Plastic Injection Molding – Types and Units

Plastic Injection Molding – Types and Units

The plastic injection process is one of the most important processes to produce plastic parts in terms of serial production manner. We can produce a very large size range of plastic products with the plastic injection molding process. The size and weight can range from a few grams to tens of kilograms.

Plastic injection molding.
Plastic injection molding diagram.

The most basic explanation of the plastic injection process can be like this; Injection of molten polymer into metal molds and solidifying inside these molds to obtain the required shapes. In each cycle of the plastic injection process, we can produce one to tens of parts. The cycle time changes according to the used material and produced part. But in general, the cycle time of plastic injection molding processes ranges from 30 seconds to a few minutes.

Injection of thermoplastic materials in injection molding processes is the most common practice. We can mold the thermosetting and elastomer polymers also if the equipment and toolings of the injection molding machine are changed according to them.

We can produce very complex parts with plastic injection molding machines. But the shapes of products must be suitable for detachment from the molds. For most plastic injection molding applications, processes are net-shape processes.

Plastic Injection Machine Parts And Working Principle

General plastic injection machines include three sections. The first section is the injection unit. Here, polymer pellets which are the starting form of molded parts, are provided from the hopper inside the barrel. Inside the barrel, there is a rotating screw that pushes the plastic material forward direction. This screw mechanism is common with the plastic extrusion process. We place the heaters also around the barrel to heat and produce polymer melt from plastic pellets.

Plastic pellets.
Plastic pellets.

Injection Unit

They adjust the rotation motion of the screw with an electric motor and gear systems at the back-side of the machinery. At the end of the screw, there is a non-return valve that prevents the backflow of polymer melt from the mold side.

There are also other types of injection units with the most used types explained above. Older plastic injection machines, use plunger systems to push the plastic melt inside the mold cavity. Another alternative is that they give the starting material from the hopper to the screw section. This screw section melts and homogenizes the polymer melt and pushes this polymer melt inside the plunger system. When the plunger system is full of the polymer melt, a shut-off valve is in a close position. Then plunger pushes the polymer to melt inside the mold. We call this system generally a screw-preplasticizer or two-stage system. We call it the first plunger-type injection molding.

Clamping Unit

Also, we call the second section of the plastic injection machine as ‘clamping unit’. In the clamping unit, there are two plates, one stationary and the other one moving. Between platens, we place the mold. The moving platen move via the hydraulic system. The movement of the moving platen creates the required pressure to hold the mold halves during injection and solidification. We can apply several tons of pressure to injection molds.

Clamping units of plastic injection machines have generally three types; toggle, hydraulic and hydromechanical systems. In toggle systems, we use crosshead mechanical systems to push the movable plate. The advantage of these systems is the application of very good pushing forces at the end of the clamping. In general, we use them in lower tonnages. We use hydraulic clamping units for higher tonnages which we explained just above. The third type of clamping unit is hydromechanical. In hydromechanical clamping systems, we make the moving action with the hydraulic system and the clamping action with a mechanical system. This is very useful when we produce very high tonnages of plastic parts.

In a typical plastic injection molding application, we clamp the mold via a hydraulic system with a moving plate. Then we supply the material from the hopper into the screw-barrel system. With the rotation of the screw, we force the molten plastic into molds. In this stage, we close the non-return valve to prevent backflow from the mold. The pressure is held up to the solidification of polymer inside molds take place. When the solidification is complete, we open molds then we remove the molded part from the mold. Then, we close and pressurize the mold again. In the solidification stage, new material is provided inside the rotating screw.

General Mold Systems in Plastic Injection Machines

We do not consider the mold side of the whole system inside the machinery system. Because we design the molds according to the part geometry. Molds are made from metals, and the cavities to inject polymer inside them are carved via machining operations. These operations are somewhat expensive. But, if we think that it is serial production, income will compensate for these machining costs.

We place the prepared molds on movable and fixed plates. In general, they produce molds in two half designs one of them placed on a movable platen and we place another one fixed on the fixed platen.

Just like ordinary metal casting processes, plastic injection molds have sprue, gating, and runner systems inside them. We perform the plastic injection from the sprue section and are directed to the internal cavities by runners. Ratings are the entrance sections of internal cavities of plastic injection molds.


Plastic injection molds.
A typical plastic injection mold(Image Source:

To separate the injected plastic parts from the mold at the end of the cycle, we require some ejection systems. We make these ejection systems generally with the application of ejector pins on the movable platen. Because of the shrinkage nature of plastic materials, after the separation of two plates, the plastic part is clung to the movable plate. With the application of ejector pins, they separate the plastic molded product from the mold.

As we stated above the two-half design is one of the most encountered designs in the plastic injection industry. Also, three half of mold designs that we use. In two half molds, we separate runner and sprue systems from mold in unity with the main part. But in the three plate mold design, we separate sprue and runner systems into a separate section. In general, a more uniform distribution of polymer melt inside cavities is done in three plate mold design.

