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Why do automotive injection molding manufacturers add glass fiber?

In order to further improve the performance of plastics injection molding manufacturers often add glass fiber (referred to as glass fiber), talc, mica, calcium carbonate, kaolin, carbon fiber, etc. as reinforcing materials in plastics, using resin as the matrix and binder to form a new composite Materials, called reinforced plastics (such as reinforced plastics in which epoxy resin is the parent resin plastic, are also called fiberglass).

   Due to the different types, lengths, and contents of glass fibers mixed with plastics, their craftsmanship and physical properties are also different.

   1. Thermosetting reinforced plastics

   Thermosetting reinforced plastics are composed of resins, reinforcing materials, additives, etc. Among them, resin serves as the matrix and binder. It requires good fluidity, suitable curing speed, few by-products, easy adjustment of viscosity and good compatibility, and must meet plastic parts and molding requirements. Reinforcement materials play the role of skeleton and come in many varieties and specifications, but glass fiber is commonly used, with a general dosage of 30% and a length of 15 to 20 mm. Additives include diluents to adjust the viscosity (to improve the bonding between glass fiber and resin), glass fiber surface treatment agents to adjust the resin-fiber interface state, to improve fluidity, reduce shrinkage, improve gloss and durability. Fillers and colorants for grindability, etc. Due to the selected resin, glass fiber specifications (length, diameter, alkali-free or alkali-containing, count, number of strands, twisted or untwisted), surface treatment agent, glass fiber and resin mixing process (premixed method or Pre-impregnation method, plastic ratio, etc. have different properties.

   1.1 Processing characteristics

   ⑴Fluidity: The fluidity of reinforced materials is worse than that of ordinary pressed plastics. When the fluidity is too high, it is easy to cause resin loss and glass fiber separation and accumulation. If it is too small, the molding pressure and temperature will increase significantly. There are many factors that affect liquidity. To evaluate the liquidity of a certain material, specific analysis must be done according to the composition. Factors affecting liquidity

   ⑵ The shrinkage rate of shrinkage-reinforced plastics is smaller than that of ordinary compressed plastics. It is mainly composed of thermal shrinkage and chemical structure shrinkage. The first factor that affects shrinkage is the type of plastic. Generally, phenolics are larger than epoxy, epoxy phenolics, unsaturated polyesters, etc. Among them, unsaturated polyester materials have the smallest shrinkage. Other factors that affect shrinkage are the shape and wall thickness of the plastic part. Thick walls will cause greater shrinkage. Plastics containing more fillers and glass fibers will have less shrinkage. High volatile content will lead to greater shrinkage. High molding pressure and large loading capacity will result in smaller shrinkage. The shrinkage is small. The shrinkage of hot demoulding is greater than that of cold demolding. The shrinkage is larger when the curing is insufficient. When the pressing time and molding temperature are appropriate, and the curing is sufficient and uniform, the shrinkage is small. The shrinkage of different parts of the same plastic part is also different, especially for thin-walled plastic parts. Generally, the shrinkage rate is 0 to 0.3%, with 0.1% to 0.2% being the majority. The shrinkage size is also related to the mold structure. In short, various factors should be considered comprehensively when determining the shrinkage rate.

   ⑶ Compression ratio: The specific volume and compression ratio of reinforced materials are larger than those of ordinary compressed plastics, and the premixes are larger. Therefore, a larger charging chamber is required when designing the mold. In addition, it is difficult to load materials into the mold, especially for premixed materials. Mixing is more inconvenient, but if the blank preforming process is used, the compression ratio can be significantly reduced.

   The charging amount can generally be estimated in advance and adjusted after pressure testing. There are four methods for estimating the loading amount:

   ① Calculation method The charging amount can be calculated according to the formula:

      A = V × G[1+(3%~5%)]

   V--Volume of the plastic part (cm3);

   G--specific gravity of the plastic used (g/cm3);

   3% ~ 5% - Compensation value for loss of material volatiles, burrs, etc.

   ②Shape simplification calculation method, simplify the complex shape plastic parts into several simple shapes, change the size accordingly, and then calculate according to the simplified shape.

   ③ Specific gravity comparison method, when imitating plastic parts based on metal or other material parts, the loading amount can be obtained by taking the ratio of the material specific gravity of the original part to the specific gravity of the selected reinforced plastic and the weight of the original part.

   ④ Casting type comparison method: Inject casting materials such as resin or paraffin wax into the mold cavity for molding, and then use the specific gravity comparison method to obtain the charging amount of the part.

   ⑷Material state Reinforcement materials can be divided into the following three states according to the way in which glass fiber and resin are mixed to make raw materials.

   ① Premix is made by mixing and drying glass fiber with a length of 15 to 30 mm and resin. It has a large specific volume and better fluidity than prepreg. The fiber is easily damaged during molding, the quality uniformity is poor, and loading is difficult. , poor working conditions. It is suitable for pressing small and medium-sized plastics with complex shapes and mass production. It is not suitable for pressing plastic parts requiring high strength. When using premix, it is necessary to prevent the material from "caking" and causing a rapid decrease in fluidity. This material has poor mutual solubility, and resin and glass fiber tend to separate and accumulate.

