An Introduction to Plastic Injection Molding

The designer should always decide whether using injection molding is the best approach to mold the item before moving further with it. Have other approaches or product designs that employ different production techniques and utilize the same, comparable, or different materials—or materials that would allow for a similar final result, or even one of higher quality and/or cheaper cost—been thought about or investigated?

This post helps you understand the plastic injection molding process and costs. Our goal is to enable our customers to manufacture satisfactory plastic parts more efficiently and at lower cost. We hope this post is useful to you.

Plastic Injection Molding

Custom Plastic Injection Molding

Plastic Injection molding is one of the most cost-effective manufacturing processes used to produce large quantities of plastic parts with consistent dimensions. It is a cyclic process of rapid mold filling followed by cooling and ejection.

The material, which is generally available as grains or powder, is plasticised in an injection unit and injected into a clamped mold under high pressure. The process also produces very little waste, and discarded thermoplastic materials can be recycled back into the machine for use in production.

The main advantage of injection molding is that it is a very economical method of mass production. Ready parts with tight tolerances can be produced in one step, often completely automatically.

Although the initial mold for plastic injection molding costs more than many processing methods, the economies of scale gained in mass production offset the initial setup costs.

RpProto provides customers with custom plastic injection molding production. Our factory in Xiamen, China, has nearly a hundred injection molding machines, making us the most efficient and price-competitive injection molding factory in China.

Advantages of Injection Molding

Produce at High-Volumes

Injection molding allows you to produce a very high number of parts (hundreds of thousands), all using the same process.

Complex Geometry

Injection molding enables you to manufacture complex-geometry parts with high tolerances and very highly detailed features.

Lower Costs and Greater Efficiency

Automated injection molding processes has very high efficiency and lower cost by removing labor cost.

Material Flexibility

There are thousands of unique types of plastics with various strength and rigidity, clarity and flexibility, or antimicrobial or biodegradable properties.

Consistency Quality of Parts

Injection molding is designed to create consistent, identical large qauntity of parts.

Injection Molding Machine

The basic parts that make up a commercially available injection molding machine are shown below.

Basic components of an injection moulding machine
  • The control unit and control cabinet allow the machine operator to control and monitor the process.
  • The injection unit is there to melt and meter the material into the tool.
  • The clamp unit holds the injection moulding tool and gives the required clamp force to hold the two sides of the mould tool together.
  • The machine base with hydraulics houses the systems that drive the movement of the machine. Each machine has a certain ‘footprint’. This is the floor space it requires for operation.

The Injection Moulding Process

In injection molding the mold and the plasticising area are separated from each other. The plasticising area, i.e., the plasticising cylinder temperature, is kept at the level of the processing temperature. The mold on the other hand, is kept cold enough for demolding of the injection molded part (thermoplastics) or warm enough for crosslinking (thermosets).

The plasticised material is injected into the clamped mold. In an injection molding machine, the clamping unit which contains the mold and the injection unit are integrated. Completely automated production is possible if the mold is installed with a vertical parting line. This enables the parts to fall down and out of the mold after demolding.

Injection molding machines are typically used for the processing of thermoplastics. There are two types of injection unit available: a piston injection unit and a screw piston injection unit (reciprocating).

The reciprocating screw method is the most common. For the processing of thermosets only screw piston machines can be used. This is because without the screw, the dwell time would be too long and the risk of early crosslinking would be too high.

Injection Mould Tooling


An injection mould tool has two major purposes:

  •  It is the cavity into which the molten plastic is injected
  •  The surface of the tool acts as a heat exchanger (as the injected material solidifies with contact)

Injection mould designs differ depending on the type of material and component being moulded. Mould tool design and component design are equally important considerations for success. Component design is beyond the scope of this book but the various tooling, gating, temperature control and ejection systems that make up the mould tool will be considered here. After parts are injection moulded they must be ejected. A variety of mechanisms can be employed such as ejector pins, sleeves, plates or rings.
The design standard for injection mould tools is the two-plate design.

Types of Moulds

Two-Plate Mold

This is the simplest mold design. mold cavities are formed in one plate only with the stationary half of the mold blank. A central sprue bushing can be placed into the stationary half of the mold or it is possible to have a direct runner system to a multi-impression mold. The moving half of the mold contains the ejection mechanism.

Two-plate injection mould

Stripper Mold

A stripper mold is very similar to the standard two-plate mold except for the ejection system. This design has a stripper plate for ejection, whereas the standard one has pins or sleeve as the ejectors. This is illustrated in below picture. The advantage of a stripper plate is the increased surface area for ejection that it offers.

Mould with stripper plate

Slide Mold

Like the two previous designs, this is also a two-plate mold. However it has slides and cam pins for additional lateral movement as shown in below Figure. This type of design is suitable for producing parts with undercuts or external threads.

Slide mould

Three-Plate Mold

These are normally used when multi-cavities are involved and semi- or fully-automatic working is required. This type of mould, as its name suggests, has an extra plate. This plate usually continues the gate on one of its sides with the complete runner system, preferably trapezoidal. The opposite side of the plate carries part of the mold form (usually the female part).

Three-plate mould

When the mold opens the plate is separated by means of a delayed action mechanism (e.g., chains or length bolts), so breaking the restricted gate. The moldings are then ejected from one daylight and the sprue and runner system are ejected from the other.

Successful ejection of moldings relies on clean separation of the molding and gate at the parting line. With this method of tooling, restricted gates of the correct design must be used.

Multi-plate molds are usually more expensive than two-plate molds and can be slower in production if an operator has to remove the sprue and runner system when the mold is open. This can usually be avoided by providing automatic ejection of sprue and runner. The distance travelled by the plates is governed by the length of the chain or the length of the bolts used to separate them.

Looking for a supplier for injection molding?

Injection molding should be an excellent option for your project if you need to efficiently and consistently create a large number of parts and are willing to invest in creating a high-quality mold to make your parts.

Talk to a RpProto  to see if your project is right for injection molding.

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