MOULDS & MOULDING METHODS
1.Mold
The primary function of a mould is to shape the finished product. In other words, it is imparting the desired shape to the plasticized polymer and cooling it to get the moulded part. It is basically made up of two sets of components.
1 The cavity & core
2 The base in which the cavity & core are mounted.
Different mould construction methods are used in the industry. The mould is loaded on to a machine where the plastic material can be plasticized, injected and ejected.
2.Functions of a mould:
a) Allow hot plastic into the mould.
b) Shape the profile of component.
c) Conduct heat from injected plastic.
d) Resist temp., Pressure, stress, etc.
e) Eject the part smooth.
f) Guide the sliding half w.r.t. fixed half.
g) Have good polished surfaces.
h) Rigid enough to prevent deflection.
i) Produce consistent quality on long run.
j) Safe in operation.
k) Display identification of part, material and project.
l) Easy to assemble, dismantle, repair & maintain.
m) Provision for lifting & loading on machine.
n) Have quick clamping facility.
o) Suit moulding machine specification.
p) Have all external edges rounded off.
3.General Types of Moulds
There are three general type of moulds and these may be sub divided in to several class. These three types are,
a) Injection moulds
b) Compression moulds
c) Transfer moulds.
Injection moulds are again divided in to several types. They are
a) Injection moulds
b) Compression moulds
c) Transfer moulds.
Injection moulds are again divided in to several types. They are
1) Two plate mould (cold runner)
2) Three plate mould (cold runner)
3) Hot runner mould
4) Insulated runner mould
5) Hot manifold mould
6) Stacked mould
Two plate mould
A two plate mould consists of two plates with the cavity and core mounted in either plate. The plates are fastened to the press platens, and moving half of the mould usually contains the ejector mechanism and the runner system. A two plate mould is the most logical type of tool to use for parts that requires large gates. This cold runner system results in the sprue, runners and gates solidifying with the cavity side plastic material.
Three plate mould
The three plate mould is made up of three plates. They are,
- Stationary plate which is attached to the stationary platen of the machine and usually contains the sprue and half runner
- The middle or cavity plate, which contains half of the runner and gate on one side and the cavity on the other side and allowed to float when the mould opens,
- A core plate which contains the moulded part and ejector system. This type of cold runner mould design makes it possible to separate the runner system from the part when the mould opens. The design makes it possible to use center pin point gating.
Hot runner mould :
With this system, the runners are kept hot in order to keep the molten material in the fluid state at all time. In effect, this is a runner less moulding process, and is sometimes called that. In runnerless mould, the runner is contained in a plate of its own. Hot runner moulds are similar to three plate moulds except that the runner section of the mould is not opened during the moulding cycle. Other than the heated plate for the runner, the remainder of the mould is a standard two plate mould. Followings are the advantages of hot runner mould.
1) No moulded side products (sprue, runner, gate)
2) No separating of gate.
3) The cycle time can be reduced.
4) Uniform melt temperature can be attained.
5) Shot size capacity and clamp tonnage required are decreased.
lnsulated hot runner mould :
It is a variation of hot runner mould. In this type of moulding, the outer surface of the material in the runner acts as an insulator for the molten material to pass through. In this type, the moulding material remains molten by retaining its own heat. Sometimes a torpedo and hot probe are added for more flexibility. This type of mould is ideal for multi - cavity center gated parts. The size of the runner is almost twice the diameter when compared to the cold runner system.
Hot manifold mould :
This also is a variation of the hot runner mould. In this mould, the runner is heated and not the runner plate. This is done by using electric cartridge insert probe in sprue, runner and gate.
Stacked mould :
Basically, a stacked mould is a multiple two plate mould with mould placed one over the other. This construction can also be used with three plate mould, hot runner mould etc. . A stacked mould construction double the output from a single moulding machine and require the same clamping force. The machine should have additional shot capacity. Stacked moulds are also being used with more than two plates.
4.Moulding methods
Various moulding methods (process ) are used depending on the configuration. They are,
1) Injection moulding
2) Compression moulding
3) Transfer moulding
4) Continuous extrusion
5) Blow moulding
6) Vacuum moulding
7) Rotational moulding
The machine used for different process is named after their process. For example, the machine used for injection moulding process is called Injection moulding machine.
Injection moulding
In this process, the plastic material is injected in to the mould through a sprue bush by means of a screw plunger. This process can be used for both thermosetting and thermoplastic materials.
Compression moulding
In this process, the plastic material is placed in the cavity and use a force for compressing the compound as the mould closes. This moulds are generally used for thermosetting materials.
Transfer moulding
In this process, the plastic material is transferred from a transfer pot and then forced in to the cavity by means of a plunger. This method is used for moulding thermosetting materials only.
Continuous extrusion
In this process, usually a full mould will not be used, instead a die is used to the specific shape. The material is forced to travel through, the die in a continuous stream.
