Mechanical Engineering: Overview of the Mold and Die

Mold and Die

What is a mold?

A mold is a production tool that can produce parts with certain shape and size requirements. In industrial production, we need special tools installed on the press that are used to make materials into parts or products of the desired shape through pressure.

In plastic processing, a mold is used to form a complete three-dimensional plastic part. Although molds are classified into many types such as casting molds, metal-plastic forming, and mold injection molds, their functions are similar. The plastics processes that use molds are compression molding, injection molding, blow molding, thermoforming, and reaction injection molding.

Typical types of molds

Based on the number of parts, the basic types of molds used in plastic processing, regardless of whether they are compression, injection, transfer, or even blow molds, are usually classified by the type and number of cavities they have. They have been classified into three mold types: single-cavity mold, dedicated multiple-cavity mold, and family multiple-cavity mold.

Single-cavity Mold

Single-cavity molds represent one of the simplest mold concepts. Single Cavity Mold can only mold a single part per production cycle. The mold design is simple and the cost is low. This type of mold is ideal for low-volume production and large plastic part designs.

Dedicated Multiple-cavity Mold

A dedicated multiple-cavity mold has cavities that produce the same part. This type of mold is very popular because it is easy to balance the plastic flow and establish a controlled process. It can produce multiple parts per production cycle, so it is ideal for high volume and smaller parts. Because of the shorter lead time per batch, using multi-cavity mold results in faster completion of the desired number of parts. This increases productivity and produces a better yield rate for higher volume runs.

Family mold

In a family multiple-cavity mold, each cavity may produce a different part, which is ideal for prototype molds because of the fast molding cycles. A family injection mold has more than one cavity cut into the mold, allowing multiple various parts with the same material to be formed in a single cycle. Family mold is ideal for low quantity parts and suitable for prototypes rather than production parts.

Historically, family mold designs were avoided because of difficulty in filling uniformly. Family mold usually isn’t balanced when they are filling out because the parts often have different shapes, which can lead to an increase in molding defects. When multiple parts come out of the mold, there is a lot more handling needed to separate the parts. That typically involves a greater level of manual labor, since automation doesn’t work as well in the separation process. However, recent advances in mold making and gating technology make family molds more appealing.

Based on the mold opening mechanism, injection molds are classified into Two Plate Mold, Three Plate Mold, and Stacked Mold.

Two-plate mold

Two-plate mold is a type of injection molding that can be used to manufacture plastic parts. It’s one of the most common types of molding, and it’s very easy to perform. 

Two-plate molds are made up of two metal plates with holes in them. The two plates are separated by a space where the plastic will be injected during manufacturing. The holes on each metal plate helps guide the plastic into the space between them. This allows for more accurate placement of your part when you’re making it in injection molding machines. These molds are usually made from steel or aluminum, which makes them highly durable and easy to clean afterward.

Three-plate mold

Three-plate molds use an additional striper plate to house the runner. It has a double-action ejector system, and the runner does not eject with the part. Since the runners are separated when the part is ejected, there is no need for any secondary operations. This reduces the overall production cycle and enables faster production. Three-plate molds are great for high-volume production, but the initial setup costs are high.

Three-plate mold often referred to as runnerless mold or hot runner molds, have the flexibility to change gate locations anywhere on the part. Since the runners are on a different plate, you can place gates anywhere on the part.

The cost of making a three-plate mold is high. But it eliminates many extra steps, and for mass production, the higher tooling costs are negligible. In addition, compared to the two-plate mold, the three-plate mold has a good appearance surface. Therefore, if the part quality is an issue, then you should choose a three-plate mold.

Stacked mold

Stacked injection mold uses a grid of cavities. This means that multiple cavities can be configured in a single machine. For example, if you have four single cavities stacked in an injection machine, each cycle will produce four identical parts rather than just one. This boosts machine efficiency.

Based on the feeding system, injection molds are further decided into Hot runner mold and Cold runner mold.