   ② Prepreg is made by immersing a whole bundle of glass fibers in resin, drying and cutting them into short pieces. It has poorer fluidity than premix, poor compatibility between bundles, small specific volume, small loss of glass fiber strength, good material quality uniformity, and easy to make reasonable auxiliary materials according to the shape and stress state of the plastic part during molding. It is suitable for pressing High-strength plastic with complex shapes.

   ③ Impregnated felt material is a felt material made by evenly spreading short fibers on glass cloth and impregnating resin. Its performance is between the above two. It is suitable for pressing large thin-walled plastic parts with simple shape and little change in thickness.

   ⑸ Hardening speed and storability Reinforced plastics can be divided into fast and slow according to their hardening speed. The rapid material solidifies quickly and the charging mold temperature is high. It is a commonly used raw material suitable for compression molding of small plastic parts and mass production. Slow-speed materials are suitable for pressing large plastic parts with complex shapes or special performance requirements and when producing in small batches. The heating speed of slow-speed materials must be carefully selected. If it is too fast, internal stress will occur, uneven hardening, and poor filling. Too slow will reduce production efficiency. Therefore, the performance and requirements of the materials used should be understood in advance when designing the mold.

   Each material has its own allowable storage period and storage conditions. Expiration or poor storage conditions will cause the plastic to deteriorate, affecting fluidity and quality of plastic parts. Therefore, attention should be paid to mold testing and production.

   1.2 Molding conditions

   Molding conditions for thermosetting reinforced plastics

   1.3 Precautions for plastic parts and mold design

   ⑴The following matters should be paid attention to when designing plastic parts.

   ① The smoothness of plastic parts can reach 7 to 9, and the accuracy should generally be level 3 to 5. However, the accuracy along the pressing direction is not easy to guarantee, and a free tolerance should be used.

   ② It is not easy to demould, so a larger demoulding slope should be used. If a larger draft angle is not allowed, the radial tolerance of the plastic part should be larger.

   ③The plastic parts should have a rotationally symmetrical shape and should not be too high.

   ④ The wall should be thick and uniform, avoiding sharp corners, gaps, narrow grooves and other shapes. All surfaces should be connected with arc transitions to prevent stress concentration, dead corners and material stagnation, poor filling, and material accumulation blocking the flow channel.

   ⑤ The hole should generally be a through hole, avoid using blind holes below Φ5 mm. The bottom of the blind hole should be a hemispherical or conical surface to facilitate material flow. The hole diameter and depth ratio are generally 1:2 ~ 1:3. Large plastic parts should be as large as possible. Small holes are not designed, the hole spacing and hole margins should be large, and small holes arranged in a high density should not be molded.

   ⑥ Screw holes are easier to form than threads. Threads below M6 are not suitable for forming. The tooth shape should be semicircular or trapezoidal. The fillet radius should be greater than 0.3 mm. The half-angle tolerance should be paid attention to. You can refer to the general plastic thread for design. When the threads of plastic parts are joined to threaded parts of other materials, the matching tension must be considered, and the length of the thread segment should be the minimum size.

   ⑦ The molding pressure is high, the insert should have sufficient strength to prevent deformation and damage, and the positioning must be reliable.

   ⑧Small shrinkage, directional, prone to poor welding, deformation, warping, shrinkage holes, cracks and stress concentration, and uneven distribution of resin fillers. Thin-walled plastic parts are fragile and difficult to demould, and large-area plastic parts are prone to ripples and material accumulation.

   ⑵The following matters should be paid attention to when designing the mold:

   ① It should be easy to load and facilitate the flow of materials to fill the cavity.

   ② The draft angle should be above 1°.

   ③ It is advisable to choose the direction with the largest projected surface of the plastic part as the direction of molding and pressurization to facilitate filling of the cavity with materials. However, it is not appropriate to use the direction perpendicular to the axis of the insert and core axis with high dimensional accuracy as the direction of molding and pressurization.

   ④The material has strong penetrating power, resulting in thick flash that is difficult to remove. Pay attention to the direction of the flash when selecting the parting surface. The upper and lower molds and inserts should have an integral structure, the assembly gap of the combined structure should not be large, and the detachable molded parts of the upper and lower molds should have a level 3 to 4 sliding fit.

   ⑤The shrinkage rate is 0 to 0.3%, generally 0.1 to 0.2%, and the material volume is generally 2 to 3 times the volume of the plastic part.

   ⑥The molding pressure is large and the material seepage and squeezing force are large. Mold core inserts should have sufficient strength to prevent deformation, displacement and damage. Especially when the gap between the slender core and the cavity is small, you should pay more attention.

   ⑦ The mold should be polished and hardened.

   ⑧The ejection force is large, the ejection pin should be strong enough, and the ejection should be uniform. The ejection pin should not also serve as the core.

