HDPE Blow Molding
HDPE (high-density polyethylene) blow molded products exhibit excellent heat and cold resistance, as well as good chemical stability and mechanical strength, due to their high crystallinity and non-polar thermoplastic resin properties.
Blow Mold Tooling Inc|Design Points & Precautions
Modern blow mold tooling Inc. has a high level of blow molding mold manufacturing and production, capable of producing highly precise products widely used in various fields. The so-called plastic blow molding is a process of compressing the softened thermoplastic placed in a mold through air, causing it to expand and deform, solidifying it into a hollow container. Blow molded products have stable quality, good environmental performance, high production efficiency, high durability and flexibility. For example, the HDPE blow molding process is widely used in fields such as food, beverage, automotive, and healthcare.
The blow-molded products produced by blow-molding molds are closely related to our daily lives and provide convenient and durable features. Among various plastic manufacturing processes, blow molding technology is very eye-catching. Since its emergence, the way people produce and use plastic products has undergone tremendous changes.
Design Points of Blow Molding Mold Blanks
When designing molds for Blow Mold Tooling Inc., the first step is to design the billet. A billet with high design accuracy can effectively ensure the quality of blow-molded products and improve production efficiency. The main design points of the billet include:
- Determine the Wall Thickness of the Billet. The most suitable value for wall thickness, except for the neck of the billet, should be around 2-5 millimeters. If the wall thickness is large, it is easy to waste raw materials, increase the difficulty of forming, and even cause a large temperature difference between the inside and outside of the billet. The maximum wall thickness on the cross-section of the billet should not exceed twice the minimum wall thickness, and on the longitudinal section, it should not exceed three times.
- Determine the Inflation Ratio. During injection blow molding, the axial tension of the billet is very small, and the blowing expansion ratio is generally less than 3:1. In special circumstances or local blowing expansion ratios of the container can sometimes reach 4:1. However, the larger the blowing ratio, the greater the possibility of uneven wall thickness of the plastic part.
- Determine the Length-to-diameter Ratio of the Core Rod. In general, the length-to-diameter ratio of the core rod should be less than 10:1. If the length-to-diameter ratio is too large, the core rod is prone to bending deformation under high-pressure injection pressure, resulting in uneven wall thickness of the billet.
- Shrinkage Rate. Manufacturers should consider that different varieties of plastics have different shrinkage rates when designing preforms and blow molding molds. The shrinkage rate of general polyethylene (PE) and polypropylene (PP) is 1.6%~2.0%; The shrinkage rate of polystyrene (PS) and polycarbonate (PC) is only 0.5%. Even for the same type of plastic, the thicker the container wall, the greater the corresponding shrinkage rate.
- Determine the Cross-section of the Billet. When designing the blank of an elliptical blow-molded product, the shape of its cross-sectional area can be determined based on the length ratio of the long and short axes of the ellipse, that is the elliptical ratio. If the ratio of the long and short axes of the ellipse is less than 1.5, the cross-section of the billet is circular; If the ratio of the long and short axes is less than 2, the core rod of the billet shall be circular, and the cavity of the billet shall be elliptical; If the ratio of the long and short axes is greater than 2, the cross-section of the billet adopts the corresponding ellipse. The larger the elliptical ratio, the more difficult it is to manufacture the mold, so the elliptical ratio is generally not greater than 3. To prevent welding marks on the container, the ratio of the maximum wall thickness to the minimum wall thickness of an elliptical container should be less than 1.5.
Notes on Material Selection for Blow Molding Molds
When selecting the manufacturing materials for molds, Blow Mold Tooling Inc. needs to consider the different properties of the materials. Common mold materials include iron, stainless steel, aluminum alloy, polymer materials, etc.
- Select Materials according to the Molding Method of the Mold
The material for making the mold can be selected based on the molding method of the mold. If laminated processing is used, low-carbon steel can be used for molding; If CNC machining is used, high-quality carbon steel and other mold steels can be used for design and manufacturing; If the method of casting and mechanical processing is adopted, aluminum alloy or zinc alloy can be used for casting and embedded with corresponding metal inserts.
