What is injection moulding

What is injection moulding

Introduction to injection molding

Injection moulding definition is one of the processing methods for mass production of plastic products.

The basic principle of injection molding is:

The hot-melted plastic raw material is pushed from the screw of the injection molding machine into the cavity of the injection mold.

After cooling and solidifying, the plastic formed product is obtained.

Injection mold composition

Injection molding step decomposition 1
Injection molding step 1

The injection mold consists of two parts:

  • Movable mold
  • Fixed mold

The movable mold is mounted on the moving template of the injection molding machine

Injection molding step decomposition 2
Injection molding step 2

The fixed mold is mounted on the fixed template of the injection molding machine

In the injection molding, the movable mold and the fixed mold close constitute the casting system and the cavity

The movable mold and the fixed mold are separated when the mold is opened

Injection molding step decomposition 3
Injection molding step 3

So that the plastic product can be separated from the plastic mold.

Although the structure of the mold differs depending on the type and performance of the plastic

The shape, and structure of the plastic product, and the type of the injection machine, the basic structure is uniform

Injection molding step decomposition 4
Injection molding step 4

Injection Molds Common Sense

The mold is mainly composed of

  • Mold gating system,
  • Mold temperature control system
  • Molded part
  • Mold structural part.

The mold gating system and the molded part are in direct contact with the plastic and vary with the plastic and the product.

It is the most complicated and most varied part of the mold and requires the highest degree of smoothness and precision.

You can also read injection moulding (Wikipedia) for reference.

About the Injection mold Feed System

The Feed System of Plastic Molds, also known as the runner system.

This refers to a set of feed channels that lead the plastic melt from the nozzle of the injection machine to the cavity.

Including the main channel, the cold material hole, the runner and the gate, etc

which is directly related to the plastic product, Molding quality, and production efficiency.

Molded parts refer to various parts that make up the shape of the product.

Including moving molds, fixed molds and cavities, cores, forming rods, and exhaust ports.

About the mould main runner

It is a passage in the mold that connects the nozzle of the injection machine to the splitter or cavity.

The top of the main flow path is concave to engage the nozzle.

The inlet diameter of the main flow channel should be slightly larger than the nozzle diameter (O.8mm) to avoid flashing and prevent the two from intercepting due to inaccurate connection.

The diameter of the inlet depends on the size of the product, generally 4-8mm.

The diameter of the main flow path should be enlarged inward by an angle of 3° to 5° so that the flow path retains the release of the plastic.

About the Cold-Slug Well

The Cold-Slug Well is a cavity at the end of the main flow path to trap the cold material generated between the two injections at the end of the nozzle to prevent the clogging of the runner or gate.

If the cold material is mixed into the cavity, internal stress is easily generated in the manufactured product.

The cold pocket has a diameter of about 8-10 mm and a depth of 6 mm.

In order to facilitate demolding, the bottom is often carried by the stripper.

The top of the demolding rod should be designed as a zigzag hook or a recessed groove so that the plastic retained in the main channel can be smoothly pulled out during demolding.

About the mould Sub-runner

The Sub-runner of the plastic mould is the passage connecting the main channel and each cavity in the multi-slot mold.

In order for the melt to fill the cavities at equal speed, the
Sub-runner on the mold should be symmetric and equidistant.

The shape and size of the cross-section of the runner have an effect on the flow of the plastic melt, the release of the product and the difficulty of mold manufacturing.

If the flow is equal to the amount of material, the flow path resistance with a circular cross-section is the smallest.

However, due to the small specific surface of the cylindrical flow path

the cooling of the residual plastic of the shunting channel is disadvantageous

and the shunting channel must be opened on the two mold halves, which is laborious and easy to align.

Therefore, a trapezoidal or semi-circular cross-section runner is often used and is placed on one half of the mold with the stripper rod.

The runner surface must be polished to reduce flow resistance to provide faster-filling speed.

The size of the flow channel depends on the type of plastic, the size, and thickness of the product.

For most thermoplastics, the cross-sectional section width is no more than 8m, and the extra-large can reach 10-12m, and the extra small is 2-3m.

The cross-sectional area should be minimized if the requirements are met, so as to avoid increasing the plastic retained by the runner and prolonging the cooling time.

Mold cavity

The mold cavity is the space for molding plastic products in the mold.

The components used to form the cavity are collectively referred to as molded parts.

Each molded part often has a special name:

The molded part constituting the outer shape of the product is called a female mold (also called a female mold)

and the inner shape (such as a hole, a groove, etc.) constituting the inner shape of the product is called a core or a punch (also called a male mold).

When designing a molded part, the overall structure of the cavity should be determined according to the properties of the plastic

the geometry of the product, the dimensional tolerances and the requirements of use.

