Plastic Injection Molding: A Complete Guide

Have you ever wondered how mineral water companies get their 3d printed bottles? Or how biscuit companies get packets for their products? The article brings forth a thorough discussion about a procedure that has revolutionized the world of plastics. It’s called the plastic injection molding process.

About Plastic Injection Molding?

Starting with the first question- what is injection molding?

Plastic injection molding definition, in simple terms, is the industrial process of creating mass identical plastic products by following certain injection moulding design guidelines. In today’s world, it is one of the most used ways for creating a number of similar plastic products at a time using CNC machines.

Plastics have been an integral part of our lives. From simple household utensils to flying planes, humans use plastics as raw material for products everywhere, and plastic injection moulding is the easiest way of getting an abundance of custom plastic consumer products.

How Does Plastic Injection Molding Work?

Following are the basic steps of creating injection molded plastic products and parts:. 

  1. The process starts with the material selection process and placing the common plastic resins in an injection molder. 

2. The machine has a heated barrel or hot runners where the plastic material melts. 

3. After that, the liquid is injected into the mold with a single cavity (which is inside the device as well) using a mold injection where it is turned into a specific shape using injection molding design. The cycle time, injection time, wall thicknesses, injection pressures, and other parameters are fixed according to the design for manufacturability.

Plastic Injection Molding Machine

Injection molding machines are a critical part of the entire molding process. These machines have various components and different configurations like the vertical and the horizontal configuration. That said, the common components in any injection molding machine include the following:

1. The Injection Unit: 

This part of the injection moulding machine works for heating as well as injecting the resins inside the plastic injection mould. The unit has various parts like a hopper, the barrel, ram injector, reciprocating crew, the screws, and the nozzle. The hopper is a large container with an open bottom that feeds the material to a barrel. The barrel, with a ram injector, forces the injection moulding material forward through a heated section. Reciprocating screws are used to propel the injection molding materials forward. The resins are then forced to enter the grooves in the screw and move towards the plastic injection mold. The heating channel melts the resins and the molten material gets injected into moulds using a nozzle at the end. 

2. The Clamping Unit:

Moulds are usually created in two halves. Just prior to the injection process, the injection molds are clamped together. Each half needs to be fixed to the platen, which is a large plate-like structure. Once the injected molten resins are allowed the desirable cooling time, the molds are opened using a clamping motor. 

The rear part of the mold has an ejection system with an ejection bar that pushed the solid plastic part out of the open cavity. 

3. Mold Base:

A mold has two main parts: the mold core and the mold cavity. These two are together fixed to the platens of a mold base. The mold base on the front part has the support plate, the sprue bushing and a locating ring that aligns the mold base with the nozzle.

The rear part of the mold base has an ejection system as well as a support plate. The ejection system has an ejection bar, as mentioned earlier. As the name suggests, it helps in pushing the solidified plastic injection molds out of the mold cavity. 

6. Mold Channels:

Mold channels are the pathways that the plastic resins take to flow into the mold cavities. These channels are integrated into the mold design to create a single piece. 

The molten plastic enters through a channel called the sprue and then goes through other channels called the runners. The runners lead the plastic resin through a gate which throws the molten resin into the cavity. 

Cooling channels are also sometimes created to allow the water to circulate along the mold walls and fasten the process of cooling the molten plastic. 

7. Mold Design and Tooling:

Various other design issues need to be considered while creating a mold, in addition to the mold base and mold channels mentioned above. The plastic injection mold design needs to include complex geometries and features like undercuts. These units are also known as slides or side actions since they are made to slide into the part cavity. Some devices may also be inserted into the injection mold along the parting direction.

Additionally, the draft angle applied to the mold walls is an important part of the injection moulding design. 

Let us understand more about injection mold tooling. Custom tooling includes molds made of steel or aluminum. Each metal has its own advantages and is used depending on the type of plastic part required. Mold core and Mold Cavity are the two main components of the part cavity. The space between these two (when the mold is closed) creates the part cavity which is injected with molten plastic resin.

8. Machine Specifications:

There are various specifications common to a plastic injection molding machine. 

The injection molders are usually classified by the tonnage of the clamp force that they offer. This is further determined by the pressure of injecting the molten resins. Materials requiring high injection pressure require high tonnage. 

Other important specifications to consider in a plastic injection molding machine include the shot capacity, the clamp stroke, mold thickness, and the platen size. 

Detailed Working of Injection Molding Machines

An injection molding machine consists of four major parts- the feed hopper, screw, heated barrel, and the mold cavity.

Injection molding involves two basic steps:

  • Melt Generation by a rotating screw.
  • The propelling movement of the screw helps to fill the mold cavity with the molten resins which passes the injected resin under high pressure.

A hopper and runner system is used to place the raw plastic resins into a heating barrel carefully. 

