Injection molding is when you heat up plastic pellets until they melt, then shoot the melted plastic into a mold with a machine, and let it cool down to make a plastic thing. Injection molding is a good way to make plastic stuff because it’s fast, it’s accurate, and you can make all kinds of different things with it, like car parts, phone cases, and food containers.

Ⅰ.Understand Injection Molding

1.1 Plastic Injection Molding Definition

Injection molding, also known as injection molding, is a molding method that combines injection and molding. The advantages of the injection molding method are fast production speed and high efficiency, the operation can be automated, there are many designs and colors, the shapes can be from simple to complex, and the sizes can be from large to small, and the product dimensions are accurate, the products are easy to replace, and can be made into complex shapes Parts, injection molding is suitable for mass production and complex-shaped products and other molding processing fields.

1.2 Historical Background and Evolution Process

In 1868, Hyatt developed a plastic material he named celluloid. Celluloid had been invented in 1851 by Alexander Parks. Hyatt improved it so that it could be processed into finished shapes. Hyatt and his brother Isaiah registered the patent for the first plunger injection machine in 1872. This machine was relatively simple compared to those used in the 20th century. It operated basically like a giant hypodermic needle. This giant needle (diffusion barrel) injected plastic into the mold through a heated cylinder.
In the 1940s, World War II created a huge demand for cheap, mass-produced products. In 1946, American inventor James Watson Hendry built the first injection molding machine, which allowed for more precise control of injection speed and quality of produced items. This machine also enables material mixing before injection, so that colored or recycled plastics can be thoroughly mixed into the virgin material. In 1951, the United States developed the first screw injection machine. It did not apply for a patent, and this device is still in use.

In the 1970s, Hendry went on to develop the first gas-assisted injection molding process and allowed the production of complex, hollow products that cooled rapidly. This greatly increases design flexibility as well as the strength and endpoints of manufactured parts while reducing production time, cost, weight and waste.

Ⅱ.Plastic Injection Molding Process

2.1 Mold design

The injection mold is mainly composed of molding parts (the parts that make up the cavity in the moving and fixed mold parts), gating system (the channel through which the molten plastic enters the mold cavity from the injection machine nozzle), guide parts (when the mold is closed, it can be accurately aligned), ejection mechanism (the device that pushes the plastic out of the cavity after the mold is separated), temperature adjustment system (to meet the mold temperature requirements of the injection process), exhaust system (to remove the air in the cavity during molding and the volatile gases of the plastic itself are discharged out of the mold, often with exhaust grooves on the parting surface) and supporting parts (components used to install, fix or support molded parts and other mechanisms), sometimes with lateral parting and core pulling mechanism.

2.1.1 Design Steps of Injection Mold

1. Preparation Work before Design
(1) Design brief.
(2) Understand plastic parts, including their shapes, how they are used, and the materials they are made from.
(3) Check how plastic parts are made.
(4) Know the model and size of the injection molding machine.

2. Develop a Molding Process Card
(1) Know the product, including a drawing, weight, thickness, area, size, and if there are any special features or parts.
(2) Know the plastic used in the product, including the name, model, maker, color, and if it needs to be dried.
(3) Know the main technical details of the injection molding machine, including the size of the machine and the mold, the type of screw, and the power.
(4) Know the pressure and distance the injection molding machine uses.
(5) Know the injection molding conditions, including the temperature, pressure, speed, and clamping force.

3. Injection Mold Structure Design Steps
(1) Figure out how many cavities you need. Think about how much plastic you can inject, how much force you can use to clamp the mold, how accurate the product needs to be, and how much money you want to spend.
(2) Decide where the mold will split. The mold should be simple, easy to take apart, and not mess up how the plastic part looks or works.
(3) Figure out how to arrange the cavities in the mold. Try to make it balanced if you can.
(4) Decide how the plastic will get into the mold. This includes the main channel, the runners, the gates, and the cold slug well.
(5) Decide how to get the plastic part out of the mold. Different parts of the mold will have different ways to get the plastic part out.

(6) Decide how to control the temperature. The temperature control system depends on what kind of plastic you’re using.
(7) Figure out how to machine and install the die or core using an insert structure. Split the inserts into sections and put them in at the same time.
(8) Figure out how to get rid of the air. Usually, you can use the mold parting surface and the gap between the ejection mechanism and the mold. But for big and fast injection molds, you need to design ways to get rid of the air.
(9) Figure out how big the injection mold should be. Use the formulas to figure out how big the molded parts should be and then figure out how thick the side wall of the mold cavity should be, how thick the bottom of the cavity should be, how thick the core pad should be, how thick the movable template should be, how thick the cavity plate of the modular cavity should be, and how tall the injection mold should be.

