Plastic Injection Molding: What Is It? How Does It Work?


This article contains everything you need to know about Plastic Injection Molding.

Read further to learn more about topics such as:

  • What is plastic injection molding?
  • Benefits of plastic injection molding
  • The plastic injection molding process and machinery
  • Polymers in plastic injection molding
  • Reaction injection molding
  • Defects in injection molded plastics
Chapter 1: What is Plastic Injection Molding?

Plastic injection molding, or commonly referred to as injection molding, is a manufacturing process used in the mass fabrication of plastic parts. It involves an injection of molten plastic material into the mold where it cools and solidifies to its final form. It is ideal for manufacturing high volumes of plastic products, which serves many industries and individuals.

The mold is customized during tooling which gives the shape of the part. It can create numerous parts that are identical and dimensionally consistent, even on parts with complex designs and parts that only allow low dimensional tolerance.

Chapter 2: Advantages and Disadvantages of Plastic Injection Molding

The injection molding process is beneficial to the manufacturer of the plastic part because of the below advantages:

  • High productivity

    The injection molding process is a quick method of forming plastic parts. It has a fast production rate and is capable of producing voluminous plastic parts in a single cycle. The mold may contain multiple cavities. The more cavities are contained in the mold, the more parts it can produce in a single molding cycle. Also, it can produce thousands of parts before the tooling needs to be maintained.

  • Highly repeatable

    Injection molding can produce multiple parts with consistent form and dimensions. The parts produced in different molding cycles are guaranteed to be identical. This advantage is necessary when product consistency is required.

  • Low scrap rates

    Injection molding generates less scrap compared to traditional manufacturing processes and CNC machining. Excess plastics coming from the sprue, runners, and gates after the injection molding process may be heated and remolded if a recyclable thermoplastic material is used.

  • Low labor costs

    Injection molding is a highly automated process where most of its steps are performed by the machine controlled by an operator and intervention by manual labor is minimal. This reduces the manufacturing cost.

However, the disadvantages of injection molding are the following:

  • Expensive tooling

    The mold tooling is designed, fabricated, and tested before mass production, which increases the investment cost. Before a conceived design is produced by injection molding, a prototype mold tool is designed and produced to mold the first parts of the new design. Several iterations and trial runs must be done to ensure that the mold produces a part with accurate dimensions, which can be very expensive and time-consuming.

  • Difficult to make changes in the part design

    Any change in the form and dimensions of the part requires modifications in the dimensions of the mold cavities. If the size of the part is to be increased, a portion of the cavity must be stripped away so that the molten plastic can occupy a larger volume. Since the tooling is made up of hard metal, metal fabrication methods must be used. If the size of the part is to be decreased, a new tool with smaller cavities is required.

    • A uniform wall thickness to allow uniform cooling and to avoid shrinkage and other defects.
    • A low wall thickness and volume, to achieve a uniform and shorter cooling time since a small amount of hot plastic is allowed to cool. As a rule of thumb, the wall thickness of the part should be kept at a minimum of 6 mm.
    • Reduced external or internal undercuts. This is to minimize tooling costs.

      Limitations in the part design

      The part design, as much as possible, must have:

Chapter 3: The Plastic Injection Molding Process and Machinery

The plastic injection molding process cycle is outlined as follows. The process takes place in an injection molding machine which mainly consists of the clamping unit, injection unit, and the mold.


In the clamping step, the mold halves are closed before injecting the molten plastic and are held after the molten material has dwelled in the cavities. It takes place in the clamping unit which is responsible for:

  • Applying sufficient clamping force to resist the injection force, keeping the mold halves closed during injection step until the dwelling step.
  • Ejecting the molded part after the dwelling step.
  • Opening and closing the mold plates between molding cycles.
  • Holding the mold plates in proper alignment.