There’s a reciprocating screw inside the barrel which creates heat with a frictional force with heater bands. The plastic melts there. 

As the machine is still moving (with the help of the screw), the plastic is then injected through a nozzle from where it gets inside a mold cavity through a gate system. It almost looks like a tunnel. 

Finally, we get the desired shape of the plastic resins, which cools as it is inside the cavity.

While the plastic is inside the cavity, the machine applies a holding pressure. If you didn’t know, particles expand when they heat. 

When the molded plastic inside the mold cavity cools down, its volume will decrease as well, so we need some pressure to compensate for that.

Ejector pins are used to take the material out from the machine after it solidifies. 

As the screw moves, another batch of granular plastic is fed to the heating barrel. The process continues from there, and the ejector pins help remove the injection mold plastics after it cools and hardens.

Types of Plastic Injection Molding

There are generally three types of injection molding machines: 

1. Hydraulic Injection Molding

These are the oldest type of injection molding machines which have been around for centuries. 

Pressure creating liquids called hydraulic oils are used to create pressure during the injection molding process. This oil is used inside a hydraulic motor, which in turn rotates the screw.


One of the major considerations in the hydraulic injection is creating leak-free systems for the molding process. The hydraulic oil often leaks inside the plastic, which creates irregularities. It hence required to consider seal design carefully to cover the motor.

The technique has been present for years and is still operated by various factories in the world. The relatively lower cost and ease of maintenance of these injection molding machines have been the two major factors for people being interested in it for insert molding.

Pros and Cons of Hydraulic Injection Molding Machine


  • They are cheap to buy and have better injection rates. 
  • They have better ejection capability given that they are more potent than other injection molding machines. 
  • They are available in abundance all over the world, so it is effortless to get spare parts. 
  • Hydraulic Injection Machines offer the capacity to accommodate a good number of products due to large shot size. 
  • It provides accurate design because of the clamping force it provides. 
  • The modern advances in this machine have introduced gas accumulators, which make this machine faster.


  • Leaks happen in a hydraulic injection molding machine. So, you can expect to clean it a lot. 
  • The same goes for your place; you will need some work to keep it tidy. 
  • The power consumption can raise concerns for many. Generally, hydraulic injection machines eat up 5.12 kWh of energy during a process. 
  • Irrespective of the multiple benefits, these machines have serious environmental drawbacks in terms of sound pollution since they make a loud noise.

2. Electrical Injection Molding

The problem with hydraulic injection molding is that even if it is effective, it is not efficient (the machine works fine, but we can’t say the same for energy consumption). 

Japan introduced the first electrical hydraulic machine in the 1980s, and it has been used as an alternative to the bulkier hydraulic machine ever since.

electrical injection molding

An electrical injection molding uses high-speed servo motors. It is easy to use and is more advanced as compared to the conventional hydraulic versions.

The machine is highly cost efficient owing to the comfort and speed of production it provides. At the same time, an electric hydraulic machine needs less maintenance and has a digital control option, which means we can set the energy and pressure credentials to how much we need.

Pros and Cons of Electrical Injection Molding Machine


  • Electrical Injection Molding is effective and works fast. Industries can create a whole lot of product in a short while. 
  • Since there are no liquids involved in the process, workplace hygiene is maintained.
  • The machines produce minimal sound and are hence environment-friendly.
  • It has lower power requirements as we can digitally control the output levels. 
  • We don’t need a lot of maintenance. Thus, it has a little operational cost. 
  • These machines are adept at producing high precision plastic parts.


  • Electric machines generate smaller clamping force as compared to hydraulic machines.
  • These machines do not need to be maintained frequently. However, in the event of unforeseen problems, the main issues arise due to non availability/ low availability of parts. 
  • It costs more than hydraulic machines.

3. Hybrid Injection Molding

This molding method is the mix of both hydraulic injection molding and electrical injection molding. 

It was produced with the addition of both molding benefits and has proven to be better than the former two types. This method of molding has a high level of flexibility and holds the selling quality of both the previous two types of molding machines. 


It can save energy like an electrical system, providing the accuracy of the hydraulic unit at the same time. It can alternate between the DC pump motors and the hydraulic pump, so both of them operate only when they are individually required. Due to this, there are no quality compromises.

Pros and Cons of Hybrid Injection Molding Machine


  • It provides excellent results with very flexible control of energy level outputs. 
  • Hybrid Injection Molding is your answer if you want to gain high-quality output, saving bills at the same time.
  • This machine is high-speed, so expect a great response time. 
  • The machine doesn’t heat so frequently and seems to need less maintenance. 


  • In most of the cases, we need to assemble the machine depending on the user’s requirement. 
  • The architecture is very specific- only the party who built it can maintain it. 
  • The workforce should be trained to use both electrical and hydraulic systems.

Advantages of Plastic Injection Molding

So, why use plastic injection molding techniques?

