PC is often referred to as polycarbonate. Due to its excellent mechanical properties, it is often referred to as bulletproof adhesive.
PC has the characteristics of high mechanical strength, wide working temperature range, good electrical insulation performance, good dimensional stability and transparency.
This article will introduce the characteristics of PC material and the matters that need attention during the injection molding process from different perspectives.
PC material injection molding
To meet the needs of various injection molding processes, polycarbonate is available in different melt index specifications. Usually, melt indices between 5 and 25g/10min can be used for injection molding.
However, the best processing conditions depend on the type of injection molding machine, the shape of the molded product and the specification of the polycarbonate, and there are considerable differences, which should be adjusted according to the actual situation.
Key points of injection molding machine selection
Requirements for plastic injection molding machines producing PC products: The maximum injection volume of the product (including runners, gates, etc.) should not be greater than 70-80% of the nominal injection volume, and the screw should use a single equidistant head thread with a check ring for gradual replacement. Compression type screw, the screw’s L/D ratio is 15-20, and the geometric compression ratio C/R is 2-3.
Clamping pressure: 0.47 to 0.78 tons per square centimeter multiplied by the projected area of the finished product (or 3 to 5 tons per square inch).
Machine size: the weight of the finished product is about 40 to 60% of the capacity of the injection molding machine is the best, if the machine is expressed in terms of polystyrene capacity (ounce), it should be reduced by 10% to use the capacity of GUANG DA. 1 ounce = 28.3 grams.
Screw: The length of the screw should be at least 15 diameters long, and its L/D should be 20:1 best. The compression ratio should be 1.5:1 to 30:1. The stop valve at the front of the screw should be of the sliding ring type, and the resin flowable gap should be at least 3.2MM.
Nozzle: The tip opening should be at least 4.5MM (diameter), if the weight of the finished product is 5.5KG or more, the nozzle diameter should be 9.5MM or more. In addition, the tip opening should be 0.5 to 1MM less than the gate diameter, and the shorter the segment channel, the better, about 5MM.
For cost and other reasons, current PC injection molding products often use modified materials, especially electrical products, but also need to improve the fire resistance. When molding flame-retardant PC and other plastic alloy products, the plasticizing system for injection molded parts requires a uniform blend and corrosion resistance that is difficult to achieve with conventional plasticizing screws.
The key points of the PC material injection molding process
a. The injection temperature must be consistent with the shape, size and mold structure of the product. Product performance, requirements and other aspects can only be made after consideration.
Usually, the temperature used in molding is 270-320℃. If the material temperature is too high, over 340°C, the PC will decompose, the color of the product will become darker and defects such as silver lines, dark streaks, black spots and bubbles will appear on the surface. At the same time, the physical and mechanical properties are greatly reduced.
b. Injection pressure has a certain influence on the physical and mechanical properties, internal stress and molding shrinkage of PC products. It has a greater impact on the appearance and demolding of the product.
Too low or too high injection pressure can cause the product to look somewhat defective. For some defects, the injection pressure is usually controlled between 80-120MPa.
For products with thin walls, long processes, complex shapes and small gates, choose a higher one in order to overcome the resistance to melt flow and fill the mold cavity in time. Injection pressure (120-145MPa) To obtain a complete and smooth surface product.
c. Holding pressure and holding time The amount of holding pressure and the length of holding time have a great influence on the internal stress of PC injection molded products.
If the holding pressure is too small, the feeding effect is poor and it tends to cause vacuum bubbles or surface shrinkage. When recessed, the holding pressure is too high and large internal stresses may be generated around the gate.
In practical processing, this is usually solved by high material temperature and low holding pressure. The choice of holding time should depend on the thickness of the product, gate size, mold temperature, etc. Usually, small and thin products do not need a long holding time.
On the contrary, large and thick products should have a longer holding time. The length of holding time can be determined by the gate seal time test.
d. Injection speed has no significant effect on the performance of PC injection molded products. Except for thin-walled, small-gate, deep-hole and long-flow products, medium or low speeds are usually used for processing, preferably with multi-stage injection. Slow speeds are usually used.
e. The temperature of the injection mold is usually controlled at 80-100°C. It can also be increased to 100-120°C for complex shapes, thinner, and more demanding products, but should not exceed the heat deflection temperature of the mold.
f. Screw speed and back pressure. Due to the high viscosity of the PC melt, which is conducive to plasticization, exhaust and maintenance of the molding machine, to prevent screw overload, the screw speed should not be too high, generally controlled at 30 -60r / min appropriate, and the back pressure should be controlled at between 10-15% of the injection pressure.
g. In the PC injection molding process, the use of release agents should be strictly controlled. At the same time, the use of recycled material should not exceed three times the amount used, and the amount used should be about 20%.
a. At the molding temperature of polycarbonate, add cleaning material (general grade polystyrene or trans-phenylene) and shoot out twenty to thirty times continuously.
b. Set the machine back and continue to empty the purge material until the purge material starts to swell and bubble.
c. Reset the barrel temperature to 200 to 230°C.
d. Continue to empty the cleaning material until the melt temperature of the cleaning material reaches 260℃ and the appearance is clean and transparent.
