Also known as plastic molding processing. It is a general term for various processes that convert synthetic resins or plastics into plastic products. It is a large production department in the plastics industry. Plastic processing generally includes plastic batching, molding, machining, joining, modification and assembly. The last four processes are carried out after the plastic has been molded into finished products or semi-finished products, also known as plastic secondary processing.
History
Plastic processing has developed along with the development of synthetic resins, and it has promoted the development of plastic machinery. Many plastic processing technologies are borrowed from rubber, metal and ceramic processing. The history of plastic processing can be traced back to the 1890s. After the birth of celluloid, due to its flammability, it could only be made into blocks by molding, and then machined into sheets, which could be processed by thermoforming. This was the earliest plastic processing. Casting molding was successfully studied with the advent of phenolic resin; injection molding began in the 1920s and was used to process cellulose acetate and polystyrene; in the mid-1930s, soft polyvinyl chloride extrusion molding was successfully developed, and a single-screw extruder specifically for plastics was introduced; in 1938, twin-screw extruders were also put into production. In the early 1940s, polyurethane foam plastics were produced, and blow molding technology was used to produce polyethyleneHollow products. The advent of the reciprocating screw injection machine in 1952 brought injection molding technology into a new stage. In the 1960s and 1970s, the newly developed plastic processing technologies included various new molding methods for reinforced plastics, such as winding, pultrusion, sheet molding, reaction injection molding, structural foam molding, profile extrusion molding, sheet solid phase molding, co- extrusion, co-injection molding, etc. In the 1980s, plastic processing developed towards high efficiency, high speed, high precision, energy saving, large-scale or ultra-small, ultra-thin, etc. Computer technology entered this field, raising the entire plastic processing technology to a new level.
Development prospects
The Chinese government has a series of policies to promote the development of the plastics industry, which will vigorously promote the stable and sustainable development of the entire industry.
In order to cope with the impact of the international financial crisis, implement the government ‘s overall requirements on maintaining growth, expanding domestic demand, and adjusting the structure, ensure the stable development of light industry, accelerate structural adjustment, and promote industrial upgrading, the government has issued the “Light Industry Adjustment and Revitalization Plan”, listing plastics as one of the three key industries in the planning.
As one of the pillar industries of light industry, the total industrial output value, total assets, number of employees, export volume and other indicators of the plastic products industry have accounted for more than 10% of the total light industry. Therefore, the adjustment and revitalization of the plastic processing industry is directly related to the revitalization and development of the entire light industry, and occupies a pivotal position.
In the process of responding to the international financial crisis, the central and local governments have taken a series of effective measures to help enterprises solve their difficulties and overcome the difficulties together with them.
The main measures are: First, support key enterprises to “go global” through various means and set up logistics centers and distribution centers in major sales markets. Second, establish economic and trade cooperation zones and actively promote the construction of overseas industrial parks and economic and trade cooperation zones. Third, continue to support the construction of foreign trade professional markets, build border trade professional markets for light industrial products in Southeast Asia, Central Asia, Northeast Asia and other regions, establish China’s light industrial product trade centers in the Middle East, Northern Europe, Russia and other regions with conditions, strengthen external publicity, and facilitate the entry and exit of goods and personnel. Fourth, give full play to the role of processing trade and support enterprises to expand processing trade. Fifth, improve the trade service system, such as establishing technical regulations and standards management service platforms and training systems for light industrial export products, as well as quality and safety case notification, return verification, early warning and emergency handling systems, to improve the quality management level of enterprises and maintain the image of Chinese products. Simplify the customs clearance and inspection procedures for the export of light industrial products, reduce relevant charging standards, improve customs clearance efficiency, and promote trade facilitation. These benefits
The introduction of policies has played an increasingly positive role in helping the plastics processing industry cope with the crisis and achieve healthy and rapid development.
