Plastic Blow Molding
The plastic blow molding process is not only suitable for producing hollow containers such as beverage bottles, cosmetic bottles, etc., but also extends to the production of large hollow containers and blow molded trays, as well as some precision components in the automotive field.
Blow Mold Tooling Design
Blow mold tooling is mainly composed of a mold base, mold core, mold cavity, and cooling system, they are important tools for blow molding and a key component in the blow molding process. Its performance and structure can directly determine the appearance and size of blow-molded products. Plastic blow molding has outstanding advantages, and its relatively automated production process provides a crucial step for the popularization of plastic products.
The Influencing Factors of Blow Mold Tooling Blanks
When providing mold manufacturing services, blow tooling suppliers first need to consider the design and manufacturing requirements of the mold blank. A good blank is crucial for the quality of the supplier’s subsequent product production.
- During the extrusion process of tubular billets, controlling the appropriate extrusion temperature can significantly improve the forming process and the quality of the billets. For example, increasing the extrusion temperature within an appropriate range can reduce the viscosity of the melt, improve the fluidity of the melt, and increase the screw speed appropriately without affecting the mixing of materials. It can also reduce the power consumption of the extruder; And it is beneficial for the strength, surface smoothness, and transparency of the final formed part. If the melt temperature exceeds a certain range, it not only does not have the above benefits, but also prolongs the cooling time, increases the shrinkage rate of the product, affects the appearance of the product, and causes the extruded billet to sag, affecting the uniformity of the longitudinal wall thickness of the billet, causing degradation of thermosensitive plastics such as PVC due to excessive temperature, and reducing the strength of engineering plastics such as PC. If the temperature is lower than the normal level, it will lead to incomplete plastic plasticization, the rough surface of the blow mold tooling, increased internal stress, lower product strength, and cracking.
- Choosing suitable raw materials largely determines the quality of the final product. The selected raw materials need to meet both the minimum usage requirements of the product and the requirements of the blow molding process. Manufacturers generally choose hollow resin. We also need to consider the melt mass flow rate of the raw materials, i.e. MFR. Resins with lower MFR are beneficial in preventing the sagging of the billet, making it easier to obtain a billet with uniform wall thickness. However, when the manufacturer increases the screw speed, the surface of the melt with lower MFR is prone to becoming rough. Therefore, for medium and large-sized blow-molded products, it is important to focus on how to prevent billet sagging. On the premise of ensuring quality, a lower MFR has more prominent advantages for large-sized products; But for small blow molded products, the MFR should be slightly higher.
- The mouth blow mold tooling determines the size and shape of the billet. Therefore, when designing the mouth mold, manufacturers need to consider the smoothness of its inner and outer surfaces to manufacture smooth billets in the future.
- An important factor affecting the quality of extruded billets is the screw speed. The higher the screw speed, the faster the extrusion speed, and the greater the output of the extruder. At the same time, it can help reduce the sag of the billet. However, too fast speed not only reduces the surface quality of the billet but may also cause some plastics to experience melt fracture, such as HDPE. Moreover, increasing the screw speed can easily generate a large amount of frictional and shear heat after long-term operation, which may lead to thermal decomposition of thermosensitive plastics such as PVC. If the screw speed is too low, it will cause the viscosity of the billet to decrease, the fluidity to deteriorate, and the extrusion speed to slow down. Due to the self-weight generated by the plastic, the billet will sag, resulting in significant differences in wall thickness and even increasing the difficulty of forming. Therefore, when selecting the appropriate screw speed, it is necessary to extrude a smooth and uniform billet as much as possible without exceeding the working capacity range of the extrusion system and use a faster screw speed as much as possible. Generally, blow molding machines from manufacturers use larger extrusion devices to keep the screw speed below 70r/min.
Assembly of Blow Mold Tooling
Blow mold tooling manufacturers generally provide assembly services to customers after providing mold manufacturing services. They not only need to ensure that all parts of the installation can fit perfectly but also have certain aesthetic requirements.