Mould and gate design
The common mold temperature is 80-100℃, and 100-130℃ with glass fiber. For small products, a needle-shaped gate is available, and the depth of the gate should be 70% of the thickest part.
The larger the gate, the better, to reduce the defects caused by excessive shearing of plastic. The depth of the venting hole should be less than 0.03-0.06mm, the runner should be as short and round as possible, and the slope of the mold release is usually about 30′-1.
What are the process conditions for PC injection molding?
PC’s molding process performance
a. The moisture absorption of PC is small, but the sensitivity to water is very strong, and a small amount of moisture can cause degradation at the molding temperature. The moisture content of PC raw material should be less than 0.2%, otherwise it should be dried.
b. PC melt moisture content is in line with the requirements, can be tested by the method of injection into the air, if the melt from the nozzle for uniform colorless, bright, no silver and bubble thin strip, that is qualified.
c. PC is amorphous plastic, good thermal stability, wide range of molding temperature, more than 300 ℃ before the serious decomposition. Decomposition produces non-toxic, non-corrosive gas.
d. The fluidity of PC is poor, the change of temperature has a greater impact on the fluidity, and the cooling speed is faster.
e. The wall thickness of the plastic parts should be uniform, avoid sharp corners, notches and metal inserts cause stress concentration, if there are metal inserts should be preheated, the preheating temperature is generally 110 ~ 130 ℃.
f. PC products are prone to stress concentration, so the molding conditions should be strictly controlled and the products should be heat treated accordingly.
g. PC pouring system should be mainly short and coarse, and should be set up with cold cavities, using direct inlet feed.
h. PC melt has strong adhesion to metal, so the barrel and nozzle should be cleaned up at the end of operation.
i. The non-Newtonian nature of PC is not obvious, and the viscosity decreases insignificantly after increasing the injection pressure, but decreases significantly after increasing the temperature.
j. The melting temperature of PC is higher and the viscosity is also higher, so it is appropriate to use screw injection molding machine to operate, and the nozzle area should be heated.
k. Drying qualified PC, placed in room temperature air for more than 15min, will lose the drying effect due to moisture absorption, for this reason, there should be a heating device in the hopper of the injection molding machine, so that the temperature of PC is not less than 100℃.
l. PC melt is not corrosive to steel, so the mold does not need to be chromium-plated, but it should have sufficient strength, rigidity and wear resistance.
m. The density of PC varies greatly with temperature, and the volume shrinkage of the melt when condensing is easy to make the plastic parts produce defects such as hollow bubbles, so the wall thickness of the plastic parts can not be too thick, preferably not more than 150mm, and the wall thickness is uniform.
n. PC melt molding shrinkage is small, such as molding conditions are appropriate, generally can be molded out of a high precision plastic parts.
Main injection molding conditions of PC
a. Barrel temperature. The barrel temperature has a great influence on the quality of plastic parts, low material temperature will cause a lack of material, no surface gloss, and silver wire; high material temperature is easy to produce overflow material, bubbles, and discoloration.
General barrel temperature control in 230 ~ 310 ℃. For thin-walled products, the barrel temperature should take a large value; for thick-walled products can take a small value.
b. Mold temperature. In general, the internal stress in the plastic parts and the cooling of the mold temperature and material temperature difference are roughly proportional to the relationship, so require a high mold temperature, mold temperature can be controlled at 80-120 ℃, thin-walled plastic parts should be taken 80-100℃.
Thick-walled plastic parts should be taken 100-120 ℃. If the mold temperature exceeds 120℃, the plastic parts will be slow to cool, easy to stick to the mold, difficult to demold, easy to warp, and the molding cycle is long.
The viscosity of PC melt is high, so it needs a higher molding pressure. When using a screw-type injection molding machine, the injection pressure is generally 70~140MPa, while the plunger-type injection molding machine is 100~160MPa, depending on the raw material, plastic part, mold, and injection molding machine.
