What is plastic injection molding?

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What is plastic injection molding?

Injection moulding (U.S. spelling: injection molding) is an industrial manufacturing technology for the mass production of identical plastic parts with good tolerances. It is the most common manufacturing process for plastic parts and products. In Injection Molding, polymer granules are melted in the injection machine and then injected under pressure into a plastic mould or mold, where the liquid plastic cools and solidifies to the configuration of the cavity. Once the part cools, the mold opens, the part is ejected, the mold closes, and the process repeats.

The materials used in injection molding are thermoplastic polymers that can be colored or filled with other additives. With the development of industry, injection moulding can be performed with a host of materials, mainly including thermoplastic, thermosetting polymers, metals (for which the process is called die-casting), glasses, and elastomers.

Injection molding technology allows plastic product development teams to quickly create large volumes of identical parts with consistently high quality. After a product is designed, usually by an industrial designer or an engineer, molds are made by a mould-maker (or toolmaker) from metal, usually either steel or aluminum, and precision-machined to form the features of the desired part. Injection moulding is widely used for manufacturing various plastic parts, from the smallest components to entire body panels of cars. Almost every plastic part around you was manufactured using injection molding, from car parts to electronic enclosures to kitchen appliances.

The injection molding process involves designing the product, making a mold, and then the manufacturing process of melting plastic resin pellets and using pressure to inject them into a mold. Injection molding is so popular because of the dramatically low cost per unit when manufacturing high volumes. Injection molding offers high repeatability and good design flexibility. The main restrictions on injection molding usually come down to economics, as a high initial investment for the mold is required. Also, the turn-around time from design to production is slow (at least four weeks).

What is the detailed plastic injection molding process?

Process Flow

The injection molding process mainly includes six stages: mold closing – filling – pressure holding – cooling – mold opening – demoulding. These six stages directly determine the molding quality of the product, and these six stages are a complete continuous process.

Let’s focus on the four stages: filling, pressure holding, cooling, and demoulding.

Filling Stage

The filling is the first step in the entire injection molding cycle, from when the mold closes to start injection molding until the mold cavity is filled to about 95%. Theoretically, the shorter the filling time, the higher the molding efficiency, but in actual production, the molding time (or injection speed) is subject to many conditions.

High-speed filling

When filling at high speed, the shear rate is high, and the viscosity of the plastic decreases due to the effect of shear thinning, which reduces the overall flow resistance; The local viscous heating effect will also make the thickness of the solidified layer thinner. Therefore, during the flow control phase, the filling behavior often depends on the volume to be filled. In the flow control stage, due to the high-speed filling, the shear thinning effect of the melt is usually significant, but the cooling effect of the thin wall is not obvious, so the impact of the rate prevails.

Fill at low speed

Heat conduction controls low shear rates, high local viscosities, and high flow resistance at low filling rates. Due to the slow replenishment rate and slow flow of hot plastic, the heat conduction effect is more obvious, and the heat is quickly taken away by the cold mold wall. Coupled with a small amount of viscous heating, the thickness of the solidified layer is thicker, which further increases the flow resistance at the thinner wall.

Due to the fountain’s flow, the plastic polymer chains in front of the flow wave line up almost parallel to the flow. Therefore, when two plastic melt’s flow meets, the polymer chains on the contact surface are parallel. In addition, the properties of the two melts are different (the residence time in the mold cavity is different, and the temperature and pressure are also different), resulting in the structural strength being poor in the fusion area of the melts microscopically. Put the parts at an appropriate angle under the light and observe with the naked eye, and it can be found that there are clear joint lines, which is the formation mechanism of the weld line. Weld lines not only affect the appearance of plastic parts but also have a loose microstructure, which can easily cause stress concentration, reducing the part’s strength and causing the fracture.

Generally speaking, the strength of the weld line that produces welding in the high-temperature zone is better. Because at high temperatures, the mobility of polymer chains is relatively good, and they can penetrate and entangle each other. In addition, the temperature of the two melts in the high-temperature area is fairly close, and the thermal properties of the melts are almost the same, increasing the welding area’s strength. On the contrary, the welding strength is poor in the low-temperature area.

