Welcome to the world of plastic injection molding – where you can create medical syringes, electrical switches, keyboards, bottles, and everything in between.
Did you know that according to Grand View Research, the revenue of the plastic injection molding market is forecasted to reach a whopping USD 397.6 billion in 2030?
In this article, we will discuss what is injection molding for beginners, which materials are best for injection molding, its types, how injection molding is performed, and much more.
Keep reading our ‘injection molding 101’ guide to become a pro at plastic injection molding!
What is Injection Molding?
Injection molding is a type of molding process that produces large volumes of parts by injecting a heated liquid resin into a mold. The material is then allowed to get cool to a solid state to form the desired shape.
It is the least expensive manufacturing process for bulk production as it only requires the initial investment for the tooling; the price per individual piece is much lower than other manufacturing processes.
The process is similar to making a pudding, except you would have to inject the powdered gelatin into a mold in an injection molding machine.
Since this manufacturing process results in good precision and produces less mechanical waste, it’s ideal for the medical industry, the automotive sector, and the electronics industry.
What is Plastic Injection Molding?
Plastic injection molding is a manufacturing process that produces large volumes of plastic parts and products. In this process, plastic pellets are melted and injected into a metal mold, which cools and solidifies into the desired shape.
The molded piece is then ejected (either manually or automatically).
The most common molding is Thermoplastic injection molding, where the material injected is thermoplastic. There is another molding type where the silicone rubber uses thermoset plastics and the final piece is manufactured by a chemical reaction.
It is expected that the injection molded plastics market will see a 4.6% increase in its compound annual growth rate (CAGR) until 2028.
History of Plastic Injection Molding
The brothers Isaiah and John Hyatt patented and invented the first molding machine in 1872. Initially, the machine was built on a simple molding mechanism and was only used to produce simple parts such as hair combs and mold buttons.
Later, German scientists Arthur Eichengrün and Theodore Becke improved the molding process by creating less flammable solutions in 1903. The solutions were then converted into a powdered form that was easier to inject.
After the invention of thermoplastics, the plastic injection molding industry saw some revolutionary changes, later followed by the growing needs of World War II.
What Products Are Made From Plastic Injection Molding?
Injection molding can produce parts with a variety of finishes and textures, making it possible to create parts that are both functional and aesthetically pleasing.
Here is a list of industries and their respective products where injection molding is commonly used:
- Automotive: Plastic injection molding is used to manufacture car dashboards, interior body, engine components and parts of the electronic system. Automotive parts require complex shape-making, and injection molding serves well for that.
- Medical: Syringes, tubing, surgical equipment, and medicines packaging; all are made by injection molding. It is an ideal manufacturing process for the medical industry as it offers precision and accuracy, the primary requirement for patient safety and care.
- Consumer Products: Plastic injection molding is a common and cheap manufacturing process for producing kitchenware products, household appliances, toys, and much more.
- Packaging: Injection molding is used worldwide for packaging materials, including bottle caps, food containers, jars, etc. The surface finish achieved by the injection molding is of excellent quality, making it a perfect option for packaging.
- Aerospace: Interior and exterior parts of spacecraft are made by an injection molding process. Durability is the main concern when developing parts for the aerospace industry, but injection molding solves it by providing high-strength, lightweight parts that can withstand the harsh conditions of space travel.
- Military and Defense: Manufacturing companies can create components for military equipment such as weapons, communication devices, and artillery. The molded material is durable enough to withstand impact forces.
- Furniture: Injection molding is commonly used in the furniture industry to create plastic parts, such as chair and table legs, door handles, racks, and knobs.
- Energy and Utilities: Solar panels, wind turbines, and electrical components; all of them require protection from environmental factors like temperature, moisture, and chemicals.
- Construction: Injection molding is an integrated manufacturing process for the construction industry. It can create various products, such as PVC pipes, fittings, and electrical conduits.
- Sports Equipment: You can create high-precision parts for the sports industry with the help of plastic injection molding. Examples: protective padding, injection molded helmets, golf balls, and tennis rackets.
Components of Plastic Injection Molding Machine
Some of the main parts of a plastic injection molding machine include:
- Injection unit
- Clamping unit
- Ejector system
- Control system
- Heating system
- Cooling system
What is the Plastic Injection Molding Process?
