Factors Affecting the Accuracy of Silicone Molding

Rapid silicone molding technology, as a kind of injection molding, offers advantages such as short production cycles, low costs, good elasticity, and ease of demolding. It is widely used for low-volume mold production for product performance testing.

Silicone molds used for rapid prototyping services will produce errors during the production process due to the physical properties as well as the casting materials themselves, such as the elastic deformation produced by silicone extrusion and the gravity effect, and the shrinkage rate of the casting materials, all of which will bring about key impacts on the casting parts.

Traditionally, accuracy is improved by estimating errors through experience data or multiple measurements of the cast parts and then compensating for these errors in the original model. 

However, current rapid injection molding analysis software primarily analyzes based on traditional injection molding processes. Since silicone molding differs in mold deformation, shear rate at the gate, shear pressure, and cooling conditions, using traditional analysis methods is ineffective, lacking specificity, and resulting in high computational loads and low efficiency.

Advantages and Disadvantages of Silicone Molding

Advantages:

1. Silicone is transparent or translucent, making it easy to cut and shape after curing.
2. Silicone has excellent fluidity and cures in a vacuum, which helps eliminate bubbles and accurately preserve the model’s structure and surface texture.
3. Silicone molds are elastic and have good demolding properties.

Disadvantages:

1. Silicone molds can compress under their own weight, affecting the accuracy of the cast parts, especially for larger components.
2. With repeated demolding, the silicone mold surface deteriorates, reducing the surface precision of the parts, thus limiting molding to low-volume.
3. Silicone molds deform elastically under external forces during casting, making process prediction difficult. Establishing molding parameters relies heavily on the operator’s experience and skills.

Analysis of Factors Affecting Silicone Molding

Silicone molding differs from conventional injection molding in several key aspects:

1. Mold Deformation: Conventional injection molds experience negligible deformation during injection. In contrast, silicone molds, especially for larger models, exhibit significant elastic deformation, impacting the accuracy of the cast parts.
2. Filling Process: In traditional injection molding, the melt is injected at high speed into a closed mold at a lower temperature, followed by pressure holding and cooling. In silicone molding, liquid ABS relies on its gravitational potential energy to fill the mold. Mixed components A and B are poured and rely on inertia to overcome mold filling resistance.
3. Cooling and Curing: Traditional injection molding uses cooling pipes, causing a temperature gradient that results in inconsistent shrinkage and internal stresses. Silicone molding occurs in an oven at 70-80°C, with minimal internal-external temperature differences, reducing residual thermal stresses.

Factors Affecting Workpiece Accuracy Post-Molding

If the bottom surface of the part shows thinner central sections compared to the original model while the edges are thicker, this is mainly due to gravitational deformation altering the mold cavity size.

Mold Cavity Wall Thickness: As the mold cavity wall thickness increases, the part shrinkage rate initially increases and then decreases. This is due to the combined effects of silicone material shrinkage, adhesion to the prototype, and internal shrinkage towards the center. For rubber products, thin parts (<3mm) shrink more than thick parts (>10mm). Beyond a wall thickness of 10mm, the shrinkage center effect diminishes, reducing overall mold cavity volume shrinkage.

Gate Design: The gate design directly affects the dimensional accuracy and quality of the parts, influencing mold manufacturing complexity. Improper gate design can cause defects like weld lines, deformation, and sink marks. For silicone rubber molds aimed at rapid resin part production, direct circular gates are preferred. Gate size, location, and shape parameters significantly impact part dimensions.

Part Wall Thickness: The shrinkage rate increases with wall thickness. For non-crystalline resins, thicker parts shrink more due to exothermic reactions during curing. For crystalline resins, thicker parts also shrink more due to prolonged cooling and crystallization times. Inconsistent curing speeds between the interior and exterior of thick parts create internal stresses, leading to significant dimensional variations.

Conclusion

Vacuum casting is an excellent method (read more) for producing functional prototypes and small batches of plastic parts, balancing high quality, end-use parts, and production lead times. The process and relatively inexpensive tooling facilitate engineering verification and design changes. It also allows for functional testing, reporting, and certification approval before mass production.