Pressure Holding Stage

The function of the pressure-holding stage is to continuously apply pressure, compact the melt, increase the density of the plastic (densification), and compensate for the shrinkage behavior of the plastic. During the pressure-holding process, the back pressure is high since the mold cavity has been filled with plastic. During the pressure-holding and compacting process, the screw of the injection molding machine can only slowly move forward slightly, and the flow speed of the plastic is also relatively slow. The flow at this time is called pressure-holding flow. Since the plastic is cooled and solidified by the mold wall during the pressure-holding stage, the viscosity of the melt increases rapidly, so the resistance in the mold cavity is enormous. In the later stage of pressure holding, the material density grows, and the plastic parts are gradually formed. The pressure-holding stage lasts until the gate is solidified and sealed. At this time, the cavity pressure in the pressure-holding stage reaches the highest value.

During the packing phase, the plastic exhibits partially compressible properties due to the relatively high pressure. In areas of higher pressure, the plastic is denser and has a higher density. In areas of lower pressure, the plastic is looser and has a lower density, causing the density distribution to change with location and time. During the pressure-holding process, the plastic flow rate is meager, and flow no longer plays a leading role. Pressure is the main factor affecting the pressure-holding process. During the pressure-holding process, the plastic fills the mold cavity, and the gradually solidified melt acts as a medium for transmitting pressure. The pressure in the mold cavity is transmitted to the surface of the mold wall through plastic, which tends to spread the mold, so proper mold clamping force is required for mold clamping. Under normal circumstances, the mold expansion force will slightly open the mold, which is helpful for the exhaust of the mold. Still, if the mold expansion force is too large, it can easily cause burrs, overflow of the molded product, and even open the mold. Therefore, when selecting an injection molding machine, an injection molding machine with sufficient clamping force should be set to prevent mold expansion and effectively maintain pressure.

Under the new environmental conditions of injection molding, we need to consider some new injection molding processes, such as gas-assisted molding, water-assisted molding, foam injection molding, etc.

Cooling Phase

In injection molding molds, the design of the cooling system is essential. Because the molded plastic product can get to a certain rigidity after being cooled and solidified, the rigidity can avoid the deformation of the plastic product due to external force after demoulding. Since the cooling time accounts for about 70% to 80% of the entire molding cycle, a well-designed cooling system can significantly shorten the molding time, improve injection molding productivity, and reduce costs. An improperly designed cooling system will lengthen the molding time and increase the price. Uneven cooling will further cause the warping of plastic products.

According to the experiment, heat entering the mold from the plastic melt is roughly distributed in two parts. 5% is transferred to the atmosphere through radiation and convection, and the remaining 95% is conducted from melt to mold. Due to the role of cooling water pipes in the mold of plastic products, the heat is transferred from the plastic in the mold cavity to the cooling water pipes through heat conduction through the mold frame and then taken away by the cooling liquid through heat convection. A small amount of heat not taken away by the cooling water continues to be conducted in the mold, dissipating in the air after contacting the outside world.

The injection molding cycle consists of clamping time, filling time, pressure holding time, cooling time, and demoulding time. The cooling time accounts for the most significant proportion, about 70% to 80%. Therefore, the cooling time will directly affect the length of the molding cycle and the output of plastic products. In the demoulding stage, the plastic product should be cooled to a temperature lower than the thermal deformation temperature to prevent the plastic product from being loosened due to residual stress or warping and deformation caused by external force from the demoulding.

Factors that affect the cooling rate of the product are:

Design of plastic products. Mainly the wall thickness of plastic products. The thicker the product, the longer the cooling time. Generally speaking, the cooling time is proportional to the square of the thickness of the plastic product or proportional to the 1.6th power of the maximum flow channel diameter. That is, the thickness of the plastic product is doubled, and the cooling time is increased by four times.

Mold steel and its cooling method. Mold material, including mold core, cavity, and mold base material, dramatically influences the cooling rate. The higher the thermal conductivity of the mold material, the better the effect of transferring heat from the plastic per unit time and the shorter the cooling time.