Hot runner mold

The hot runner mold system is generally composed of hot nozzles, manifolds, temperature control boxes, and accessories. Hot runner molds use a preheated runner system that is usually heated by rods, coils, or heating elements. The runner system is usually housed in a fixed plate, which we usually call a three-plate mold. The runner system maintains high temperatures even when the part gets cold. This is why the runner is not ejected from the part, we call it a runnerless mold. This saves any extra process needed to remove the runner.

Hot runner molds are usually expensive because we need a heating mechanism to heat the runner. It also adds a separate plate to the mold, which adds to the cost. Besides, cleaning the runner system is a problem because the runners are hidden in the mold. Compared with cold runner molds, hot runner molds are more complicated to operate and maintain, If you want to change the material of the part, the mold needs to do a lot of work to adopt the new material.

Cold runner mold

The mold is called cold runner mold because they don’t have a heating mechanism for runners and parts are ejected with runners attached. Cold runner molds usually consist of two or three plates that are held within the mold base. The molten thermoplastic is first injected into the mold from a nozzle via the sprue, which fills the network of runners that lead to the mold cavities. In this system, the runners are unheated and act as a delivery system that distributes the molten plastic to the individual molding cavities. The cold runner system subsequently cools the sprue, runner, and gate along with the molded part.

Cold runner molds require a separate process to remove the runners. These additional processes add extra time and cost and are not suitable for mass production. The cold runner mold will also cause some waste due to the repair of the runner. While it is possible to regrind these runners, this affects part quality and nobody wishes to do so. However, cold runner molds are inexpensive to manufacture, and changing the material of a part does not require extensive maintenance. It’s really a good option for low-volume parts.

What materials does the mold use?

There are dozens of materials that can be used to make molds for producing plastic products, including many types of aluminum, brass, copper, epoxy, and many others, as well as combinations of these. Below are some of the more common materials and the role they play in the making of molds.


1020 carbon steel. This steel is used for ejector plates and ejector retainer plates and is easily machined and welded. Not usually hardened because of distortion and warp, this material must be first carburized if hardening is preferred.

1030 carbon steel. Used for mold bases, ejector housings, and clamp plates, this steel has 25% greater tensile strength than 1020 and can be easily machined and welded. It can be hardened to Rockwell hardness C scale (Rc ) 20 to 30.

1040 carbon steel. Commonly used for support pillars, this tough steel has good compressive strength and can be hardened to Rc 20 to 25.

4130 alloy steel. This is a high-strength steel used primarily for cavity and core retainer plates, support plates, and clamping plates, and is supplied at 26 to 35 Rc.

6145 alloy steel. The primary use of this type of steel is for sprue bushings and it is supplied at 42 to 48 Rc.

S-7 tool steel. Shock-resistant with good wear resistance, this steel is used for interlocks and latches and is hardened to 55 to 58 Rc.

O-1 tool steel. This is general-purpose, oil-hardening steel used for small inserts and cores and hardened to 56 to 62 Rc.

A-2 alloy tool steel. This steel has good dimensional stability and abrasion resistance, is used for hobs and slides, and is hardened to 55 to 58 Rc.

A-6 tool steel. A-6 tool steel is a general-purpose oil-hardening steel with good dimensional stability and high hardness. Its primary use is for optical quality cavities and cores, and it is hardened to 56 to 60 Rc.

D-2 tool steel. This steel has a high chromium and high carbon content, and is difficult to grind, but has excellent abrasion resistance. It is used for gate inserts, lifters, and slides, and is hardened to 58 to 60 Rc.

H-13 tool steel. This is a very high toughness, low-hardness steel used for high-quality cavity and core requirements. It is primarily used for ejector pins, return pins, sprue pullers, leader pins, and slide-actuating angle pins, and is supplied annealed at 15 to 20 Rc , but can be hardened to 60 Rc with little distortion.

P-20 tool steel. This is a modified 4130, commonly referred to as prehard. It is supplied at a Rc hardness of 28 to 40, which provides moderately high hardness, good machinability, and exceptional polishability. It is used primarily for cavities and cores, as well as stripper plates.

420 stainless steel. Used in applications requiring exceptional chemical resistance (such as molding PVC resins), this steel is usually supplied in an annealed condition (15 to 25 Rc ) but can be hardened to 55 to 60 Rc. Its primary use is as steel for cores and cavities.