- Mold Appearance Assembly
The edges of the nonworking area of the blow mold tooling should be chamfered; When assembling the mold, it is necessary to ensure that each parting surface fits tightly; The supporting surfaces between each component should be aligned; For the convenience of installation, manufacturers usually equip large or medium-sized molds with hooks or rings; The fitting size, ejection form, closing height after assembly, and blow mold tooling opening distance of the installation site should all meet the design requirements and the technical level of the selected equipment; The assembled mold needs to be marked with the direction, number, drawing number, and equipment model of the moving and fixed molds.
- Assembly of Ejection Mechanism
The actions of each ejecting part need to cooperate, achieving unity, stability, and no jamming; When working, it is necessary to have good strength, stiffness, stability, and uniform force distribution to ensure that the parts and pouring system can be smoothly demolded and removed during mold opening, and accurately return to the specified position during mold closing.
- Assembly of Formed Parts and Pouring System
The formed parts and pouring system should have a smooth surface, no dead corners or pits, and no scratches on the surface; Non integral cavities or cores that are pieced together or embedded should ensure consistency in the splicing surface, and should be firm, seamless, and have a smooth surface; The dimensional accuracy of the formed parts should be highly consistent with the design requirements; When the formed plastic is corrosive, the formed surface should be treated with chrome plating, polishing, etc. to prevent corrosion; After assembly, the positions of the formed parts that need to cooperate with each other should be consistent to ensure the shape and dimensional accuracy of the formed products; The parting surface, pouring system, and feeding valve of the cavity should not be designed as rounded arcs, but should maintain sharp edges; There should be appropriate gaps or pressure areas between the core and cavity, extrusion ring, plunger, and feeding chamber that come into contact with each other, to prevent scratches or damage caused by direct compression of parts during mold closing.
- Assembly of Locking and Reinforcing Parts
The locking force of the locking parts should be strong, accurate, and reliable; The reinforced parts must be firmly fastened and must not become loose; The positioning parts should be matched with appropriate elasticity and there should be no looseness.
- Assembly of Heating and Cooling System
The most important thing about cooling devices is that they cannot leak, so manufacturers need to ensure the firmness and reliability of the cooling devices; The most important thing for heating devices is to ensure insulation after installation, and there should be no leakage of electricity; After the installation of the heating and cooling system is completed, it is necessary to check whether the actions are accurate, flexible, timely and coordinated.
- Assembly of Moving Parts
The fitting clearance of each sliding component should be appropriate, and the positioning of the start and stop positions should be accurate and reliable; The movement of the guiding parts of the moving parts should be smooth, flexible, and coordinated with each other, without any jamming or obstruction.
- Assembly of Guide Mechanism
During assembly, the guide pillar and guide sleeve should be checked promptly to ensure that they are perpendicular to the mold seat, can slide flexibly, and are stable without jamming; The guidance accuracy must meet or exceed the design requirements to have good guidance and positioning effects on the moving and fixed molds; The inclined guide pillar should be able to closely fit with the slider and have sufficient strength, stiffness, and wear resistance; Check whether the elasticity of the slider and groove is moderate, whether the movement is flexible, and whether there is no jamming phenomenon.
The Causes of Damage to Blow Mold Tooling
During the long-term use of blow mold tooling, various reasons may lead to mold damage, causing irreparable losses to customers and manufacturers. The main causes of damage are:
- The material used to manufacture molded parts has poor performance and is not suitable for this working condition, which can easily lead to deformation, corrosion, and even severe wear of the mold after working for some time.
- The hardness of the mouth mold and core rod is relatively low, causing severe wear on the smoothness of the working surface, resulting in surface roughness.
- When installing or disassembling parts in blow mold tooling, the operator may use their hands or other tools to strike the parts, causing deformation or scratches on the mold parts.
- When the manufacturer adjusts the mold, errors in the work program debugging can cause the mold adjustment screw to break, the mouth mold or the sizing sleeve to deform, and it cannot continue to be used.
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