Injection molding

Plastics can be roughly classified into three major categories based on their properties, applications and prices: general-purpose plastics, engineering plastics and special-purpose plastics. 

General-purpose plastics are the most common type of plastics in daily life. They have large production volumes, low prices, and wide applications. Typical examples include PE (polyethylene, used for plastic bags), PP (polypropylene, used for lunch boxes), PVC (polyvinyl chloride, used for water pipes), PS (polystyrene, used for disposable cups), and ABS (used for toy shells). Their characteristics are low cost and easy processing, but they have low mechanical strength, average heat resistance, and the typical operating temperature does not exceed 100℃. 

Engineering plastics are materials that have reached a new level in terms of performance, featuring higher mechanical strength, heat resistance, and dimensional stability. They can be used as structural components instead of metals. Common examples include PA (nylon), PC (polycarbonate), and POM (polyoxymethylene). They can operate for a long time at temperatures above 100°C and even 150°C, and their load-bearing capacity is much stronger than that of common plastics. Their prices are also higher. In situations where performance is required, such as for automotive parts, electronic and electrical enclosures, and mechanical gears, engineering plastics are usually chosen. 

Special plastics are like the "special forces" among plastics, possessing one or more extremely outstanding properties - such as extremely high heat resistance (able to withstand temperatures above 200℃ for a long time), excellent corrosion resistance or self-lubrication, but at the cost of being very expensive. Typical representatives include PPS, PEEK, PTFE, etc., which are widely used in high-end fields such as aerospace, medical devices, and semiconductors. 

Summary: General-purpose plastics address "daily needs", engineering plastics address "structural needs", and specialty plastics address "extreme needs". When choosing materials in practice, one needs to consider the working conditions of the product (temperature, stress, environmental corrosiveness) and the cost budget, and find the most suitable balance point between performance and price.