This article examines the types of plastic injection molds and their preparation and use methods. that plastics are divided into two groups, 1. Thermoplastic molds 2. Thermoset molds (bakalite):
The group of thermoplastics or thermoplastics, which are bent due to heat and get their hardness by reducing the amount of heat, and chemical changes do not take place in them, and after injection, they take the shape of the mold chamber.
In plastic injection molding, hot thermoplastic material fills the mold chamber. In this method, the thermoplastic material is hot and the mold chamber is cold.
From another point of view, thermoplastic materials are materials that can be reused in the production process after one or more uses. These materials are poured into the injection machine in the form of granules or powder.
Construction of injection molds:
Plastic molds are generally divided into two types:
1- Plastic injection molds with cold channel
2- Plastic injection molds with hot channel
And also in terms of construction, they are of two types:
1- Double-sided plastic injection molds
2- Three-panel molds, the number of mold plates and their separation line are based on factors such as the number of mold holes, the shape of the plastic part, the type of injection machine, the type of consumables and the air outlet system, etc. are determined basically in every injection mold. There are two main parts:
A- The fixed part of the mold (semi-fixed) in which hot plastic injection materials are injected.
B- The moving part (semi-drive) where the traces of the moving part of the injection machine are closed and the system and mechanism of ejecting most of the parts are located in it.
Determining the number of cavities and mold compartments is one of the important points of designing injection molds, and plastic molds in this field are of two types:
1- Single-cavity plastic injection molds
2- Multi-cavity plastic injection molds
Single-cavity plastic injection molds:
In some cases, single-cavity molds are used, which limit the amount of production of plastic parts; Therefore, the design and manufacture of single-hole molds will be cheaper in terms of construction time and economic issues.
Multi-cavity plastic injection molds:
If the number of manufactured products is large, especially in cases where the piece is small, the method of designing and manufacturing multi-hole plastic injection molds is used.
Molds of thermoset plastics (bakalite):
The group of thermoset or bakelite or thermosetting plastics, which hardens due to heat and causes chemical changes in these materials, which are called thermoset or bakelite.
In this method, the mold is cold and the material is cold, and after feeding, the mold is heated and the material takes the shape of the mold and hardens.
Thermoset plastics or Duroplasts are produced under the influence of pressure and heat of 170 c. At first they soften and become plastic, but after a while they harden and the main feature of these materials is that they cannot be softened and reused after hardening, and they cannot be dissolved in any type of solvent and after hardening becoming, important chemical changes occur in them.
Types of molds for thermoset materials (bakalite)
In the method of molding thermoset materials, the materials in the mold compartment are gradually heated and then fed into the hot mold, and these softened materials take the shape of the cavity and the mold compartments due to the pressure of the mold and The chemical changes are cooled and thrown out of the mold.
Molding of thermoset materials is done with three specific methods, of course, other methods such as Hodeidai and… are used.
1- Transfer molding
2- Molding under pressure
3- Molding under piston pressure
1- Transfer molding:
In this method, materials are injected into the cavity of the mold through one or more channels, under pressure through the efficiency chamber, and the mold is coupled and closed before starting work.
2- Pressure molding method:
In the pressure molding method, powder or pellets or tablets are poured into the mold chamber, and when the mold is closed, it takes the shape of the desired piece under pressure and heat.
3- Molding method under piston pressure:
In the method of molding under piston pressure, the thermoset material under the pressure of the piston, which forms the shape of the top of the work piece, enters into the chamber and cavity of the mold and takes the necessary form under pressure and temperature.
In the die casting process, molten materials (which can be materials such as aluminum, copper, etc.) are directed to the mold chamber under a certain pressure, and by using this method, parts with high precision and complex and clean forms can be produced. Usually, after production, there is no need for other operations such as machining and polishing, and only pleats and waste parts should be removed.