PC material melting temperature
PC material melting point temperature of 230 ~ 240 degrees, can be injected into the air to determine the processing temperature of high and low. General PC plastic processing temperature of 270-320 ℃, some modified or low molecular weight PC plastic materials for 230-270 ℃.
a. Injection barrel temperature: 250 ~ 320, 290 ~ 320, 270 ~ 300, 250 ~ 280, 80 ~ 90 to increase fluidity, the need for mold temperature: 100 ~ 120.
b. PC material is highly absorbent; drying operation is needed before injection; 120 degrees for 2~4 hours.
c. PC material fluidity is not very good, so the injection pressure of 160MPA or more.
Most of the injection speed is fast, such as playing electrical switch parts. It is common to use slow-speed-fast molding.
The injection speed has no significant effect on the performance of PC injection molded products. Generally, the injection speed is medium or slow, except for products with thin walls, small gates, deep holes, or long runners.
It is better to use multi-stage injection, especially the slow-fast-slow multi-stage injection moulding process.
The back pressure of about 10 bar, can be reduced if there is no air pattern and color mixing.
Staying too long at high temperatures, the material will degrade, but also CO2 and turn yellow. Do not use LDPE, POM, ABS, or PA to clean the barrel. Apply PS cleanup.
Some modified PC plastic materials, due to the number of recycling too many times (molecular weight reduction) or a variety of components mixed unevenly, easy to produce dark brown liquid bubbles.
PC plastic physical properties
PC is a non-crystalline engineering material with exceptionally good impact strength, thermal stability, gloss, bacterial inhibition properties, flame retardant properties, and pollution resistance. The notched Izod impact stregth of PC is very high and the shrinkage rate is very low, generally 0.1%~0.2%.
1.High impact strength and wide range of operating temperatures.
2.High transparency and free staining.
4.Excellent electrical properties.
5.Tasteless, odorless, and harmless to human body in accordance with health and safety.
6.Low shrinkage rate and good dimensional stability.
PC plastic chemical properties
PC has good mechanical properties, but poor flow characteristics, making the injection molding process for this material difficult. When choosing which quality of PC material to use, the final expectation of the product should be the benchmark.
If the plastic part requires high impact resistance, then use a low flow rate PC material; conversely, you can use a high flow rate PC material, which can optimize the injection molding process.
PC plastic material it is a new type of thermoplastic, transparent degree of 90%, known as the transparent metal. It is rigid and tough, with high impact strength, high dimensional stability and a wide range of operating temperatures, good electrical insulation properties and heat resistance and non-toxic, can be injected, extrusion molding.
PC plastic has excellent thermal properties and can be used for a long time between -100℃ and 130℃, and the embrittlement temperature is below -100℃.
PC plastic advantages and disadvantages
Advantages of PC plastics:
a. High impact strength and good creep resistance.
b. Good heat resistance, low brittleness temperature (-130℃) can resist the influence of sunlight, rain and temperature change.
c. Good chemical properties, high transparency.
d. Good dielectric properties.
e. Good dimensional stability.
Disadvantages of PC plastic:
a. Poor solvent resistance.
b. Stress cracking phenomenon.
c. Easy hydrolysis when immersed in boiling water for a long time.
d. Poor fatigue strength.
Application areas of PC
Food processing machinery
Polycarbonate resin is mainly used in the production of various food processing machinery, power tool housings, bodies, brackets, refrigerator freezer drawers and vacuum cleaner parts.
Moreover, polycarbonate materials have also shown great value in the use of important parts in computers, video recorders and color televisions, which require high precision parts.
Optical lenses made of optical grade polycarbonate can be used not only for cameras, microscopes, telescopes and optical test instruments, but also for film projector lenses, copier lenses, infrared auto-focus projector lenses, laser beam printer lenses, and various prisms, multi-faceted mirrors, and many other office equipment and home appliances.
Polycarbonate injection moulding products are widely used in artificial kidney hemodialysis equipment and other medical devices that need to be operated under transparent and intuitive conditions and need to be repeatedly sterilized. Such as the production of high-pressure syringes, surgical masks, disposable dental appliances, blood separators, etc.
Through this article, we have learned about PC materials and the issues that should be paid attention to during the injection molding process, as well as the product applications of PC materials.
However, in the actual project implementation process, there are still many factors involved in making a good PC plastic injection molding product.
If you have a plastic product that needs to be made of PC material, it is recommended to choose an experienced injection mold factory and injection molding manufacturing factory to ensure the smooth implementation of the project.