   ⑨ The rapid forming material can be demolded at the molding temperature, and the mold of the slow forming material should be equipped with heating and forced cooling measures.

   2. Thermoplastic reinforced plastics

   Thermoplastic reinforced plastics are generally composed of resin and reinforcing materials. Currently commonly used resins are mainly nylon, polystyrene, ABS, AS, polycarbonate, linear polyester, polyethylene, polypropylene, polyformaldehyde, etc. The reinforcing material is generally alkali-free glass fiber (there are two types of long and short fiber materials, the long fiber material is generally the same as the pellet length of 2 to 3 mm, the short fiber material length is generally less than 0.8 mm) and is formulated with resin after surface treatment. The glass fiber content should be selected according to the proportion of resin, and the most reasonable ratio should be selected, generally between 20% and 40%. Due to the different resins used in various reinforced plastics, the length and diameter of the glass fiber, the presence or absence of alkali and surface treatment agents, the reinforcing effects and molding characteristics are also different.

   As mentioned earlier, reinforcements can improve a series of mechanical properties, but they also have a series of disadvantages: low impact strength and impact fatigue strength (but increased notched impact strength); reduced transparency and welding point strength, shrinkage, strength, and thermal expansion coefficient , the anisotropy of thermal conductivity increases. Therefore, this plastic is currently mainly used for small, high-strength, heat-resistant, poor working environment and high-precision plastic parts.

   2.1 Process characteristics

   ⑴The melt index of reinforced materials with poor fluidity is 30% to 70% lower than that of ordinary materials, so they have poor fluidity and are prone to defects such as poor filling, poor welding, and uneven distribution of glass fibers. In particular, long fiber materials are more likely to have the above defects, and they are also prone to damage the fibers and affect the mechanical properties.

   ⑵ Small molding shrinkage and obvious anisotropy. The molding shrinkage is smaller than that of unreinforced materials, but the anisotropy increases. The shrinkage along the direction of material flow is small and large in the vertical direction. It is small near the feed inlet and large at a distance. Plastic parts are prone to occur. Warping and deformation.

   ⑶Poor demoulding, excessive wear, difficulty in demolding, and large wear on the mold. During injection, the material flow also wears heavily on the pouring system, core, etc.

   ⑷ Gas is prone to occur during molding because the fiber surface treatment agent is easy to volatilize into gas and must be discharged. Otherwise, defects such as poor welding, material shortage, and burns may easily occur.

   2.2 Precautions for molding

   In order to solve the above-mentioned technological disadvantages of reinforcement materials, the following matters should be paid attention to during molding:

   ⑴ It is suitable to use high temperature, high pressure and high speed injection.

   ⑵ The mold temperature should be high (crystalline materials should be adjusted as required). At the same time, resin and glass fiber should be prevented from accumulating separately, glass fiber exposure and local burns.

   ⑶ Pressure maintaining and compressing should be sufficient.

   ⑷ Plastic parts should be cooled evenly.

   ⑸ Changes in material temperature and mold temperature have a greater impact on the shrinkage of plastic parts. High temperatures lead to greater shrinkage. Increased holding and injection pressures can reduce shrinkage but have less impact.

   ⑹ Due to the good rigidity of the reinforcement and high heat distortion temperature, it can be demoulded at a higher temperature, but attention should be paid to uniform cooling after demoulding.

   ⑺Appropriate release agent should be selected.

   ⑻ It is advisable to use a screw injection machine for molding. In particular, long fiber reinforcements must be processed with a screw injection machine. If there is no screw injection machine, they should be processed on a plunger injection machine just like short fiber materials after granulation.

   2.3 Molding conditions

   Commonly used thermoplastic reinforced plastic molding conditions.

   2.4 Precautions for mold design

   ⑴ The shape and wall thickness of the plastic part should be designed to facilitate the smooth flow of material to fill the cavity, and try to avoid sharp corners and gaps.

   ⑵ The draft angle should be large, 1° to 2° for a mold containing 15% glass fiber, and 2° to 3° for a mold containing 30% glass fiber. When the demoulding slope is not allowed, forced demoulding should be avoided and a transverse parting structure should be used.

   ⑶ The cross-section of the pouring system should be large and the flow process should be straight and short to facilitate the even dispersion of fibers.

   ⑷ When designing the feed inlet, consideration should be given to preventing insufficient filling, anisotropic deformation, uneven distribution of glass fibers, and easy generation of weld marks and other adverse consequences. The feed port should be thin, wide, fan-shaped, annular or multi-point in order to make the material flow turbulent and the glass fiber to be evenly dispersed to reduce anisotropy. It is best not to use a needle-shaped feed port. The mouth section can be appropriately increased, and its length should be short.

   ⑸ The mold core and cavity should have sufficient rigidity and strength.

   ⑹ The mold should be hardened, polished, and wear-resistant steel should be selected. The parts that are prone to wear should be easy to repair.

   ⑺The ejection should be even and powerful to facilitate replacement and repair.

   ⑻ The mold should be equipped with an exhaust overflow groove and should be located in a location prone to weld marks.


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