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Plastic Injection Molding Process - Start Manufacturing in China

Plastic injection molding process in China – Guide to understanding the process along with what to consider for getting started successfully in China

Vaughn Cook RockWell Window Wells
Editorial Team
April 7, 2021

Plastic injection molding is one of the most versatile and effective manufacturing methods used in China. Over the years, the industry has grown by leaps and bounds. China has the ability to produce excellent quality products at a fraction of the cost which will allow you to create high-quality goods within your budget. Here’s a look at everything you need to know about the plastic injection molding process. We will also look at why Chinese plastic injection molding companies are the most reliable.

Plastic injection molding is a manufacturing process used to produce high volumes of the same object. By this method, plastic is melted and then force-injected into a mold cavity via high pressure. After the molten plastic takes the desired shape, it is then cooled until hardened to obtain the molded product.

The benefits of plastic injection molding

Plastic injection molding can be used to create a variety of products, including toys, one-piece chairs, storage containers, plastic parts, and most types of plastic products available today.

The process is useful when you are required to mass-create a product of particular dimensions and mold complexly-shaped products of any size.

Another advantage of plastic injection molding is that it can be used to create both metal and non-metal inserts.

It is possible to create thin-walled parts with high precision and accuracy that other molding processes cannot imitate.

The injection molding cycle itself is short, has high production efficiency, and the entire process is automated.

The final product of the plastic injection molding process is stable.

The plastic injection molding machine maximizes raw material use, unlike traditional manufacturing processes that cut away substantial amounts of the original plastic. There is minimal wastage in the plastic injection molding process.

Manufacturing components and other plastic products in China via the plastic injection molding process is very cost-effective. The machines are highly sophisticated and can produce a large quantity of the product without compromising on quality.

The molds are reusable and the process is very repeatable which means that the same product can be produced repeatedly without sacrificing the quality of the product. As a result, you can guarantee your consumers get the same high-quality parts time and again.

5 Types of injection molding technology

1. Thin-wall molding

This technology is used to create plastic parts consisting of very thin walls – for example, electronics, tubes, and test apparatuses.

2. Gas-assisted injection molding

Thick plastic injection-molded parts run the risk of distortion as they cool. Gas-assisted injection molding technology shoots gas into a polymer-filled injection mold, allowing the plastic on the outside of the mold to remain smooth and have a finished appearance. In contrast, the inside remains porous/hollow.

3. Metal injection molding

This technology is used to injection mold metals. It is commonly used in the cell phone market to shield the electronics from micro and radio waves.

4. Liquid silicone injection molding

This technology is used when you need the part created to withstand extremely high temperatures or chemical agents. For example, some car parts and medical devices. These make use of liquid silicone.

5. 3D Printing

3D Printing technology plays an important role in prototyping an injection molded part.

How Plastic Injection Molding works

The plastic injection molding process requires three components – raw plastic material, a mold, and a plastic injection molding machine.

  • Creating the mold

The first step to creating a product via the plastic injection molding process is to design a mold. This requires a great deal of skill, precision, and time. Design engineers undertake the process of creating the mold. Various computerized versions of the mold design are created. Each version includes the pattern, logo, finish, graphics, and layered textures.

Once the final design has been decided, it is crafted according to the final product’s exact dimensions. The molds are made of steel to ensure they can withstand the heat and pressure associated with mass production.

Molds can consist of a single cavity, or they can have multiple cavities. In multiple cavity molds, each cavity can either be identical or unique to form multiple shapes or parts during a cycle.

The molds are made of strong, durable, and heat resistant materials like stainless steel and aluminum. While aluminum molds are cost-effective, they are not typically suited to produce high volume products or parts that comprise narrow dimensional tolerances.

Plastic injection molding process - Spray bottle
Injection molded parts can be found everywhere. Make sure the correrct material is used for the application.
  • Choosing the material

While there are several thousands of plastic materials available, some are better than others. Choosing the right plastic affects the molding operation and the performance of the final product. Here is a list of some of the best plastic injection molding materials and where they can be used.

Acrylic: Used to create transparent parts. Examples of this would include transparent walls, windows, and lighting equipment. It can be used as an alternative to glass. Acrylic possesses high tensile strength, is scratch and weather resistant.

Nylon (polyamide): Used to produce mechanical parts like bearings, gears, and bushings. Nylon is a very strong material and melts only at a very high temperature. It is resistant to chemicals and has a good wear.

Polycarbonate: This is a very durable material and is much stronger than acrylic. It can be used for compact discs.

Polyoxymethylene (POM): Can be used to make automotive parts and mechanical parts that are typically made of metal. The material is rigid, tough, and strong. It is used to make products like ball bearings, knife handles, fasteners, and gears. It is also highly resistant to solvents.

Polystyrene (PS): Both General Purpose Polystyrene (GPPS) and High Impact Polystyrene (HIPS) can be used in the plastic injection molding process. While GPPS is transparent, HIPS is opaque. Hard cases for toolboxes and power tool bodies are some of the products that can be made with this material.

Polyethylene (PE): This lightweight material is one of the most common materials used in the plastic injection molding process and is known for its high elasticity, electrical insulating properties, and its elasticity. Although it is not especially strong, it is ideal for making various bottles, trash cans, and plastic bags.

