Detailed Explanation Of Shrinkage Rate Of SMC Composite Material Molds

The shrinkage rate of SMC (Sheet Molding Compound) composite material molds is a core technical parameter in mold design and mass production of products, directly determining the dimensional accuracy, surface quality, and assembly compatibility of the products. Essentially, it is the ratio of the difference between the final size of the product and the size of the mold cavity at room temperature to the size of the mold cavity at room temperature after the SMC material cools from the molding temperature. Precisely controlling this parameter is a key prerequisite for achieving precise molding of SMC products and ensuring production stability.

I. Core Definition and Calculation Method of SMC Composite Material Mold Shrinkage Rate

The mold shrinkage rate of SMC composite materials (usually referring to linear shrinkage rate in industrial scenarios) is defined as the percentage of the difference between the mold cavity size at room temperature and the actual size of the product to the mold cavity size at room temperature. The core calculation logic revolves around the volume change law of the material during temperature changes and curing reactions, and can be specifically divided into two application scenarios: theoretical calculation and actual molding measurement.

1.Theoretical calculation formula: Shrinkage rate α = (c - b) / c × 100%, where c represents the cavity size of the mold at room temperature (mm), and b represents the actual size of the product at room temperature (mm).

2. Actual molding shrinkage calculation: The dimensional change effect at the pressing temperature must be included, that is, α = (a - b) / a × 100%, where a is the size of the mold cavity or the product at the pressing temperature (mm). This calculation method is more in line with the actual temperature fluctuations in mass production processes and has higher reference value for the calculation results.

It is important to note that the shrinkage of SMC products is composed of two core parts: one is the structural shrinkage caused by the densification of molecular cross-linking during the resin curing reaction (irreversible), and the other is the thermal shrinkage from the molding temperature to room temperature after molding (reversible). Among them, the influence of thermal shrinkage on the final shrinkage rate is usually greater than that of structural shrinkage, and its variation pattern is closely related to the type of resin system, the type and dosage of low shrinkage additives.

II. Typical Range of Shrinkage Rate of SMC Composite Molds

The curing shrinkage rate of ordinary unsaturated polyester resin-based SMC can reach 7% to 10%, but by adding low shrinkage additives (such as PVC, PS, PE, etc. thermoplastic resins), the actual molding shrinkage rate can be significantly adjusted to a reasonable range. In industrial mass production scenarios, the molding shrinkage rate of conventional SMC products is usually controlled at 0.03% to 0.1%, while low shrinkage SMC (LS-SMC) can achieve near-zero shrinkage (below 0.01%), meeting the requirements of high-precision products.

The shrinkage rates of SMC materials with different formulations vary significantly. For instance, the shrinkage rate of SMC materials specifically designed for electrical components (such as RL-SMC-6006) can be as low as 0.03 ± 0.03%, while that of general-purpose SMC materials typically falls within the range of 0.07% to 0.08%. In practical applications, the technical parameters provided by the material supplier should be taken as the core reference to avoid dimensional deviations caused by formulation differences.

III. Key Factors Affecting the Shrinkage Rate of SMC Composite Molds

The shrinkage rate of SMC composite molds is not a fixed constant but is influenced by multiple factors such as raw material formulations, mold structures, product shapes, and molding processes. Among these, the characteristics of raw materials and process parameters are the core variables for controlling the shrinkage rate and require close attention:

1. The Impact of Raw Material Formulations

Raw material formulations are the fundamental factors determining the shrinkage rate. Differences in their components can lead to fundamental changes in shrinkage characteristics. The specific impacts are as follows:

1.Resin system: The shrinkage rate of SMC based on epoxy resin is much lower than that of SMC based on phenolic resin and unsaturated polyester resin. The core reason is that the density of epoxy resin is relatively high before curing, and no small molecule substances are released during the curing process; while phenolic resin releases water molecules during curing, and the molecular spacing of styrene crosslinking agent in unsaturated polyester resin changes significantly during curing and is accompanied by partial volatilization, both of which lead to an increase in the shrinkage rate.

2.Low shrinkage additives: The type and dosage of additives have a significant impact on the shrinkage rate. For instance, when the dosage of additives such as chlorinated vinyl acetate copolymer and polystyrene is increased from 15 parts to 30 parts, the linear shrinkage rate can be reduced from 0.25% to 0.01%. However, it should be noted that excessive additives will prolong the curing cycle and reduce the mechanical strength of the product. In industrial mass production, the dosage is usually controlled at around 5%.

3.Fibers and fillers: The shrinkage rate of inorganic fillers such as glass fibers and calcium carbonate is much lower than that of the resin matrix. Increasing their content can effectively suppress the overall shrinkage rate. For instance, when the mass content of glass fibers is raised from 15% to 30%, the shrinkage rate can be reduced by more than 30%. However, it is necessary to balance the fiber length and material fluidity to avoid the negative impact of overly long fibers on the filling effect of the mold cavity.