(10) Use a standard mold base. Use the main dimensions of the injection mold that you designed and calculated to pick out a standard mold base for the injection mold and try to pick out standard mold parts.
(11) Sketch out the mold structure. It’s important to draw a complete structural sketch of the injection mold and draw the mold structure diagram.
(12) Check the mold and injection machine dimensions. Check the parameters of the injection machine used: including the maximum injection volume, injection pressure, clamping force, and the size of the mounting part of the mold, the mold opening stroke and the ejection mechanism.
(13) Review the mold structure design. Do a preliminary review and get the user’s consent. At the same time, confirm and modify the user’s requirements.
(14) Create the mold assembly drawing. Clearly show how the parts fit together, the dimensions you need, the part numbers, the schedules, the title blocks, and the technical requirements for each part of the injection mold (the technical requirements include things like how the mold should be built, like how the ejector system should work, how the slides should work; how the mold should be put together, like how the parting line should fit together, how the top and bottom of the mold should line up; how the mold should be used; how the mold should be treated to keep it from rusting, how the mold should be numbered, how the mold should be engraved, how the mold should be sealed with oil, how the mold should be stored; and any testing or inspection requirements you have).

(15) Make drawings of mold parts. Take apart the mold and make drawings of the parts in this order: start with the inside, then the outside; start with the complicated parts, then the simple ones; start with the parts that shape the product, then the parts that hold everything together.
(16) Look at the design drawings. The last thing you do when you design an injection mold is look at the design drawings again. This time, you look at them to make sure you can make the parts.

2.2 Material Selection

2.2.1 Plastic Material Types Commonly Used in Injection Molding

1. Polypropylene (PP)
Polypropylene is a common plastic material that is widely used in plastic injection molding. It is lightweight, resistant to acids and alkalis, and has a low density. It also has excellent wear resistance and impact resistance. Because the melting temperature of polypropylene is relatively low, it is necessary to pay attention to controlling the melting temperature and injection pressure during the injection molding process to avoid problems.

2. Polyamide (PA)
Polyamide is a high-performance plastic material with high strength, high toughness, and high wear resistance. Therefore, it is widely used in the automotive, aerospace, electronics, and other industries. When injecting polyamide, it is necessary to inject it at a higher melting temperature, so it is necessary to pay attention to controlling the injection temperature and time to avoid problems such as material burning.

3. Polyurethane (PU)
Polyurethane is a great plastic material with great wear resistance, oil resistance, UV resistance, etc., so it’s used a lot in industry, construction, and other fields. When you’re doing injection molding with polyurethane, you need to inject it at a higher temperature, and you need to control the injection pressure and time so you don’t have problems with the material separating from the mold.
Besides polypropylene, polyamide, and polyurethane, there are a lot of other kinds of plastic materials that you can do injection molding with, like polyethylene (PE), polycarbonate (PC), etc. Different plastic materials have different things you need to pay attention to when you’re doing injection molding, and you need to adjust things based on the specific situation.

2.2.2 Factors Affecting Material Selection

Physical Properties. The physical properties of a material (like strength, hardness, toughness, corrosion resistance, etc.) determine how it performs when you use it. For example, if you need a part that can handle heat, take a hit, or has to be really strong, you need to pick a material with the right physical properties.

Chemical Properties. The chemical properties of the material (like whether it’s easy to rust, whether it can handle acid or base, etc.) are also important, especially if the part is going to be in an environment where it might react with chemicals.

Production Cost. The cost of making stuff is also a big deal, including the cost of the stuff you start with and how hard it is to make the stuff.

Eco-friendly. When you’re thinking about sustainability and saving the planet, it’s important to pick stuff that’s good for the planet.

2.3 Mold Clamping

Mold closing is the first step in the injection molding process. The opening and closing of the mold of the injection molding machine are completed by the mold closing system. When closing the mold, it can provide reliable clamping force to the mold to withstand the huge opening force of the mold caused by the high-pressure injection and filling of the molten plastic during the injection molding process.

2.4 Injection Molding (Injection/Filling)

The injection process usually has three steps: feeding, plasticizing, and injecting. At a certain temperature, the fully melted plastic material is mixed by the screw and injected into the mold cavity with high pressure. (Note: Temperature control and pressure control.)

2.4.1 Temperature Control

1. Barrel Temperature
During the injection molding process, the temperatures that need to be controlled include barrel temperature, nozzle temperature, and mold temperature, etc. The first two temperatures mainly affect the plasticization and flow of plastic, while the latter temperature mainly affects the flow and cooling of plastic. Each plastic has a different flow temperature. The same plastic has different flow temperatures and decomposition temperatures due to different sources or grades. This is due to different average molecular weights and molecular weight distributions. Plastics in different types of injection molding have different flow temperatures and decomposition temperatures. The plasticizing process in the machine is also different, so the temperature of the barrel is also different.

