Hollow blow molding is a molding method for manufacturing hollow plastic products. Borrowing from glass container blowing technology, it developed into plastic blow molding technology in the 1930s. Hollow blow molding is a plastic molding method that uses gas pressure to inflate a hot preform closed in a mold into a hollow product.
Hollow blow molding dies can generally be divided into two main categories based on the preform forming process: extrusion blow molding dies and injection blow molding dies. They can also be divided into two categories based on the different stretching conditions: ordinary blow molding dies and stretch blow molding dies.
Extrusion blow molding involves melting and plasticizing thermoplastics, extruding a preform through an extruder die, placing the preform in a mold, inflating it with compressed air, and then cooling it to obtain the finished product. Injection blow molding uses an injection molding machine to inject molten material into a mold to prepare a preform, which is then placed in a blow molding mold while still hot, and inflated by introducing air. Stretch blow molding involves heating a preform to a suitable temperature, placing it in a mold, stretching it axially with a stretching rod, and then stretching it laterally with air. Stretch blow molding can be further divided into one-step (hot preform) and two-step (cold preform) processes, depending on the preform preparation method.
Extrusion blow molding dies mainly consist of two female mold halves, as shown in the figure below. Because blow molding dies operate under high loads during mass production, inserts are needed in easily worn and damaged areas such as the mold opening, neck, and cut edges. This facilitates the replacement of these parts, ensures the mold's durability, and also makes mold manufacturing and product size adjustment easier for products with complex appearances.
Blow molds made of aluminum are lightweight, have fast heat transfer, good thermal conductivity, good machinability, and low density. However, aluminum has low hardness, is easily worn, has poor weldability after damage, and is relatively expensive. Furthermore, when using aluminum alloy to directly form cooling channels, the quality requirements for the cooling water are high, and the threaded connection strength is relatively weak.
2. Structural steel materials
Generally, if blow-molded products are produced in large batches with long production times, and considering the need for high strength and hardness in the mold cuts, structural steel is a better choice for mold material. Although its heat transfer performance is not as good as aluminum alloys and beryllium copper alloys, a well-designed cooling channel structure can effectively improve heat transfer.
(a) Low carbon steel
Using low-carbon steel for the cavity portion of molds offers unique advantages for applications with limited mold-making equipment. Low-carbon steel has good weldability, facilitating welding processes. For large and extra-large blow molding molds with deep and long cooling channels that are difficult to machine using drilling methods, low-carbon steel can be used for layered welding to form the cooling channels and even the cavity. This method is sufficient for products with less stringent precision requirements. Blow molding molds made of low-carbon steel can also undergo surface treatment using carburizing (nitriding). For low-carbon steel molds that have been used for some time, carburizing (nitriding) can extend their service life.
(b) Medium carbon steel or medium carbon alloy steel
These materials are typically used directly to form mold cavities, mold base plates, and other components. For blow molding molds where cooling channels can be directly formed using machining methods, medium carbon steel or medium carbon alloy steel is suitable for direct fabrication.
(c) High-carbon steel or high-carbon alloy steel
High-carbon steel or high-carbon alloy steel is commonly used to make guide and positioning parts for blow molding dies, as well as some special wear-resistant parts.
3. Stainless steel material
Stainless steel is widely used in molds for small to medium-sized blow-molded products that require high surface quality and are produced in large batches. Stainless steel molds offer unique advantages for products requiring high surface quality and large production volumes due to their good dimensional stability, high surface finish, and excellent corrosion resistance.
4. Beryllium copper alloy materials
Beryllium copper alloys possess excellent comprehensive properties, making them particularly suitable for manufacturing small to medium-sized blow-molded products and blow molds for large-volume production. Currently, beryllium copper alloys are widely used in aluminum alloy blow molds to create preform inserts with good results. Beryllium copper alloys typically contain 2.75% beryllium and 0.5% cobalt, with the remainder primarily being copper. Changing the beryllium content can impart different strengths, hardness, thermal conductivity, corrosion resistance, and wear resistance. However, beryllium copper alloys are relatively expensive. Beryllium copper powder is harmful to human health; therefore, protective measures should be taken during processing.
Precautions for material selection
1. Select materials according to the batch size of blow-molded products.
For products requiring high surface quality and large production volumes, it is advisable to select high-grade mold steel to manufacture blow molding molds.
For products with low surface quality requirements and high production volume, ordinary carbon steel can be used to make blow molding molds.
The product surface quality requirements are not high, and the output is not large, so general materials such as aluminum alloy and ordinary carbon steel are used to make blow molding molds.
2. Select materials according to the mold processing method.
The mold cavity is machined using CNC machining methods and can be made of high-quality carbon steel or other mold steels.
The mold is manufactured using a layered processing method and can be made of low-carbon steel.
The mold is made by casting and machining, and aluminum alloy can be used for casting.
3. Select materials according to the structural components of the mold.
When the cavity strength requirement of a blow mold is not high, aluminum alloy can be selected. However, when the strength requirement of the mold cut and threaded parts is high, beryllium copper alloy, high-quality carbon steel, and other high-quality steel materials can be used to make inserts.
When the venting components and inserts of the mold require high precision and surface quality, high-grade mold materials such as beryllium copper alloy and stainless steel can be selected.
The guiding and positioning parts of the mold have high requirements for strength and wear resistance, and can be made of materials such as high-carbon steel with high hardness.
Heat treatment of mold materials
Heat treatment of blow molding molds is also crucial. For blow molding molds made of steel, tempering should generally be performed after rough machining to reduce deformation. After tempering, the mold should be left to stand for 24–48 hours before finishing. For the cut edges, local quenching can be used to increase their hardness.
Parts such as guides and positioning components of molds can be hardened. Some molds made using aluminum alloy casting methods should undergo a certain aging treatment to extend their service life.
For blow molding molds that require a long service life and produce large batches of products, a second nitriding treatment can be performed after the trial molding is successful to ensure their durability.