As we know, plastics are an integral part of the human world (And yes, even though we say that, we do advocate effective and sustainable use of plastic products owing to the environmental concerns). The invention of plastic injection molding has changed the production ideas of this sector. The technique has made it possible to produce a large number of plastic products through a relatively easy and cost-efficient process.

Following are some key advantages of plastic injection moulding:

 I. Accuracy

Plastic molding is accurate and holds the capacity of manufacturing almost every type of parts using plastics. 

The efficient process and the practicality of the procedure makes it possible to get any types of shapes with complex geometries. 

 II. Fast method

As compared to traditional plastic molding techniques, injection molding is a faster and more convenient process of creating plastic goods. In most cases, the cycle of production lasts for 15 to 20 sec. However, the speed of work is dependent upon the structure of injection molding materials.

 III. Low Operating Cost

Plastic injection molding usually works automatically, which helps in minimizing the cost of supervision. It additionally keeps operation well optimized and production in progress.

So, instead of having to hire different people for different tasks, you can teach one of your employees to operate the machine.

 IV. Sustainable Method

It is necessary for injection moulding manufacturing companies to look over the environmental cons and pros through their production. 

Plastic moulding is not only practical but also a sustainable method that decreases ecological consequences. 

Injection plastic molding process only uses limited resources for production and focuses on recycling more than a new product.

 V. Flexibility

Needless to say,  injection molding techniques have advanced a lot over the last few years.

There are plenty of options for technology and machines. At the same time, as injection molding is common throughout the world, it is easy to get the human resources to fix in case something goes wrong.

Plastic resins

The number of plastic polymers being invented keeps growing. Plastic part manufacturers and designers hence need to consider various important features and properties peculiar to each plastic resin along with the part that is required to be produced. 

Additionally, various fillers like silica, mica, calcium carbonate, glass, clay, alumina trihydrate are used to enhance the properties of resins. 

Some key properties which need to be assessed before choosing a plastic resin for a particular plastic part include:

  • Hardness
  • Flexibility
  • Weight
  • Cost

Some of the most commonly used plastic resins for plastic injection molding include the following:

  • Acrylonitrile Butadiene Styrene (ABS)- used in applications like appliances, computer parts, housing equipment, sports and musical instruments, medical devices, helmets, toys, electronics, automobiles, and various other injection moulding products
  • Low-Density Polyethylene (LDPE) and High-Density Polyethylene (HDPE)- Used for production of medical products, electrical wirings, computer components, food industry (like juice boxes), toys, agricultural products, furniture, compost bins or kitchenware, refrigerator shelves, plastic bags, plastic containers, water bottles, etc.  
  • Polycarbonate (PC)- Used in the manufacture of eyeglasses, lenses, medical devices, mobile phones, DVDs and car parts
  • Polyamide (Nylon)- Used in the production of medical parts, automotive industry, sports gear, and other industrial tools and equipment
  • High Impact Polystyrene (HIPS)- Used for injection moulding products like display boards, garden equipment, packaging material, TV parts, toys, cold drink cups, computer housings, etc
  • Polypropylene (PP)- used in food and beverage industries, kitchen and tableware, toys, medical components, etc
  • Acrylic (PMMA)- Used in production of car screens, picture frames, beauty products, paints, eye lenses,
  • Polyetheretherketone (PEEK)- Used to make mold parts for the automotive industry, aerospace and medical parts like implants
  • Acetal/ Polyoxymethylene (POM)- Used in food and beverage industries, automotive industries, household appliances, plastic gears, guitar picks, valve parts, etc
  • Thermoplastic Polyurethane (TPU)- Commonly used for cable insulations, sports gear, etc.

Plastic Part Decoration Techniques

Plastic parts are no longer considered complete if left bare. Plastic part decoration is an important aspect of value addition to the mold parts.

Various techniques of plastic part decoration are used by injection molding manufacturers these days. Some common techniques include:

Common Plastic Injection Molding Defects

Plastic injection molding is a very precise process. Small errors during the process or even the wrong selection of resins can lead to huge losses and quality issues. It is hence important to carefully understand the types of injection molding defects and their causes in order to have suitable prevention options. 

Following are some commonly observed defects during the plastic moulding process:

Defects within the process – Various factors like flow rate of the resins, the pressure, or the temperature settings need to be carefully determined in order to avoid injection molding defects. When not considered appropriately, these could lead to defects like Flow marks, Burn marks, Warping, Vacuum voids, Sink marks, Weld lines, and Jetting. 

Defects due to resin choice or storage– this includes defects like discoloration, surface delamination, short shot, flash, silver streaking, products stuck in the mould, etc. 

How to Choose an Injection Molding Device?

It is fair to say that only the technician working in a molding company has an idea to distinguish between right and wrong device. So, your best bet would be to visit a company and ask for recommendations.

