Types of Molds

Types of Molds

This chapter explains the major types of molds and molding methods.

A mold is a device made of metal to produce a product faster, less expensive, and more consistently.
There are various types of molds for molding different products. Here are the major types of molds.

For example, a waffle maker is a kind of mold.

Casting Mold

Molds parts by pouring melted metal (liquid metal).


(1) Assemble a mold.		(2) Pour liquid metal.		(3) Open the mold after cooled down.

Typical Products

Engine (Cylinder block), Manifold, Wheel, Figurines

Forging

Forms parts by striking a material with a die.


(1) Place a material.		(2) Strike.		(3) Strike more.		(4) Complete.

Typical Products

Products that require intensity such as crank shaft, conrod, or knuckle.

More details
The forging method of softening a material by heat before striking is called hot forging. On the other hand, the method of striking a material without heating it up is called cold forging. Cold forging is used to mold a soft material such as aluminum.

Press Mold

Molds by pressing a sheet against a mold (normally used for metal sheets)


(1) Place a material.		(2) Press a mold.		(3) Complete.

Typical Products

body, gate, steel wheel, or attachable steer of automobile, a pull top of a can, aluminum ash tray, mug cup

More details

Press type of mold includes bending (molds a shape), trimming (trims off unnecessary parts), edging (processes edge), and drawing (bends the parts that cannot be bent by a bender). Some parts are molded through several procedures, and a progressive press molding is used to perform these procedures sequentially.

Die Cast Mold

Molds by applying pressure to melted metal (liquid metal) and inject it into a mold.


(1) Assemble a mold.		(2) Inject liquid metal with pressure.		(3) Open mold after cooled down.

Typical Products

Aluminum products such as parts of an engine, precision products

Plastics Mold

A plastics mold is used to mold parts by melting a molding material such as plastics with heat and pressure into the mold.
A plastics mold is used for various molding methods such as injection molding, compression molding, and extrusion. You will learn about injection molding later on. Now, let's discuss the other methods here.

Typical Products

Plastics products such as household goods and home appliances, automobile interiors, or general plastics products like a plastics bottle.

Compression Molding

Molds by putting material in a mold first and compressing it.


(1) Place a material.		(2) Press, close mold and compress.		(3) Open mold.

More details

Procedure: Put a proper amount of molding material of thermosetting plastics powder into the mold.; Close the top mold, and then heat up and pressure the mold.; The material will be softened and plasticated, filling up the mold.; Completely harden the material with more heat and pressure.; Open the mold and remove the part using a take out pin.
Advantages: As a molding material is put in the mold, it will not move around and distortion of parts can be reduced.; As the pressure for mold clamping is directly applied to the molding material, precise parts can be achieved.; As a gate is not required, there is no limitation on the type of molding material (granule, powder, etc.).; Therefore, it is used for molding of thermosetting plastics.; As the structure is simple, equipment cost can be reduced.
Disadvantages: If heating up when the mold is not completely closed or excessively pressured, the molding material may leak out of the mold.; If too much molding material is put in, it may overflow.
Many flashes will be generated.

Extrusion

A continuous extrusion is formed by pushing material (that is fed in through a hopper) with a screw, that also mixes the material, out of the device's exit. The material will have the same cross section as the device's exit.


(1) Put material in a hopper.		(2) Push out while stirring with screw.		(3) Complete.

More details

Procedure: Put a molding material in a hopper (material feed container).; Plasticate it by stirring and mixing it with a screw while heating it up.; Push the molding material out by the screw through a small hole of the apex mold (a clasp used to give the material a desired shape).; Finish molding by cold solidification.
Advantage: Performs molding continuously and effectively.
Disadvantage: Limited application as this method only molds parts that only have a cylindrical or tube-shaped section.

Blow Molding

Molds by putting a tube-shape material (parison) in the mold and blowing in air.


(1) Put tube-shaped material (parison) in mold.		(2) Blow in air.		(3) Complete

More details

Procedure: Pinch a part of a molding material that has been molded into a tube shape with a separate mold.; Blow compressed air into the molding material, causing it to expand until it conforms to the shape mold to mold the part.
Advantage: Widely used for molding of bottle containers.

Vacuum Molding

A sheet material is heated and soften conform to the shape of the mold using a vacuum.
Either a concave or convex mold can be used.


(1) Set material sheet.		(2) Vacuum air out.		(3) Allow air in again and remove part.

More details

Procedure: Soften a sheet of thermoplastics molding material with a heater.; Suck the air out of the mold through the vent hole to form a vacuum, causing the molding material to conform to the mold and assume its shape.; Allow air in again to remove the part.
Advantages: As molding pressure can be lowered than the atmosphere pressure (10,333kg/), plaster, wood, or thermosetting plastics can be used as a mold.
Large size parts can be molded with relatively low equipment cost.
Disadvantage: This method is generally not used for parts with a complicated shape.

RIM (Reaction Injection Molding)

Molds by mixing two or more reactive materials in the mold. The reaction produces a new substance that fills the mold.

More details

Procedure: Inject a molding material consisting of a mixture of a catalyst and an activator into the mold.; Cause a copolymerization in the mold.
Advantages: As this molding requires lower pressure than regular injection molding, an aluminum or fiber mold can be used.; Molding large sizes and complicated shapes is possible.
Disadvantages: A copolymerization generates gas, which compresses the air left in the mold and is likely to cause burns.
Molding cycle is extended.

FRP (Fiber Reinforced Plastics) Molding

Uses the fiberglass or carbon fiber as reinforcement.
There are two types of this molding - SMC (sheet molding) method and hand lay up method.

More details
Let's look at the detailed procedure now.

SMC (Sheet Molding Compound) method, or Sheet molding method

Procedure: Place a sheet molding material of fiber mat wrapped by polyester in the mold.; Press the material with heat and pressure.
Advantages: Thickness is uniform and complicated parts can be molded. Also, both sides of the parts can be finished smoothly.

Hand Lay Up Method

Procedure: Place a fiber mat along the mold shape.; Spread liquid resin over and harden at room temperature.
Advantages: This is used to mold relatively large FRP parts.; No machine equipment is necessary.
Disadvantages: Manually done, it is difficult to make thickness uniform, making the molding cycle extremely long.

Transfer Molding

Soften the material by heating in the cavity of the mold and then push it into the mold.


(1) Soften material in the heating cavity.		(2) Push into mold.		(3) Open the mold.

More details

Procedure: Soften a molding material in the heating cavity.; Push softened plastics into the mold by applying pressure.; Harden the molten plastics.; Open the mold and remove the part.
Advantages: The molding procedure is similar to injection molding; however, transfer molding involves heating of a molding material in the heating cavity to melt it into a molten plastics.
Widely used for molding of thermosetting plastics.; Transfer molding was developed to mold parts that are difficult to mold with compression molding, but currently is used for limited types of parts.; Best used to form a complicated shape or a thick molding.
Disadvantage: The production cost of mold is high.