2. Nozzle Temperature
The nozzle temperature is usually a little lower than the maximum temperature of the barrel. This is to prevent “drooling” that can happen with a straight-through nozzle. The nozzle temperature can’t be too low, or the molten material will solidify too soon and clog the nozzle, or the solidified material will get injected into the mold cavity and mess up the product.

3. Mold Temperature
Mold temperature has a great impact on the intrinsic performance and apparent quality of the product. The mold temperature depends on the crystallinity of the plastic, the size and structure of the product, performance requirements, and other process conditions (melt temperature, injection speed and pressure, molding cycle, etc.).

2.4.2 Pressure Control

The pressure during the injection molding process includes plasticizing pressure and injection pressure, and it directly affects the plasticization of plastic and the quality of the product.

1. Plasticizing Pressure (Back Pressure) :
When using a screw injection machine, the pressure on the melt at the top of the screw when the screw rotates and retreats is called plasticizing pressure, also known as back pressure. The size of this pressure can be adjusted through the relief valve in the hydraulic system. In injection, the size of the plasticizing pressure remains unchanged with the screw speed. Increasing the plasticizing pressure will increase the temperature of the melt, but will reduce the plasticizing speed. In addition, increasing the plasticizing pressure can often make the temperature of the melt uniform, the color materials can be mixed evenly, and the gas in the melt can be discharged. In general operations, the plasticizing pressure should be determined as low as possible while ensuring excellent product quality. The specific value varies with the type of plastic used, but it usually rarely exceeds 20kg/cm².

2. Injection Pressure
In current production, the injection pressure of almost all injection machines is based on the pressure exerted by the plunger or the top of the screw on the plastic (converted from the oil line pressure). The role of injection pressure in injection molding is to overcome the flow resistance of the plastic from the barrel to the cavity, give the molten material a filling rate, and compact the molten material.

Injection pressure is divided into injection pressure and holding pressure, usually 1 to 4 stages of injection pressure and 1 to 3 stages of holding pressure. Generally, the holding pressure is smaller than the injection pressure and should be adjusted according to the actual plastic material used to achieve the best result, physical properties, appearance, and size requirements.

2.5 Cooling and Solidification

When it comes to injection molding, injection cooling is one of the most important factors that determine the quality of the molding and the efficiency of production. The principle of injection molding cooling is mainly based on the effect of the cooling medium on the plastic parts. The cooling system of injection molding usually uses flowing media, such as water or oil. When the cooling medium flows through the cooling water channel of the injection mold, it takes away the heat in the mold through heat transfer, so as to quickly cool the plastic part. The injection molding cooling system can also adopt different cooling methods, such as horizontal heating and oblique heating, according to the specific shape and material of the mold, so as to achieve the best cooling effect.

2.5.1 Cooling Process and its Importance

1. Reduce the Temperature of Plastic Parts
Plastic parts are made by injection molding, which means injecting hot plastic into a mold. When the plastic is injected into the mold from the injection molding machine, it gets hot in some places and cools down in others. So, the mold has hot spots at different temperatures. If you don’t cool them down, the plastic will get too hot in some places and not hot enough in others. This causes problems like the plastic shrinking and getting warped. Cooling the mold quickly cools down the hot spots to the right temperature. This makes the plastic cool down more evenly. It also makes the plastic parts have less stress inside them. This gives you more time to stop the plastic parts from getting warped.

2. Speed Up Cooling
When the plastic flows back into the mold, the heat of the injection mold gradually dissipates along the direction of the plastic flow. So, the front section of the flow will not fit along the long axis. Do you think so? The latter part will gradually cool down and solidify. The injection molding cooling system can increase the spread of mold temperature in a timely manner, make the temperature distribution uniform, and accelerate the cooling speed. This allows the plastic to be shaped faster, shortens the mold opening cycle, and improves production efficiency.

3. Avoid Deformation
If the plastic part cools unevenly after injection molding, it will cause stress inside the plastic part, which will cause deformation or even rupture. Through injection cooling, the internal temperature of the plastic part can be made uniform, and the internal stress can be reduced, so as to avoid deformation of the plastic part and improve the stability and reliability of the product.

2.5.2 Effect of Cooling Rate on Final Product

Injection cooling is a method to control the dimensional accuracy of injection molded parts. During the injection molding process, the molten plastic will cool and solidify quickly after being injected into the mold. The cooling process is very important for the dimensional stability of the injection molded parts. By designing the cooling system properly, the injection molded parts can shrink evenly during the cooling process, which ensures that the dimensional accuracy of the final product meets the requirements.
The cooling of injection molding has a great influence on the performance and quality of injection molded parts.

