The Influence Of Mold Accuracy And Release Agent On The Surface Quality Of SMC Products
Mar 10, 2026
Sheet molding compound (SMC) molded products are widely used in automotive, aerospace and other fields. The surface quality directly determines their performance. Mold accuracy and the type of release agent are the core factors affecting the surface quality, and their combined effect has not yet formed a systematic pattern. This paper adopts the single-variable method, fixes the molding process parameters, and explores the influence of different mold accuracies and release agent types on the surface roughness (Ra), gloss, defect rate and size deviation of the products. The synergy mechanism is analyzed and the optimal process combination is clarified. The results show that mold accuracy is the dominant factor, and the type of release agent is the optimization factor. High-precision molds and fluorocarbon release agents in synergy can achieve the best surface quality. This research provides theoretical and process guidance for the high-quality production of SMC products.
1. Introduction
SMC is made of unsaturated polyester resin as the matrix and can be molded to produce complex-shaped and dimensionally stable products. With the upgrading of industry demands, the market has higher requirements for the smoothness, gloss and subsequent processing performance of SMC products, and it is necessary to avoid defects such as bubbles and scratches.
The molding process parameters have some influence on the surface quality, but the mold accuracy and the type of release agent play a more crucial role. The mold accuracy directly determines the replication effect of the product's shape, and insufficient accuracy may lead to rough surfaces and dimensional deviations; the release agent, as an isolation medium, an inappropriate type can cause problems such as mold adhesion and surface residue.
Most existing studies focus on a single factor or other composite materials, and there are relatively few systematic studies on SMC. The interaction rules between the two have not been clearly defined. Based on this, this paper explores the influence of the two on the surface quality of SMC products and their interaction mechanism, optimizes the process combination, and provides support for quality control.
2. Experimental Scheme Design
2.1 Experimental Raw Materials and Equipment
The specific parameters of the raw materials, equipment, molds and release agents used in the experiment are as shown in the table below:
|
Category |
Type / Specification |
Kernel Parameter |
|
SMC composite material |
technical grade |
Main components: Unsaturated polyester resin, non-alkali glass fiber (length 30mm, content 30%), calcium carbonate filler (content 40%); Room temperature density 1.8 - 2.0 g/cm³; Curing temperature 120 - 140℃ |
|
test facility |
Four-column hydraulic press |
Model 1000kN, used for SMC molding process |
|
surface roughometer |
Model TR200, used for measuring the surface roughness of the product |
|
|
gloss meter |
Model HG60, used for measuring the surface glossiness of products |
|
|
coordinate measuring machine |
Used for detecting the size deviation of the product |
|
|
High-definition microscope |
Used for observing surface defects of the product |
|
|
Flat plate mold |
high precision |
Cavity dimensions: 400mm × 200mm × 3mm; Ra ≤ 0.2μm; Hardness ≥ HRC55; Guidance error ≤ 0.03mm (after polishing and cleaning) |
|
Medium precision |
Cavity dimensions: 400mm × 200mm × 3mm; Ra = 0.4 - 0.8 μm; Hardness HRC 45 - 55; Guidance error 0.03 - 0.05mm (after polishing and cleaning) |
|
|
low precision |
Cavity dimensions: 400mm × 200mm × 3mm; Ra ≥ 1.0μm; Hardness ≤ HRC45; Guidance error ≥ 0.05mm (after polishing and cleaning) |
|
|
external releasing agent |
Silicone type |
Model KL-200; General type; Coating amount 10g/m² |
|
Fluorocarbons |
Model: FC-302; PTFE type; High temperature resistant with low residue; Coating amount: 10g/m² |
|
|
Waxy type |
Model W-401; Synthetic polyethylene wax type; Economy model; Coating amount 10g/m² |
2.2 Test Process Parameters
Using the single-variable method, the mold pressing process parameters were fixed: temperature 130℃, pressure 600kN, holding pressure time 720s, mold closing speed 15mm/s. After 20 minutes of curing, the mold was cooled and demolded. Each group of tests was repeated 5 times, and the average value was taken to reduce errors.
2.3 Surface Quality Evaluation Indicators
Refer to industry standards and select 4 core indicators: surface roughness (Ra), gloss (60° angle), defect rate (statistical bubbles, scratches, etc.), and dimensional deviation (conforming to ±0.3% standard). All were measured by corresponding instruments and the average value of multiple points was taken.
Surface roughness (Ra): Measured by a roughness meter. The smaller the value, the smoother the surface.
Luster: Measured by a luster meter. The higher the value, the better the luster.
Defect rate: Observed through a microscope, calculated based on the proportion of defective products.
Dimensional deviation: Measured by a three-coordinate measuring machine. The smaller the deviation, the higher the dimensional accuracy.
3 Test Results and Analysis
3.1 The influence law of mold accuracy on the surface quality of SMC molded products
When the fluorocarbon release agent is fixed, the test results show that mold accuracy dominates the surface quality, and the higher the accuracy, the better the effect:
(1) Surface roughness: High precision products have Ra = 0.22 μm, medium precision 0.65 μm, and low precision 1.85 μm. Due to the replication effect of the mold cavity shape, low precision molds are prone to causing surface roughness and exposing fibers.