A typical plastic injection molding cycle takes between two seconds to two minutes. Here are the four stages a plastic injection molding process goes through:
Clamping is the very first step of plastic injection molding. Molds for plastic injection are manufactured in two halves (clam-shell style pieces). Before injecting the molten liquid, the two mold halves are closed by a ‘clamping unit’ so the liquid doesn’t escape between them.
Machines that need more clamping power take more time for the clamping process, increasing the overall production time per single piece.
After the mold is clamped, the ‘hopper’ feeds the plastic pellets (or granules) into the injection system. These pellets are conveyed through the machine barrel and heated up through temperature and compression until melted into a complete liquid.
You should maintain the temperature of the liquid throughout the process, as abrupt changes can cause defects in the final piece. In some machines, pressure may be applied directly to the mold so that it fills all the mold cavities adequately.
The molten plastic is injected into the mold cavity through the screw once the injection unit is attached to the mold. The amount of plastic injected into the mold is the exact portion that will become part of the final piece, which is also called a ‘shot.’
The time required to fill a shot is called ‘injection time.’ It’s hard to calculate injection time as the flow of molten plastic changes with every cycle. A ‘hydraulic unit’ controls the injection pressure, which is typically between 35-140 MPa.
As soon as the plastic makes contact with the interior mold surfaces, it starts to cool down. The molten plastic starts to convert into a solid form by taking the shape of the mold.
Since the liquid’s density is less than solid, there might be a little ‘shrinkage’ during solidification.
The cooling shouldn’t be too fast as it may cause warping or distortion of the final piece.
The total time of solidification is called ‘cooling time,’ and it varies according to the molten material, mold material, and walk thickness of the final piece. Once the cooling time is completed, the mold can be opened to proceed to the next stage.
The final stage of the plastic injection molding machine is ejection. The clamping motor will open the mold into two halves. The ejection unit in place pushes the final product out of the mold with the help of ejecting force.
Without the ejector unit, the final part won’t move out of the mold as it shrinks after the solidification and often sticks to the interior surface.
Here are the following post-processing steps done on the final piece removed from the mold:
- Gate Trimming: In many cases, excess material, known as flash or runners, will be present on the part after molding.
- Deburring: When the molten material solidifies in the mold, parting lines or gaps around the ejection pins, it causes ‘burrs’ (a type of defect) on the surface. This can be removed manually using hand tools or through automated processes such as tumbling, sandblasting, or vibratory finishing.
- Ultrasonic Welding: Using ultrasonic welding, the plastic molded parts may be joined without the need for fasteners, bolts, or adhesives.
- Secondary Operations: The final part may be required to be painted, plated, heat staked, and assembled, or there may be a need for the addition of fasteners or inserts.
Types of Plastic Injection Molding
There are different types of plastic injection molding, including:
1. Standard Molding
This is the most basic type of plastic injection molding in which a single color mold and material are used to produce the final piece.
Bottle caps, toys, and beverage bottles are common applications of this type of molding.
Overmolding is a manufacturing process where two or more materials are molded together to create a single part. In this type of molding, a two-step process is followed, First, the base material ‘substrate’ is produced.
Then a second material is molded, usually a soft thermoplastic elastomer (TPE), over the first part. Toothbrushes, electrical connectors, and some medical devices are manufactured by overmolding.
3. Insert Molding
Insert molding is similar to overmolding – The only difference is that, In insert molding, the insert is placed into the mold before the plastic material is injected, while in overmolding, the insert is placed on top of a previously molded substrate or base part.
Insert molding is commonly used in the manufacturing of products such as electrical connectors, automotive components, medical devices, and consumer goods.
4. Two-Shot Molding
The two-shot molding (also known as ‘dual-shot molding’) process typically involves injecting one material or color into a mold cavity to create the first shot, then immediately injecting a second material or color into the same cavity to create the second shot.
Thus, instead of two molds, two-shot molding does the job with a single mold only.
Two shot molding process can create parts with complex geometries, multiple colors, or different materials, all in a single molding cycle.