Additionally, particular products need to be equipped with special devices such as accumulators, closed circuits, injection, compression, etc.  The critical factors that often affect the selection include molds, plastic resins, products. 

Below are some factors that need to be considered before choosing the device for injection molding: 

  1. Mold type, its weight, height, design, etc. 
  2. Type of plastics, quality 
  3. Size of the product 
  4. Molding requirements, product quality, production speed 
  5. Model of the injection molding machine

While the aforementioned are the basic considerations that hold true when selecting a plastic injection molding machine, there could be various other additional factors depending on the uniqueness of the project and the product. 

It is one of those things that you must talk to an expert about to get right. On top of that, buying a factory machine with just a little information by yourself might turn your money into a bad investment.

Injection molded parts and applications

The process of plastic injection molding is highly versatile and is hence used to produce a variety of plastic parts for a huge range of applications. 

Various common uses of injection mold plastic parts derived from the technique include:

  • Plastic bottles and other consumer goods like toilet seats, skateboards, etc.
  • Electronic housings used in remote controls, medical equipment, etc. 
car stereo
  • Toys like children’s furniture, collectible models, legos in various shapes, colors, and sizes are made out of injection molded plastics.
  • Agricultural components
  • Household parts like containers, drinkware, tableware, kitchenware, cabinet closures, flower pots, air freshener units, etc. 
  • Healthcare/ Medical industry products like plastic syringes, medical tools, implants, medication trays, sharp disposal bins, wall mounts, etc. use injection molded plastics. 

  • Use in automotive industry applications like interiors of a vehicle.
  • Use in the construction industry for conduits for concrete beams, raised flooring panels, roofing vents, deck fasteners, etc. 
  • Sports goods
  • Various other custom injection molded parts

Injection molding cost factors

All plastic injection molding projects are unique in terms of complexity, design, material choice, and sustainability issues.  These are also the key factors that contribute to the different quotes for each project. 

Some key plastic injection molding cost driving factors include:

Material Cost

The material cost can be attributed to the choice of resins, fillers, and the unit price of the material. Weight of the material is often a determining factor for cost. Additionally, the maximum wall thickness of the plastic part plays a significant role in determining the quantity of material required. The size of channels, the volume of the mold cavity, the amount of material is closely related to this thickness of the part. 

Cost of Production

Productions costs are largely dependent on the size of injection molding machines and the mart design. This helps in determining the hourly rate and the cycle time. 

As mentioned earlier in the article, plastic injection molding machines are classified based on the tonnage of the clamping force which is further calculated based on the pressure with which resins are to be injected. The larger the part, the higher the clamping force and more the production costs. 

The clamping force is also dependent on the choice of resins. Certain resins require high clamping force for injecting. Further, the size of the part also determines the type of machines required. 

Cycle time is basically the sum total of the injection time, solidification time (cooling) and the resettling time. Lower wall thickness parts have reduced injection time as well as decreased cooling time. 

Again, the choice of materials and its properties play a significant role in determining the factors like cooling time. 

Finally, the quantity of plastic moulded parts required determines the overall quote of the project.

Cost of Tooling

Tooling costs include the mold base and the cavity size. Needless to say, larger parts requires a bigger mold base which results in higher costs. Similarly, the size of the cavity is an important contributor to the costs. When a larger cavity size with more depths is required to be machined, the costs are increased compared to a smaller cavity size. 

Tooling is also dependent on production quantity. Higher production volumes require better quality and stronger molds that can work till project completion. 

To sum up, various injection molding cost driving factors include :

  • Material cost
  • Cost of production
  • Quantity of parts required
  • Tooling cost
  • Complexity of the design
  • Quality requirements
  • Redesign factors

Read up our special article on tips to reduce plastic injection molding costs. 

Plastic Injection molding manufacturers

Manufacturing of plastic parts has various considerations, ranging from choosing and sourcing injection moulding materials, understanding the injection moulding machine specifications, avoiding defects, considering the type of process suitable for a particular part, mold design and tooling and plastic part decoration. 

It is hence important to research up some crucial factors when choosing a plastic injection molding company

When considering a plastic injection molding manufacturers, be sure to check them up against the following criteria:

  • Design team
  • Availability of technology and machinery
  • Production volume and capacity
  • Range of services
  • Product application
  • Quality assurance procedures
  • Budget and time considerations
  • Background experience and customer reviews

Unipipes is the best injection mold manufacturer and precision molding team in Malaysia offering a complete end to end solution for all services related to plastic injection moulding. 


Plastic injection molding is the easiest way to make large volumes of identical products from raw plastic resins. It is an economical method when we look at what it does. 

This factory technology is in a very advanced state, and hence the consumers can get an abundance of options.

 We are the best precision molding team in Malaysia. Call now and get a quote!