TRANSFER MOULDING
1. Introduction:
Transfer moulding is the process of heating the thermoset material in a separate chamber provided in the mould and then transferring the liquid into the impression under sufficient pressure, through runner and gate. There are two methods of transfer moulding.
2. Types of Moulds:
a) Pot transfer mould:
In this design the resin is heated and liquefied in a pot or transfer chamber provided in the mould. The plunger which forces the liquid resin into the cavity is the part of the upper section of the mould and is actuated by the platen of an ordinary compression moulding press. The volume of the pot must always exceed the area of all cavities, sprues and runners by at least 15% since the total force exerted by the pot is required to keep the mould from opening and flashing when the liquid resin is injected into the cavities.
b) Plunger transfer mould:
In this design the material is injected by an auxiliary ram or plunger which is part of the transfer press. The press platen here is used only to hold the mould halves together at the required clamping force. The clamping force must always be 20 - 25% above the total force which might be exerted by all cavities, sprues and runners when operating under full auxiliary ram pressure. Otherwise the mould will tend to open during injection.
3) Transfer mould design:
For the best results in transfer moulding, the design should take into account the type of mould, number of parts to be moulded and the material to be used. Other important considerations are.
Gates: The gate should always be as small as possible so that the degating mark is less. However, they should not be small that they cause a severe restriction in flow.
Runner: Runner should be as short as possible to minimize the flow restrictions. The best and most expensive runners are full round. The most popular are half round or modified trapezoidal.
Vents: Vents for air removal are very important. They most often are in the form of small grooves in the parting line of the mould from the edge of the cavity to the outer mould edge.
4) Advantages of transfer moulding:
- Moulding cycle: In going through the runners and gates, the flow restrictions can be large enough to add a sensible quantity of heat to the liquid resin, thus saving time in the chamber.
- Tighter dimensional tolerances: Since the material is injected to a closed mould, there is little or no flash, and dimensions across the parting line are held closely. Also, since there is little flash, the finishing cost are less.
- Insert moulding: Because the material enters the cavity as a liquid the pressure
- against fragile inserts is less, and there is less tendency for pins or thin mould section to bend.
- Encapsulation: Transfer moulding is the only feasible method to encapsulate delicate electronic parts such as capacitors and transistors, where maximum density is required with low moulding pressure.
Family moulds:
It is well suited for producing parts of different sizes that must be assembled together. Here one shot allows production of several parts all connected together by runners and all cured at the same time. In compression moulding each part should probably be moulded separately and cured for different times, which might result in slight colour differences.
Mould cost:
The mould cost in most of the cases is less compared to the compression mould. The wear and tear is also less compared to a compression mould so that dimensional accuracy is kept longer and re finishing is not required as often.
5) Disadvantage of transfer moulding:
Mould cost: In some cases the cost of the transfer mould will be more compared with a compression mould of the same piece part.
Material cost: The material cost per piece part will be more in transfer moulding since the sprue runner and gate cannot be re used.
Mechanical properties: Under certain circumstances, the physical properties of transfer moulded parts can be less in particular directions. An example might be a tendency to crack around inserts or weld lines.
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| Plunger Transfer Mould |
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| Pot Transfer mould |
6) Transfer moulding problem check list:
The common moulding faults and their causes are based on the assumptions, that the mould in question has adequate runner and gating provisions.
Incomplete fillings, Knit - lines, burning blisters, can all result from excessive frictional heat developed when transferring the moulding compound through runners and gates which are too small. A correct mould design is very important.
7) Parameters (specifications):
The specification of SP 130V/CTS Compression/Transfer Moulding Machine.
The machine will have a 130 tones (1.3 MN) clamping unit with a down stroking movement and will be mounted on a suitable MS fabricated base structure.
7.1) General specification
Working height (top of fixed mould plate ) from ground = 940mm
Estimated floor space = 1600 x 2300mm
Height of top platform on power-pack from ground = 1290..
Locking unit specification will be as follows
Mould locking force 130 tones by directly hydraulic clamping unit , which can be adjusted up to, the maximum tonnage to suit the size of the mold
- Size of the plates = 880 x 730mm
- Distance between tie bars = 650 x 500mm
- Minimum mould height = 200mm
- Maximum mould opening = 550mm
- Maximum daylight = 750mm
- Diameter of tie bars =80mm
- Mould opening force main ram = 9.5 tones (95 KN)
- Opening and closing speed = 150mm/second
- Slow opening and closing speed = 6mm/second
7.2) Types of operation of Locking Unit with control circuit:
The control circuits will be so arranged that locking unit can be run in the following four manners by selecting the sequence with a selector switch.
1. Inching or hand control:
The hand controls for the moving mould plate, the top ejector up down and the transfer cylinder up/down are arranged for push button operations. The hand controls are so arranged that it will be possible to operate the machine with safety gate open during mould setting operations. However, this feature is provided with customer’s approval unless otherwise informed accordingly.
2. Compression moulding sequence without breathing:
Close the safety gate by hand. With gate closing as a signal, the following operations will take place in an automatic sequence
- Moving mould platen fast advance
- Moving mould platen slows approach
- Pressure builds up and hold
- Curing interval by process timer
- Moving mould platens open fully and stop
- Open safety gate by hand
- Eject component by transfer cylinder and/or top ejector by manual operations.
3. Compression moulding sequence with breathing features:
Close the safety gate by hand. With the gate closing as a signal, the following operations will take place in an automatic sequence.
- Moving mould platen faster than advance
- Moving mould platen slow approach
- Pressure build up to a pre-moulding value
- Pressure hold as per time set on timer
- Moving platen goes up by an amount adjustable by limit switch
- Moving platen comes down after a pre-determined interval set on process timer
- Pressure build up to a locking pressure and hold
- Curing intervals by process timer
- Moving platen opens fully and stops
- Open safety gate by hands
- Eject component by operating transfer cylinder and/or top ejector by manual operations
(The pre-moulding tonnage value will be adjustable between minimum of 45 tones to maximum of 80% of locking force.
4. Transfer moulding:
Close the safety gate by hand. With the gate closing as a signal, the following operations will take place in an automatic sequence.
- Moving mould platen fast advance
- Moving mould platen slows approach
- Pressure builds up and hold
- Transfer rams up and hold as per the time set on timer
- Curing time by process timer
- Moving platen opens fully and stops
- Eject component by operating transfer cylinder and/or top ejector by manual operation
- Clean transfer chamber by manual operations.
(The transfer ram will have an adjustable stroke limiting its upward movement)
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