AISI-SAE Tool Steel Grades

Defining property AISI-SAE grade Significant characteristics
Water-hardening W  
Cold-working O Oil-hardening
A Air-hardening; medium alloy
D High carbon; high chromium
Shock resisting S  
High-speed T Tungsten base
M Molybdenum base
Hot-working H H1–H19: chromium base
H20–H39: tungsten base
H40–H59: molybdenum base
Plastic mold P  
Special purpose L Low alloy
F Carbon tungsten


While there are many grades of aluminum available for making molds, the most common, and most efficient to work with, is the aircraft-grade 7075 (T6). This wrought aluminum alloy is produced by hot rolling cast aluminum to the desired thickness of the plate. The entire mold can be made of the same material (including cavity and core) and an anodizing process can be utilized to impart a surface hardness of up to 65 Rc for wear resistance.

However, due to the smoothing tendency of the normal aluminum surface, it is possible to mold with no surface treatment. The microscopic hills and valleys of the aluminum surface tend to even themselves out without galling. The use of 7075 aluminum can result in mold build times being reduced by up to 50% (due to faster machining times) and the injection molding cycle is reduced by up to 40% (due to faster heat dissipation), depending on the size and complexity of the product being molded.

Until recently, aluminum was considered a mold-making material only for low-volume production or prototype molds. The use of 7075 alloys has created opportunities to use aluminum for high-volume production in up to millions of cycles. Even glass-reinforced and high-temperature plastics can be molded successfully in aluminum molds.

Different Grades of Aluminum

Series Alloying Element Heat Treatable* Extrusion Rolled Cast
1XXX (e.g. 1050) None – 99% Al No No Yes Yes
2XXX (e.g. 2014) Copper – Cu Yes Yes No Yes
3XXX (e.g. 3103) Manganese – Mn No No Yes No
4XXX (e.g. 4015) Silicon – Sn No No Yes No
5XXX (e.g. 5251) Magnesium – Mg No No Yes Yes
6XXX (e.g. 6063) Magnesium & Silicon – Mg & Sn Yes Yes Yes No
7XXX (e.g. 7075) Zinc – Zn Yes Yes Yes Yes

Beryllium-copper Alloys

High strength and high levels of thermal conductivity make the beryllium-copper alloys excellent selections for making cores and cavities for injection molds. They are commonly used as components that are fitted to steel mold bases, but also can be used in conjunction with aluminum mold bases for greater economy.

They are particularly useful for situations where the placement of cooling channels in the mold makes heat removal difficult, such as in deep draw parts or parts with unusual contours. Strategically placed beryllium-copper components will assist in dissipating the heat from these areas without using complicated water line channels.

The types of beryllium-copper most commonly used for cores and cavities are CuBe 10, CuBe 20, and CuBe 275. They differ mainly in tensile strength; the higher numbers having the greater strength. In addition, the higher number of grades allows for higher levels of hardness. This ranges from a low of Rb 40 for CuBe 10 to a maximum of Rc 46 for CuBe 275.

Other Materials

There are other materials that can be used for making molds for plastic injection molding, including epoxy, aluminum/epoxy alloys, silicone rubbers, and even wood. However, these are usually all reserved for very small volumes, such as under 100 pieces. In most cases, these represent low-volume prototyping, and the molds are not expected to meet the demanding requirements of higher-volume production levels.

What is a Die?

A die is a specialized machine tool used in manufacturing industries to cut and/or form material to a desired shape or profile. Unlike mold shaping complete parts directly, a die is used to form two of the three dimensions of a part. The third dimension, usually thickness or length, is controlled by other process variables.

Like molds, dies are generally customized to the item they are used to create. Die is mostly used in the forming or stamping process, in which the desired product shape is made in the die.  Die is usually made from tool steel (a kind of carbon steel and alloy steel that is particularly well-suited to be made into tools and tooling, including cutting tools, dies, and hand tools). Tool steels are specially alloyed steels designed for high strength, impact toughness, and wear resistance at room and elevated temperatures. Dies are useful because they can cut many objects at once, increasing productivity.