Cooling water pipe configuration. The closer the cooling water pipe is to the mold cavity, the larger the pipe diameter, and the more the number, the better the cooling effect and the shorter the cooling time.

Coolant flow. The greater the cooling water flow rate (generally, it is better to achieve turbulent flow), the better the effect of cooling water taking away heat by heat convection.

The nature of the coolant. The coolant’s viscosity and heat transfer coefficient will also affect the heat transfer effect of the mold. The lower the viscosity of the coolant, the higher the thermal conductivity, and the lower the temperature, the better the cooling effect.

Plastic option. Plastic refers to how quickly plastic conducts heat from a hot place to a cold place. The higher the thermal conductivity coefficient of the plastic, the better the heat conduction effect. Or the lower the specific heat of the plastic, the temperature is easy to change, so the heat is easy to dissipate, the heat conduction effect is better, and the required cooling time is shorter.

Processing parameter setting. The higher the material temperature, the higher the mold temperature, the lower the ejection temperature, and the longer the required cooling time.

Design rules for the cooling system:

The designed cooling channel should ensure a uniform and rapid cooling effect.

The purpose of designing the cooling system is to maintain proper and efficient mold cooling. Cooling holes should use standard sizes to facilitate processing and assembly.

When designing the cooling system, the mold designer must determine the following design parameters according to the wall thickness and volume of the plastic part.

  • the location and size of the cooling hole;
  • the length of the pit;
  • the type of hole;
  • the configuration and connection of the hole;
  • the flow rate and heat transfer properties.

Demoulding Stage

Demolding is the last step in an injection molding cycle. Although the product has been cold-set and molded, demoulding still significantly impacts the quality of the product. Improper demoulding methods may cause uneven force on the product during demoulding, and defects such as product deformation when ejected. There are two main ways of demoulding: ejector pin demoulding and stripping plate demoulding. When designing the mold, the appropriate demoulding method should be selected according to the structural characteristics of the product to ensure product quality.

For molds that use ejector pins for demoulding, the setting of the ejector pins should be as uniform as possible. The position of ejector pins should be selected where the demoulding resistance is the largest and the strength and rigidity of the plastic parts are the largest to avoid deformation and damage of the plastic parts.

The stripper plate is generally used for the demoulding of deep-cavity thin-walled containers and transparent products that do not allow traces of push rods. This mechanism is characterized by large and uniform demoulding force, smooth movement, and no clear leftover scraps.

Injection Molding Guide

Process Parameters

Injection Pressure

The hydraulic system of the injection molding system provides the injection pressure. The pressure of the hydraulic cylinder is transmitted to the plastic melt through the screw of the injection molding machine. Under pressure, the plastic melt enters the vertical channel of the mold through the nozzle of the injection molding machine (for some molds, it is also the main channel), the main channel, and the shunt. Road, and through the gate into the mold cavity, is the injection molding process, or the filling process. The existence of pressure is to overcome the resistance in the flow process of the melt, or conversely, the opposition in the flow process needs to be offset by the force of the injection molding machine to ensure the smooth progress of the filling process.

During the injection molding process, the pressure at the nozzle of the injection molding machine is the highest to overcome the flow resistance throughout the melt. Afterward, the pressure gradually decreases along the flow length to the wavefront of the front of the melt. If the cavity is well exhausted, the final pressure at the front of the melt is atmospheric.

There are many factors affecting the melt-filling pressure, which can be summarized into three categories:

  1. Material factors, such as the type of plastic, viscosity, etc.
  2. Structural factors, such as the type, number, and location of the gating system, the cavity of the mold, shape, thickness of the product, etc.
  3. Forming process elements.

Injection Time

The injection molding time mentioned here refers to the time required for the plastic melt to fill the cavity, excluding extra time such as mold opening and closing. Although the injection time is very short and has little impact on the molding cycle, the injection time adjustment dramatically affects the pressure control of the gate, runner, and cavity. A reasonable injection time is helpful for the ideal filling of the melt, and it is essential for improving the surface quality of the product and reducing the dimensional tolerance.

