This article examines the types of plastic injection molds and the methods of preparing and using them. Plastics are divided into two groups: 1. Thermoplastic molds 2. Thermoset molds (Bakelite):

Thermoplastic Molds:

The thermoplastics or thermoplastic group, which become pliable upon heating and regain their hardness as the temperature decreases, undergo no chemical changes and take on the shape of the mold cavity after injection.

In plastic injection molding, the heated thermoplastic fills the mold cavity; in this method, the heated thermoplastic and the mold cavity are cold, and after injection, the material takes the shape and form of the mold and hardens.

From another perspective, thermoplastics are materials that can be reused after one or several uses in the production process. These materials are poured into the injection machine in the form of granules or powder.

Structure of Injection Molds:

Plastic molds are generally divided into two types:

1- Plastic injection molds with cold runners

2- Plastic injection molds with hot runners

And also, structurally, they are of two types:

1- Two-plate plastic injection molds

2- Three-plate molds, where the number of mold plates and their separation line is determined based on factors such as the number of mold cavities, the shape of the plastic part, the type of injection machine, the type of materials used, and the air outlet system, etc. Essentially, every injection mold has two main sections:

A- The fixed part of the mold (half-fixed), where the heated plastic injection material is injected.

B- The movable part (half-movable), which is closed in the movable section of the injection machine, and the system and mechanism for ejecting parts are mostly located there.

Determining the number of cavities and mold chambers is an important aspect of designing injection molds, and plastic molds in this regard are of 2 types:

1- Single-cavity plastic injection molds

2- Multi-cavity plastic injection molds

Single-cavity plastic injection molds:

Single-cavity molds are used when the production quantity of the plastic part is limited; therefore, the design and construction of single-cavity molds will be cheaper in terms of production time and economic issues.

Multi-cavity plastic injection molds:

If the number of produced products is high, especially in cases where the part is small, the design and construction of multi-cavity plastic injection molds are used.

Thermoset Plastic Molds (Bakelite):

The group of thermoset plastics or Bakelite or heat-hardening plastics, which harden upon exposure to heat and cause chemical changes in these materials, are referred to as thermosets or Bakelite.

In this method, the mold is in a cold state, and the materials are also cold; after feeding, the mold is subjected to heat, and the materials take the shape and form of the mold cavity and harden.

Thermoset plastics or duroplasts are produced under pressure and heat of 170 degrees Celsius. They first soften and become plastic but harden after a while, and the main characteristic of these materials is that after hardening, they cannot be softened again or reused and are not soluble in any type of solvent, and significant chemical changes occur in them after hardening.

Types of Thermoset Material Molds (Bakelite)

In the thermoset molding method, the materials gradually heat up in the mold cavity and are then fed into the heated mold, and these softened materials take the shape and form of the mold cavities due to the pressure of the mold and cool down due to chemical changes and are ejected from the mold.

The molding of thermoset materials is carried out by three specific methods; however, other methods such as extrusion and ... are also used.

1- Transfer molding

2- Pressure molding

3- Piston pressure molding

1- Transfer molding:

In this method, materials are injected into the mold cavity under pressure from one or more channels through the output chamber, and the mold is paired and closed before starting work.

2- Pressure molding method:

In the pressure molding method, powders, pellets, or tablets of materials are poured into the mold cavity, and with the closing of the mold, they take the desired part shape under pressure and heat.

3- Piston pressure molding method:

In the piston pressure molding method, thermoset materials are introduced into the mold cavity and chambers under the pressure of the piston that shapes the surface of the workpiece and take the necessary form under pressure and heat.

Die Casting Process:

In the die casting process, molten materials (which can be materials like aluminum and copper, etc.) are directed into the mold cavity under a certain pressure, and using this method, parts with high precision and complex and clean shapes can be produced. Usually, after production, there is no need for further operations such as machining and finishing, and only burrs and excess parts need to be removed.

