How to choose plastic molding process for your product?

Different plastic materials are being used to produce different objects in the manufacturing industry nowadays, and the manufacturing of each part needs a unique plastic molding process that can form the part on basis of its specs. Before you produce a plastic product, you need to determine the specific plastic molding process that would best address your production needs. In this article, we offer a general overview on 5 different plastic molding types, their strengths and specific applications.

Blow Molding – applicable for production of hollow items, e.g. bottles

Blow molding is often applied to glass blowing, in which a parison (heated piece of plastic, usually a tube) is air inflated. The plastic material is pushed by air against the mold to get the desired shape. After cooling, the plastic part will be ejected.

This molding process is mainly used to produce one-piece hollow items in a high volume. It is the right process for you to make a lot of plastic bottles. The blow molding process is able to create thin-walled containers in a very consistent and yet economical way.

Compression Molding – applicable for production of larger items, e.g. automotive spare parts

As its name indicates, in this molding process, the heated plastic material will be placed in a heated mold, which will compress it into the desired shape. Though often coming in sheets, the plastic material can also be in bulk. The integrity of the final product is maintained by curing (heating). Like other molding processes, after the part is shaped, it will be ejected. In the circumstance where plastic sheets are used, the plastic material needs to be trimmed in the mold before part removal.

Compression molding is designed to deal with high-strength compounds, e.g. thermosetting resins, fiberglass and reinforced plastic materials. In the compression molding process, the application of the materials with outstanding strength properties makes it the most appropriate process for the production of auto parts.

Extrusion Molding – applicable for production of long hollow objects, such as tubes, pipes and straws

Different from other molding processes which get the plastic materials into a mold through extrusion, extrusion molding, as its name indicates, extrudes the molten plastic into a die, instead of a mold, to determine the shape of the final product.

The extrusion molding process forces the molten plastic material through a die to get the desired shapes with a fixed section. Many different shapes can be obtained through this process efficiently. Due to the fact that the plastic material needs to be melted and then solidified, only thermoplastics is strong enough to undergo the extrusion process. After cooling, the final extruded part can be cut or rolled for easier shipment.

Injection Molding – applicable for production of high-quality, high-volume parts

Among all molding types, injection molding by far serves the most versatile purposes. Rated on basis of tonnage or pressure, the presses applied in the process differ in size, among which, the larger ones can be used for production of auto parts, while the smaller ones can be applied to produce very precise parts, such as those for surgical purposes. Besides, a lot of different plastics and additives can be used to increase the flexibility of injection molding for designers and engineers.

With regard to the process itself, it is very straightforward, but a lot of enhancements and customization technologies are added for production of parts with the desired surface finish and structure. The injection molds are generally made from steel and designed with cavities to form the part. During this process, molten plastic is injected into the mold, and filled into mold cavities. After cooling, the part will be ejected by ejector pins.

In this method, the cost of the mold itself is relatively high, but the cost of every part is rather low. Cost effective part, diversified resin and finish options have made the injection molding process a popular technique in the present manufacturing industry.

Rotational Molding (AKA Rotomolding) – applicable for production of large, hollow, one-piece items

Rotational molding coats the mold interior to form the desired part by using high temperatures and rotational movements. In this process, the mold rotates constantly to create the centrifugal force that is used to form even-walled parts. Though not a very fast moving process, it is an ideal method for manufacturing of large hollow containers, e.g. tanks. In addition, it is a more cost effective way for production of particular parts, when compared with other molding types. In this very process, little material is wasted and excess material can be recycled, making it not only economical, but also environmentally friendly.

 

Every plastic molding method has its unique advantages and disadvantages. Designers and engineers need to keep these differences and options in mind when planning for the most appropriate solution. Mold makers should be able to provide more in-depth insights into what best fits a particular project, be it applications or materials.