Sprue and runner systems that we produce can be recycled or used in another process. But, if the used polymer melt must be ‘virgin’, sprue and gating systems are discarded. Also, they design hot-runner systems to hold the runner and sprue systems in the liquid stage, which we can use in another cycle of production.

Plastic injection molding diagram.
A typical plastic injection molding diagram(Image Source:

To solidify and cool the polymer melt inside molds, we design a water circulation system inside molds. Also, we open air vents on molds to evacuate the entrapped air inside mold cavities. These vents are in the diameters that polymer cannot fill, but air can pass.

General Defects That Encountered In Plastic Injection Processes And Recommended Solutions

Weld Lines

When they inject the polymer inside the mold, weld lines take place around the cores. This is because the polymer melt goes around the core, then meets at the other side of the core. At this meeting point, weld lines can occur.

Solution Of Weld Lines: Smart placement of cores and gating systems, higher melting temperatures and injection pressures, and better venting systems can solve this problem.

Flashing Problem

Flashing of polymer melt occurs around the parting surfaces of halves of molds.

Solution Of Flashing: Flashing generally occurs because of the insufficient tolerances between touching surfaces of mold halves. Too high injection pressures compared with the clamping force is also another reason. Uneven design of air vents can lead to flashing problems in injection molding processes. So, designing and fixing these problems smartly can solve the flashing problem.

Voids And Sink Marks On Products

The main reason for sink marks and voids is the internal shrinkage at the thick sections of parts. Voids are the internal spaces inside thick sections and sink marks are the depressions of surface sections because of the internal voids.

Solution Of Voids And Sink Marks: Proper design of parts that has no very thick sections. Uniform thickness along the designed part is very important to solve this problem.

Short Shot Problem

Short shots are the problems in plastic injection processes that solidification of polymer melt without filling the mold cavity.

Solution Of Short Shot Problem: Short shots take place because of insufficient shot pressures and injection temperatures. The operator must consider these parameters to solve the short shots problem in plastic injection processes.

Shrinkage In Plastic Injection Processes

Shrinkage is a very important parameter of polymer injection processes that we need to consider professionally. We define shrinkage as the linear contraction of a polymer when it cools from a molten state to a solid state. Because of this issue, we must make a mold design by considering the shrinkage of injected material.

Plastic mold shrickage calculation.
Plastic mold shrinkage calculation via software(Image Source:

Shrinkage of injected plastic inside mold depends on some factors;

  • When the injection pressure increases, resultant shrinkage is reduced. But there is a limit for injection pressure that changes machinery to machinery.
  • Maintaining pressure has also the same effect as injection pressure on shrinkage.
  • Compaction time is also reducing the effect on shrinkage.
  • If the part is thicker, the part tends to be more shrink.
  • Increasing the difference between molding temperature and room temperature increases shrinkage.

To compensate for the shrinkage in plastic injection, they design molds bigger than the usual part in volume. So, consideration of shrinkage required very good mastery to evaluate the parameters defined above. But, you can make shrinkage calculations for specific dimensions of mold by using the calculator below.

Mold Dimension Shrinkage Calculator


The use of the above calculator is very simple. You just need to enter the required values; Dimension which must have a length value, and the unitless shrinkage value of a typical polymer. Click on the ‘Calculate!’ button to see the mold design value of this dimension. If you want to do another calculation, just click on the ‘Reset!’ button, then enter new values.

You can find shrinkage values of different polymers from polymer suppliers. And also you can find some polymer shrinkage values below;

PVC: 0.005

Polystyrene: 0.004

Nylon 6-6: 0.02

Polycarbonate: 0.007

ABS: 0.006

Polyethylene: 0.025

Types of Plastic Injection Processes

These are the general types of plastic injection processes.

Thermoset Injection Processes

Thermoplastic and thermoset injection processes.
Illustration of difference between thermoset injection and thermoplastic injection molding processes(Image Source:

You know that polymer materials are classified into three groups; thermoplastics, thermosettingand elastomers. They produce most plastic injection machines for thermoplastics. But also, make plastic injection machines for thermoset plastics. There are major differences in the tooling and design between thermoplastic injection machines and thermosetting injection machines. But the most important differences between them are the length of the barrel and screw, and heating temperatures.

This difference is related to the curing of thermoset materials at certain temperatures. We do not want to cure thermoset material outside of the mold. We do not desire the curing of thermosets inside the barrel or between the barrel and mold.

So, we hold screw and barrel temperatures at 50 degrees to 125 degrees. This temperature changes according to the thermoset material that molds.

The most important thermoset plastics that injection mold are; urea-formaldehyde, epoxies, melaminesunsaturated polyestersand phenolics. They inject most thermoset plastics with fillers. So, composite materials are also injection molded.

Typical mold temperatures range from 150 degrees to 230 degrees. Injection cycle times change according to the curing time of injected polymer.

Multi Injection Molding Process

Multi injection molding process.
Multi-injection process explanation(Image Source:

The multi-injection process is a common process to produce unitary parts made from different kinds of plastics. When they combine these different kinds of plastics, they produce useful products that include different plastics combined.