(2) Glossiness: High-precision products have a glossiness of 98GU, medium-precision products have 82GU, and low-precision products have 58GU. The smoother the cavity, the more uniform the light reflection, and the higher the glossiness.
(3) Defect rate: High precision 0.1%, medium precision 0.8%, low precision 4.7%. The low precision molds have large guide errors and rough cavities, which are prone to entrapping air and generating bubbles and scratches.
(4) Dimension deviation: High precision ±0.12%, medium precision ±0.23%, low precision ±0.38% (exceeding standard), insufficient mold accuracy resulting in misalignment during closure and dimension deviation.
3.2 The influence pattern of mold release agent types on the surface quality of SMC molded products
When the high-precision mold is fixed, the effects of different mold release agents vary significantly. The comprehensive performance ranking is: fluorocarbon type > silicone type > wax type:
(Fluorocarbon type): The demolding effect is the best, with no residue, Ra = 0.22 μm, glossiness 98GU, defect rate ≤ 0.2%, high temperature resistance, suitable for high-end products, only with higher cost and strict coating requirements;
(2) Silicone type: It has strong versatility and moderate cost, allowing for multiple demolding processes. However, the surface is prone to residual silicone film, resulting in a slight decrease in glossiness, with Ra = 0.35 μm, a defect rate of 0.5%, and requires secondary cleaning. It is suitable for mid-range products.
(3) Wax-based: The cost is the lowest, but the demolding performance is poor, the film formation is uneven, and residues are easy to occur. The glossiness is approximately 59GU, Ra = 0.82 μm, the defect rate is 1.8%, and it has good heat resistance, but it is only suitable for low-end structural components.
3.3 Synergistic influence law between mold accuracy and demolding agent type
There is a significant synergistic effect between the two. Mold accuracy is the dominant factor, and the demolding agent is the optimizing factor. The core combination effect is as follows:
(1) High precision + fluorocarbon type: The optimal combination, Ra = 0.22 μm, glossiness 98GU, defect rate ≤ 0.1%, meeting the requirements of high-end products;
(2) High precision + silicone/wax-based: The effect is poor. The residual mold release agent cancels out the advantages of the mold, preventing the full realization of the high precision value.
(3) Medium Precision + Fluorocarbon Type: Balances cost and quality, Ra = 0.65 μm, glossiness 82GU, defect rate 0.8%, suitable for mid-range products;
(4) Medium Precision + Silicone/Wax Type: The effect is average, with a defect rate of 1.2% to 2.5%. It is suitable for mid-range products with relatively low requirements for appearance.
(5) Low precision + arbitrary release agent: The effect is the worst. Ra ≥ 1.8 μm, defect rate ≥ 4.5%. It is only suitable for structural components with no quality requirements.
4 Mechanism Analysis
4.1 Mechanism of Mold Precision's Morphological Replication and Dimension Control
The cavity of the mold determines the surface smoothness of the product through the morphological replication effect. High-precision molds enable smooth resin flow and uniform fiber distribution, resulting in a smoother surface; through the size transfer effect, the dimensional accuracy is controlled. High-precision molds have small guiding errors and can avoid uneven mold closure and bubble residue, reducing dimensional deviations; low-precision molds are prone to various defects.
4.2 Mechanism of Interface Isolation of Release Agents
Release agents achieve demolding by reducing interfacial tension through the formation of an isolation layer: Fluorocarbon-based release agents have low surface energy, form dense films without residues, and are resistant to high temperatures, ensuring surface quality;
silicone-based release agents have good lubricity but are prone to residual silicon films, affecting subsequent processing; wax-based release agents form uneven films and have poor high-temperature resistance, prone to residue and causing defects.
4.3 Mechanism of Synergistic Effect of the Two
The core of synergy is "precision dominance and optimized demolding": High-precision molds provide the foundation for surface quality, allowing the release agent to spread uniformly; High-performance release agents compensate for the lack of mold precision, reducing friction and mold sticking. The matching of the two can synergistically enhance efficiency, while the opposite results in mutual cancellation of advantages and reduced surface quality.
5. Conclusion and Recommendations
5.1 Core Conclusion
Mold accuracy is the dominant factor. High-precision molds can significantly improve surface quality, while low-precision molds are unable to meet the demands of high-end applications.
The type of release agent is an important optimization factor. The comprehensive performance is as follows: fluorocarbon type > silicone type > waxy type, suitable for products of different grades.
The synergy effect of the two is significant. The optimal combination is high precision + fluorocarbon type, while the combination of medium precision + fluorocarbon type is the ideal choice that balances cost and quality.
The mold morphology replication effect and the interface isolation effect of the release agent jointly determine the surface quality. The matching of these two effects can achieve synergistic enhancement.
5.2 Production Process Suggestions
For high-end products: Use high-precision molds and fluorocarbon release agents. The coating amount should be controlled at around 10g/m². Regular maintenance of the molds is necessary.
Mid-range products: Use medium precision + fluorocarbon type, or high precision + silicone type. For the silicone type, an additional secondary cleaning process is required.
Low-end structural components: Select low-precision + wax-based materials, control the molding parameters, and meet the basic usage requirements will suffice.
During production, the accuracy of the molds is regularly checked, the cavities are cleaned, and the coating process of the release agent is standardized. A reasonable matching of these two factors is adopted to achieve high-quality and low-cost production.