As we all know, China’s plastic processing industry has a high degree of marketization and strong adaptability. Its products also have a high cost-effectiveness in the international market. The further expansion of the domestic demand market provides a broad market space for the development of light industry. As long as we seize the opportunity, make full use of the market -driven mechanism, and make up our minds to actively take comprehensive measures, we can achieve the adjustment and revitalization of the plastic processing industry. Both the government and enterprises realize that this international financial crisis is an opportunity for a reshuffle. As long as we have firm confidence, seek opportunities in crisis, judge the situation, and focus on the present and the long-term, we can turn crisis into opportunity and gain greater development space. Such as the following aspects:
Inorganic chemicals, chemical printing and dyeing, organic raw materials, polymers /resins, plastics and rubber, chemical equipment, coatings and paints, chemical reagents, exhibitions, exhibition information, insulating electrical materials and equipment, epoxy flooring, electronic materials, adhesives, paints and coatings, fiberglass and construction industry.
Precautions
Plastic processing shrinkage is one of the most common problems in plastic processing. For plastic products with high surface quality requirements, shrinkage is an even more difficult problem. Therefore, with the continuous improvement of plastic processing technology, it is imperative to minimize plastic processing shrinkage and improve product quality.
In plastic processing, the shrinkage formed at thicker locations of injection molded plastic parts, such as ribs or protrusions, is more serious than that at adjacent locations. This is because the cooling rate of the thicker area is much slower than that of the surrounding area. The different cooling rates cause depressions to form at the joint surface, which is the familiar shrink mark. This defect seriously limits the design and molding of plastic products, especially large thick-walled products such as the slope of a TV.Case and display shell, etc. In fact, shrinkage marks must be eliminated on products with strict requirements such as daily electrical appliances, while plastic processing shrinkage marks are allowed for products with low surface quality requirements such as toys.
Plastic processing shrinkage marks may be caused by one or more reasons, including processing methods, part geometry, material selection, and mold design. Geometry and material selection are usually determined by the raw material supplier and are not easy to change. However, there are many factors related to mold design on the mold manufacturer’s side that may affect the plastic processing shrinkage link. Cooling runner design, gate type, and gate size may have a variety of effects. For example, a small gate such as a tube gate cools much faster than a tapered gate. Premature cooling at the gate will reduce the filling time in the cavity, thereby increasing the chance of shrinkage marks. For molders, adjusting processing conditions is a way to solve the problem of plastic processing shrinkage. Filling pressure and time significantly affect shrinkage. After the part is filled, excess material continues to fill the cavity to compensate for the shrinkage of the material. Filling the stage too short will lead to increased shrinkage, and ultimately more or larger shrinkage marks. This method of solving plastic processing shrinkage may not reduce shrinkage marks to a satisfactory level by itself, but molders can adjust filling conditions to improve shrinkage marks.
Processing procedures
Plastic ingredients
In addition to polymers, the raw materials used in plastic processing generally also need to add various plastic additives (such as stabilizers, plasticizers, colorants, lubricants, reinforcing agents and fillers, etc.) to improve the molding process and the performance of the product or reduce the cost of the product. The additives and polymers are mixed and evenly dispersed into powder, also known as dry blends. Sometimes the powder needs to be processed into pellets by plastic refining. Such powders and pellets are collectively called compound materials or molding materials.
Plastic molding
The key link of plastic processing. Plastics in various forms (powder, granules, solutions or dispersions) are made into products or blanks of the desired shape. There are more than thirty molding methods. Its selection is mainly determined by the type of plastic (thermoplastic or thermosetting), the initial form, and the shape and size of the product. Common methods for processing thermoplastics include extrusion, injection molding, calendering, blow molding and thermoforming, while thermosetting plastics are generally processed by compression molding, transfer molding, and injection molding. Lamination, compression molding and thermoforming are the processes of molding plastics on a plane. The above plastic processing methods can all be used for rubber processing. In addition, there are casting methods using liquid monomers or polymers as raw materials. Among these methods, extrusion and injection molding are the most commonly used and basic molding methods.