5. Gas-Assisted Injection Molding
In gas-assisted injection molding, a pressurized gas (typically ‘Nitrogen’) is injected into a mold cavity after the plastic material has been injected. The gas pressure helps maintain a smooth finish for the final piece.
The gas is then vented out of the mold, and the part is allowed to cool and solidify.
6. Thin Wall Molding
Thin wall molding is a manufacturing process that produces plastic parts with walls that are thinner than 1mm. This process involves the use of specialized injection molding machines that are capable of injecting plastic material at high speed and pressure.
Common applications can be plastic lids, syringes, cell phone components, etc.
What Plastic is Used for Injection Molding?
There are endless possibilities for injection molding materials.
Let’s take a look at some important examples:
1. Acrylonitrile Butadiene Styrene (ABS)
Acrylonitrile Butadiene Styrene (ABS) is a widely used thermoplastic polymer known for its toughness, impact resistance, and heat resistance against various chemicals and oils.
However, it’s not recommended for use under direct sunlight.
It is made from three different monomers: acrylonitrile, butadiene, and styrene. ABS is commonly used in plastic injection molding due to its ease of processing, good dimensional stability, and low cost.
- Wheel covers
- Phone adaptors
- Keyboard keys
2. Polyetherimide (PEI)
Polyetherimide (PEI) is a high-performance engineering thermoplastic known for its excellent mechanical, thermal, and electrical properties. It has the potential to replace metals in several applications.
It is used where high tolerances and low warp distortions are needed.
- Lamp sockets
- Throttle bodies
3. Polycarbonates (PC)
Polycarbonate (PC) is a transparent and amorphous thermoplastic polymer that is known for its high impact strength, heat resistance, and optical clarity. It is comparatively more expensive than other plastic materials.
You shouldn’t use it for the food and beverage industry as it’s bad for the human body.
- Mobile devices
4. Acrylic (PMMA)
Acrylic, also known as polymethyl methacrylate (PMMA), is another transparent and rigid thermoplastic polymer that has good weather resistance and scratch resistance.
It can be dyed and colored so you can make great aesthetic effects while using it. Acrylic is also safe for food storage as it is BPA-free. Hence, it’s an ideal choice if you belong to the food industry.
- Solar panels
- Lighting applications
- Decorative showcases
5. Polystyrene (PS)
There are two types of Polystyrene (PS) used in injection molding machines; High Impact Polystyrene (HIPS) and General Purpose Polystyrene (GPPS). They are cheap and therefore, a great option for bulk production.
HIPS is less brittle than GPPS and is opaque while GPPS is more transparent.
6. Nylon (PA)
Nylon (PA) is a strong and durable synthetic polymer that is widely used in plastic injection molding due to its high strength, noise dampening, toughness, and abrasion resistance.
Although Nylon has many industrial applications, it has very high shrinkage limits. Thus, you have to keep necessary allowances while molding Nylon products.
- Electrical connectors
7. High-Density Polyethylene (HDPE)
High-density polyethylene (HDPE) is a strong and durable thermoplastic polymer that is widely used in plastic injection molding due to its high strength-to-density ratio, chemical resistance, and ability to withstand high temperatures.
It’s affordable and reliable – therefore, used by a variety of consumer goods producers. However, it’s highly flammable and non-biodegradable.
- Recycling bins
- Shampoo bottles
- Flower pots
8. Low-Density Polyethylene (LDPE)
Low-density polyethylene (LDPE) is a flexible and durable thermoplastic polymer in comparison to HDPE. Due to its soft nature, it has poor temperature capability, making it highly flammable near fire.
- Plastic bags
- Plastic wraps
9. Polyoxymethylene (POM)
Polyoxymethylene (POM), also known as acetal, is a strong and rigid thermoplastic polymer that is widely used in plastic injection molding due to its high stiffness, low friction, dimensional stability, and opaque nature.
It’s naturally white in color and has good lubricity. It’s one of the plastics whose properties remain the same at low temperatures.
- Lock systems
10. Thermoplastic Polyurethane (TPU)
TPU is known for its flexibility, durability, and resistance to wear and tear. Due to its tough nature, it might be too hard for some mold products.