- Select Materials according to the Purpose and Requirements of Structural Components
The selection of materials can be based on the purpose and technical requirements of the structural components of the mold. When the strength requirements for the cavity of a general blow molding mold are low, aluminum alloy and zinc alloy materials can be chosen for casting. For the cutting and threaded parts of the mold with high strength requirements, high-quality steel materials such as beryllium copper alloy and high-quality carbon steel can be made into inserts and embedded in the mold; The exhaust components of the mold have high requirements for accuracy and surface smoothness, so high-quality advanced mold materials such as beryllium copper alloy and stainless steel can be selected for production; The guide pillar and guide sleeve of the mold have high requirements for strength and wear resistance, and can be formed using materials such as high carbon steel with high hardness. If low hardness and low strength materials are used, it will lead to substandard strength of the mold.
- Select Materials in Batches based on Blow-molded Products
If large-scale production is required and the surface quality of the produced products is high, advanced and high-strength mold steel can be selected as much as possible to make blow molding molds; If only small-scale production is required but the surface quality requirements of the product are not high, ordinary carbon steel can be chosen for production; If the production batch is small and the surface quality requirements of the product are not high, general materials such as aluminum alloy, zinc alloy, and ordinary carbon steel can be used to make blow molding molds, without the need to use advanced materials for production.
Design Points for Container Wall Thickness
When designing the wall thickness of plastic parts, Blow Mold Tooling Inc. needs to consider the type of plastic and the shrinkage rate. To avoid or reduce the disadvantage of uneven wall thickness, manufacturers should consider the following key points when designing:
- Consider the influence of blow molding orientation degree. The tensile strength and elastic modulus of plastic materials are significantly different from the mechanical properties of finished container wall thickness materials, and the longitudinal and transverse mechanical properties of container walls are also different. The combination of the longitudinal stretching ratio and radial expansion ratio determines the degree of blow molding orientation. Therefore, the calculation of the ultimate stress of wall thickness strength should be obtained by cutting samples in both directions of the product. Thin-walled hollow products with the same material and a blowing expansion ratio greater than the stretching ratio have stronger transverse mechanical properties than vertical longitudinal mechanical properties of their wall thickness materials.
- Uneven-formed wall thickness. The inspection standards for blow-molded products are composed of average wall thickness and minimum wall thickness, respectively. In general, Blow Mold Tooling Inc. calculates the volume of materials and billets based on the average wall thickness. Meanwhile, the wall thickness of the formed product is related to factors such as the shape and size of the billet, the fluidity of the material, the temperature of the billet, and the blowing expansion ratio; It is also related to the design and manufacturing of blow molding molds, such as the smoothness of the mold cavity surface, the setting of rounded arcs, the position and quantity of exhaust holes, cooling speed, etc. The larger the volume of the container, the more likely the wall thickness is to be uneven. A large container with an average wall thickness of 5mm and a minimum wall thickness of 2mm has a minimum wall thickness of 40% of the average wall thickness, which is already a qualified blow-molded product.
- The effect of infiltration on wall thickness. Permeability needs to be tested at a certain temperature and pressure, which represents the ability of plastic materials to resist the penetration of various gaseous and liquid substances. Not only does the wall thickness of plastic containers affect permeation, but the surface area and volume of the container also affect the permeation process. The packaging penetration rate of carbon dioxide is an important technical indicator in drinkable carbonated beverages. For example, if the polyester bottle wall is too thick, carbon dioxide will be absorbed by the bottle wall material, but if the bottle wall is too thin, carbon dioxide will permeate and evaporate outside the bottle.
Blow mold tooling Inc. has many design points in the manufacturing of blow molding molds. With precise control over these points, its blow molding products and molds have been exported to the UK, Germany, South Africa, the Philippines, and some Southeast Asian countries.
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