4.Volatile content: The higher the content of volatiles such as styrene in SMC materials, the more voids are left after the volatiles escape during the molding process, and the shrinkage rate increases accordingly. Therefore, in industrial production, the volatile content of the molding compound must be strictly controlled to avoid exceeding the critical value of 5%.

2. The influence of mold structure and product shape

1.Mold stiffness: When the mold stiffness is insufficient, elastic deformation is prone to occur during the pressing process, resulting in larger product dimensions and indirectly reducing the measured

shrinkage rate. Especially for thin-walled products, the influence of mold stiffness is more prominent. The stiffness can be improved by increasing the mold wall thickness or using high-strength materials such as P20 mold steel.

2.Complexity of product structure: The shrinkage rate of thick-walled products is usually higher than that of thin-walled products. The core reason is that the resin in thick-walled areas cures and dissipates heat slowly, resulting in more thorough structural shrinkage. Products with complex cavities, fillets or ribs are prone to uneven shrinkage, with local shrinkage rate deviations possibly exceeding 10%. Local shrinkage compensation should be targeted during the mold design stage.

3.Flow direction difference: During the flow of SMC material in the mold cavity, fibers align along the flow direction, causing the shrinkage rate in the flow direction to be typically higher than that in the direction perpendicular to the flow. This anisotropy is more pronounced in large products. The flow state should be balanced by optimizing the gate position of the mold to reduce shrinkage differences.

3. The Influence of Molding Process Parameters

Molding process parameters directly affect the final shrinkage rate by regulating the curing reaction process and the degree of material densification. Among them, temperature, pressure, and holding time are the core control points that need to be precisely matched with the material properties.

1.Mold pressing temperature: Within the conventional molding temperature range (135~155℃), an increase in temperature leads to an increase in shrinkage rate. The core reason is that the higher the temperature, the greater the volume of thermal expansion of the material, and the amount of thermal contraction upon cooling to room temperature also increases accordingly; meanwhile, high temperatures accelerate the curing reaction, which may result in insufficient structural contraction, but overall, the effect of thermal contraction is dominant.

2.Molding pressure: Within a reasonable range, increasing the molding pressure (typically from 10 MPa to 30 MPa) can reduce the shrinkage rate, as high pressure can make the product more compact and minimize the voids formed during resin curing. Adopting a two-stage compression system of "high-pressure filling + low-pressure curing" can further optimize the shrinkage rate. Moreover, the smaller the pressure in the second stage and the longer the holding time in the first stage, the better the shrinkage control effect.

3.Pressure holding and heat preservation time: Extending the pressure holding and heat preservation time can increase the crosslinking density of the resin, reduce the coefficient of linear expansion, and thereby lower the shrinkage rate. Under normal circumstances, when the heat preservation and pressure holding time is extended from 5 minutes to 15 minutes, the shrinkage rate can be reduced by 20% to 30%. However, it is necessary to avoid overextension which may lead to a decline in production efficiency. The time should be reasonably set in combination with the thickness of the product and the characteristics of the material.

4.Demolding method: The shrinkage rate of cold demolding (demolding when cooled to below 60℃) is lower than that of hot demolding (demolding at over 100℃), as cold demolding can reduce the secondary cooling shrinkage of the product after demolding; however, cold demolding will prolong the production cycle, and the choice should be made based on a comprehensive balance of the product's precision requirements and production efficiency goals.

IV. Control Methods for Shrinkage Rate of SMC Composite Molds and Key Points of Mold Design

To ensure the dimensional accuracy of the products, the shrinkage rate should be managed comprehensively from three aspects: material formulation, process optimization, and mold design. Among them, shrinkage compensation in the mold design stage is a crucial step to guarantee the precision of mass production:

1. Optimization Control of Material Formulation

Preferentially use low-shrinkage resin systems (such as vinyl ester resin), or add 5% to 8% of low-shrinkage additives (such as vinyl acetate copolymer) to keep the basic shrinkage rate below 0.05%, thereby reducing the risk of shrinkage fluctuations from the source.

By reasonably adjusting the content of glass fiber (25% to 30% by mass fraction) and its length (3 to 6 mm), the shrinkage rate can be reduced while ensuring the mechanical strength of the product. Additionally, 10% to 20% of inorganic fillers (such as talc powder) can be added to further enhance the shrinkage inhibition effect.

1. Strictly control the volatile content of the molding compound and its storage environment and time to avoid abnormal increase in shrinkage rate due to styrene evaporation and ensure the stability of material performance.

2. Optimization control of molding process

Adopt a two-stage compression system: in the first stage, high pressure (20~25MPa) is applied for filling the mold cavity to ensure complete filling of the material; in the second stage, the pressure is reduced to low pressure (5~10MPa) for curing, reducing structural shrinkage stress, which can lower the shrinkage rate by 15%~25% and improve shrinkage stability.

