Plastic products can be seen everywhere in our daily life, and different plastic products are made through different processing techniques. Plastics manufacturing is a process of making plastic into semi-products or products with practical value. Usually, plastics manufacturing includes primary processing and secondary processing of plastics.
Basic Knowledge of Plastic Materials:
Plastic raw materials are solid or elastomers at room temperature, and the raw materials are heated during processing to make them a fluid, molten liquid. Plastics are classified into "thermoplastic plastics" and "thermosetting plastics" according to their processing characteristics.
"Thermoplastic" can be heated and shaped many times, and it is recyclable. Its fluidity is like mucus and is a slow melting state. Commonly used are PE, PP, PVC, ABS, etc.; "Thermosetting" plastics will solidify after heating. The molecular chains form a chemical bond and become a stable structure, so even if heated again, it cannot reach a molten liquid state, such as epoxy resin and rubber.
So....What Are the Common Plastics Manufacturing Processes?
The followings are some common types of plastic processing processes and details:
Plastics Casting (Dip Molding, Slush Molding, Rotational Molding)
Plastic Thermoforming (Compression Molding, Vacuum Forming)
Plastic Injection Molding
Plastics Welding (Friction Welding, Laser Welding)
Not only metal can be casted, but plastics can also be. Pour the liquid plastic material into a mold, cure it at room temperature or low temperature, and then take out the finished product to produce a smooth surface object, which is also often called casting or plastics casting. The materials used in plastics castings are acrylic, phenolic resin, Polyester, and Epoxy. It is often used to make hollow products, plates, etc., and the extended plastic processes include Dip Molding, Slush Molding, and Rotational Molding.
(1) Dip Molding:
Soak the high-temperature mold in the molten plastic liquid, then slowly take it out, dry it, and finally peel off the finished product from the mold. The speed of removing the mold from the plastic needs to be controlled. The slower the speed, the thicker the plastic layer. This process has cost advantages and can be produced in small quantities. It is commonly used in the production of hollow objects such as balloons, plastic gloves, hand tool handles, and medical equipment.
(2) Slush Molding:
Pour the molten plastic liquid into a high-temperature mold to make a hollow product. After the plastic forms a layer on the mold, the excess material is poured out. After the plastic solidifies, the mold can be opened to take out the parts. The longer the plastic stays in the mold, the thicker the shell. This is a relatively high-degree-of-freedom process, and can produce more complex shapes and obtain good appearance details. Commonly used car interiors made of PVC and TPU, such as the surface of the instrument panel and doorknob.
(3) Rotational Molding:
Put a certain amount of plastic molten liquid into the heated two-piece closed mold, and then rotate the mold to distribute the material evenly on the mold wall. After solidification, the mold can be opened to take out the finished product. In the process, air or water is used to cool down, and the finished product must have a hollow structure, and because of the rotation, the finished product has a soft curve. In the beginning, the amount of plastic liquid determines the wall thickness, and it is mostly used to make axially symmetrical circular objects, such as pottery flower pots, children's play equipment, lighting equipment, water tower equipment, and so on.
Blow molding is also called hollow molding. The middle of the machine is covered with a blowing device. When the raw material is heated, the plastic that passes through the die will be extruded, and it will appear as a hollow tube strip. When the two molds are closed, they will cut off the remaining material, and then send air to the mold. At this time, the plastic will fill up the entire inner wall of the mold like a blowing balloon, and finally, the finished product will be taken out after cooling. The materials used in blow molding are ABS, LDPE, HDPE, PP, PC, PS, etc., and the products are mostly various bottles and cans, and liquid-filled parts used in automobiles and medical treatment.
Plastics extrusion is also called extrusion molding. After heating and softening the thermoplastic material, it is extruded into the molding die for molding. The plastic will form a cross-section consistent with the die. After a long section of extrusion, it is cooled, and then according to the need to be cut to the required length. Plastics extrusion equipment have low cost and are widely used. Most of them adopt automated production, mainly used to produce pipes, plates, rods, films, water pipes, parts, plastic plates, plastic strips, cables, etc.
