30.0 INJECTION MOULDING MACHINE FOR THERMOSET MATERIALS:
30.1 Introduction:
Thermoset materials were only compression moulded or transfer moulded in the early days. But lately the machine manufacturer modified the injection moulding machine for thermoset materials to injection mould. The machine numbers were limited while comparing with injection moulding machines.
30.2 Economical Advantages
• Use reciprocating screw machine – replace conventional method.
• Considerable cycle time saving
• Increase productivity
• Compound more homogeneously fused by the screw action
• Compound transferred to the mould at high temperature
• More uniform curing
• Void free compression
• Can be moulded larger variations in part thickness
• Complex articles having very thin walls sections can be moulded
• Greater dimensional accuracy and stability
• Very thin flash appears which can be trimmed easily
• Compound need not weigh or palleting
• Because of screw rotation,
– The material starts melting
– Its viscosity is reduced
– Because of the heat received from barrel the heat plasticizes the material faster
the viscosity drops quickly polymerization starts and viscosity increases.
30.3 Direct Screw Injection Machine (DST):
The most popular type of machine used for injection moulding Thermoset is the line screw machine. It is similar to injection moulding machine used for thermoplastic materials. The machine produced by various manufacture differ the such details as method of barrel heating, screw and toggle. The moulding cycle time for the conventional moulding of thermosets, are fairly long. This is also known as the direct in line screw machine and is basically similar to that used for thermoplastic moulding. The thermosetting material, fed to the machine, is pre-heated to a plastic state as the screw rotates with in the heated barrel. It is then injected through the nozzle at the front of the barrel directly into the heated mould and curing takes place in the heated mould. The main difference between thermoplastic and thermosetting injection moulding machine is that the screw has a low or no compression length to diameter ratio in the later. The behavior of the Thermoset is to be considered in the machine and mould design and in the operation of the machine. The later factor is opposite to thermoplastic practice in that barrel temperatures are lower and the temperature of the mould is higher.
Although the material cost was less because of the comparatively longer cycle time, it had difficulties in comparing with thermoplastics
30.4 Advantages and Disadvantages of Direct Screw Transfer:
The principles saving which occur from DST techniques are those which arise from the shorter cycle times. These savings can arise only of the shortening of the cycle time is sufficient to outweigh the higher machine hour rate of the DST process. The higher rate is due to the longer capital, higher maintenance costs and the greater complexity of operation. In all this time saving are available but on thinner sections, the saving is much less.
a) Mould design:
Mould design for thermosetting materials generally follow the similar principles used for thermoplastic materials. But mould must be heated to 130o to 300oCelsius. Instead of being cooled, thermosetting materials are more abrasives than unfilled thermoplastics. Therefore hardened replaceable gates should be used and runners should be chromium plated. Cavities and flash lands should be chrome plated to reduce the tendency of mouldings to stick in the cavities and thus obstruct automatic ejection.
b) Mould heating:
Small mould up to 50mm thickness are heated by conduction heat from hot plates. Larger moulds are heated directly by electricity. Care should be taken to ensure even heating of all mould cavities. Stripper plates should be avoided in the design because they make even heating of the tool difficult and mould locking may occur.
c) Sprue and runner:
Sprue and runners should be as short as possible. Sprue pullers of the Z or cone type are normally used. But Z type should be locked to prevent rotation. To minimize the wear the sprue bush must be hardened to 55 HRC.
Runners should preferably be fully round and about 6 mm in diameter. The diameter increase by 30% for runners over 100mm length.
d) Gates and venting:
Gating generally follow thermoplastic practice. Cross sectional areas can be calculated roughly from the recommended formula for injection moulding of thermoplastics.
Gate is (sq.mm) = 23/100 of the mass of moulding in grams. The gates should be large enough to allow filling of the mould at about 40 to 85 gram / second. Special gates are used for separate types of moulding. Adequate venting is essential to obtain full mouldings free from blisters and porosity. Vents (0.0012 to 0.05mm) with relieved ejector pins and inserts should be used to achieve maximum venting and to case mould filling.
Liberal draft angle on moulds and an adequate number of evenly spaced ejector pins are essential to reduce distortion of mouldings on ejection. Ejector pins should have surface hardness of about 60 HRC and should be of as large as possible.
30.5 Injection moulding Conditions
To obtain short cure time the material temperature to be as high as practicable when it emerges from nozzle. It is usually in the range of 100-200oC, approximately 30oC higher than barrel temperature. The most important factor affecting the temperature are
• Cylinder temperature
• The back pressure on the rotating screw
• Screw speed.
Nozzle & cylinder heating zone are the most sensitive means of controlling the material temperature.
30.6 Pre Plasticizing/Moulding Pressure
There are three pressure parameters which is given below
• Back or counter pressure
• Injection pressure
• Dwell or hold on pressure.
a) Back Pressure:
The back pressure is the hydraulic pressure applied to the screw during plasticizing to prevent the screw from moving through the plastic or semi plastic moulding material. This ensure that the forward zone of barrel is fully packed with pre plasticized material and correct quantity of material deliver for next shot. While screw rotates, any volatiles forces back to the feed section and can be escaped. If high back pressure exerts, high load on screw drive may cause excessive wear of the screw and barrel liner.
b) Injection Pressure:
The injection pressure affects on mould dimensions, mould material, plasticity and injection machine. It is to be controlled to give mould filling rate of about 40 to 85gm per second. If the pressure is excess, it will cause burn marks, mould wear and nozzle wear. It also may result in air trapping and porous moulding due to injected material curing before the mould cavity is completely filled.
c) Dwell or Hold on Pressure:
It is the reduced pressure approx. one sixth of injection pressure and it is applied after the selected shot volume has been injected until sprue is cured. It will prevents porosity and blisters. The exact duration of application vary with
• Type of machine
• Size of moulding
• Nozzle and sprue diameter
• Type of moulding material
d) Cure Time:
The Variable depends on the moulding material, size of moulding, moulding temperature and moulding pressure.
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