The clamping unit consists of:

  • The platen holds the mold halves when it is attached to the injection molding equipment.
    • The stationary platen holds the front mold half and houses the nozzle of the injection unit, which is directly aligned with the front mold half. The movable platen moves the rear mold half by sliding on the tie bar during the opening and closing of the mold.
  • The tie bar supports the movable platen during translation. It aligns the mold plates together. The size of the tie bar limits the size of the mold that can be placed in an injection molding machine.
  • The clamping system is responsible for translating the movable platen towards the stationary platen. There are three types of clamping systems of an injection molding machine:
    • Toggle clamps are suitable for injection molding machines with low tonnage requirements. It is equipped with an actuator that moves the crosshead forward, thus extending the crosshead links that have the movable platen attached to its end.
  • Hydraulic clamps can be easily set and regulated at a wide tonnage ranging from 150-1,100 tons. Hydraulic pressure is used to translate the movable platen and to develop the force required to secure the mold halves during the injection step.
  • Hydromechanical clamps can provide a larger clamping tonnage of above 1,000 tons. The operation of hydromechanical clamps is a combination of the toggle and hydraulic clamping systems. A hydraulic cylinder is used to translate the movable platen, then it is fixed mechanically in its position. Once the mold halves are closed, a high-pressure hydraulic cylinder is used to build the required tonnage.
  • The ejection system, which will be discussed later on.


In the injection step, the raw plastic pellets are melted and then delivered to the mold which takes place in the injection unit. The injection unit is responsible for:

  • Supplying molten plastic to fill the mold cavities. The volume of plastic injected into the mold is called a shot. The shot is dependent on the volume of the part.
  • Applying heat to melt and homogenize the plastic pellets before injecting them into the mold.
  • Applying sufficient injection pressure and speed to push the molten plastic and fill the mold cavities.

The injection unit consists of:

  • The hopper is a large container where the raw plastic pellets are fed. It has an opening at the bottom where the pellets are introduced to the threads of the reciprocating screw inside the barrel.
  • The barrel contains the reciprocating screw and has heaters jacketed on its periphery.
  • The heaters provide thermal energy to melt the plastic pellets to their molten, viscous state.
  • The reciprocating screw pushes the plastic through the length of the barrel by both rotating and sliding axially. A hydraulic cylinder supplies the injection pressure. As the plastic moves along the length of the barrel, they gain fluid properties due to the combined heat, pressure, and friction. The molten plastic is accumulated in front of the screw and its backflow is prevented by a non-return valve. The use of reciprocating screw is the most common injection system in modern injection molding machines.Another injection mechanism is the use of a screw pre-plasticizer. This injection system has separate barrels for melting and injecting the plastic into the mold. The first barrel has a mechanism similar to the reciprocating screw. Once the plastic passes through the first barrel, it proceeds to the second barrel that uses a plunger to transfer the molten plastic to the mold.

    Older injection molding machines use a single barrel, plunger-type injection system to melt and inject the plastic.

  • The nozzle introduces the molten plastic to the mold cavities. It is located in the stationary platen and is directly aligned with the front mold half.

Dwelling and Cooling

Once the molten plastic is transferred into the mold, it is allowed to dwell inside the cavities. The injection pressure is replaced by the holding pressure in this step to compact the molten plastic during its solidification.

Cooling starts once the molten plastic comes in contact with the surface of the cavities. Cooling is facilitated by a coolant system inside the mold to remove heat. Shrinkage of the part may occur during cooling. and additional melt is allowed to flow to compensate for shrinkage that occurs during cooling. After cooling it for a sufficient time, the mold halves are separated and the molded part is ejected.


In the ejection step, the cooled part is separated from the mold. The ejection system, which is contained in the clamping unit, facilitates the removal of the molded part from the mold cavities.

The ejection system consists of an actuating ejector bar that pushes the ejector plate with ejecting pins. The ejecting pins push the solidified part out of the open mold plates at the end of the molding cycle. Sufficient ejecting force must be applied because the part adheres to the mold during cooling.

mold release agent is used to aid the removal of the molded parts from the mold cavities. It can be reapplied before the start of the clamping step after a few molding cycles, or it may be fixed permanently on the surface of the mold cavities.


The last step in the production of injection-molded plastics is trimming, wherein excess plastics resulting from the flow of the molten plastic are cut from the molded part and each molded unit is separated from the rest of the molded parts. Trimming takes place in separate equipment.

During injection of the molten plastic, the mold channels (sprue, runners, and gates) are filled. The molten plastic in those channels also solidifies together with the melt inside the cavities. Flashes may also be present on the edges of the part. After cooling, the excess plastic materials adhere to the part which needs to be cut.

