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Mold Physics: Micro-Vibrations Against Bubbles and Managing Structural Shrinkage

354 Views 0 Liked Posted on: May 31st 2026
By: Stefano Bettollini

In the previous chapter, we saw how to read plastisol in the cup or container. Today we take a step forward and enter the most critical phase of the process: the behavior of the matter when it meets the mold, whether it is open-pour (silicone) or injection. When liquid plastisol is introduced into a mold, it must collide with complex geometries: crevices, millimetric cavities, and undercuts. At these points, air constantly risks being trapped, creating structural and aesthetic defects that force the lure to be discarded.

The Deception of Shaking and the Micro-Vibration Solution The reflex of many makers, especially with silicone molds, is to take the mold and tap or bang it on the workbench to make the air rise. This method, however, is a double-edged sword: the rubber in contact with the external walls of the mold cools down much faster than the central core. By rough-handling or banging the mold, the peripheral thermal shock locks and traps the air instantly before it can rise. The solution developed in our laboratory is purely mechanical: the high-frequency vibrating table. We are not talking about shaking the mold, but placing it on a mechanically isolated base, driven by a small motor with an eccentric weight. The secret lies in the nature of the vibration:

  • Infinitesimal (faint) displacement: The mold must not bounce.

  • Very high frequency and repetition: A dense micro-vibration that acts directly on the fluid's viscosity.

This continuous movement breaks the surface tension of the rubber solely and exclusively through vibration, constantly pushing the air (which is lighter) upwards, allowing the plastisol to occupy every single detail of the mold without thermal shocks.

Structural Shrinkage: A Ticking Time Bomb The second major obstacle is structural shrinkage. This is an inevitable physical law: the plastisol polymer expands when hot and contracts when it cools down. If we inject a high-hardness compound at the highest possible temperature, the shrinkage will be massive. In an open-pour mold, the shrinkage will manifest as a central depression on the exposed surface. In injection molding, however, the shrinkage will mathematically concentrate at the point of the mold that remains hot the longest. This explains why in large swimbaits annoying dents or structural voids often form in specific spots: the geometry of the lure creates volumetrically thicker areas that retain heat, becoming the epicenter of the shrinkage.

How to Solve Shrinkage: Timing and Pressure To limit this defect, we must work in the opposite way, taking advantage of the material's thermal transition times:

  • Exploiting Gelation: Instead of pouring or injecting plastisol at the peak of its heat (when it is extremely fluid), you can wait for the moment it begins to cool down and become denser. In this transition phase, much of the structural shrinkage is already physiologically exhausted, reducing the final geometric impact on the lure.

  • Direct Pressure Injection: On certain complex lures, the only solution is to keep the material under constant pressure. To do this efficiently, it is necessary to modify the geometry of the mold, creating a direct and articulated pouring channel (or sprue) exactly at the point where the shrinkage concentrates, so as to continue feeding the cavity while the rubber contracts.

Understanding these thermal and mechanical dynamics distinguishes our scientific approach from simple "DIY". In future posts from the Tycoon Technical Lab, we will dive even deeper into these transitions, guiding you toward total control of the matter.

Tags : Fluid dynamics, Gelation phase, Injection molding, Mold physics, Structural shrinkage, Vibrating table
Posted in: TycoonLures, The Physics of Plastisol

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