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Sheet moulding compound (SMC) 101

Join Date: 2022-03-26

What is Sheet moulding compound(SMC):

Sheet moulding compound, a kind of glass fibre reinforced plastic, is the abbreviation of SMC. The main raw materials are GF (special yarn), UP (unsaturated resin), MD (filler) and various low shrinkage additives and auxiliary agents. It first appeared in Europe in the early 1960s. Around 1965, the United States and Japan developed this technology one after another. SMC composite material is a reinforced polyester containing glass fibres. Glass reinforcement is between 10% and 60%, and the length is slightly longer than bulk moulding compound (BMC) - between 6mm and 50mm. Long fibres and strong resin combine to produce a strong, cost-effective material.

What is SMC process? The production method of SMC ensures the complete bonding of fibres and resins. The top and bottom of the material are wrapped with polyethylene or nylon plastic film to prevent self-adhesion. The resin is spread evenly on the film in a paste form; next, the glass fibres are cut and added to the paste. The substance is then squeezed between two film layers and rolled to a predetermined thickness. SMC must be stored for 48 hours before being sold or used.

SMC sheet: The sheet material prepared by the extrusion of unsaturated polyester resin glass fiber filler thickener is called SMC sheet, also known as sheet film. Plastic SMC is divided into three types:

1) SMC-R: Random fiber sheet molding compound. The length of glass fiber is generally less than 5cm, and the distribution in the resin is random. The number after R indicates the content of glass fiber, for example, SMC-R50 indicates that the content of glass fiber is 50%.

2) SMC-C: Continuous glass fiber sheet molding compound. The reinforcing material in plastics is; continuous long glass fiber, arranged in a certain direction. The molding fluidity in the direction of the plastic fiber is small, and the mechanical strength is high.

3) SMC-D: oriented glass fiber sheet molding compound. Reinforcement materials in plastics are non-continuous glass fibers with a length of more than 10 cm arranged in a certain direction. Since the fibers are not continuous, the molding fluidity along the fiber arrangement direction is improved.

Sheet moulding compound

* It is important to note that the SMC material does not fully cure upon storage. The final cure takes place in SMC mold.

What is SMC compression molding?

In this process, they are cut into small pieces, and heat and pressure are applied to them by a hydraulic press to create anything from simple designs to complex parts. Cycle time is around 1-5 minutes, but depends on SMC part size and thickness.

(1) SMC molding temperature

The molding temperature depends on the curing system of the resin paste, the thickness of the SMC product and the complexity of the structure. If the thickness of the product is 25 ~ 32mm, its molding temperature is 135-145 ℃. And thinner products can be formed at 171 ℃. The molding temperature should be selected between the highest curing speed and the best molding conditions. It is generally believed that the SMC molding temperature is between 120-155 ° C.

(2) SMC molding pressure

Smc molding pressure is determined by product structure, shape, size and smc material formula. Simple shape SMC products only need 5-7mpa molding pressure; complex shape products, molding pressure can reach 7-15mpa. The determination of molding pressure should consider many factors. Generally speaking, smc molding pressure is between 3-7mpa.

(3) Curing time

SMC curing time (also called holding time) is related to its material characteristics, molding temperature, product thickness and color and other factors. The curing time is generally calculated at 40s / mm. For thick products above 3mm, for each additional 4mm, the curing time increases by 1min.


What are the characteristics of Sheet moulding compound?

  • Have excellent electrical insulation properties and maintain good dielectric properties at high frequencies, free from electromagnetic effects, and do not reflect electromagnetic waves.
  • Chemical corrosion resistance. The sheet moulding compound is a reinforced polyester material with good corrosion resistance characteristics such as acid, dilute alkali organic solvents and seawater. In contrast, metal materials can't do it at this point.
  • Lightweight and high strength. The specific modulus is equivalent to steel's, but the strength can reach four times that of steel. However, the typical 25% weight reduction achieved at similar strength to steel is the main benefit.
  • Long service life. The simulated aging test shows that their service life is more than 20 years, exceeding traditional materials such as metals.
  • SMC molded parts with an accurate dimension, excellent finish surface, and low cost for modification.

What fields are Sheet moulding compound used for?

Automobile field:

Due to their lightweight and high strength, SMC materials have gradually become the leader in the automotive field. On the one hand, the use of a lightweight and high-strength materials can reduce the car's energy consumption during driving; on the other hand, the price of SMC materials is cheaper than traditional metal materials.

SMC mold used in Automobile field


High-speed rail field:

The strength, fatigue performance, lightweight, and manufacturability of the materials should have a higher standard for the fast-developing high-speed rail industry. Therefore, more and more new materials are used in SMC materials.

Sheet moulding compounds have begun to be used in vehicles, and the amount has been increasing, representing the future development trend. Fibre-reinforced resin matrix composites (FRP) have the advantages of high specific strength (stiffness), fatigue resistance, corrosion resistance, heat insulation, flame retardancy, and strong designability.

Nowadays, high-speed rail passenger interior sidewall panels, inner roof panels, assembled glass fibre reinforced plastic toilets, integral glass fibre reinforced plastic toilets, integral glass fibre reinforced plastic washrooms, glass fibre reinforced plastic air-conditioning ducts, waste exhaust ducts, seats or seat frames are all made of this material.

SMC mold used in High-speed rail field


Bathroom field:

The integral bathroom's bottom plate, wall panel, ceiling, bathtub, etc., are mostly made of sheet moulding compound. SMC material is a special material for aircraft and spacecraft. It has compact material, smooth surface, heat insulation, anti-aging, long service life, etc. Excellent characteristics. Compared with ordinary bathroom walls, it is easy to absorb moisture, and the surface is rough and difficult to clean. The advantages of the overall bathroom are quite obvious.

SMC Mold used in bathroom field

Since its establishment, MDC Mould has been mainly engaged in designing and manufacturing various composite material hot compressor moulds. So far, the moulds manufactured by the company have fully met the quality requirements of the world's top 500 companies, and the technology has been able to synchronize with the international. MDC's extensive experience in compression moulding allows us to produce innovative and cost-effective parts for our OEM customers. Our experienced engineers will select the best method for your project based on production volume, part size, budget and other requirements.

In addition to mould, MDC provides a one-stop solution for SMC molding, and we have more than 100 proprietary material formulations. Using off-the-shelf SMC material can eliminate or reduce development time, resulting in cost savings. In other cases, custom formulations can provide additional cost savings through weight savings, shorter cycle times, or improved design efficiency. Either way, our dedicated SMC laboratory is ready to support you.

If you want to produce high quality products of SMC mould, then MDC need to have high quality SMC mould manufacturing equipment. Now SMC molds are our core business and our customers are Mercedes Benz, Audi, GM, etc.

SMC MouldSMC Mould

how to choose unsaturated polyester resin for smc?

The selection of unsaturated polyester should meet the following requirements:

  1. 1. Low viscosity or solid powder, convenient for glass fiber impregnation or mixing.
  2. 2. Easy to react with thickener to meet thickening requirements.
  3. 3. It can cure quickly and improve production efficiency.
  4. 4. It has high thermal rigidity to ensure that the part will not be damaged when it is demolded.
  5. 5. It has sufficient toughness and does not crack when the workpiece is deformed.

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