Injection Molding: What It Is? How It Works? Who Is It For?

Injection molding is considered to be the mainstream technology in the world’s manufacturing industry, and more than 30% of the world’s products are manufactured using injection molding technology.

plastic parts manufactured by Injection Molding

Because the equipment is expensive, injection molding is often used in the production of bulk plastic machining based on economies of scale – the more parts, the lower the unit cost.

Other advantages of injection molding include high tolerances, repeatability, wide material selection, low labor costs, and standard post-processing.

plastic parts manufactured by Injection Molding

Injection molding is a very mainstream way of manufacturing plastic parts in the market, and the market value of this technology in the world has reached 260 million US dollars. Injection molding is widely used in aerospace, medical and other fields, and many complex parts are manufactured through this advanced technology with extremely high precision.

What is Injection Molding?

Injection molding refers to a method in which a material melted by heat is injected into a mold cavity by high pressure, and after cooling and solidification, a molded product is obtained. This method is suitable for the mass production of complex-shaped parts and is one of the important processing methods.

plastic parts manufactured by Injection Molding

As early as 1872, brothers John and Isaiah Hyatt patented injection molding technology. However, the “golden age” of injection molding came after World War II in the mid-20th century. Due to the high demand for low-cost, mass-produced parts, injection molding, on the one hand, can produce a large number of parts, and on the other hand, the cost of mass production is relatively low, so it has become a good choice.

How Does Injection Molding Work?

Injection molding is mainly divided into 6 stages, namely:

  • Compound die;
  • Injection;
  • Dwell;
  • Colling;
  • Opening Mould;
  • Product Take Out.

First of all, the injection molding machine is composed of three main parts: the feeding hopper, the screw, and the heating barrel. When the plastic material appears in the hopper, it will use the friction of the screw to heat the plastic. When the plastic reaches the desired temperature, the next step is injected into the mold. After the final cooling treatment, the desired form is produced.

plastic parts manufactured by Injection Molding

Second, if necessary, multiple parts and shapes can be molded repeatedly. Plastic parts can also be combined with parts of other materials using insert molding. The basic principle of this injection molding technology seems very simple to us, but this process is very complex, requiring suitable equipment and corresponding expertise.

Injection Molding Conditions

The injection molding conditions refer to the barrel temperature of the molding machine, the injection speed, and the mold temperature.

Types of Injection Molding

Rotational Moulding: Using this technique, the material is placed in a mold that is slowly rotated over a heating furnace. The material begins to melt and adhere to the inner surface of the mold, forming the object layer by layer. Rotational molding allows simple to complex shapes, such as industrial tanks, to be manufactured without the need for gluing or welding.

Extrusion Molding: This method is to compress the molten material (usually plastic) into a two-dimensional hole. It goes through a series of molds, where it melts to get its shape. When the product is ready, it forms a long two-dimensional shape that can be cut into multiple parts.

inject molding products manufactured by YIJIN Hardware

Metal injection molding: A metal processing process. After forming a block from metal powder and binder material, injection molding techniques are used to shape and cure, and then the part is post-processed at a high temperature. Finally, the binder is removed and the powder is sintered to obtain the product.

Reaction Injection Molding: Using thermosetting polymers rather than plastics, the blocks are pressed into a mold and mixed with a catalyst, which is then left to cure. The most common reaction injection molding material is polyurethane.

plastic parts manufactured by Injection Molding

Liquid Silicone Injection Molding: Liquid silicone injection molding is very similar to reaction injection molding: liquid silicone (rubber) is injected into the mold and then cured. This technique can be used to evaluate different material configurations prior to full production.

Injection Molding Material

There are thousands of materials used for injection molding, mainly composed of various polymers and plastics (some thermoplastics and elastomers). All materials produce extremely low viscosity melts that rapidly fill complex mold cavities at low injection pressures. Combined with low injection pressure, the possibility of producing highly stressed parts is minimized. Commonly used injection molding plastic machining materials include:


Polyethylene (PE)

Polyurethane (PU)

Polycarbonate (PC)

Polypropylene (PP)

Nylon (Polyamide)

How to Solve These 4 Common Problems Encountered in Injection Molding?

1. The Solution to the Bubble Phenomenon in the Injection Molding Process

According to the causes of bubbles, the countermeasures are as follows:

When the wall thickness of the product is large, the cooling rate of the outer surface is faster than that of the central part. Therefore, as the cooling progresses, the plastic resin in the central part expands to the surface while shrinking, resulting in insufficient filling in the central part.

plastic parts manufactured by Injection Molding

This situation is called a vacuum bubble, and the solutions mainly include:

  • Determine the reasonable sprue gate and sprue gate size according to the wall thickness. The general sprue gate height should be 50% ~ 60% of the product wall thickness.
  • Leave some supplementary injection material until the sprue gate is sealed.
  • The injection time should be slightly longer than the sprue gate sealing time.
  • Reduce injection speed and increase injection pressure.
  • Use materials with high melting viscosity.