It’s good for the ozone-exposed environments and has the adequate load-bearing capability. A disadvantage is that it has a short shelf life.
- Inflatable rafts
11. Thermoplastic Rubber (TPR)
It is technically a mixture of rubber and plastic – which allows it to have tremendous weather and chemical resistance. However, it would lose its rubbery properties at very high temperatures. It’s also known as an elastomer.
It’s a non-toxic type of plastic. Hence, it’s safe for the environment.
- Wires and cables
12. Polypropylene (PP)
Being the second most commonly used plastic in the world, Polypropylene is a major part of the food and packaging industry.
PP is a semi-crystalline polymer with a high melting point, which allows it to be molded into various shapes and sizes. Its flexibility allows it to bend several times without breaking.
- Storage containers
- Power tool bodies
Benefits of Plastic Injection Molding
Plastic injection molding has been dominating as one of the most used manufacturing processes in the world. There are several reasons behind it:
Industries require bulk production for sustainability and to remain competitive. This allows manufacturers to sell their goods at a lower cost. This is where plastic injection molding comes in.
There is only an upfront investment needed for the tools and injection molding system. Once it is set up, the recurring costs can be significantly reduced.
By using injection molding, you can automate your manufacturing tasks and produce large quantities of parts quickly and efficiently, which leads to economies of scale and lower production costs per unit.
Plastic injection molding produces low waste, which saves you heaps of money overall. Furthermore, this waste can be recycled by melting it and running it through the molding process again. So there will actually be no waste at all.
Since the process is highly precise and accurate, it ensures that there is very little waste. The parts manufactured through plastic molding are nearly finished. Hence, they often don’t need any additional materials.
Another advantage of plastic injection molding is that it can produce very complex shapes that can’t be produced by other manufacturing methods (without producing tons of waste).
This includes producing parts with intricate features such as undercuts, hollow spaces, thin walls, and complex geometries.
When you’re using plastic injection molding, the room for error is minimal. Despite making complex structures, it produces parts with consistent quality and size. The tolerances are minimal and the final piece requires very little post-processing.
This results in a good manufacturer reputation and increased positive feedback among customers.
All you need is a good quality control unit that can ensure that the final product meets the required specifications.
Improved Surface Finish
You’ll find an improved surface finish (texture and appearance of the outer layer of the part) by using injection molding.
If you want to improve the surface finish of your part, consider slow injection speeds and longer cooling times. It will significantly reduce the likelihood of warping or distortion.
Disadvantages of Injection Molding
While the advantages of injection molding are countless, it comes with some disadvantages as well. Here are some of them:
High Upfront Costs
If you don’t have an upfront investment, it will be difficult to set up injection molding plants. Also, if you don’t need mass production, this method won’t be cost-effective for you.
The upfront costs are directly proportional to the complexity of your object.
You should select an experienced manufacturer like Kemal that knows how to make an injection mold efficiently.
Once the mold is designed and manufactured, it cannot be easily modified or adjusted. This limits the flexibility of the manufacturing process, particularly for smaller changes or customization.
It’s difficult to select the best material for your job out of many options. Some may be more difficult or expensive to mold. This can limit the material options for certain applications.
Kemal offers a wide range of plastic materials, including commodity plastics, engineering plastics, and specialty materials such as thermosets and elastomers. We can advise you on the pros and cons of each material option, helping you make an informed decision.
What Do Injection Molding Manufacturers Do?
Injection molding manufacturers specialize in the production of plastic parts and products made by the injection molding process.
Here are some of the key tasks that injection molding manufacturers typically perform:
- Mold Design
- Mold Fabrication
- Part Design
- Material Selection
- Injection Molding Operation
- Quality Control
Kemal’s Custom Plastic Injection Molding Services
We hope you liked our injection molding guide. If you have any questions, don’t hesitate to contact us!
Kemal is the leading injection molding manufacturer that offers creative solutions for all your injection molding needs. Whether you need mold design and fabrication, material selection, injection molding, quality control, or post-processing – We have got you covered.
With 28+ years of experience and 2000+ companies served, we are committed to delivering top-quality products and services that meet your specific product requirements.
Contact us today to learn more about their injection molding services and how we can help you achieve your production goals.