What is Plastic Injection Molding?

Plastic injection molding is one of the main and most popular production processes in our modern days. Manufacturing is a highly flexible technology that can be used for the production of many kinds of plastic components (from the smallest gears and clips to the largest automotive panels and housings) with high accuracy, high speed, and efficiency. 

To begin with, the thermoplastic material is melted and injected into a prefabricated mold in order to cool down and solidify into the final part. As such, plastic injection molding is pivotal in a wide range of industries, from automotive, electronics, medical devices, packaging, and consumer goods, to just name a few.

History of Plastic Injection Molding Process

Plastic injection molding has a history that starts back at the end of the 19th century. In 1872, John and Isaiah Wesley invented the first injection molding machine in order to inject celluloid to make things like combs and buttons. It was archaic for us in the era, but the start of a revolution.

The scope of injection molding expanded wholesale to electrical insulators and to handles and other durable products with the advent of new synthetic polymers in the early 20th century, for instance, Bakelite. 

But the injection molding rise had hardly begun until the post WWII years and even the 1940s. 

During war, the requirement to develop military equipment and consumer goods, rapidly and cheaply, at less than the cost, and at increased speed led to better machines, more efficient processes, and hence the development of plastics of higher performance.

The reciprocating screw injection molding machine was a major step forward in the 1950s with better mixing, melting, and consistency than the plunger- type machines of the past. This enabled larger and more complex parts.

However, with the passage of time, companies have made their own great contribution to improving the technology behind injection molding. GV Mold, being a market leader, has helped manufacturers achieve a new level of performance and quality in applications like automotive, healthcare, and many other industries by using precision tooling, computer-aided design (CAD), and advanced materials.

Types of Injection Molding

There are various types of injection molding. Let’s talk about a few: 

Conventional Injection Molding

Conventional injection molding is the type most commonly used and handles most of the molded plastic parts used every day. In this technique, the resin is melted, and thermoplastic resin is injected into a closed mold to shape it. Most repeatable, simplest, and at the same time, most scalable process, which is essential for mass production of various packaging components, automotive clips, household items, etc.

Multi-Shot Injection Molding

The second type of injection molding is called multi-shot, two-shot, or multi-component molding, wherein two or more materials are injected into the same mold in successive stages. The parts that are made by this technique are made of multiple colors (such as a toothbrush, which has a hard plastic core and a soft rubberized grip) or materials or properties. The use of multiple materials in one process leads to more efficiency, fewer assembly requirements, more functionality of the product, and increased product aesthetics.

Insert Injection Molding

During insert molding, the preformed components (metal inserts, fasteners, bushings, etc.) are combined into the plastic part. The integrated components produced by this process have increased strength, conductivity, or functionality. Electrical connectors, threaded parts, and medical devices are common insert molding products. This process helps reduce the secondary assembly needs and make the final product more durable.

Gas-Assisted Injection Molding

Gas-assisted molding is a process where an inert gas (mostly nitrogen) is injected through the molded part while molding and hollows out portions of the part during the injection. 

With the gas pushing the molten plastic against the walls of the mold, a hollow section is created, and less material is needed. It is suitable for large, thick-walled parts, where weight must be reduced and warp must be prevented, but the surface quality must be improved. In use in automotive bumpers, handles, furniture frames, and appliance applications.

Steps Involved in Plastic Injection Molding Process

Step 1: Designing the Mold and Product

Injection molding is successful based on the product design and the mold design. Using sophisticated CAD software, engineers and designers use the software to create 3D models of the product; wall thickness, draft angles, ribs, and undercuts are all accounted for. 

Besides these, the mold should also have to consider the number of cavities, the cooling systems, gate types, venting, ejection mechanisms, etc. This step helps predict the flow patterns, cooling behavior, and possible defects prior to the mold being created.

Step 2: Selecting the Appropriate Thermoplastic Material

The right choice of the right material is necessary to achieve the required performance, durability, and cost. The strength, flexibility, temperature resistance, and chemical compatibility of the selected material must match your requirements. 