Plastic thermoforming is to place material with uniform thickness in a mold, heat it to soften the material and cover it on the surface of the mold, and then extrude it with an external force. After the cooling stage, it solidifies to obtain a finished product, which can be subdivided into hot press molding (compression molding) and vacuum molding. The difference lies in the different ways of applying pressure.
(1) Compression Molding:
Compression molding is a method in which plastic granular materials are placed on a heated mold and formed by a downward pressure mold at the same high temperature. It is also called hot compression molding and compression molding. The wall thickness of the finished product depends on the gap between the molds. When the molds are separated, the finished product will be pushed out by a thimble. Because there is no pouring port and runner system in the process, raw materials are not likely to be lost, and any plastic materials can be formed, such as PF, MF, UF, and EP. Post-processing removes unnecessary places. Compression molding is commonly used to make thermosetting plastic products and plastic products with glass fiber reinforced materials. Among them, it is especially suitable for objects with simple shapes, no inner chamfers, and thickness, such as heavy parts, sockets, cups, and plates, etc.
(2) Vacuum Forming:
Vacuum forming can also be called vacuum thermoforming. After the plastic sheet is evenly heated to soften, the machine will contact the plastic sheet with the mold, and draw the air out to form a vacuum state, allowing the plastic sheet to tightly cover the mold for molding. Wait for the finished product to cool down. Because vacuum forming is a low-pressure process, there are few restrictions on the mold material, and it can be considered to damage the mold when there is a barbed shape. Vacuum forming technology is suitable for proofing and mass production. Wood and plaster can be used in small quantities, while epoxy resin or wear-resistant aluminum can be used as molds for mass production. All thermoplastic plastic sheets can be used, the most common are PS, ABS, acrylic, PC, common applications such as baking utensils, bathtubs, packaging materials, furniture, car interiors, etc.
Plastic Injection Molding:
In plastic injection molding, granular raw materials are fed into the machine, then molten plastic is injected into a stainless-steel mold with high temperature and high pressure, and then the temperature is lowered, and the finished product is ejected with a thimble. The range of materials and applications that can be used is wide because steel molds can produce complex, high-precision, and diverse finished products, but the relative cost of mold opening is also high, so the output is usually increased to share the mold cost. Plastic injection molding can produce a wide range of items, including daily necessities, auto parts, medical equipment, electronic products, baby toys, and so on.
Plastic welding is the process of using heat to melt thermoplastics to join plastic objects. The methods are divided into contact and non-contact, contact types such as vibration friction welding, non-contact types such as ultrasonic, laser, infrared, gas convection welding, etc.
(1) Friction Welding:
Under certain pressure, vibration amplitude, and frequency, two plastic objects quickly rub against each other to generate heat, so that the plastic is in a molten state on the joint surface, and then the two are clamped for joint action. Suitable for thermoplastic materials and semi-crystalline resins (such as HDPE, PP, TPO), common applications such as car intake manifolds, instrument panels, car lights, and other aerospace applications. Usually, the molten plastic will flow out from the joint to produce flash, but this kind of process does not require high-cost molds to achieve a good air-tight joint effect.
(2) Laser Welding:
When laser welding, the two objects must be superimposed together. The upper layer is the object that can be penetrated by laser light. When the laser light is irradiated, it will penetrate the upper layer and illuminate the lower layer to absorb the laser light. The lower surface melts and conducts heat to the upper object. In the process of joining, clamps are used to determine the tightness between objects, and similar plastics have a high degree of connection stability.
The difference between plastic foam molding and other moldings is that the material needs to be expanded first, and then the material is poured into the mold for molding. Most thermoplastics and thermosetting plastics can be processed into foamed materials, such as PUR, EPS, PVC, EPE, EPP. The spherical raw material will be thermally expanded to 40 times its original size through pentane and hot steam. The material needs to be allowed to stand for about 12 hours before it can be heated to the injection mold or the injection machine, and the particles are fused and molded at high temperatures. It can greatly reduce the weight of the finished product, has the effects of buffering, thermal insulation, electrical insulation, and sound absorption. Plastic foaming is suitable for mass production. Plastic foaming can be processed in a wide range of sizes and ranges, such as fruit packaging, surfboards, bicycle helmets, car interiors, etc.