Molding process: add powder or granular raw materials into the barrel of the injection machine [hot water heating outside the barrel, screw shearing and rotary friction - plastic plasticizing and melting - injection (mold temperature is far higher than the barrel temperature) - plastic crosslinking reaction in the cavity - curing molding - mold opening - take out the plastic parts
Differences between thermosetting injection molding and thermoplastic injection molding: 1) the former has cross-linking reaction
2) Mold needs to be heated (not cooled)
3) Process conditions are completely different
Requirements for plastics 1. Thermosetting plastics should have high fluidity
2. There are special requirements for the curing time of plastics. The residence time in the barrel shall not be solidified (i.e. the time for 80-900c to keep the flowing state shall be more than 10min; the time for 75-850c shall be more than 1h). However, it is expected to be solidified as soon as possible after injection into the mold cavity
3. Injection molding can be used for almost all thermosetting plastics, but the largest amount is phenolic plastic.
Requirements for the injection machine 1. Strictly control the heating temperature and heating time
2. The screw driving device of injection machine should adopt the structure of hydraulic motor with overload protection device or reducer with cycloidal pin wheel to realize stepless speed change of 0-90r / min.
3. The length diameter ratio and compression ratio of the screw of the injection machine are smaller than those of the thermoplastic injection machine. (length diameter ratio is usually 14-20, compression ratio is 0.8-1.4)
Key design points 1. Mold material
It should be made of materials with high strength, good wear resistance and heat resistance
2. Requirements for parting surface
(1) Reduce the contact area of parting surface to improve the mold closing state
(2) Minimize cavities or pits on the parting surface
(3) The parting surface shall have enough hardness (the hardness of the parting surface shall not be lower than 30hrc)
3. Requirements for sliding parts
(1) The fit clearance of various sliding parts shall be less than 0.03mm (basically no overflow)
(2) The surface roughness Ra of mating parts shall be less than 0.2 μ M
(3) To improve the surface hardness of parts, it is generally required that the surface hardness is 54-58hrc, which can be increased to more than 60HRC under special circumstances
(4) Shorten the fit length. (the fitting length is generally 2-3 times of the diameter)
4. Requirements for insert placement
Generally, the insert hole is designed as a step hole, and the fitting accuracy of the insert rod and the socket is improved, so that the clearance value is within 0.01-0.02MM. Generally, the inserts shall be placed outside the mold as far as possible.
5. Design of gating system
(1) Main channel
As the plastic enters the mold from the nozzle with lower temperature and the temperature of the material is expected to rise as soon as possible, it is better to have a smaller cross-sectional area of the main channel, so as to increase the heat transfer area and friction heat generation. Section shapes are common: circle, trapezoid, square, semicircle, etc.
(2) Shunt channel
The fillet radius at the junction of the shunt channel and the main channel is 5-10 mm. The sectional shape of the shunt channel is usually semicircle or trapezoid, which is good for heat transfer
(3) Draw bar
Widely used conical pull hole with push rod at the bottom
6. Design of exhaust tank
When thermosetting plastics solidify in the mold for polycondensation reaction, a large number of gas substances will be released, plus the original air in the mold, and because of its good fluidity, it is easy to block the gap, etc., so a special exhaust slot must be opened. The depth of the exhaust groove is 0.1-0.3mm, the width is 5-10 mm, and it can be deepened to 0.8 mm beyond 6 mm from the cavity.
7. Forming parts consideration
Thermosetting plastics are easy to enter into small gaps (0.01-0.02MM), so it is better to avoid the use of the combination of inlay forming parts and the use of the integral forming parts. When calculating the working size of the moulded parts, the burr value generated on the parting surface should be taken into account, which is generally 0.05-0.1mm.
8. Demoulding method
Generally, push plate and push tube are not used for demoulding, because it is difficult to remove the plastic chips drilled into the fitting gap. The push rod has a wide range of forms, because it has the advantages of convenient manufacture, reliable coordination and easy removal of waste materials. The clearance between push rod and formwork shall be 0.01-0.03mm.
9. Mold heating
The mold temperature has a direct impact on the molding quality of the plastic part, so the surface temperature of the mold cavity must be accurate and uniform, and the temperature difference should generally be less than 50c; generally, the electric heating rod or electric heating sleeve is used for heating.