The performance of injection molded parts is often closely related to factors such as its crystallization structure and molecular chain orientation, and these factors are related to the cooling rate of the injection molded parts. By controlling the cooling rate of the injection molded parts, the degree of crystallization and molecular chain orientation of the injection molded parts can be adjusted, thereby improving its mechanical properties, heat resistance, chemical resistance, etc., and reducing the internal stress of the injection molded parts and improving its overall performance. quality.

2.6 Ejection of Molded Parts

The ejection principle of the injection molding machine is to eject the injection molded products from the mold through mechanical force or air pressure for further processing. Generally, a mechanical ejection mechanism is used to drive the ejection plate through steel balls, springs or cylinders to eject the product.

2.6.1 Common Problems and Solutions

1. Ejection Force is Too Small or Too Large
If the ejection force is too small, the product will not be ejected from the mold. If the ejection force is too large, the product may be damaged or deformed. In this case, you need to adjust the ejection force, air pressure, or ejection mechanism.

2. Product Sticking to the Mold
If the product sticks to the mold, it may be because the mold is not cooled enough or the ejection time is too short. You need to increase the ejection time or improve the cooling.

3. Incomplete Ejection of the Product
If only part of the product is ejected, you need to increase the ejection force and check whether the mechanical structure or ejection cylinder is normal.

Ⅲ.Application of injection molding

Injection molding is a common plastic processing method that is widely used in various fields. It produces plastic products by injecting molten plastic into a mold and then cooling and hardening it into the desired shape. Injection molding has the advantages of fast molding speed, high production efficiency, and high product precision. Therefore, it is widely used in automobiles, electronics, home appliances, medical equipment, and other fields.

1. Automotive Industry
Injection molding is used to make plastic parts with complex shapes, such as dashboards, door panels, and center consoles. It meets the automotive industry’s requirements for part quality, appearance, and performance, and it costs less to produce, so automakers like it.

2. Electronics Industry
As electronic products get better and better, the requirements for their cases get higher and higher. Injection molding can make fine, high-precision cases for electronic products like cell phones and computer keyboards. It meets the electronics industry’s requirements for appearance quality, part precision, and production efficiency, and it helps electronic products keep getting better.

3. Home appliance Industry
Home appliances usually need to be durable and look good, and injection molding can do that. Injection molding can make home appliances with complicated shapes and structures, like washing machine shells, refrigerator handles, and so on. Injection molding can make sure home appliances are good quality and work well, and make them more competitive.

4. Medical Device Industry
Medical devices have to be really safe and clean, and injection molding can do that. Injection molding can make products that meet medical device standards, like syringes, IV sets, and so on. Injection molding can make sure medical devices are good quality and work well, and keep patients healthy.

Ⅳ. Advantages and Disadvantages of Plastic Injection Molding

4.1 Advantages

1. High Production Efficiency
Injection molding process can be highly automated, reducing reliance on labor and improving production efficiency.

2. High Precision and Good Repeatability
Due to the precise control of parameters such as temperature, pressure, and speed during the injection molding process, the product has high dimensional accuracy and good repeatability, making it suitable for mass production.

3. Wide Applicability
Injection molding is suitable for processing a variety of materials, such as thermoplastic materials, thermosetting materials, rubber, etc., and can meet diverse product needs.

4. Energy Saving and Environmental Protection
Injection molding equipment is usually powered by electricity, which is more energy-saving and environmentally friendly than traditional machines, and the waste materials can be recycled.

5. Cost Advantage
Injection molding is suitable for mass production and can reduce the cost of a single product.

4.2 Disadvantages

1. Molds are Expensive
Injection molding needs precise molds, so you have to spend more money at the beginning.

2. Long Processing Cycle
Injection molding needs to cool and shape, so it takes a long time. This affects efficiency and makes the process longer.

3. High Technical Requirements
Injection molding needs skilled operators and technical knowledge. It’s also hard to maintain and fix.

4. Limitations on Scope of Applicatio
Injection molding can’t make complex or big things. It’s better for small things.

Conclusion

Yo, so check it out. The global plastic injection molding market is gonna get bigger. It’s supposed to go from 4.56 million tons in 2023 to 5.95 million tons in 2028. The main reasons why the injection molding market is growing are because more people want to use injection molding in cars and more people want to use it in packaging. Also, more people want to buy stuff and more people want to buy electronics. That’s why the injection molding market is growing. But, it’s expensive to get into the injection molding market and there are other ways to make stuff like 3D printing. That’s why the injection molding market isn’t growing as fast as it could.

But the move toward making lighter, more electric vehicles and new uses in healthcare could bring more bright spots for the growth of the plastic injection market. Plastic injection molding offers a range of solutions, from bulk packaging to thin-walled container and bottle molds. These solutions are widely used in various end-user industries for packaging purposes. In addition to offering a versatile packaging solution, it also reduces plastic consumption and proves to be ideal both economically and ecologically.