" Pressing

It is a common error to assume that maintaining high, constant pressure is sufficient to consolidate the material.

However, constant pressure locks air inside rather than forcing it out. Skipping the cyclic release leads to specimens that may look solid on the surface but are structurally compromised internally.

Compromised Test Reliability

For researchers, the primary output of the molding process is often data transparency.

Specimens produced without a degassing cycle will exhibit high variability in mechanical properties. This unreliability makes it impossible to accurately characterize the material, rendering test results invalid.

Making the Right Choice for Your Goal

To ensure your PLA and starch biocomposites perform as intended, apply the pressure release strategy based on your specific objectives:

If your primary focus is mechanical testing: Strictly adhere to the degassing cycle to eliminate defects and ensure your data reflects the true material properties, not processing errors.
If your primary focus is material longevity: Use cyclic pressure release to minimize porosity and internal stress, which are the primary drivers of delamination and premature failure.

The degassing cycle is not merely a procedural formality; it is the deciding factor between a flawed sample and a high-performance biocomposite.

Summary Table:

Feature	Purpose of Cyclic Pressure Release	Impact on Final Composite
Gas Management	Expels trapped air and volatiles	Eliminates internal voids and gas pockets
Material Density	Allows particles to settle compactly	Increases density and reduces porosity
Structural Bond	Promotes PLA and starch adhesion	Prevents delamination and internal defects
Data Accuracy	Removes processing variables	Ensures reproducible mechanical test results
Stress Relief	Releases pressurized gas pockets	Prevents warping and internal residual stress

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References

Yokiushirdhilgilmara Estrada-Girón, Francisco Javier Moscoso‐Sánchez. Characterization of Polylactic Acid Biocomposites Filled with Native Starch Granules from Dioscorea remotiflora Tubers. DOI: 10.3390/polym16070899

This article is also based on technical information from Kintek Press Knowledge Base .