The injection molding time is much lower than the cooling time, about 1/10 to 1/15 of the cooling time. This rule can be used as the basis for predicting the entire molding time of plastic parts. When doing mold flow analysis, only when the screw rotation completely drives the melt to fill the cavity does the injection time in the analysis results equal the injection time set in the process conditions. If the pressure switch of the screw occurs before the cavity is full, the analysis result will be greater than the setting of the process condition.

Injection Temperature

Injection temperature is an essential factor affecting injection pressure. The injection molding machine barrel has 5 to 6 heating sections, and each raw material has its suitable processing temperature (for detailed processing temperature, please refer to the data provided by the material supplier). The injection molding temperature must be controlled within a specific range. If the temperature is too low, the plasticization of the molten material will be poor, affecting the quality of the molded parts and increasing the difficulty of the process; if the temperature is too high, the raw materials will quickly decompose. In the actual injection molding process, the injection temperature is often higher than the barrel temperature, and the higher value is related to the injection rate and the characteristics of the material up to 30°C. It is due to the high heat generated by the shearing of the melt through the gate. There are two ways to compensate for this difference in mold flow analysis. One is to measure the temperature of the melt when injected into the air, and the other is to include the nozzle when modeling.

Holding Pressure and Time

At the end of the injection molding process, the screw stops rotating and moves forward. Now the injection molding enters the pressure-holding stage. During the pressure-holding process, the nozzle of the injection molding machine continuously replenishes the cavity to fill the vacated volume due to the part’s shrinkage. If the pressure is not maintained after filling the cavity, the amount will shrink by about 25%. Especially the ribs will form shrinkage marks due to excessive shrinkage. The holding pressure is generally about 85% of the maximum filling pressure. Of course, it should be determined according to the actual situation.

Back Pressure

Back pressure refers to the stress that needs to be overcome when the screw reverses and backs up the storage. The use of high back pressure is conducive to the dispersion of colorants and the melting of plastics. Still, at the same time, it prolongs the retraction time of the screw, reduces the length of plastic fibers, and increases the pressure of the injection molding machine. Therefore, the back pressure should be lower, generally no more than injection molding 20% of the force. When injecting foam, the back pressure should be higher than the pressure formed by the gas. Otherwise, the screw will be pushed out of the barrel. Some injection molding machines can program back pressure to compensate for the reduction in screw length during melting, which reduces heat input and lowers the temperature. However, because the result of this change is difficult to estimate, it is not easy to make corresponding adjustments to the machine.

For more injection molding guides relative to plastic product design, please check this blog:

Injection Molding Wall Thickness Guidelines and Design Specifications

Typical Plastic Injection Molding Parameters Chart as Shown Below:

plasticLDPEHDPEEthylene Propylene Copolymer PPPPGlass fiber reinforced PPPSHIPSABS
injection parameters        
1, Injection molding machine typescrewsscrewsscrewsscrewsscrewsscrewsscrewsscrews
2, screw type111111111111111
Rotating speed, r/min/30-60/30-6030-60/30-6030-60
3, gate typeDirect typeDirect typeDirect typeDirect typeDirect typeDirect typeDirect typeDirect type
temperature ℃150-170150-180170-190170-190180-190160-170160-170180-190
4, barrel temperature front section ℃170-200180-190180-200180-200190-200170-190170-190200-210
middle section ℃/180-220190-220200-220210-220/170-190210-230
rear section ℃140-160140-160150-170160-170160-170140-160140-160180-200
5, mold temperature,℃30-4530-6050-7040-8070-9020-6020-5050-70
6, injection pressure, MPA60-10070-10070-10070-12090-13060-10060-10070-90
7, holding pressure, MPa40-5040-5040-5050-6040-5030-4030-4050-70
8, injection time, s0-50-50-50-52-50-30-33-5
9, pressure holding time, s15-6015-6015-6020-6015-4015-4015-4015-30
10, cooling time, s15-6015-6015-5015-5015-4015-3015-4015-30
11, cycle time, s40-14040-14040-12040-12040-10040-9040-9040-70
12, drying equipmentHorizontal boilingHorizontal boilingHorizontal boilingHorizontal boilingHorizontal boilingHorizontal boilingHorizontal boilingHorizontal boiling
temperature ℃90-10090-100100-120100-120100-12090-10090-110100-110
time, h<0.5<0.5<0.5<0.5<0.5<0.50.5-1.00.5-1.0

About more plastic…

Common Injection Molding Defects

Most defects in injection molding are related to either the flow of the melted material or its non-uniform cooling rate during solidification.