Blow moulding
Blow moulding is very much like compression moulding because the blow mould generally closes on a hollow shape that has been deposited in between the halves of the mould. Air or gas pressure is introduced at the center. This internal pressure causes a flow of the heated material in to intimate contact with the relatively cold mould sections. Here the plastic material solidifies and is subsequently ejected.
Vacuum moulding
The mould used for this process is similar to that of the female half of the compression or blow mould. Auxiliary equipment on the machine heats the material and drags it over the cavity as indicated by the precise technique chosen.
Rotational moulding
The machine provides the means of holding the mould and rotating it about two axis at the same time. This rotation causes the powdered material that was loaded in to the mould before it was clamped in to the machine to solidify and cling to the wall of the mould. Thickness of the deposit depends on the time allowed for rotation and the amount of material placed in the mould.
5 Injection mould components
Fixed half and moving half
A mould is generally divided into two parts. Fixed half and moving half. The half that is attached to the stationary platen of the machine is termed as fixed half. The other half of the mould attached to the moving platen of the machine is known as the moving half. Generally the core is situated in the moving half for easiness of providing an ejector system.
Impression
The injection mould contains within it an impression into which plastic material is injected and cooled. The impression gives the moulding its form. Therefore the impression is defined as that part of the mould which imparts shape to the moulding. The impression is formed by two mould members.
The cavity
It is the female portion of the mould and gives the external form to the moulding.
The core
It is the male portion of the mould and forms the internal shape of moulding.
Cavity and core plates
The basic mould consists of two plates. The plate in which the cavity is formed is known as cavity plate. Similarly the plate from which a core projects is termed as the core plate. When the mould is closed, the two plates come together forming a space between the two plates which is the impression.
Sprue bush
During the injection moulding process, the plastic material is delivered to the nozzle of the machine as a melt. It is then transferred to the impression through a passage. This passage is a tapered hole with in a bush. The material in this passage is known as sprue and the bush is called sprue bush. Thus the sprue bush can be defined as that part of the mould in which the sprue is formed. Sprue bush is the connecting member between the machine nozzle and the mould face. Sprue bush should be made from 1.5% nickel chrome steel and should be hardened to withstand the stresses. The rearward movement of the sprue bush is prevented by stepping the end fitting a register ring which served a dual purpose of securing the sprue bush and mould location. The internal hole of the sprue bush has an included taper of 3 degrees to 5 degrees which facilitate the removal of the sprue from the mould. The taper should be highly polished.
There are two basic types of sprue bush.
- The sprue bush with a spherical recess which is used in conjunctions with a spherical front end nozzle and.
- Sprue bush with a perfect flat rear face and a corresponding nozzle is used. Starting diameter is equal to the nozzle diameter plus 0.5mm.
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| Spherical seating (2o-3o included angle) |
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Flat seating (nozzle aperture+0.5mm) |
If the alignment between the nozzle and bush hole is perfect, a leak free joints is achieved with the spherical seating. However a misalignment results in gap and leakage occurs. But in case of a flat-faced nozzle, no leakage can occur, if the two apertures are slightly out of line.
Register or locating ring
The nozzle and the sprue bush must be correctly aligned if the material has to pass without interference into the mould. So by including a register ring, the mould can be aligned to the machine. The register ring is a flat or stepped circular member fitted on to the front face of the mould. Its purpose is to locate the mould in its correct position on the injection machine platen. When the mould is mounted on the machine the register ring fits into a circular hole which is accurately machined in the injection platen on the cylinder-nozzle axis. This ensures that the small aperture in the nozzle is in direct alignment with the sprue bush hole. The register ring forms a direct connection between the sprue bush and the hole in the injection platen of the machine.
Register ring is located on the outside diameter of the sprue bush. It can also be located on the step end sprue bush where the ring secures the sprue bush in position. In case of a mould which necessities a long-reach nozzle, the register cannot be located on the sprue bush. The register ring is locate in a recess in the rear of the injection bolster.
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| Location of mould on Platen by register ring |
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Register ring being used for dual purpose of mould location and holding sprue bush down |
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Alternative arrangement for Use with long reach nozzle |
Runner and gate system
The material can be directly injected into the impression through the sprue bush or for multi-impression moulds. It can pass through the runner and gate system before entering the impression.
The runner is a channel machined into the mould plate to connect the sprue with the entrance to the impression. The gate is a channel connecting the runner with the impression. It has a small cross-sectional area when compared with rest of the feed system.
Guide pillars and bushes
To mould an even - walled moulding it is necessary to ensure that the cavity and the core are kept in alignment. This is achieved by providing guide pillars and bushes on the mould plates.
