Differences and Similarities between Injection Blow Molding and Extrusion Blow Molding

The difference between extrusion blow molding and injection blow molding of plastic bottles is that in injection molding, the mold used to mold the plastic bottle body should include male mold and female mold. The injection machine injects the plastic melt into the mold cavity under high pressure. When opening the mold cavity, the male mold must be removed from the female mold to eject the bottle body. In extrusion blow molding, the extrusion die is used for molding. Blowing mold is mainly composed of two halves of female mold. Generally, male mold is not required, and the parison is inflated with injected compressed air. Let’s look at the characteristics of the two production processes.

The characteristics of extrusion blow molding are:

(1) The cost of blow molding machines (especially blow molding molds) is low. For example, when molding the same products, the cost of blow molding machines is about 1/3 or 1/2 of that of injection molding machines, and the production cost of products is also low.

(2) In blow molding, the parison is formed by the extrusion die under low pressure and inflated under low pressure, so the residual stress of the product is small, and the resistance to tensile, impact, bending and environmental strains is high, with good performance. In injection blow molding, the plastic melt must pass through the mold runner and gate under high pressure, which will lead to uneven stress distribution.

(3) The molecular weight of blow molding grade plastics (such as PE) is much higher than that of injection molding grade plastics. This blow molding product has high impact toughness and high resistance to environmental stress cracking, which is very beneficial to the production of large capacity plastic bottles.

(4) As the blow molding die is only composed of female die, the wall thickness of the bottle body can be changed by simply adjusting the die opening clearance or extrusion conditions of the die head, which is very beneficial for products that cannot accurately calculate the required wall thickness in advance. For injection molding, the cost of changing the product wall thickness is much higher.

(5) Blow molding can form products with very thin wall thickness, which cannot be molded by injection method.

(6) Blow molding can form complex, irregular and integral products. When injection blow molding is adopted, two or more pieces should be produced first and then combined together through buckle fitting, solvent bonding or ultrasonic welding.

Features of injection blow molding:

(1) When the plastic melt is injected into the parison mould, it is subject to certain orientation effect in the circumference of the cavity. In addition, the temperature of parison blow molding is lower than that of extrusion blow molding, and the orientation effect produced by blow molding can be retained more, which helps to improve the strength and other properties of plastic bottles.

(2) The plastic bottle size (especially the neck thread size) has high precision, so it is easy to ensure the uniformity of the bottle wall thickness as required, and the bottle size is uniform.

(3) There is no joint on the surface of the bottle body, and it does not need to be trimmed after molding. In general, there is no leftover material. The leftover material produced by extrusion blow molding is usually 5%~30%.

(4) The glossiness of the bottle body is high, and the transparency of the bottle body formed by transparent polymer is obvious.

(5) A sliding bottom module can be set on the blow molding die, so the shape design of the bottom of the bottle body is flexible. It is more suitable for forming hard plastic bottles and wide mouth plastic bottles.

(6) Injection blow molding requires parison mold and blow molding mold, and the mold requirements are high, so the cost of injection blow molding mold is high, about 2-3 times that of extrusion blow molding. Because the pressure on the melt in the parison mold cavity is high, the residual stress of the blow molding bottle body is high, and the mold closing force required by the parison mold is high. Most of the bottles formed by injection blow molding are of small volume (generally 10-300 mL), and products with complex shapes cannot be formed, so the energy consumption is significantly higher.

In drug packaging, the sealing degree of the bottle mouth directly affects the drug safety within the validity period of the drug, while the injection blow molding method is used to ensure the dimensional accuracy of the bottle mouth and thread and the dimensional accuracy of the appearance, and the wall thickness of the bottle body is uniform, which meets the drug packaging requirements and can prevent the drugs in the bottle from being polluted by external substances, Therefore, the State Drug Administration has proposed the injection blow molding method to produce high-quality medical plastic bottles as early as the Ninth Five Year Plan period.