Secondly, the position of the parting surface, the gate and the vent hole, and the demolding mode are selected according to the determined structure.

Finally, the design of each part is carried out according to the size of the control item and the combination between the parts is determined.

The plastic melt has a high pressure when it enters the cavity, so the molded parts should be properly selected and checked for strength and stiffness.

To ensure the smooth and beautiful surface of plastic products and easy release

Where the surface in contact with plastic has a roughness Ra > 0.32 um and is resistant to corrosion.

Molded parts are generally heat-treated to increase hardness and are made of corrosion-resistant steel.

About Plastic Mould Gate

The gate is the channel that connects the main runner (or the sub-runner) to the cavity.

The cross-sectional area of the channel can be equal to the main runner (or sub-runner)

but it is usually reduced. So it is the smallest part of the cross-sectional area of the entire runner system.

The shape and size of the gate have a great influence on the quality of the product.

Mould Gate effect

  • Control flow velocity
  • In the injection, the molten material stored in this part can be solidified early to prevent backflow.
  • The passing melt is subjected to strong shearing to raise the temperature, thereby lowering the apparent viscosity to improve the fluidity
  • Easy to separate the product from the runner system

The shape, size, and location of the gate depending on the nature of the plastic, the size and structure of the product.

Generally, the cross-sectional shape of the gate is rectangular or circular, and the cross-sectional area should be small and the length should be short.

This is not only based on the above effects, but also because the small gate becomes larger and easier, and the large gate is difficult to shrink.

The gate location should generally be selected where the product is thickest and does not affect the appearance.

Gate size design should take into account the nature of the plastic melt.

Temperature control system

In order to meet the mold temperature requirements of the injection process

a temperature adjustment system is required to adjust the temperature of the mold.

For injection molds for thermoplastics, the main purpose is to design a cooling system to cool the mold.

The common method of mold cooling is to open a cooling water channel in the mould

and use the circulating cooling water to take away the heat of the mold.

The heating of the mold can be done by using hot water or steam in the cooling water passage

and electric heating elements can be installed inside and around the mold.

Molded part

The molded part is composed of a core and a die and refers to various parts constituting the shape of the product, including a movable mold

a fixed mold, and a cavity, a core, a forming rod, an exhaust port, and the like.

The core forms the inner surface of the article and the die forms the outer surface shape of the article.

After clamping, the core and cavity from the cavity of the mold.

According to the process and manufacturing requirements

sometimes the core and the die are made up of several pieces, sometimes made into a whole

and only the inserts are used in the parts that are easily damaged and difficult to process.

Mold exhaust port

The mold exhaust port is a trough-shaped air outlet opened in the mold to discharge the original and the gas brought in by the melt.

Otherwise, the product will have pores, poor connection, filling mold dissatisfaction

and even the accumulated air will burn the product due to the high temperature caused by compression.

In general, the vent hole can be disposed at the end of the flow of the melt in the cavity or on the parting surface of the mold.

The latter is a shallow groove with a depth of 0.03-0.2mm and a width of 1.5-6mm on one side of the die.

During the injection, the vent hole will not have a lot of melt oozing out, because the melt will cool and solidify there to block the channel.

Do not open the position of the exhaust port to the operator, in case the fuse is accidentally sprayed and wounded.

In addition, the matching gap between the ejector rod and the ejector hole

the matching gap between the top block and the stripper and the core may be utilized to exhaust.

Mould Structural part

Structural part refers to the various parts that make up the mold structure

including guiding, demolding, core pulling and various parts of the parting.

Such as front and rear splint, front and rear buckle template, bearing plate, pressure column, guide column, stripping plate, stripping rod and return rod.

Guide member

In order to ensure accurate alignment of the moving and fixed molds during mold clamping, guide parts must be provided in the mold.

In the injection mold, four sets of guide columns and guide sleeves are usually used to form the guide.

Sometimes it is necessary to set the inner and outer cones that match each other on the movable mold and the fixed mold to assist the positioning.

Launch agency

In the mold opening process

an extrusion mechanism is required to push or pull out the plastic product and its aggregate in the flow channel.

Introducing a fixed plate and a push plate for holding the pushrod.

A reset lever is generally fixed in the push rod, and the reset lever resets the push plate when the mold is fixed and fixed.

Side core pulling mechanism

Some plastic products with undercuts or side holes must be laterally typed before being pushed out.

After the lateral core is pulled out

the mold can be smoothly demolding. At this time, aside core-pulling mechanism needs to be provided in the mold.

Standard mould base

In order to reduce the heavy mold design and manufacturing workload, most of the injection molds use standard mold bases.

We are an injection moulding manufacturer who specialized in plastic molding, if you have any questions about plastic injection molding, please feel free to contact us!

2019年6月11日 Industry News ,