In the field of industrial production of plastic products, injection molding and blow molding stand as twin pillars, jointly supporting the vast system of modern manufacturing. Although both processes fall under the category of thermoplastic processing, they exhibit distinct technical paths in terms of molding mechanisms, application scenarios, and product characteristics. A deep understanding of their differences is crucial for rationally selecting production processes and optimizing product design.
In the field of industrial production of plastic products, injection molding and blow molding stand as twin pillars, jointly supporting the vast system of modern manufacturing. Although both processes fall under the category of thermoplastic processing, they exhibit distinct technical paths in terms of molding mechanisms, application scenarios, and product characteristics. A deep understanding of their differences is crucial for rationally selecting production processes and optimizing product design.I. Fundamental Differences in Molding Mechanisms
The core of injection molding lies in "injection-pressing molding." Molten plastic is injected into a closed mold cavity at high pressure through a screw, and the product is removed after holding pressure and cooling. The entire process resembles precision casting, where the plastic takes shape inside the mold. For example, in automotive bumper manufacturing, raw material pellets are melted at 200–300°C and injected into a steel mold with complex rib structures at a high pressure of 80–150 MPa; the cooling time is typically controlled within 30–60 seconds.Blow molding, by contrast, follows the principle of "pneumatic shaping." First, a plastic parison (preform) is heated to a viscous flow state, placed in a mold, and expanded by compressed air to fit the mold cavity wall. The production of mineral water bottles perfectly illustrates this process: a tubular parison with a diameter of about 100 mm, under the action of 0.3–0.6 MPa air pressure, forms a thin-walled container with precise volume in just 2–3 seconds. This molding method is essentially gas-assisted forming of soft materials.
II. Technical Divergence in Equipment and Molds
Injection molding machines are known as "pressure giants" in the industrial field. Their core components include an injection system (with a screw diameter of up to 120 mm) and a clamping mechanism (with a maximum clamping force of 6,500 tons). Taking the large injection molding machine from KraussMaffei in Germany as an example, it is equipped with a hydraulic servo system that can achieve a repeat positioning accuracy of ±0.1 mm. Mold design must consider melt flow balance; large appliance shell molds often use hot runner systems, which can increase raw material utilization to over 95%.The core of blow molding equipment lies in the parison control system. The die diameter adjustment accuracy of a hollow blow molding machine must reach 0.01 mm, and the wall thickness control system can achieve a thickness uniformity of ±0.02 mm. A six-layer co-extrusion blow molding production line for automotive fuel tanks, through die rotation and multi-layer extrusion technology, can simultaneously complete the composite molding of barrier layers, adhesive layers, and structural layers. Molds are mostly made of aluminum alloy, weighing only one-third of injection molds, but require surface polishing with a roughness Ra ≤ 0.2 μm to ensure product glossiness.
III. Differentiated Logic in Material Selection
Injection molding has strict requirements for the melt strength of materials. ABS resin, due to its excellent fluidity (melt flow rate [MFR] 15–25 g/10min), is the first choice for electronic shells. In contrast, polycarbonate (PC) requires strict control of moisture content (<0.02%) to avoid hydrolytic degradation. Engineering plastics like POM (polyoxymethylene) need to maintain a barrel temperature of 190–210°C during injection molding to prevent molecular chain breakage and subsequent decline in mechanical properties.Blow molding focuses more on the ductility and elastic recovery of materials. High-density polyethylene (HDPE) has become the mainstream material for packaging containers due to its good blow-up ratio (up to 4:1); its melt strength index (MSI) needs to be controlled between 0.3–0.8 N. In multi-layer blow molding, the EVOH barrier layer requires the material to maintain an oriented structure during the blowing process. The vinyl acetate content of ethylene-vinyl acetate copolymer (EVA) must be strictly controlled between 18–28% to achieve optimal adhesion performance.
IV. Significant Differences in Product Characteristics
Injection molded products have precise geometric dimensions and complex internal structures. The tolerance of injection molded parts for smartphone middle frames can be controlled within ±0.05 mm, and internal reinforcing ribs with a thickness of only 0.8 mm can withstand a static load of 50 kg. This process enables integrated molding of multiple components—for example, the injection molded part of an automotive instrument panel integrates more than 20 functional structures such as speaker holes and air conditioning vents.The advantages of blow molded products lie in lightweight and sealing. The weight of a 500 ml PET beverage bottle has been reduced to 9.9 g (a 30% reduction compared to ten years ago), while the burst pressure remains above 1.2 MPa. The hollow structure endows products with excellent thermal insulation performance; the thermal conductivity of blow molded chemical storage tanks is only 1/500 that of metal materials. However, blow molded parts have poor dimensional stability—a 1°C change in ambient temperature may cause a 0.03% change in volume.