The guide pillar has its working diameter smaller than the fitting diameter and flange is provided on the fitting diameter side. This stepped pillar has certain advantages over a constant diameter pillar. If the working diameter is smaller, it can be easily remove without damaging and no damage on the fitting hole guiding diameter, but the attempts to remove a constant diameter pillar will damage the fitting hole as well as the guiding diameter
A guide bush is incorporated in the mould to provide a suitable wear resisting working surface for the guide pillar and to permit replacement in the event of wear or damage.
Heel blocks or rails
Heel blocks are the support blocks mounted on to the bottom plate to support the mould plate. Heel blocks provide a Space into which the ejector plate assembly can be fitted and operated.
Ejector Plate
Ejector plate is used for serving the purpose of transmitting the ejector force from the actuating system of the injection machine to the ejector element.
Ejector retainer plate
Ejector retainer is a plate securely attached, to the ejector plate for the purpose of retaining the ejectors and push back pins
Electors
Ejectors are the part of ejector system which apply the ejection force to the mouldings. There are different types of ejectors used like ejector pins, ejector plates, ejector sleeves. The type of ejector is selected depends on the component shape.
Push back pins
The push back pins are incorporated in a mould to push the ejector N assembly to its rear position as the mould is closed.
Rest button or stop pad
With a large ejector plate system, it is preferable to provide rest buttons or stop pin on the lower side of the ejector plate or on bottom plate. This reduce the effective seating area. The possibility of the ejector elements remaining slightly above due to the foreign matters being trapped behind ejector plate, is avoided by using the rest buttons.
Sprue puller or sprue breaker
It is necessary to remove the sprue from the sprue bush after each injection for further injection of material into the mould. Sprue breakers or hookers or pullers are used to pull the sprue when the mould opens. There are different types of sprue pullers generally used.
Attachment of mould to machine platen
There are two way of clamping the mould halves to the platen of the injection moulding machine and they are either directly, or means of bolt and clamps. For direct bolting clearance hole or slot provided in each mould half to correspond with the holes tapped in the machine platen. In indirect clamping mould is clamped on the platen by means of flat clamps and bolt or special clamps and bolts.
3.6 Mould Material Specification:
Description | Materials | Hardness | Remarks | |
NTTF | Others | |||
Locating ring | MS/EN2A | Mild steel or equivalent | No force is applied on to this plate. | |
Sprue bush | EN30B | 1.5% Nickel chrom steel | HRC 48 - 50 | High polish required and no scratch marks |
Top clamp plate | MS/EN2A | Mild steel or equivalent | Only clamping and supporting cav. Plate | |
Cavity Plate | MS/P20 | Mild steel or equivalent | Only holding inserts | |
Cavity inserts | See note | Assab Orvar Supreme or equivalent | HRC 45 - 50 | Hardened, high polish/matt finish |
Stop button | EN 31 | H13 or equivelant | HRC 55 - 58 | Resist the impact |
Guide Pillar | EN 230 | EN 32 | HRC 60 - 62 | Case hardening to a depth of 0.8- 1.0mm |
Guide Bush | EN 230 | EN 32 | HRC 58 60 | Case hardening to a depth of 0.8- 1.0mm |
Core inserts | See note | Assab Orvar Supreme or equivalent | HRC 45 - 50 | Hardened, high polish/matt finish |
Cavity Plate | MS/P20 | Mild steel or equivalent | Only holding inserts | |
Core back plate | MS/EN2A | Mild steel or equivalent | Only clamping and supporting | |
Sprue puller bush | EN30B | H13 or equivalent | HRC 48 - 50 | Ejector pin moves inside |
Rails | MS/EN2A | Mild steel or equivalent | Only supporting | |
Ej. Retainer plate | MS/EN2A | Mild steel or equivalent | Only retaining ejector elements | |
Ejector plate | EN31 | H13 or equivalent | HRC 45-50 | Ejector force acting |
Ej. Guide bush | E230 | Phosphor bronze | Self lubricated | |
Ej guide pillar | E230 | EN 32 | HRC 60-62 | Case hard to a depth of 0.8 – 1.0mm |
Return pin | EN30B | H13 or equivalent | HRC 55-58 | More tempered at collar |
Ejector pin | EN30B | H13 or equivalent | HRC 48-50 | More tempered at collar |
Bottom plate | MS/EN2A | Mild steel or equivalent | Only supporting | |
Stripper plate | EN30B | H13 or equivalent | HRC 48-50 | |
Wedge | EN31 | H13 or equivalent | HRC 48 50 | |
Centering bush | EN8 | H13 or equivalent | HRC 48 50 | |
- For cavity and core insert, the material selection will be depending on the type of plastic and the production rate. It can be either EN30, P20, W302, H13, Stavax etc.. P20 is used if the cavity & core is directly machined in the plate itself.
For a long run mould, to get more strength and toughness for the mould base assembly, use ASSAB 760 or equivalent instead of mild steel.
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