Due to various functional requirements, environmental conditions, regulatory standards, and aesthetics, the material selection is a critical step. When the materials are chosen right, the end product can have the right heat-resistance and impact-resistant, while still being biocompatible and sterilizable, etc.

Step 3: Preparing and Conditioning the Raw Material

The raw thermoplastic pellets need to be conditioned, especially dried properly, to be compatible for the injection molding process. Nylon and PET are among a group of many hygroscopic resins that can help absorb excess moisture from the air. 

Not drying the moisture sufficiently during molding will result in the moisture becoming steam, which will cause cosmetic as well as structural defects. Desiccant dryers or vacuum ovens dry the pellets, wo that they can be consistently processed by the manufacturers.

Step 4: Feeding and Melting the Thermoplastic

In this step, the machine hopper is loaded with conditioned pellets, which feed into a heated barrel. Material is conveyed forward by a rotating screw, which melts it gradually through friction as well as external heat. Temperature control is precise across many heating zones to ensure that the plastic gets to the correct melt viscosity to fill the mold without degrading the material.

Step 5: Injecting the Molten Plastic into the Mold

As the screw advances in this process, it forces the molten plastic at high pressure into the enclosed mold cavity. The cavity has to be fed as fast and as completely as possible, without voids, weld lines, or incomplete filling, taking into account the injection pressure and speed. In some cases, the mold has to withstand pressures of over 10,000 psi.

Step 6: Packing and Holding Under Pressure

After the mold is filled with the material, more pressure is applied against the moving mold to tightly pack the material into the cavity (to make up for the shrinkage of the plastic as it cools). It is important to have a holding phase in order to achieve dimensional accuracy and to prevent sink marks, especially in thicker parts.

Step 7: Mold Opening and Part Ejection

Finally, the mold opens, and ejector pins push the part out of the cavity once it is cool enough and has solidified. In some systems, robotic arms or air blasts assist removal. For complex or cosmetic parts, ejection must be well controlled, to avoid damage to the delicate features or surfaces.

Step 8: Trimming, Finishing, and Quality Inspection

After ejection, the parts usually undergo secondary processing, e.g., cutting off material, cutting off excess sprues or runners, and surface finishing when required. In the meantime, dimensional accuracy, surface defects, color consistency, or functional performance, are checked through automated or manual inspections. 

Advantages of Plastic Injection Molding

The dominance of plastic injection molding in manufacturing industry can be explained by its numerous advantages:

• Cost efficiency at scale: Once a mold is made, additional parts are very cheap, and that is what makes this ideal for high-volume production.

• Unmatched precision and repeatability: Modern machines and molds produce parts of consistent tolerances.

• Material versatility: A vast amount of thermoplastics and additives (such as glass fibers, UV stabilizers, flame retardants, etc.) can be used to tailor parts to specific needs.

• Fast production cycles: Cycle times are measured in seconds or minutes, making a rapid throughput to meet tight production schedules.

• Low labor requirements: Automated systems involving much less manual handling, assembly, and inspection result in a much lower production costs.

• Sustainability potential: Today, recycled materials are incorporated in many processes, and mold design innovations have significantly reduces waste and energy consumption. These advancements have made injection molding a very sustainable process.

These advantages have made it quite clear why plastic injection molding is a cornerstone of industrial revolution today.

GV Mold, a leader in injection molding solutions, produces automotive components, electronic housings, and molds with cutting-edge equipment, expert engineering, and strict quality control to fulfill the most accurate specifications.

Conclusion

Over time, plastic injection molding has transformed to become the foundation of many production processes around the world. Incredible parts and high precision would not be possible without this; it has enabled the cost-effective production of modern life, cars, medical devices, smartphones, laptops, and so on. The potential of injection molding will only grow as new materials, technologies, and practices of sustainability are released.