Optimize the temperature control curve: adopt a gradient heating mode (preheat at 135℃ for 5 minutes first, then increase the temperature to 150℃ for curing), to avoid local overheating causing uneven shrinkage; after curing, use a slow cooling method to reduce the thermal shrinkage gradient and minimize shrinkage stress.

Precise matching of pressure holding and heat preservation time: Different settings are made based on the thickness of the product. For thin-walled products (<5mm), the pressure holding time is 5 to 8 minutes, and for thick-walled products (>10mm), it is 12 to 15 minutes. This ensures the resin is fully cured and reduces the fluctuation of shrinkage in the later stage.

3. Key points of shrinkage compensation in mold design

Principle of shrinkage rate selection: When designing the mold, the compensation shrinkage rate should be determined in combination with the material formula parameters, product structure characteristics, and molding process conditions. Generally, a margin of 10% to 20% is added to the measured shrinkage rate. For complex-structured products, targeted local compensation should be made according to the shrinkage differences in different parts (for example, the compensation rate at thick-walled areas can be increased by 5%).

Mold structure optimization: Enhance the overall rigidity of the mold to prevent elastic deformation during the pressing process; set the exhaust grooves reasonably (with a depth controlled between 0.03 and 0.05mm) to efficiently expel volatile substances and air, reducing shrinkage voids; optimize the position and number of gates to ensure uniform material flow and minimize anisotropic shrinkage.

Subsequent corrective measures: For high-precision products, a cyclical optimization process of "trial molding - dimensional measurement - mold correction" should be adopted. The actual shrinkage rate is inferred from the dimensional deviation of the first article, and then the cavity dimensions of the mold are locally fine-tuned through methods such as electrical discharge machining to ensure the final precision.

V. Relevant Standards and Testing Requirements

The testing and classification of the shrinkage rate of SMC composite material molds must strictly adhere to the national standard GB/T 15568-2024 "General Sheet Molding Compound (SMC)" (this standard has replaced GB/T 15568-2008). The standard clearly stipulates the classification requirements for shrinkage performance and the corresponding test methods:

1. Classification by shrinkage performance: The standard classifies SMC into four grades, S1 to S4, based on their shrinkage performance. Each grade corresponds to a specific range of shrinkage rates. When making actual selections, it is necessary to match the material with the corresponding grade based on the precision requirements of the product to avoid dimensional deviations caused by improper material selection.

2. Testing Method: Mold pressing is carried out using disc-shaped specimens. The linear shrinkage rate is calculated by measuring the difference between the diameter of the mold cavity and the diameter of the specimen at room temperature. During the testing process, the test conditions must be strictly controlled. The mold pressing temperature is set at 141℃ and the holding time is 10 minutes to ensure that the test results are comparable to the actual production scenarios.

VI. Common Issues and Countermeasures

During actual mass production, abnormal shrinkage rates can easily cause quality defects such as warping of the products, dimensional deviations, and surface cracks. The specific countermeasures for common issues are as follows:

Excessive shrinkage rate leads to smaller dimensions. Core countermeasures include: increasing the content of low-shrinkage additives from 5% to 8%, raising the molding pressure by 5 to 10 MPa, and extending the holding time by 3 to 5 minutes. If it is detected that the volatile content of the material exceeds the standard, qualified molding materials should be replaced in a timely manner to solve the problem at its source.

2. Uneven shrinkage causes warpage deformation: The key solutions are: keeping the temperature difference between the upper and lower molds within 5℃, using cooling fixtures to restrict the free deformation of the product after demolding, optimizing the product structure (such as adding ribs to enhance rigidity), and adjusting the gate position to ensure uniform material flow and balance the shrinkage rate of each part.

3. Anisotropic shrinkage causes assembly deviation: The optimization solutions include: adjusting the mold cavity design to keep the material flow direction consistent with the assembly reference direction of the product; increasing the fiber length from 3mm to 6mm to reduce the fiber orientation degree; adopting a multi-point gate design to balance the material flow state within the mold cavity and reduce the shrinkage difference.

Summary

The shrinkage rate of SMC composite material molds is a comprehensive reflection of material properties, mold design, and molding processes. The core control logic is as follows: reducing the basic shrinkage rate through low-shrinkage formula design, regulating the curing and thermal shrinkage processes through optimization of process parameters, and offsetting dimensional deviations through mold shrinkage compensation. In industrial mass production, it is necessary to precisely determine the shrinkage rate compensation value based on the precision requirements of the product, material formula parameters, and production process conditions to achieve precise and stable production of SMC products. For high-precision products (dimensional tolerance ±0.05mm), it is recommended to adopt a combined solution of "customized material formula + two-stage pressing process + secondary mold correction" to ensure that the shrinkage rate is strictly controlled below 0.03%.