Bulk (Dough) molding compounds (BMC/DMC)

October 29, 2024

wanplas

Industry Knowledge, Plastic Materials

Bulk molding compounds are thermoset plastics that are mixed with various inert fillers, fiber reinforcements, catalysts, stabilizers and pigments to form a sticky “putty-like” composite for compression or injection molding.

Introduction

Bulk molding compound (BMC) is a thermoset plastic that is blended with various inert fillers, fiber reinforcements, catalysts, stabilizers and pigments to form an adhesive “putty-like” composite for compression or injection molding. Bulk Molding Compound (BMC) is highly filled and reinforced with short fibers, 10% to 30% glass fiber reinforcement, and lengths typically range from 1/32 inch to 1/2 inch (12.5mm).

A brief history

BMC (DMC) material is the abbreviation of Bulk (Dough) molding compounds, that is, bulk molding compound. It is often called unsaturated polyester bulk molding compound in China. Its main raw materials are GF (chopped glass fiber), UP (unsaturated resin ), MD (filler) and various additives that are fully mixed into a dough-shaped prepreg. DMC materials were first used in the former West Germany and the United Kingdom in the 1960s, and then developed greatly in the United States and Japan in the 1970s and 1980s respectively. Because BMC bulk molding compound has excellent electrical properties, mechanical properties, heat resistance, chemical corrosion resistance, and is adaptable to various molding processes, it can meet the performance requirements of various products. Therefore, it is becoming more and more popular among users. favorite.

Depending on the end application, composites are formulated with precisely controlled dimensions, good flame and tracking resistance, high dielectric strength, corrosion and stain resistance, excellent mechanical properties, low shrinkage and Color is stable. Bulk Molding Compound (BMC) has excellent flow characteristics and excellent insulating and flame retardant properties, making it ideal for a variety of applications where precise details and dimensions are required. The material is available in 75 colors and can withstand powder spraying or water-based paints.

Nature

A dough-shaped prepreg made of chopped glass fibers mixed with unsaturated polyester slurry. It is suitable for molding by molding, transfer molding, injection molding and other processes. The products produced have good mechanical properties, high dimensional stability, good surface finish, excellent water resistance, oil resistance, corrosion resistance, heat resistance and excellent electrical properties, especially The arc resistance can reach about 190s. The resin slurry is prepared according to the formula. The main components are the same as the sheet molding compound, but with more fillers. Generally, no thickener is used. The resin slurry is then fully mixed with chopped glass fibers (length is about 3 to 25 mm). Mainly used in electrical appliances, motors, radios, instruments, machinery manufacturing, chemical equipment, construction, transportation, national defense and other sectors.

1. General performance: BMC (DMC) has a larger specific gravity, between 1.3 and 2.1; the product has a bright appearance, good hand feel, and a hard and heavy feel; heating with fire will produce a lot of oil smoke and the smell of styrene; some Varieties of BMC (DMC) are refractory to flames, but some varieties are extremely flammable, leaving inorganic substances behind after burning.
2. Dimensional stability: The linear expansion coefficient of BMC (DMC) is (1.3~3.5) × 10-5K-1, which is smaller than ordinary thermoplastics, so BMC (DMC) has high dimensional stability and dimensional accuracy. Temperature has little effect on the dimensional stability of BMC (DMC), but humidity has a more serious effect. BMC will expand after absorbing moisture. The linear expansion coefficient of BMC (DMC) is very close to that of steel and aluminum, so it can be compounded with them.
3. Mechanical strength: BMC (DMC) has higher tensile, bending, impact strength and other properties than thermoplastics, and its creep resistance is also better than thermoplastics.
4. Water and solvent resistance: BMC (DMC) has good corrosion resistance to water, ethanol, aliphatic hydrocarbons, grease, and oil, but is not resistant to ketones, chlorine hydrocarbons, aromatic hydrocarbons, acids and bases, etc. BMC (DMC) has low water absorption and its insulation performance is still very good after being soaked for one day.
5. Heat resistance: The heat resistance of BMC (DMC) is better than that of general engineering plastics. The heat deformation temperature HDT is 200~280℃, and it can be used at a temperature of 130℃ for a long time.
6. Aging resistance: BMC (DMC) has very good aging resistance and can be used indoors for 15 to 20 years. Its strength retention rate is over 60% after 10 years of outdoor exposure.
7. Electrical properties: BMC (DMC) has the most outstanding arc resistance, which can reach about 190 seconds.
8. Low odor: The styrene cross-linking agent used in BMC (DMC) will still have 0.1% residue after curing, and will emit an odor when heated. Therefore, BMC (DMC) used for food utensils (such as microwave tableware) should use UP resin with no residual styrene monomer.