The used injection nozzles are more complex than other conventional injection molding processes. There are different methods to produce different parts;

  • Bi-injection molding: Multi-injection process that combines different colors of plastics. Automotive taillights are the most important examples. In general, we use two positioned molds. When we inject one type of plastic inside the first section of the mold, the second section opens. Then the injection process goes on with other kinds of plastic. After solidification, these two plastics combined to constitute the whole part.
  • Sandwich Injection Molding: In sandwich molding, we produce structural foams. In general application, we mold the foam first inside mold as core material. And also, they inject another thermoplastic coating material to surround the core material. We use different nozzle configurations to accomplish this kind of plastic injection process.

Explanation of Reaction Injection Molding Process

Reaction injection molding process.
Reaction injection molding basic explanation(Image Source:

As you understand from its name, in reaction injection molding, there is a reaction of mixed polymers before the injection to mold. This reaction takes place in relatively short times with the addition of catalysts. Epoxies, urethanes, and urea-formaldehyde are the most important examples.

In the general application of reaction injection molding, we hold two reactive ingredients in separate containers. At the beginning of the injection cycle, we mix required precise amounts of reactive materials inside the mixing head. Then we pump this mixed and reacted polymer inside the mold cavity. The reaction also continues inside the mold cavity.

The most important advantage of the reaction injection molding process is the requirement of lower injection pressures compared with other plastic injection processes. Because of that, the requirement for tooling is also lower and cheaper.

The automotive industry is one of the most important markets of this reaction injection molding industry that various kinds of automotive parts such as produce bumpers with reaction injection molding processes.

Thermoplastic Foam Injection Molding Process

Inert gas production.
Application of inert gas in the production of structural foam molding(Image Source:

Thermoplastic foam injection is a very basic process that polymer melt is mixed with gas, and injected inside the mold. This gas and polymer melt mixture creates a cellular internal structure, which we also call foam.

This foam is lightweight. And the outer surface of this foam is formed when injected foam touches the cold mold surface. The surface characteristics of produced foam are not generally good. Some voids can occur on the surface and because of this, we need some additional surface improvement processes.

Because of these additional processes, the production cost can increase. But in thermoplastic foam injection molding, there is no requirement for high injection pressures like other plastic injection machines. This factor can reduce the costs of the thermoplastic foam injection molding process.

Also because of low injection pressures in thermoplastic foam injection molding, we can produce large parts. So, they produce structural foam generally with thermoplastic foam injection molding processes. Machine housings and structural foams are important application areas of the thermoplastic foam injection process.

Injection Molding Of PMCs

PMC injection molding process.
A very basic illustration of PMC injection molding(Image

For polymer matrix composites, both thermosetting and thermoplastic materials are used as matrix materials. So, there are thermosetting and thermoplastic variations of PMC injection molding. 

For thermoplastic injection molding, the whole process is much more basic than the thermosetting matrices. Because there is a fact that cross-linking occurs for thermosetting materials. Because of this cross-linking occurrence, we must establish very complex heating for all the sections of injection molding machinery to prevent unwanted cross-linking. 

We must inject fiber reinforcements and particle reinforcements with the matrix polymer. For fiber reinforcements, there can be rotating blades in the nozzle section to cut the fibers into intended lengths. 

Fiber reinforcement inside the polymer melt that is going through the nozzle, is oriented in one direction because of the shape of the nozzle. This gives the advantage of designs of oriented reinforcements in certain directions, to designers. 

Reinforced Reaction Injection Molding

Reaction injection molding which is also called ‘RIM’ is the process that two reactive ingredients are mixed inside a chamber and then instantly injected inside the mold. With this reaction, molded component occurs via reaction cross-linking.

Also, fiber reinforcements or glass-fiber reinforcements are added to these reaction injection molding processes. This process is also called ‘RRIM’. 

Conclusion For Plastic Injection Processes

We provided the most important data about the conventional plastic injection processes. This article can be a very good guide for plastic injection processes. We included a calculator that you can calculate the mold dimensions by considering the shrinkage inside a mold.

Mechanicalland does not accept any responsibility for the calculations done with calculators by the user. A good engineer must check the calculations again and again.

You can find other useful engineering calculators available in Mechanicalland!

Do not forget to leave your comments and questions below about the plastic injection machine and processes below.

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External Link: Sciencedirect

FAQs About Plastic Injection Molding

What is the plastic injection molding process?

The plastic injection molding process is the process where the molten plastic material is pushed inside a mold cavity to take the shape of the mold.

What are the 5 steps of plastic injection molding?

The first step is giving the granular plastic material from the hopper of the plastic injection machine. And in the second step, plastic material is squeezed to obtain a plastic melt in the chamber. With the application of a plunger mechanism, the plastic is forced into the mold. The fourth step is the solidification step of them. And in the last step, solidification of the plastic material takes place and the mold is opened.

What are plastic injection molds used for?

The general purpose of plastic injection molds is, they provide the plastic injection cavity to give the required shape to the molten plastic material.

What type of plastics is injection molded?

In general, we are using thermoplastic materials for the injection molding processes. But also, use of the thermosetting materials is common.

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