Blow Molding Machines: An Overview of Types and Applications

Introduction to Blow Molding Technology

Blow molding, also known as blow moulding, is a rapidly developing plastic processing method. The process involves using air pressure to inflate a hot plastic preform against a mold cavity, creating hollow products. This technology became prominent during World War II for producing low-density polyethylene bottles and expanded in the late 1950s with the advent of high-density polyethylene and advanced blow molding equipment . Today, blow molding machines manufacture containers ranging from a few milliliters to over 10,000 liters in capacity, serving industries from packaging to automotive .

Main Types of Blow Molding Machines

Blow molding machines are primarily categorized into three types:

1. Extrusion Blow Molding Machines: These combine an extruder, blow molding unit, and clamping mechanism. Plastic is melted and continuously or intermittently extruded as a tube-like parison (preform). When the parison reaches sufficient length, the mold closes around it, and compressed air inflates it against the mold walls. After cooling, the hollow part is ejected. Extrusion blow molding is ideal for producing complex, irregularly shaped, seamless items and is commonly used for HDPE and PP rigid plastic bottles . Its advantages include lower machinery costs (approximately one-third to one-half that of injection molding) and products with low residual stress, offering high tensile, impact, and bending resistance .

2. Injection Blow Molding Machines: These integrate plasticizing, hydraulic systems, control electronics, and blow molding mechanisms. A preform is first injection molded. This preform is then transferred to a blow molding station where it is inflated into its final shape. Common configurations include three-station (120° apart) and four-station (90° apart) machines. Injection blow molding is suitable for HDPE and PP medical bottles . It produces containers with high dimensional accuracy, no need for secondary processing, and excellent bottle mouth flatness and sealing, preventing gas permeation—a crucial requirement for pharmaceutical packaging . However, mold costs are higher, and it's generally used for smaller containers (10–300 ml) .

3. Special Structure Blow Molding Machines and Stretch Blow Molding Machines: This category includes machines using sheets, molten material, or cold preforms to produce specially shaped hollow items. Stretch blow molding machines (like "injection-stretch-blow" or "extrusion-stretch-blow") can belong to either extrusion or injection types. They involve stretching the preform biaxially (longitudinally and radially) during blowing, significantly enhancing the container's mechanical properties, barrier properties, and transparency. This is ideal for PET and PP bottles for liquid pharmaceuticals or beverages . Stretch blow molding can be one-step (integrated) or two-step (separate preform and blowing) 

Extrusion Methods: Continuous vs. Intermittent

Extrusion blow molding machines can be further divided based on parison extrusion:

Continuous Extrusion Blow Molding: The extruder continuously produces the parison. Methods include reciprocating, alternating output, or rotary table systems, where molds or outputs alternate to maintain continuous production. This method, with relatively simple equipment and lower investment, is common among small and medium-sized enterprises .

Intermittent Extrusion Blow Molding: This method uses an accumulator head. Molten plastic is continuously plasticized and fed into a storage chamber. Once a set amount accumulates, a piston rapidly pushes the melt through the die to form the parison. This is suitable for large containers over 10 liters and typically includes parison wall thickness control systems for high-quality, material-efficient production 

Conclusion

Blow molding machines offer versatile solutions for creating diverse hollow plastic products. Understanding the distinctions between extrusion, injection, and stretch blow molding, as well as continuous versus intermittent operation, helps in selecting the right technology for specific application needs, ensuring optimal product quality and production efficiency.