SMC mold

SMC is sheet molding compound.

The main raw materials of SMC are GF (special yarn), UP (unsaturated resin), low shrinkage additive, MD (filler) and various additives.

SMC has superior corrosion resistance, soft and easy engineering design, flexible and other advantages, its mechanical properties can be comparable to some metal materials, its products have good rigidity, resistance to deformation, the advantages of large temperature range.

At the same time, the size of SMC products is not easy to deform, with excellent heat resistance; in cold and hot environments can drink well to maintain its performance, suitable for outdoor anti-ultraviolet and waterproof function.

Widely used, such as car front and rear bumpers, seats, door panels, electrical appliances, bathtubs, etc.

BMC Mold

BMC is short for (Bulk Molding Compounds), that is, bulk molding compound.

BMC is a kind of thermosetting plastic, which is mixed with various inert fillers, fiber reinforcing materials, catalysts, stabilizers and pigments to form a kind of adhesive "putty-like" composite material for compression molding or injection molding. It is often made into granular, log or long strip by extrusion method to facilitate subsequent processing and molding.

BMC has many unique characteristics, such as high hardness, light weight, corrosion resistance, UV resistance, good insulation, and thermal characteristics, which make BMC more satisfactory than thermoplastics. At the same time, since many components can be molded at the same time as these parts, no post-processing is required, which is more economical from the production point of view.

At present, BMC molds have been used in automotive, energy, electrical appliances, catering services, household appliances, optical instrument components, industrial and construction supplies and other areas. Such as the rear lamp shade, electrical box, meter box, etc.

 

Preparation before 1. pressing

(1)SMC/BMC quality inspection: The quality of SMC sheet has a great influence on the molding process and product quality. Therefore, before pressing to understand the quality of the material, such as resin paste formula, resin paste thickening curve, glass fiber content, glass fiber sizing agent type. Single weight, film peeling, hardness and quality uniformity.

(2) Cutting: According to the structural shape of the product, the feeding position and the process, the shape and size of the sheet cutting are determined, the template is made, and then the material is cut according to the template. The shape of the cut is mostly square or round, and the size is more than 40%-80% of the projected area of the surface of the product. In order to prevent the contamination of external impurities, the upper and lower films are removed before loading.

Molding Process Flow Chart

Preparation of 2. equipment

(1) familiar with the operating parameters of the press, especially to adjust the working pressure and press speed and table parallelism.

(2) The mold installation must be level, and ensure that the installation position is in the center of the press table. Before pressing, the mold must be cleaned and the release agent must be applied. Before adding, wipe the release agent evenly with clean gauze to avoid affecting the appearance of the product. For new molds, oil must be removed before use.

3. Feeding

(1) Determination of the feeding amount: the feeding amount of each product can be calculated according to the following formula when pressing:

Feeding amount = product volume × 1.8g/cm³

(2) Determination of the feeding area: The size of the feeding area directly affects the compactness of the product, the flow distance of the material and the surface quality of the product. It is related to the flow and curing characteristics of SMC, product performance requirements, mold structure, etc. Generally, the feeding area is 40% ~ 80%. If it is too small, the glass fiber will be oriented, the strength will be reduced, the waviness will be increased, and even the mold cavity will not be filled. Too large, is not conducive to exhaust, easy to produce cracks in the product.

(3) Feeding position and method: Feeding position and method directly affect the appearance, strength and direction of the product. Under normal circumstances, the feeding position of the material should be in the middle of the mold cavity. For asymmetric complex products, the feeding position should ensure that the material flow reaches each end of the mold cavity at the same time during molding. The feeding method should be conducive to exhaust. When the multi-layer sheets are stacked, the material blocks are stacked in a pagoda shape according to the upper small and the lower large. In addition, the blocks should not be added separately as far as possible, otherwise air wrapping and welding areas will be generated, resulting in a decrease in the strength of the product.

(4) Others: Before feeding, in order to increase the fluidity of the sheet, preheating at 100°C or 120°C can be used. This is particularly advantageous for forming deep-drawn articles.

4. molding

When the block into the mold cavity, the press quickly down. When the upper and lower molds are in agreement, the required molding pressure is slowly applied, and after a certain curing system, the product molding ends. In the molding process, various molding process parameters and press operating conditions should be reasonably selected.

(1) Molding temperature: The molding temperature depends on the curing system of the resin paste, the thickness of the product, the production efficiency and the complexity of the product structure. The molding temperature should ensure the smooth progress of the curing system initiation and crosslinking reaction, and achieve complete curing. In general, the thickness of the article should be selected to form a lower temperature than the thin-walled article, so as to prevent excessive temperature in the thick product internal heat accumulation. If the thickness of the product is 25~32mm, the molding temperature is 135~145 ℃; and thinner products can be molded at 171 ℃. The increase of molding temperature can shorten the corresponding curing time; on the contrary, when the molding temperature decreases, the corresponding curing time needs to be extended. The molding temperature should be selected in a trade-off between curing speed and molding conditions. It is generally believed that the SMC molding temperature is between 120 and 155°C.

(2) Molding pressure: SMC/BMC molding pressure varies with product structure, shape, size and SMC thickening degree. Simple shape products only need 5-7MPa molding pressure; complex shape products, molding pressure up to 7-15MPa. The higher the degree of thickening of the SMC, the greater the molding pressure required. The size of the molding pressure is also related to the mold structure. The molding pressure required by the vertical parting structure mold is lower than that of the horizontal parting structure mold. A mold with a smaller fit gap requires a higher pressure than a mold with a larger gap. Products with high appearance performance and smoothness requirements require higher molding pressure during molding. In short, the determination of molding pressure should consider many factors. In general, the SMC molding pressure is between 3-7MPa.

(3) Curing time: The curing time (also called holding time) of SMC/BMC at the molding temperature is related to its nature and curing system, molding temperature, product thickness and color. The curing time is generally calculated as 40 s/mm. For products with a thickness of 3mm or more, it is believed that for each increase of 4mm, the curing time increases by 1 minute.

(V) Molding Production Process Control

(1) Process control

The viscosity (consistency) of SMC should always be consistent during pressing. After the carrier film of SMC is uncovered, it cannot be left for a long time. It should be pressed immediately after the film is uncovered. It should not be exposed to air to prevent excessive volatilization of styrene. Keep the feeding shape and feeding position of SMC sheet in the mold consistent; Keep the temperature of the mold at different positions uniform and constant, and check regularly. Keep the molding temperature and molding pressure constant during the molding process, and check it regularly.

(2) Product testing

The following aspects of the product shall be tested:

Appearance inspection: such as gloss, flatness, spots, color, flow lines, cracks, etc;

Mechanical properties test: bending strength, tensile strength, elastic modulus, etc., the whole product performance test; other properties: electrical resistance, resistance to medium corrosion.