Polypropylene (PP): This material is commonly used in the packaging and food storage industries. It does not degrade even when it is washed with hot water. It is tough, elastic, and highly durable.

Acrylonitrile Butadiene Styrene (ABS): This opaque material is very tough and resistant to scratches, breaking, and tearing. It is inexpensive and is commonly used in keyboards, phone adapters, and wall socket plastic guards.

Thermoplastic Polyurethane (TPU): The material is soft and pliable but also very strong. It is used to create products that require rubber-like elasticity, like mobile phone covers, power tools, and sporting equipment.

Thermoplastic Rubber (TPR): The material is a mix of plastic and rubber and therefore exhibits both these materials’ properties. It is used in products like headphone cables and medical catheters.

  • Testing

Before a product is mass-produced by the plastic injection molding method, the machine used for the process is tested to ensure all the various parts are in working condition and functioning properly. The mold and the polymer mixture are also tested to ensure they are the right fit.

A pre production mold is used to ensure that it is perfect. If it is not, adjustments are made.

The testing process cannot be skipped as the quality of the final product depends on it.

The six stages of plastic injection molding

The process of injection molding undergoes six stages. The stages have to be completed in order and determine the quality of the final product.

Stage 1: Mold closing

The process begins by placing the final mold inside the plastic injection molding machine.

The mold is divided into two halves that are attached to platens (steel clamps). A hydraulically powered clamping unit pushes the two halves of the mold together. It exerts enough force to ensure the mold is closed tight when the molten plastic is injected into it.

Stage 2: Feeding

Once the machine is started, the chosen plastic pellets or polymer granules are fed into the hopper which is a large container with an open bottom. The pellets or granules move from the hopper into a barrel, which contains a heating mechanism. Feeding the pellets or granules into the hopper can be done either automatically or manually.

A reciprocating screw slowly moves the plastic pellets forward by way of sliding and rotating movements. The reciprocating screw is usually hydraulically operated. As the material enters the grooves of the screw, heating takes place through friction and by using high amounts (10,000 to 30,000) of pressure and heat.

Once the granules have melted, the molten plastic is very quickly injected through a nozzle and fed into the mold cavity.

Stage 3: Pressure holding

The buildup of pressure packs and holds the molten plastic material within the mold. Holding pressure is consistently maintained to compensate for any material shrinkage.

Stage 4: Cooling and solidifying

At this point the plastic is allowed to cool and harden. The molten plastic within the mold begins to cool as soon as it makes contact with the mold’s interior surfaces. As it cools, the plastic solidifies in the shape of the mold.

Stage 5: Mold opening

During the cooling time, the mold remains closed and cannot be opened. The cooling time depends on the thermodynamic properties of the plastic material. Once it has hardened, the platens are opened up.

Stage 6: Demolding

The product is removed from the mold by an ejection mechanism that pushes the product out of the mold. Sometimes, a mold release agent is sprayed on the mold cavity’s inner surfaces before the injection of the plastic to help facilitate easy mold removal. Once the product has been removed, the mold can be clamped again for the next injection.

The entire cycle can take anywhere between 2 seconds and 2 minutes. The process can be repeated as many times as required, ensuring consistently high quality molds.

Some considerations regarding plastic injection molding

When considering to produce a part or product via the plastic injection molding process, keep the following things in mind:

1. Financial considerations

Entry cost: While plastic injection molding is a cost-effective process, bear in mind that it requires a large initial investment.

Production quantity: Determine the number of pieces produced at which plastic injection molding becomes most cost-effective for you. Also, determine the number of pieces that can be produced for you to break even on your investment. Consider all the costs – including the costs associated with designing, testing, production, assembly, marketing, distribution, and expected price for sales. Ensure you have a good margin.

2. Design considerations

Part design: Ensure that your final design takes into account the injection molding process. By simplifying the shape, geometry, and number of parts, you can be more successful in creating a high-quality product.

Tool design: Ensure the mold tool is designed so that it does not cause defects in the product.

3. Production considerations

Cycle time: Minimize the processes/cycle time as much as possible. A well-designed tool and excellent hot runner technology can reduce the time drastically. Even small changes that might not seem necessary at first, can reduce your cycle time and save you a lot of money when producing several thousands of parts.

Assembly:  Minimize assembly by designing thoughtfully.

How to choose a reliable China plastic injection molding company

Some important factors must be considered when choosing a China plastic injection molding company. For instance, consider what you want the company to help you achieve. This could include precision or any other critical factor that will help you improve your production activities.

Consider punctuality for a strong turnaround in the manufacture of your products.

Keep in mind that a trustworthy China sourcing agent must be accustomed to working with various global clients. This can ensure your plastic molds and parts can be sent easily to any location around the world.

Conclusion

The world of manufacturing is rapidly changing, noticeably due to the advancements in technology. Thanks to the efficiency of the highly streamlined Chinese plastic injection mold companies, molding plastic is now quite efficient and cost-effective. From hydraulic injection molding machines to all-electric injection molding machines – China offers the best choice of machines. There are several other advantages as well and by choosing China to manufacture your plastic injection molding parts you will reap the benefits of low costs, high quality, quick transportation, and reliable delivery.


Sourcing Allies is a team of expert China sourcing agents that has helped western customers manufacture and source products from low-cost regions since 2006.

For more on China sourcing visit our website or write to us at info@sourcingallies.com.

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