Among the advantages of pressure casting and diecasting, the following can be mentioned:
1- Production of precise parts with complex forms
2- Making parts with thin and narrow walls
3- Good finishing of parts and their smoothness
4- No need for machining after production
5- Strength of parts due to cooling speed
6- The accuracy of muscle placement in die cast molds
7- Mass production in the production stage due to the long life and strength of these molds
The general mechanism of extrusion consists of a spiral that takes its movement from a motor and a gearbox and moves in a cylinder that is heated by external heaters, and plastic materials are poured into the machine in the form of grains from a hopper. After the material is melted, it passes through the form of the mold under pressure, and as it cools down, it takes the form of the mold. Various shapes of plastic parts in hollow and solid states are produced by this method.
Plastic materials in the form of powder or granules (granules) are poured into the funnel of the machine, the soft and heated materials are melted by the spiral and the elements around the cylinder, and they come out with pressure through a hole (form) that has the shape of the cross section of the manufactured product. and after cooling, it takes the form and shape of the hole (mould), which is used to produce parts such as wires, rods, pipes, sheets, etc.
After introducing the types of templates, we will examine each step of making a template. To make a template, the first step is designing the templates.
There are different types of parts, the shape of these parts is the basis for the design of the part. It should be noted that the design of the desired part is done before the design of the mold and includes unique calculations. The design of the parts includes the following.
1- depressions and projections (group of fishes)
2- depressions and protrusions around the hole
3- Curved edges
5- Cutting holes
6- Right holes
7- Protruding holes
8- The relationship between holes and bends
9- Gaps (crotches)
10- Bends and pipes
After designing the piece, you should start designing the mold. According to their information and experience, the designer makes a mental design of the desired template and puts it on the paper and controls each item, then they make detailed drawings of the desired template and taking into account The separation of different parts of the mold refers them to the machining unit.
All calculations such as pressure tolerance, breathability, breaking limit, bending, resistance in critical conditions such as heat, cold, impact and other work factors are the duties of the mold designer and control factors. He must have complete knowledge of all types of templates and their design.
Material of plastic molds
After the mold is prepared theoretically, that is, by using the formula and map, it is time to make the mold in depth, but before making it, there are other steps, which include the selection of the material and material of the mold, which depends on the type of part that is ultimately desired. is determined.
The choice of metal for pure use depends on the characteristics of the desired piece, its manufacturing cost and the availability of the metal. The technical regulations of the parts are different. In the case of one part, strength may be the first condition, and in the case of another part, the appearance is the first condition.
For example, for cutting molds, the matrix of the cutting mold should be made of steel with higher carbon (harder) and dewatering mold; Like Spk
The reason for using cold-worked steels for cutting molds is that they have the ability to absorb water to felt with minimal warping, and this characteristic causes high impact resistance and hardness. The steel used for cutting molds depends on the thickness of the sheet it takes. Have a hardness between 60 and 56 RC.
Or in another example, the steel that is used for plastic molds must have high moldability, so the presence of cream with a high percentage in that steel is necessary. The piece is from the surface of the mold.
Cost estimation – economic justification – mold productivity
A very important and significant point in each step and process of modeling is economic justification. Estimating mold manufacturing costs such as materials, construction and other factors is directly related to the number of pieces in demand.
In this way, if it is possible to produce parts with a small circulation by hand or by a method other than having a mold, it is better to avoid making a mold, otherwise it is necessary to make a mold.
An important point in mold making is the issue of productivity, for example, the desired quality level indicates the number of holes in plastic molds, so that a mold with one hole in many cases does not even respond to the electricity, friction and manpower of the injection machine. If the same mold with more holes can completely change the above article.
Plastic mold making
But the last step to produce a template is making the template. The production of plastic mold is done by determining the working time, the type of machine tool required, and the expert force. All the components of the mold should be checked in terms of working time so that the specific time of making the mold can be determined approximately.
In the next step, the types of tools needed for proper machining of mold parts should be determined.
The machine tools required to make molds are usually:
So, in making each mold component in the machining unit, the key parts are ready for assembly.
In the assembly unit, after dimensional control of the parts and precision in the requested tolerances, they are put together in the mold felt.
After assembly and final preparation of the mold, the last step is mold testing and sampling.
After checking the test piece, if there are any defects in the prepared plastic mold, it will be returned to the machining unit for correction.