Here is a list of defects to keep in mind while designing a plastic part for injection molding.


When certain sections cool (and as a result shrink) faster than others, then the part can permanently bend due to internal stresses.

Parts with non-constant wall thickness are most prone to warpage.

Sink Marks

When the interior of a part solidifies before its surface, a small recess in an otherwise flat surface may appear, called a sink mark.

Parts with thick walls or poorly designed ribs are most prone to sinking.

Drag Marks

As the plastic shrinks, it applies pressure on the mold. During ejection, the walls of the part will slide and scrape against the mold, which can result to drag marks.

Parts with vertical walls (and no draft angle) are most prone to drag marks.

Knit Lines

When 2 flows meet, small hair-like discolorations may develop. These knit lines affect the parts aesthetics, but also they generally decrease the strength of the part.

Parts with abrupt geometry changes or holes are more prone to knit lines.

Short Shots

Trapped air in the mold can inhibit the flow of the material during injection, resulting in an incomplete part. Good design can improve the flowability of the melted plastic.

Parts with very thin walls or poorly designed ribs are more prone to short shots.

The 10 Most Popular Injection Molding Materials

Acrylonitrile-Butadiene-Styrene (ABS) – ABS resin is an opaque thermoplastic polymer and an engineering grade plastic. There are many advantages to using ABS. It’s tough, has good dimensional stability, it resists impacts and scratching, and is hard to break. Also, the low melting temperature makes it easy to mold. It’s commonly used to produce electronic parts such as phone adaptors, keyboard keys, and wall socket plastic guards. Why is this? Because ABS is a good insulator and won’t conduct electricity or give off fumes if it’s exposed to fire. These are important considerations for product developers working on electrical devices.

Nylon (PA) – Nylon is often used to produce strong mechanical parts like bushings, gears, and bearings. It’s very common in automotive applications because not only is it tough but it helps to reduce weight and lower production costs compared to a metal analogue. You should be aware that, although it’s a strong plastic, it tends to absorb water. It’s not the ideal choice for marine applications. Nylon is also known by its chemical designation PA (Polyamide).

Acrylic – We use acrylic to produce transparent parts such as windows, view screens, and various lighting equipment. It’s often used as an alternative to glass due to its high tensile strength and weather and scratch resistant nature. It takes dyes and colorants very well so you can produce many aesthetic effects. On top of its optical and transparent properties, acrylic is odorless and tasteless and doesn’t contain Bisphenol A (BPA). BPA is a harmful organic compound, so plastic injection molding resins like acrylic are considered safe for food storage.

Polycarbonate (PC) – Polycarbonate is another clear injection molding resin that has excellent optical properties and is extremely durable. When molding with this amorphous thermoplastic material, precise dimensional control can be maintained as it has predictable and uniform mold shrinkage. We use polycarbonate when we need something substantially stronger than acrylic. However, be aware that if you’re making optically clear plastic parts the mold tool must be highly polished, which in turn implies the use of a higher grade of stainless steel that costs more. Now, you can see that your choice of plastic resin can very much influence the appropriate mold tool material as well.

Polyoxymethylene (POM) – Polyoxymethylene (POM) is a type of acetal resin used to make mechanical and automotive parts that would usually be made with metal. This engineering thermoplastic material is very strong, tough, and rigid. It’s often used to produce gears, fasteners, knife handles, and ball bearings. Although POM has high resistance towards solvents such as alcohols, gasoline, detergents and motor oils, it shouldn’t be exposed to hydrochloric acid and nitric acid.