Raw material

1. Resin: unsaturated polyester resin, vinyl ester, phenolic, melamine and other resins.
2. Fiber: Glass fiber roving for BMC.
3. Fillers: calcium carbonate, aluminum hydroxide, etc.

Advantages

1. The molding cycle is short, it can be molded or injected, and it is suitable for mass production.
2. A large amount of fillers can be added to meet the requirements of flame retardancy and dimensional stability, and the cost is low.
3. Complex products can be integrally formed, and inserts, holes, platforms, ribs, grooves, etc. can all be formed at the same time.
4. Compared with ordinary thermoplastics, the products have higher heat resistance, insulation, elastic modulus and other properties.
5. It does not require high skills for workers, is easy to automate, and saves labor.
6. The product size is accurate.
7. Good working environment.

Shortcoming

• Only suitable for making products with small size and low strength requirements (generally BMC strength is about 30% lower than SMC).
• The injection machine price is higher.

Typical products

Electrical components (insulators, switch 29, meter boxes, circuit breaker casings, terminal blocks, casings of various household or commercial mechanical and electrical products), automotive parts (headlight reflectors, rear doors, speaker casings, etc.), meter casings, audio equipment housing.

BMC molding method

The BMC injection molding machine is mainly a process in which BMC lump materials are forced into the BMC barrel through the BMC feeder, then transported to the front of the barrel through the screw, and then injected into the mold cavity for solidification and molding.

BMC is a thermoset material. The barrel temperature of the injection molding machine should not be too high. It is generally set to 60 degrees to facilitate the flow of materials.

BMC injection molding machine is mainly a professional equipment for producing thermosetting pellet materials. There is a hydraulic system in the feeding part, which forces the pellets into the barrel when the screw is expected. BMC injection molding machine is suitable for molds with complex structures and processed by injection molding., high product efficiency and low labor intensity.

There are three BMC molding methods: compression molding, transfer molding, and injection molding, with injection molding being the main method.

1. Compression molding method: See SMC molding method.
2. Transfer molding method: The equipment includes Pot type and auxiliary piston type, with the auxiliary piston type being the main type.
3. Injection molding of BMC: BMC is the abbreviation of glass fiber reinforced unsaturated polyester thermosetting plastic. It is currently the most widely used type of reinforced thermosetting plastic.

Characteristics and applications of BMC

BMC has good physical, electrical and mechanical properties, so it has a wide range of applications. For example, it can be used to make mechanical parts such as gearbox components, intake pipes, valve covers, bumpers, etc.; in applications that require earthquake resistance, flame retardancy, aesthetics, Durable aviation, construction, furniture, etc. are also widely used; in its traditional electrical field, its uses are becoming more and more extensive.

BMC’s injection molding equipment

In BMC, due to the addition of glass fiber, the equipment used is somewhat different from the usual thermosetting plastic injection molding equipment. The injection molding machines used can be both plunger type and screw type.

1. Feeding system: Whether it is a screw type or a plunger type, an extrusion type feeding device must be attached to force the material into the barrel. This feeding device mostly uses a plunger type pressurized feeding device.
2. Injection molding system: Because the injection volume of the plunger injection molding machine is accurate and constant, the glass fiber is dispersed in the melt with less damage. Therefore, the plunger injection molding machine is used more often, but the exhaust is inconvenient.
3. Heating system: In BMC injection molding, it is very important to control the temperature of the barrel. There must be a control system to control the temperature to ensure that the temperature from the feeding section to the nozzle is optimal. Constant temperature water or constant temperature oil is often used for heating. Electric heating can also be used.
4. Mold clamping device: Several types of mold clamping devices are mostly used, such as mechanical, hydraulic and fully hydraulic.
5. Mold: BMC injection molding is somewhat different from usual thermoplastic and thermosetting injection molding. The main reason is that BMC has a lower shrinkage rate, so the mold cavity size should be changed.