Bubbles are caused by the production of volatile gas, the main solutions are:

  • Pre-dry thoroughly.
  • Reduce resin temperature to avoid decomposition gas.
  • The bubbles caused by poor fluidity can be solved by increasing the temperature of the resin and the mold and increasing the injection speed.

2. Analysis of the Reasons for the Main sprue Sticking to the Mold During Injection Molding

Causes and elimination methods of the main sprue sticking to the mold during injection molding:

  • The cooling time is too short, and the sprue has not yet solidified.
  • The slope of the sprue is not enough, and its demolding slope should be increased.
  • Improper matching of sprue bushings and nozzles causes leakage.
  • The sprue is rough and there is no cooling well in the sprue.
  • The nozzle temperature is too low, the temperature should be increased.

3. Analysis of Reasons for Slow Production During Injection Molding

The reasons and solutions for slow production during injection molding are as follows:

  • The plastic temperature and the mold temperature are high, resulting in a long cooling time.
  • If the glue melting time is long, the back pressure should be reduced, the recycled material should be used less to prevent overhead, and the cooling of the feeding section should be sufficient.
  • The action of the machine is slow, and it can be properly accelerated by adjusting the oil circuit and circuit.
  • Mold design should be easy to demoulding, as far as possible designed to be fully automatic operation.
  • Too much wall thickness, resulting in too long cooling time.
  • Nozzles drool, hindering normal production. A self-locking nozzle should be used, or the nozzle temperature should be lowered.
  • If the heat supply of the barrel is insufficient, the machine with a large plasticizing capacity should be replaced or the preheating of the material should be strengthened.

4. Cause Analysis of Insufficient Injection Material in Injection Molding Products

The main reasons for insufficient injection of injection molding products are lack of material and improper injection pressure and speed (including excessive pressure loss caused by resistance). It may be caused by the following reasons:

Reason for injection molding machine

  • The plasticizing amount or heating rate of the machine is uncertain, so the machine with a large plasticizing amount and heating power should be selected;
  • The plasticizing amount or heating rate of the machine is uncertain, so the machine with a large plasticizing amount and heating power should be selected;
  • The wear of the screw and the cylinder or the glue head causes the material to return and the actual filling modulus is not in the middle;
  • The actual temperature of the barrel is too low due to the failure of the heating system such as thermocouple or heating coil;
  • The wear of the sealing element of the injection cylinder causes oil leakage or backflow, and the required injection pressure cannot be achieved;
  • The nozzle inner hole is too small or the nozzle center is improperly adjusted resulting in excessive resistance and pressure consumption.

Reasons for injection mold

  • If the temperature of the mold part or the whole is too low, it is difficult to feed the material, and the mold temperature should be appropriately increased;
  • The distribution of the cavity of the mold is unbalanced, and the wall thickness of the part is too thin, resulting in excessive pressure consumption and poor mold filling. The wall thickness of the whole part or part should be increased or the auxiliary flow or gate can be set up in the vicinity of the insufficient filling.
  • The mold sprue is too small resulting in pressure loss; Too large will appear weak glue shooting; Too rough will result in inadequate product injection.
  • The size of the sprue should be appropriately set, and the transition between the main sprue and the branch sprue, the transition between the gates, or the turn of the gate should be applied with an appropriate arc transition.
  • The poor exhaust of mold. The material entering the cavity is blocked by the gas pressure that is too late to be discharged, resulting in the insufficient filling. We can make full use of the gap of the screw to exhaust or reduce the clamping force to use the parting surface to exhaust, and open exhaust sprue or pores if necessary.

Analysis of the Advantages and Disadvantages of Injection Molding

Advantages of Injection Molding:

  • Can form complex shapes and details
  • Excellent surface finish
  • Perfect accuracy
  • High productivity
  • Low labor cost
  • The waste can be recycled

Disadvantages of Injection Molding

  • Limited to thin-walled parts
  • High tool and equipment cost
  • Long lead times are possible

What is Injection Molding Used for?

The application of the injection molding process is very extensive:

  • Can be used in various automotive electronic products, such as tire pressure monitoring systems, seat occupant sensors, seat belt lock sensors, ECU/air quality sensors for motor vehicles, antennas for RF devices, smart key systems, etc.
  • Can be used for mobile phone batteries, antennas, flashlights, and other vulnerable parts that need waterproofing, protection, or packaging.
  • Can be used for PCB packaging
  • Retaining ring and wiring harness
  • And waterproofing of connectors/sensors in other industries, etc.


The injection molding process is being used in more and more industrial fields such as aviation, automobiles, mobile phones, medical devices and so on. The low injection pressure can ensure the quality of the product, the short cycle improves the efficiency of the work, and the recyclability of the glue can maximize the use of raw materials to save costs.

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