What Is The Difference Between Blow Molding And Injection Molding?
First, the process is different
Blow molding uses gas pressure to inflate the hot parison closed in the mold into a hollow product, or the tube parison is inflated into a tube film without a mold.
Injection molding is to melt plastic materials, and then inject them into the film cavity to form.
- The process of injection molding and blow molding is different. Blow molding is injection + blowing; injection molding is injection + pressure; blow molding must have the head left by the blowing tube, and injection molding must have a gate section
- Generally speaking, injection molding is a solid core body, blow molding is a hollow core body, and the blow molding surface is uneven. Blow molding has a blowing port.
- Injection molding, that is, thermoplastic injection molding. This method is to melt the plastic material and then inject it into the film cavity.
Once the molten plastic enters the mold, it is shaped into a certain shape according to the mold cavity. The resulting shape is often the final product, and no other processing is required before installation or use as the final product.
Many details, such as protrusions, ribs, and threads, can be formed in one step of injection molding. The injection molding machine has two basic components: an injection device and a mold clamping device for melting and feeding plastic into the mold.
The function of the mold clamping/closing
1) Make the mold close under the injection pressure;
2) Take the product out of the injection device to melt the plastic before it is injected into the mold, and then control the pressure and speed to inject the melt into the mold. The injection device currently used has two designs: screw preplasticizer or two-stage device, and reciprocating screw.
The screw pre-plasticizer uses a pre-plasticizing screw (first stage) and then injects molten plastic into the injection rod (second stage). The advantages of the screw preplasticizer are constant melt quality, high pressure and high speed, and precise injection volume control (using mechanical thrust devices at both ends of the piston stroke).
These advantages are required for transparency, thin-walled products, and high production rates. Its disadvantages include uneven residence time (causing material degradation), higher equipment costs, and maintenance costs. The most commonly used reciprocating screw injection device does not require a plunger to melt the plastic
Blow molding also is known as hollow blow molding, a rapidly developing plastic processing method.
The tubular plastic parison obtained by extrusion or injection molding of the thermoplastic resin is placed in a split mold while it is hot (or heated to a softened state). After the mold is closed, compressed air is injected into the parison to blow the plastic parison.
It expands and clings to the inner wall of the mold, and after cooling and demolding, various hollow products are obtained. The manufacturing process of blown film is very similar in principle to blow molding of hollow products, but it does not use molds.
From the perspective of plastic processing technology classification, the molding process of blown film is usually included in extrusion.
Second, different raw materials
The raw materials for blow molding are usually polyethylene, polyvinyl chloride, polypropylene, polystyrene, thermoplastic polyester, polycarbonate, polyamide, cellulose acetate, and polyacetal resin, among which polyethylene is used for most.
The raw material for injection molding is usually thermoplastic or thermoset.
Third, different applications
Blow molding is widely used in industries such as bottles, jars, baby products, and sporting goods.
Injection molding is widely used in mobile phones, laptop computers, a variety of plastic shells, communications, micro motors, computers, electrical appliances, electronics, toys, watches, lighting, motorcycles, and other industries.
Blow molding will have the following development trends in materials, machinery, auxiliary equipment, control systems, and software as the market demands its products.
- Raw materials In order to meet the function and performance (medicine, food packaging) requirements of blow-molded products, blow molding grade raw materials will be more abundant and have better processing performance.
Such as PEN materials, not only have high strength, good heat resistance, strong gas barrier properties, transparency, and ultraviolet radiation resistance, they can be applied to blow all kinds of plastic bottles, and they have high filling temperature and excellent barrier properties to carbon dioxide and oxygen. And resistant to chemicals.
(2) Product packaging containers and industrial products will have a large growth, and injection blow molding and multi-layer blow molding will have rapid development.
(3) The precision and efficiency of blow molding machinery and equipment; the automation of auxiliary production (operation) equipment.
Precision and high efficiency not only refers to the high speed and high pressure of mechanical equipment in the production and molding process, but also requires that the products produced can achieve high stability in terms of appearance size fluctuations and part weight fluctuations.
That is to say, production The size and geometry of each part of the product have high precision, small deformation, and shrinkage, and the appearance, internal quality and production efficiency of the product must reach a high level.
Auxiliary operations include flash removal, cutting, weighing, drilling, leak detection, etc. Process automation is one of the development trends.
(4) The research on blow molding simulation blow molding mechanism is more in-depth, the reasonable construction of the mathematical model of blow molding simulation, and the fast and accurate numerical algorithm are the key to the simulation.
Blow molding simulation will play a role in product quality prediction and control. An increasingly important role.
Generally speaking, there are three main aspects to evaluate the performance of plastic products:
- The appearance quality, including completeness, color, gloss;
- The accuracy between size and relative position, namely size accuracy and position accuracy;
- The mechanical properties, chemical properties, electrical properties, etc. corresponding to the purpose, namely functional
Therefore, if there is a problem in any of the above three aspects, it will lead to the production and expansion of product defects.
Classification of common defects in injection molded products
Appearance defects: silver streak discoloration, welding marks
Process problems: flash, shrinkage, lack of glue
Performance issues: warpage, embrittlement