Plastic parts joining
There are two methods for joining plastic parts: welding and bonding. Welding methods include hot air welding using welding rods, hot melt welding using hot electrodes, high frequency welding, friction welding, induction welding, ultrasonic welding, etc. Bonding methods can be divided into flux, resin solution and hot melt adhesive bonding according to the adhesive used.
Plastic parts assembly
The process of assembling plastic parts into complete products by bonding, welding and mechanical connection. For example, plastic profiles are assembled into plastic window frames and plastic doors through sawing, welding, drilling and other steps.
Machining
Borrowing the processing methods of metal and wood, etc., to manufacture plastic products with very precise dimensions or small quantities, it can also be used as an auxiliary process for molding, such as sawing of extruded profiles. Because the properties of plastics are different from those of metals and wood, plastics have poor thermal conductivity, low thermal expansion coefficient and elastic modulus. When the clamp or tool is pressurized too much, it is easy to cause deformation, easy to melt when heated during cutting, and easy to adhere to the tool. Therefore, when plastics are machined, the tools used and the corresponding cutting speeds must adapt to the characteristics of plastics. Commonly used mechanical processing methods include sawing, shearing, punching, turning, planing, drilling, grinding, polishing, threading, etc. In addition, plastics can also be cut, punched and welded by laser.
Surface modification
The purpose is to beautify the surface of plastic products, which usually includes: mechanical modification, that is, using filing, grinding, polishing and other processes to remove burrs and burrs on the parts, and correct the size, etc.; coating, including coating the surface of the parts with paint, using solvents to brighten the surface, and covering the surface of the products with patterned films, etc.; coloring, including painting, printing and hot stamping; metal plating, including vacuum coating, electroplating and chemical silver plating. Among them, hot stamping is to transfer the colored aluminum foil layer (or other patterned film layer) on the hot stamping film to the parts under heating and pressure. Many household appliances, building products, daily necessities, etc. use this method to obtain metallic luster or wood grain patterns.
How to operate
Plastic processing and molding
The commonly used methods for processing thermoplastics are extrusion, injection molding, calendering, blow molding and thermoforming, while the commonly used methods for processing thermosetting plastics are compression molding and transfer molding.
Mechanical processing of plastics
Uses the plastic processing methods of metals and wood to manufacture plastic products with very precise dimensions or in small quantities. It can also be used as an auxiliary process for molding, such as sawing of extruded profiles. Because the properties of plastics are different from those of metals and wood, plastics have poor thermal conductivity, low thermal expansion coefficient and elastic modulus. When the clamp or tool is pressurized too much, it is easy to cause deformation, and it is easy to melt when heated during cutting, and it is easy to adhere to the tool. Therefore, when plastics are mechanically processed, the tools used and the corresponding cutting speeds must adapt to the characteristics of plastics. In addition, plastics can also be cut, punched and welded by lasers.
Joining
There are two methods of joining plastic parts: welding and bonding.
Surface modification of plastic processing
Plastic processing hot stamping is to transfer the colored aluminum foil layer (or other patterned film layer) on the hot stamping film to the workpiece under heating and pressure.
Related faults
In plastic machinery processing, various plastic processing failures are often encountered. What kind of professional terms are more accurately expressed in the plastic machinery industry ? Let’s analyze the professional terms of plastic processing failures one by one.
1. Under-injection: In plastic processing, the appearance of the plastic part is incomplete due to insufficient filling of the cavity.
2. Flash: During the plastic processing and molding process, the remaining material overflows into the gap between the mold surfaces and remains on the plastic part.
3. Weld mark: A linear mark on the surface of a plastic part. It is caused by the divergence and convergence of several streams of material in the mold during injection or extrusion. The molten materials are not completely fused at the interface and cannot be fused into one. This causes a fusion mark, which affects the appearance quality and mechanical properties of the plastic part.