Polystyrene (PS) – When it comes to injection molding resins, there are two types of polystyrene that are commonly used: High Impact Polystyrene (HIPS) and General Purpose Polystyrene (GPPS). GPPS is transparent, while HIPS is opaque. Hard cases for toolboxes and bodies of power tools are also made using High Impact Polystyrene. As with so many things there is a tradeoff to be aware of. On the one hand, PS is tough and durable. It can take a lot of abuse in the field. But that also means it’s not very environmentally friendly.

Polypropylene (PP) – This thermoplastic injection molding material is widely used in the food storage and packaging industry because it doesn’t let chemicals mix with food products. Polypropylene (PP) can be washed in hot water without degrading, and it has high chemical and moisture resistance. PP has incredible impact strength, elasticity, and toughness. Designers should also note that PP is easy to recycle, and because of its flexibility, it can be used to make live hinges that can be bent many times without tearing.

Polyethylene (PE) – Polyethylene (PE) is a lightweight thermoplastic molding material that has high chemical resistance, elasticity, and electrical insulating properties. It’s not especially strong or hard, but it’s inexpensive. You’ll find it everywhere in consumer plastic parts, milk bottles, medicine and detergent bottles, plastic bags, and trash cans. PE is also the most common injection molding resin for making toys because it’s non-toxic and can take a beating without complaint.

Thermoplastic Polyurethane (TPU) – Thermoplastic Polyurethane (TPU) is soft and elastic, with great tensile and tear strength. That’s why it’s often used to make parts that demand a rubber – like elasticity. You should know that TPU is more expensive than other resins but for many applications, like protective wire and cable sheaths, there really is no substitute. Another advantage is that TPU improves the grip for products that need to be held securely in the hand.

Thermoplastic Rubber (TPR) – Thermoplastic Rubber (TPR) resin is actually a mixture of plastic and rubber, and it’s easy to use in the injection molding process. It has outstanding chemical and weather resistance and high impact strength. Because of this, TPR is used in many types of fluid dispensers, flexible hoses, catheters, and other places containing different liquids, including acid.

You can find this recyclable material in medical catheters, suspension bushings and headphone cables. Thermoplastic rubber is also known as thermoplastic elastomer (TPE).

To learn more about plastic material for injection molding and its application, check the blog as shown below:

Plastic injection molding material and application


Injection moulding is used in many industries. From automotive to home appliances and medical devices, components in various plastics are used to protect, enhance and build a massive range of products. Yet only some manufacturers know precisely how their plastic parts are produced.

Injection molding is used to create many things such as wire spools, packaging, bottle caps, automotive parts and components, toys, consumer electronics, some musical instruments (and parts of them), health care products and beauty instruments, storage containers, mechanical parts (including gears), and most other plastic products available today. Injection moulding is the most common modern method of manufacturing plastic parts; it is ideal for producing high volumes of the same object.

Manufacturers worldwide use injection molding to mass-produce all kinds of goods. You can use injection molding machines to make anything from a large object like a garbage can to a small thing like a mobile phone case.

TDL Custom Injection Molding Service

TDL mold factory specializes in plastic injection molding, dual molding, and over-molding parts. The range ranges from caps to some of the most complex injection molded automotive fastening, connecting, suspension components, engine parts, and exterior/interior parts found throughout a vehicle. Our capabilities include plastic product design, engineering, simulation, prototyping, testing, mold manufacturing, and injection molding to give our customers the best overall injection molding services.

We take the time to understand the plastic component’s role in the customer’s manufacturing process or plastic product development to recommend a solution that meets the customer’s needs confidently – and surpasses the customer’s expectations.

Plastic injection molding: Right for your part?

Injection molding allows manufacturers to produce parts of consistent quality in massive quantities — an essential capability for modern production. Partner with TDL mold for your next mold project, and we’ll help you get the most out of your production run. Contact us today to get started.

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Gary Liao

Gary Liao

Gary Liao is the Engineering Manager of TDL Company and has more than 20 years of mold design experience.

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