The Mold Tool

  • The mold mainly consists of two plates, which are attached to the clamping plates. The front mold half is adjacent to the injection unit. The rear mold half is attached to a movable plate which allows opening and closing of the mold and is also adjacent to the ejection system of the clamping unit. Before the start of the molding cycle, the two plates must be cleaned and free from contamination.
  • The mold cavity is the shaped section in the mold plates which give the plastic parts its final form. When the molten plastic flows into the cavity, it takes up the shape of the hollow space and acquires its volume. Most of the volume is contained in the front mold half. A mold can have one or more cavities.
  • The parting line is a line found in the closed mold halves which indicates their separation. A parting line may be a straight line or a curve in complex tooling designs. Air is easiest to vent in the parting line; thus, the molten plastic tends to move in this region. Line or a curve may be visible to some finished part which reveals that the two sides of the part are formed on different plates.

Mold channels:

  • The locating ring aligns the nozzle to the front mold plate.
  • The sprue bushing centers the opening of the nozzle to the front mold plate. This is where the nozzle is seated.
  • The sprue is the first passageway of the melt from the nozzle of the injection unit and the front half of the mold. It is the main channel that has several runners connected.
  • The runner distributes the molten plastic to the mold cavity.
  • The gate directs the flow of the molten plastic into the cavities by narrowing its flow path. It is located at the end of each runner wherein the molten plastic is introduced. A cavity may contain one or more gates.

Other features of the mold tool include air vents which eliminate entrapped gases inside the mold and the cooling channel which facilitates the dissipation of heat to a coolant.

Injection Molding Parameters

  • Clamping pressure, or also referred to as the tonnage, is the pressure required to hold the mold halves during the injection step. It complements the applied injection pressure. The part surface area, part depth, and size of the mold are also considered when calculating and optimizing the clamping pressure.
  • Insufficient clamping force applied can result in leakage of the molten plastic and the development of flashes. Excessive clamping force applied can result in not only part defects but also failures on the mold and the equipment itself. Fractured hydraulic cylinders, cracked platen and mold plate and crushed mold vents are some of the potential damages on the equipment in the long run induced by excessive clamping force.
  • Injection pressure is the pressure applied by the screw or plunger to force the molten plastic through the cavities until it is 95% filled. The flow characteristics of the molten plastic, such as viscosity and shear rate, also influence the required injection pressure. Molten plastics with higher viscosity have more resistance to flow, hence higher injection pressure is required to maintain the volumetric flow rate of the shot.
  • Injection pressure should also be controlled. Insufficient injection pressure can result in part defects and early solidification of the molten plastic in the mold channels. Excessive injection pressure causes pressure build-up because the internal pressure inside the cavities spikes when it is 95% filled, which can lead to the premature opening of the mold.
  • Holding pressure is applied after the cavity is 95% filled until the solidification of the gates. It is about half of the injection pressure. Holding pressure is necessary to improve the compactness of the molded part and to control shrinkage and cooling of the part.
  • Injection speed is the rate at which the screw or the plunger rotates to transfer the molten plastic to the mold cavities. In most applications, it is best to maximize the injection speed to fill the cavities with molten plastic in the shortest possible time. 

Chapter 4: Polymers in Plastic Injection Molding

Thermoplastic polymers are more common than thermosetting polymers in injection molding. Thermoplastics are plastics that can be repeatedly molten or softened by heating and solidified by cooling, making them highly recyclable material. Excess materials from a previous molding cycle are re-grinded and added back to the injection chamber along with virgin pellets, but its addition is limited to a maximum of 30% of the bulk material as it can degrade the original physical properties of the plastic.

Thermosetting plastics, on the contrary, can only be formed once after the initial application of heat because of the cross-linking of its polymeric chains. In the process, the molten form of thermosets must be transferred immediately to the mold to avoid settling into the screws and valves, which potentially damages the injection unit. However, these plastics are valued for their strength and rigidity. They are extremely resistant to high temperatures.