4. Wave flow marks: Due to the improper flow of the molten material in the mold cavity, the surface of the plastic part will produce annual ring-shaped, spiral or cloud-shaped wave-shaped uneven defects.
5. Surface turbidity: refers to cracks with gaps on the surface of plastic parts and the resulting damage. The phenomenon of cracks on the outside or inside of plastic parts due to long-term or repeated application of stress lower than the mechanical properties of plastics is called stress cracking; the phenomenon of sudden and complete rupture of plastic parts due to constant load at a certain temperature for a certain period of time is called stress cracking; the cracks and ruptures of some thermoplastic parts due to excessive exposure to high temperatures are called thermal stress cracking.
6. Compression cracking: refers to the obvious cracks in one or several layers of reinforcing materials on the outside of the laminated plastic that can be seen through the resin layer covering the surface.
7. Wrinkle cracking: A defect in which the surface of laminated plastic is cracked and obviously separated.
8. Wrinkles: During the plastic processing process, creases or wrinkles appear on one or more layers of the surface of the plastic part.
9. Cracking and whitening: The more obvious fine cracks on the surface of plastic parts are called cracks, and the frost-like fine cracks similar to cracks are called whitening. Cracking and whitening are both fine cracks without cracks. When plastic parts are exposed to a certain chemical environment or under stress conditions, environmental stress cracking will occur.
10. Silver streak: Needle-shaped, silvery-white, frosty fine lines on the surface of plastic parts along the direction of material flow.
11. Streaks: Linear streak defects existing on the surface or inside of plastic parts.
12. Marking: Dark mica flake-like spots on the surface of plastic parts due to poor dispersion or mixing of pigments and other reasons.
13. Orange peel texture: An appearance defect on the surface of plastic parts that is as uneven as orange peel.
14. Cell streaks: refers to the cell layer in foam plastics that is very different from its inherent cell structure.
15. Black spots: During the plastic processing and molding process, the molten material overheats and decomposes under high temperature and high pressure conditions, resulting in black carbonized spots on the surface of the plastic parts.
16. White spots or bright spots: In transparent or translucent plastic films, sheets or plastic parts, there are particles that are not fully plasticized. When light is transmitted through them, white spots can be seen. These spots are called ” fish eyes “. If the material is opaque or colored, these spots are called white spots or bright spots.
17. Pits: Regular or irregular small pits appearing on the surface of plastic parts, usually with a depth and width roughly the same.
18. Filler spots: Obvious spots in plastic parts caused by the presence of fillers such as wood powder or asbestos.
19. Dark spots: Dark stains that appear in laminate structures with fabric as the base material.
20. Burnt and scorched spots: During the plastic processing, under high temperature and high pressure molding conditions, the molten material carbonizes due to overheating decomposition. The carbonized charred material is mixed into the molten material, forming defects such as scars on the surface and inside of the plastic parts.
21. Bubbles: During the plastic processing and filling process, if a large amount of gas remains in the melt, or the air in the mold cavity is not completely exhausted, defects such as small volume or a series of pores will be formed inside the plastic part after molding.
22. Vacuum bubbles or dark bubbles: When plastic parts are cooled and solidified during plastic processing, due to the different cooling speeds inside and outside, sometimes the outer surface has been cooled and solidified, but the inside is still in a hot melt state. Once the center part cools and shrinks, vacuum holes will be generated inside the plastic part. These holes are generally called vacuum bubbles or dark bubbles, also known as shrinkage holes.
23. Pinhole: A pinhole -sized hole defect in plastic sheets or films.
24. Cell collapse: A defect in foam plastics caused by the destruction of the cell structure during the manufacturing process, resulting in increased local density.
25. Depression and shrinkage: During the cooling process of plastic processing, the surface of the plastic part is cooled and solidified first, and the inside or thick wall part is cooled and solidified later. When the volume shrinks, the shrinkage speed inside and outside is inconsistent. The surface of the plastic part is stretched by the inside to form depressions, resulting in shallow pits or depressions.