Some advantages of the pressure casting and die casting method include:

1- Production of precise parts with complex shapes

2- Manufacturing parts with thin and narrow walls

3- Good surface finishing of parts and their smoothness

4- No need for machining after production

5- Strength of parts due to rapid cooling

6- Accuracy of core placement in die casting molds

7- Mass production in the production stage due to the long life and durability of these molds

Extrusion Process

The general mechanism of extrusion consists of a screw that derives its motion from a motor and gearbox and moves in a cylinder that is heated by external heaters. Plastic materials in the form of granules are poured into the hopper of the machine. After the materials melt and are pushed through the mold form, they gradually take the shape of the mold as they cool. Various shapes of plastic parts in hollow and solid states are produced using this method.

Plastic materials are poured into the machine's hopper in the form of powder or granules. The softened and heated materials are melted by the screw and the heating elements around the cylinder and are extruded through a hole (form) that has the cross-section of the produced product, and after cooling, they take the shape and form of the hole (mold) which is used for producing parts like wires, rods, pipes, sheets, etc.

Mold Design

After introducing the types of molds, we will examine each step of mold construction. The first step in making a mold is the design of the molds.

There are various types of parts; the shape of these parts is fundamental for designing the part. It should be noted that the design of the desired part is done before the mold design and includes its own calculations. Part design includes the following:

1- Indentations and protrusions (fish group)

2- Indentations and protrusions around the hole

3- Bent edges

4- Tolerance

5- Hole cutting

6- Straight holes

7- Protruding holes

8- Relationship of holes with bends

9- Slots (tabs)

10- Bends and tubes

After designing the part, the mold design should begin. The designer, based on their information and experiences and using specific patterns, mentally designs the desired mold and sketches it on paper, checking each detail, then proceeds to create detailed drawings of the desired mold and, considering the separation of different parts of the mold, refers them to the machining unit.

All calculations regarding pressure tolerance, breathing, breaking point, bending, resistance under critical conditions such as heat, cold, impact, and other working factors are the responsibilities of the mold designer and control factors. It is necessary to mention that the design of different types of molds varies, and the designer must have complete knowledge of the types of molds and their designs.

Material of Plastic Molds

After the mold is theoretically prepared using formulas and drawings, the next step is to construct the mold in depth. However, there are other stages before construction, which include selecting materials and the type of mold based on the type of part that is ultimately desired.

The choice of metal for pure applications depends on the characteristics of the desired part, the cost of its construction, and the availability of the metal. The technical specifications of parts vary; it may be that for one part, strength is the first condition, while for another part, appearance is the primary condition.

For example, for cutting molds, the punch and die of the cutting mold should be made of high-carbon steel (harder) and have water-cooling capability; like Spk.

The reason for using cold-work steels for cutting molds is their water-cooling capability with minimal warping, and this property provides resistance to impact and very high hardness. The steel used for cutting molds should have a hardness between 60 to 56 RC, depending on the thickness of the sheet being cut.

Or in another example, the steel used for plastic molds must have high coating capability; thus, the presence of a high percentage of chromium in that steel is necessary. The reason for choosing this type of steel for these molds is the high surface smoothness for the plastic part and easy separation of the part from the mold surface.

Cost estimation – Economic justification – Mold efficiency

A very important and noteworthy point in each step and process of mold making is economic justification. The cost estimation for mold construction, including materials, manufacturing, and other factors, is directly related to the number of parts required.

Thus, parts with low production volumes should be avoided if it is possible to produce that part manually or by a method other than having a mold; otherwise, mold construction is necessary.

An important point raised in mold making is the issue of efficiency. For example, the desired quality level indicates the number of cavities in plastic molds, so that a mold with one cavity in many cases may not even meet the electrical and friction demands and the human force of the injection machine, while the same mold with a greater number of cavities can completely change the above situation.

Plastic Mold Construction

However, the final step in producing a mold is mold construction. The construction of plastic molds is carried out by determining working time, the type of machine tools required, and skilled labor. All components of the mold must be examined in terms of working time to approximately determine the specified time for mold construction.

In the next step, the types of tools required for the proper machining of mold parts should be prepared.

The machine tools required for mold construction typically include:

Lathe - Milling machine - Drill - Spark machine

Thus, in constructing each component of the mold in the machining unit, the parts are prepared for assembly.

In the assembly unit, after dimensional control of the parts and accuracy in the requested tolerances, the mold is assembled.

After assembly and final preparation of the mold, the last step is mold testing and sampling.

After controlling the test piece, if there are any defects in the prepared plastic mold, it is returned to the machining unit for correction.

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