The following are some of the commonly used materials in plastic injection molding:

  • Acrylonitrile Butadiene Styrene (ABS) is an opaque and amorphous thermoplastic. It is known for its lighter weight, rigidity, and resistance to impact, heat, and corrosive chemicals. Due to its low melting point, it consumes less heat energy and is easily processed in the injection molding machine. ABS plastics are used in automotive parts, sports and recreational equipment, and piping materials. The famous toy Lego Bricks are made from this material.
  • Polycarbonates are transparent thermoplastics that contain carbonates in the structure of their polymeric chains. They are known to be strong, tough, and impact resistant. The application of polycarbonates includes eyewear lenses, bulletproof glass, automotive components, and containers.
  • Nylon is a thermoplastic made of polyamides. It is durable, flexible, and resistant to impact and chemicals. It is sometimes reinforced with glass fibers to increase its tensile strength. It is used in applications where low friction is required. It has a high melting point which makes it an alternative to metals in high-temperature environments, but it is also flammable. However, it is one of the polymers that are difficult to mold due to its hygroscopic nature, shrinkage, and tendency to gassing at high temperatures.
  • Propylene is an elastic, tough, and fatigue-resistant semi-crystalline thermoplastic. It is also a good electrical insulator. It is used in packaging materials, automotive parts, and household and office items. Despite its semi-crystalline nature, it has low melt viscosity, hence it can easily flow out of the injection chamber which makes molding easier.
  • Polyethylene has several types which are differentiated based on their densities; these are low-density polyethylene (LDPE), medium density polyethylene (MDPE), high-density polyethylene (HDPE), and ultra-high molecular weight polyethylene (UHMWPE). Polyethylene is also a thermoplastic. They are lightweight, have good chemical resistance, and impermeable to liquids and gases. Polyethylene with higher densities have higher tensile and flexural strength and toughness, but have poorer elongation and exhibit brittleness in low temperatures. They are used in a wide range of applications which include packaging materials, medical devices, rigid containers, and bulletproof vests.
  • Liquid Silicone Rubber (LSR) is a common synthetic thermosetting resin. It is a "liquid rubber" solution with relatively low viscosity, which makes it easier to flow into the mold cavities. It is ideal for molding parts with tight dimensional tolerances. It is also resistant to UV degradation. Applications of LSR include materials for automobile parts, heat insulation, and medical apparatuses, and infant feeding bottles that can withstand high temperatures during sterilization or autoclaving.

Reaction Injection Molding

Reaction injection molding (RIM) is a reactive molding process that utilizes low viscosity liquid thermoset polymers, unlike the conventional plastic injection molding that uses thermoplastic resins as the feed material. The liquid thermoset polymer expands, thickens, and hardens over a heated mold tooling through a series of chemical reactions. The raw materials, tooling design, and reaction mechanisms can be selected and modified to achieve the desired properties, such as hardness, strength, and density, of the finished part.

Lightweight thermoset polyurethane is the most frequently produced part by RIM, though RIM can also be employed in producing nylon, polyesters, and others. The RIM of polyurethane starts with the polymer liquids polyol and isocyanate stored in large reservoirs. These polymers are continuously recirculated within the system; they are pumped from the reservoir to a multi-stream mix-head on the mold tooling and then back to the reservoir. When a part is to be made, the polymers will be injected to the tooling by retracting the plunger or piston inside the multi-stream mix-head. This action breaks the recirculation loop of the polymer streams, and the polymers are combined by impinging mixing at high velocity. The resulting mixture is then allowed to cure and settle over a mold tooling at relatively low pressures and temperatures. Curing time depends on the size, thickness, and complexity of the part.

Variations of RIM:

Reinforced reaction injection molding(RRIM) is a RIM process in which reinforcing agents such as glass or carbon fibers are mixed with the liquid polymers. The reinforcing agents impart additional strength, rigidity, and impact resistance to the resulting part. In the RRIM of polyurethane, chopped or milled fibers are added to the polyol before injecting into the mold. RRIM is typically employed in the manufacturing automobile parts such as body panels, bumpers, and fascia.

Structural Reaction Injection Molding(SRIM) is a RIM process in which glass mats, fiber meshes, and preforms are placed and arranged inside the mold prior to injecting the liquid polymers. The reinforcing agents impart additional strength and structural stiffness to the resulting part. SRIM is typically employed in the manufacturing of doors, shelves, and panels.

Advantages of Reaction Injection Molding

RIM requires lower pressures (~100 psi) and temperatures (~80-150°C) than conventional, non-reactive plastic injection molding. Hence, low-cost mold tooling (i.e., made from aluminum) can be used for this method. Reinforcements can be easily included in the part’s structural matrix for enhanced mechanical properties.

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