BMC injection molding process

1. Barrel temperature and mold temperature. During processing, BMC is required to maintain low viscosity flow behavior for a long time at the barrel temperature. Generally, the barrel temperature should be able to meet the lower limit of BMC. The temperature of the barrel is generally divided into two or three stages of control. The temperature near the hopper is lower and the temperature near the nozzle is higher. Generally the difference is 20~60℃, the mold temperature is generally controlled at 135~185℃.
2. Injection pressure. Since BMC has poor fluidity, fast solidification, and complex mold structure, the injection pressure should be higher, generally 80~160MPa.
3. Injection speed. Increasing the injection speed helps to improve the surface quality of plastic parts and shorten the curing time, but it is not conducive to exhaust and increases the degree of glass fiber orientation. Therefore, a lower injection speed should be used while ensuring the surface quality of the plastic part, usually 1.8~3.5m/min.
4. Screw speed and back pressure. If a screw injection molding machine is used, the screw will cause greater damage to the glass fiber when injecting BMC. In order to minimize the damage to the glass fiber, the screw speed should be selected at a low value, generally 20~50r/min. According to the viscosity of BMC, it is appropriate to use low back pressure, generally 1.4~2.0MPa.
5. Molding cycle. Due to the different size and complexity of plastic parts, the process time of each stage is also different. Generally, the injection time is 2~20s and the heat preservation time is 10~20 (thickness).

Processing characteristics of BMC (DMC)

1. Liquidity: BMC (DMC) has very good liquidity and can maintain good liquidity under low pressure.
2. Curability: BMC (DMC) has a very fast curing speed. When the molding temperature is 135~145℃, the curing time is 30~60 seconds/mm.
3. Shrinkage rate: The shrinkage rate of BMC (DMC) is very low, between 0 and 0.5%. The shrinkage rate can also be adjusted by adding additives as needed; it can be divided into no shrinkage (shrinkage rate < 0.05%), low shrinkage rate ( There are three levels: 0.05~0.3%) and high shrinkage (0.3~0.5%).
4. Coloring properties: BMC (DMC) has good coloring properties.
5. Disadvantages: long molding time, large burrs on the product.

Preparation of BMC (DMC) molding compound

BMC (DMC) is a mixture of liquid UP resin, solid powder filler, chopped glass fiber and other ingredients. In order to facilitate operations such as feeding during molding, the premix is ​​usually extruded into strips or pellets. shape.

The preparation of BMC (DMC) is divided into two steps:

1. First add the resin, initiator, colorant, release agent, part of the filler, etc. into a high-shear mixer and stir evenly. The mixing temperature reaches about 50°C, and then slowly Add the remaining filler and mix evenly to prepare a resin premixed slurry.
2. Add the premixed slurry to the Z-type reamer mixer or planetary mixer, add the dried chopped glass fiber, and stir After 10 to 15 minutes, pour out the dough and use an extruder to extrude it into strips and pellets. After drying, seal and package with polyester film and store for later use.

Press molding of BMC (DMC)

The press molding of BMC (DMC) is to add a certain amount of molding compound into a preheated mold, pressurize and heat it and then solidify it. The specific process is adding materials (adding solid molding compound into the preheated steel mold) → close Mold → Mold filling (molding material flows under pressure and fills the entire cavity) → Curing (fully solidified after being maintained at the set pressure and temperature for a certain period of time) → Open the mold and take out the product.

BMC (DMC) press molding process conditions are as follows:

1. Molding pressure: 3.5~7MPa for ordinary products, 14MPa for products with high surface requirements;
2. Molding temperature: The mold temperature is 145±5℃, and the fixed mold temperature can be adjusted lower by 5~15℃ to facilitate demoulding;
3. Mold closing speed: Mold closing must be completed within 50 seconds;
4. Curing time: The curing time of a product with a wall thickness of 3mm is 3 minutes, the curing time of a product with a wall thickness of 6mm is 4~6 minutes, and the curing time of a product with a wall thickness of 12mm is 6~10 minutes.