HDPE Blow Molding
When using HDPE material for blow molding, it is necessary to consider the molding temperature, extrusion speed, and blow molding pressure. Only products produced in this way have certain hardness and tensile strength advantages, and are less prone to defects such as aging and brittleness in some high-precision industries.
Blow Mold Companies | Common Structures of Molds
Blow mold companies can provide professional blow molding services to customers, whether it is mold design or production, which will bring satisfactory results. Due to the stable quality, recyclability, high production efficiency, good durability, and diversity of products produced by blow molding, they are widely used in packaging, cosmetics, furniture manufacturing, agriculture, automotive, and aerospace fields.
Setting up Exhaust and Cooling Systems for Blow Mold Companies
When designing mold for HDPE blow molding companies, the internal cold cutting and exhaust system directly determine the degree of curing of the mold and whether it can be demolded normally. Therefore, blow mold companies should pay special attention to:
- Exhaust System Settings for Molds
When molding containers of the same volume, the amount of air discharged by blow molding is much greater than that of injection molding. After the air is discharged, especially for large volume blow molded products, there will still be some air residue in the blank and mold cavity. In addition, due to the lower pressure inside the blow molding mold, the blow molding mold requires higher exhaust performance (especially for polished molds). If the residual air cannot be completely or quickly discharged from the mold cavity and billet, the mold damage cannot be quickly blown and expanded, resulting in poor contact with the mold cavity, blurring the text and pattern on the surface of the product, resulting in defects such as roughness and dents, which affect the performance and aesthetics of the product. Poor exhaust systems can also prolong the cooling time of products, reduce mechanical performance, and produce defects such as uneven wall thickness. Therefore, the exhaust system of the mold is crucial.
- The setting of Mold Heating and Cooling System
During blow molding, the plastic melt will continuously transfer heat to the mold, causing the temperature of the mold to continuously rise. Excessive mold temperature can seriously affect the productivity of the product. To maintain the mold temperature within an appropriate range, the mold supplier should reasonably set up a cooling system. The main point of the setting is that the cooling water channel should be kept at a consistent distance from all parts of the mold cavity, to ensure uniform cooling shrinkage in all parts of the product. For larger molds, a sealed water tank can be directly installed behind the blow molding mold, with an inlet and outlet on the tank to improve the circulation of the cooling medium and enhance the cooling effect; For smaller molds, cooling water channels can be directly set on the template, with cooling water flowing in and out from below, which can effectively avoid the generation of bubbles. In larger cooling water channels on the mold surface, baffle plates can be installed to promote turbulence, guide the flow of water, avoid dead corners during cooling water flow, and reduce the flow rate.
Blow Mold Companies' Setting of Parting Surface and Cavity
When manufacturing molds, blow mold companies should focus on the design of cuts and inserts. Reasonable cuts and inserts can directly affect the forming quality of the parts.
- The Setting of Mold Cuts
The function of the mouth of the blow molding mold is to cut the blank, so it should be designed with a sharp cut. The clamping mouth of the cutting blank should also be set according to the actual situation. If it is too small, it will reduce the thickness of the container joint, thereby affecting its joint strength; If the clamping mouth is too large, it will not be possible to cut off the tailings, resulting in the mold not being able to fully close. The shape of the incision is generally triangular or trapezoidal. Blow mold companies often use metal inserts embedded in molds to effectively prevent cutting wear. The angle at which the end of the incision expands towards the surface of the mold varies depending on the type of plastic used. The opening and closing of molds can usually be controlled by compressed air, and to adapt to different plastics, it is best to adjust the closing speed. This requires balancing speed and locking effect so that the clamping part can be fully fused without the occurrence of difficult-to-remove burrs. Set up a tailings groove located on the parting surface of the mold below the clamping edge. For billets with thin walls and large diameters, the depth of the tailings groove greatly affects the blow molding process and automatic product trimming. If the groove depth is too small, the extrusion pressure on the tailings will be too high, causing the stress on the mold, especially the clamped edge, to be too high, and the mold may not even be able to fully close; If the groove depth is too deep, the tailings cannot come into contact with the sufficient groove wall, resulting in a slower cooling rate. Heat will be transmitted to the joint of the container, causing it to soften, and the joint will be stretched during subsequent repair.
- The Setting of Inserts in Molds
At the bottom of the blow molding mold, a separate insert is usually set up to squeeze one end of the sealing blank and cut off the tailings. When designing the bottom insert of the mold, it is important to focus on the edge of the billet and the tailings groove. During the repeated mold closing process, the insert needs to withstand the pressure generated by the extruded billet melt, so the mold ground insert generally needs to have sufficient strength, rigidity, and wear resistance. Due to the accumulation of heat in the billet area, the thickness is generally larger than the product wall. Therefore, the material used to manufacture sandwich blocks must have good thermal conductivity. At the same time, blow mold companies need to consider the durability of the clamping block and can choose copper-beryllium alloy for production. For soft plastics, aluminum can generally be used to manufacture clamping blocks and can be integrated with the mold body; The joint is usually the weakest part of the blow molded container. To increase its thickness and strength appropriately, a small amount of molten material should be squeezed into the joint before cutting off the tail material after molding; It should be able to cut off the tailings and form a neat cut. The mold neck ring and shear block are the main inserts for forming the neck of the container, and the shear block helps to cut off the remaining material in the neck and reduce the wear of the mold neck ring; The opening of the shear block can be conical, and the included angle is generally taken as 60 °.
Mold Parting Surface and Cavity Settings
When manufacturing blow molding molds, the design of the parting surface and cavity is crucial for blow mold companies, as both directly determine the shape and size of the blow molded parts.
- Mold Parting Surface Settings
When blow mold companies design the structure of hollow blow molding molds, companies should set symmetrical mold parting surface positions to reduce the blowing expansion ratio and thus reduce the difficulty of demolding. Therefore, the shape of blow-molded products usually determines the position selection of the parting surface. Most blow molding molds are two-and-a-half molds with a parting surface as the boundary. For complex containers with irregular shapes, determining the parting position is particularly important, otherwise, it will cause surface scratches or even an inability to de-mold. In this case, it is even necessary to use a multi-parting surface mold composed of three or more movable parts to protect the product and reduce demolding difficulty. For containers with a circular cross-section, the parting surface is set by diameter; For elliptical containers, the parting surface should pass through the major axis of the ellipse; The parting surface of a rectangular container can pass through the centerline or diagonal, but the wall thickness at the corner opposite the parting surface is smaller.
- Setting of Cavity
The cavity of the blow molding mold directly determines the shape, size, and appearance performance of the product. The mold cavity of PE blow molding is prone to residual gas, resulting in defects such as poor exhaust, orange peel, and uneven shrinkage. Therefore, during design, the surface should be slightly rough, and sandblasting treatment can be applied to the mold cavity. But for different plastics, the sandblasting particle size also needs to be different. For example, for HDPE blow molding molds, a coarser particle size can be used, but for LDPE, a finer particle size should be used. In addition, the etching mold cavity can also form a rough surface, and even patterns can be formed on the surface of the product. When blow molding products with high transparency, it is necessary to polish the mold cavity. For blow molding of engineering plastics, the mold cavity generally cannot be sandblasted.
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