Typical types of dies

Generally, dies are classified according to their use, like stamping dies are used in press working, casting dies used in molding processes, and drawing dies are used in the manufacture of wires. For different purposes, dies are divided into the below types of dies.

#1 Simple Die

Simple die also known as a single operation die, the simple die is a shaping tool that performs one operation per press slide stroke. This type of die is typically used for smaller applications in the workplace. For example, it may be useful for the manufacturing of simple metal parts.

A simple die can be further classified according to functions such as cutting and forming. Cutting dies are used in operations like trimming, notching, blanking, and more. The forming die is used for bending, curling, etc.

#2 Compound Die

A compound die performs multiple operations like cutting or punching can be done in one stroke. For more demanding or complex blanking and piercing applications, a compound die can do the job faster. Rather than handling just one operation at a time, the compound die can simultaneously complete the blanking and piercing processes. This is a very effective method for high-volume parts. 

The compound die is less useful for bending and forming operations, and it tends to require a higher level of force than some of the other options. It’s a more cost-effective option than the simple die when it comes to manufacturing washers and other flat metal parts. If you’re looking for types of punches and dies that you can use in general cutting applications, the compound die could be the solution you need.

#3 Combination Die

The combination die is similar to the compound die in terms of design and efficiency. It can handle more than one operation at once, which allows it to deliver faster, more reliable results. As an added bonus, the combination die is well-suited for both cutting and shaping applications.

If you need to complete a blanking or punching operation combined with a bending operation, the combination die will have you covered. This versatile tool can play a role in all types of metalwork applications, from mining equipment manufacturing to electronics and appliance development.

#4 Transfer Die

The transfer die uses a single press to operate multiple tools. In high-volume production work, transfer dies come with more advantages. It can freely transfer the work by adding more shape to the part until the metal workpiece achieves the final shape.

Earlier, these processes were performed using individual presses and the workpiece moved from press to press and die to die by hand. The automation of the transfer process streamlines the operation in a single press.

#5 Progressive Die

While a progressive die can handle more than one operation at a time, it does so in several stages throughout multiple work stations. The progressive die’s main advantage is that it’s more efficient thanks to its high work speed and the reduced level of force.

While the multi-station design is more challenging to manage than the single-station unit, it’s easier for the progressive die to maximize punching productivity. That’s why engineers use progressive dies to make automotive parts, electronics and similarly complex components.

What materials does the die use?

The cutting die and forging die mainly use the alloy steels and tool steels like the mold use. When selecting tool steels for different applications, the metallurgical factors that ultimately affect tool performance must be considered. These include hardness and strength, toughness and wear resistance. Basically, the higher the hardness, the higher the strength. Higher hardness generally also corresponds to higher wear resistance.

However, high hardness and strength also generally correspond to lower toughness, or in other words, lower resistance to cracking under impact or sudden shock loads. The specific hardness, wear and toughness requirements of the application dictate which tool steel should be used.

What is the difference between a mold and a die?

Molds and dies are are tools for shaping materials into parts or products and they are very important elements in product manufacturing. Molds are usually used in injection molding to shape melted resin or cast molten metal. Dies are mainly used to cut and shape metal to a desired shape or profile. Dies and molds used similar alloys materials in most cases. but sometimes, some projects don’t need a robust mold made from high-strength alloys. Then we may use some other materials to make molds like plastics, silicone, woods instead. 

  Material Process
Mold Steels, aluminum, copper alloys and other materials that can be formed Take more time, but refined and strong
Die Alloy steels and tool steels and other high-strength alloys Faster and more efficiency


What is molding/moulding(manufacturing)?

Molding is the manufacturing process of shaping raw material using a rigid frame called a mold or matrix. The mold is made using a pattern or model of the final products.

What is tooling?

Tooling, also known as machine tooling, is the process of acquiring the manufacturing components and machines needed for production. Fixtures, jigs, gauges, molds, dies, cutting equipment, and patterns are all examples of machine tooling.

What is the difference between a mold and a die?

A mold is a hollow form or matrix for giving a particular shape to raw material in a molten or liquid state. Die is any kind of device tool for cutting or forming material in a press or a stamping or forging machine.