Different plastic molding process for plastic product

Plastic refers to the numerous organic synthetic materials that are mostly polymers of high molecular weight. Currently, it is one of the 3 synthetic materials that are indispensable for our day-to-day life, while the other 2 are synthetic rubber and synthetic fiber. Natural or synthetic resin, the main ingredient of plastic, is often mixed with a variety of additives, so that the material can be molded into desired shapes under certain temperature and pressure conditions, and maintain the shape at room temperature. As a high molecular compound, plastic molding can be processed with various methods, each of which is a blend of pros and cons, and suitable for specific applications. Now, let’s see how plastic is molded through some relevant interesting animations.

Injection Molding

Injection molding is a parts manufacturing process by injecting plastic molding material into a mold, of which the main method applied is injection molding. In this process, the plastic is loaded into a hopper, which will then feed the plastic into a heated injection unit. After being pushed through a long chamber with a reciprocating screw, the plastic material is melted into fluid. An injection nozzle is located at the end of the chamber, through which molten plastic is forced into a cold, closed mold. After the plastic is cooled and solidified, the semi-finished product will be ejected from the mold.

Plastic Extrusion

Plastic extrusion is a process for massive production, in which the plastic material is melted into a continuous shape. The extrusion process is usually applied to produce films, continuous sheets, pipes, profiles, bars, coating wires, filaments, cords and cables, etc. Together with the injection molding machine, the dried plastic material is loaded into a hopper and fed into a long heating chamber. At the end of the chamber, the material is forced out from a small opening or into a mold that’s in the shape of the desired product. As the plastic leaves the mold, it will be cooled down on a conveyor belt. A blower is sometimes involved in this process to facilitate cooling.

 Blow Molding

Blow molding process is applied to produce hollowed plastic products. Making use of air pressure, this post-forming technology blows the rubbery state parison in a closed cavity into a hollowed product.

Thermoforming

Thermoforming is a special manufacturing process which shapes the thermoplastic material into various products. The plastic sheet fixed on a frame is heated, softened, and then pressed against the molding surface under an external force, so as to mold it into the desired shape. After being cooled and trimmed, the desired product is obtained. This process is also applicable to rubber manufacturing. In recent years, thermoforming has witnessed all-new progress, e.g. the continuous production technology from sheet extrusion to thermoforming.

The 2nd animation shows the thermoforming process that involves 2 plastic sheets.

Compression Molding

Compression molding is the most commonly applied process for thermosetting materials. Usually, it is not applied to thermoplastics. In this process, the material is compressed into the desired shape. Plastic molding powder and other additives are added into the mixture to create special properties. When the mold is closed and heated, the material will be hardened to form the desired shape. The appropriate temperature, pressure and time duration applied in the process is dependent on the desired result.

Calendering

Calendering is a finishing process of heavy leather. When heated, the plasticity of the fiber is utilized to smoothen textile surface or create fine-grained parallel diagonals on the surface, so as to improve textile glossiness. After feeding, the material is heated and softened, then shaped into a sheet or film, and finally cooled down and rolled up. The most commonly calendered material is PVC.

Pultrusion

Pultrusion refers to the process where the blank is put under an uneven pressure from 3 different directions, and then extruded from mold opening or crevice, so as to reduce the sectional area and increase the length for production of the desired product. This blank forming process is referred to as pultrusion.

Vacuum Forming

Often known as suction forming, vacuum forming is a plastic processing technology by which a hard plastic sheet is vacuum-compressed against the mold surface after being heated and softened. After cooling, the desired product is produced. This process is widely applied in industries like plastic packaging, lighting, advertising, decoration and more.

Rotational Molding

Also known as rotational moulding or rotomolding, the rotational plastic molding process firstly feeds the plastic material into the mold, which will keep rotating along the 2 vertical axes while being heated. Under the influence of both gravity and heat, the plastic inside the mold will gradually and evenly spread over and stick to the entire surface of the mold cavity. After cooling and ejection, the desired product will be made.

 

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