PVC is inherently anti-inflammatory, strong and sturdy, and has good chemical resistance. The shrinkage rate is 0.2-0.6%. The product is widely used in electrical appliances, machinery, construction, daily necessities, toys, and packaging. According to the characteristics of PVC materials The analysis of the product injection molding process is as follows:
First, the characteristics of PVC material
PVC has poor thermal stability, the molding temperature is close to the decomposition temperature, the fluidity is poor, the appearance is easy to form defective defects, the PVC material has poor heat resistance, it is the easiest to scorch, generate acid gas and corrode the mold, and it can be plasticized during processing Additive to increase its fluidity, generally need to add additives to use, its strength, electrical insulation, chemical resistance is good.
Design of mold and gate
In order to shorten the injection molding cycle, the shorter the nozzle, the better. The cross section should be round. The diameter of the nozzle mouth is 6 mm, tapered, and the inner angle is 5 degrees. It is best to add a cold well. The feed well can prevent the poorly-melted semi-solid materials from entering the mold cavity, and these materials will affect the surface modification and product strength.
The draft angle should be between 0.50 and 10 to ensure that there is sufficient exhaust equipment in the cavity. The commonly used vent hole size is 0.03-0.05mm deep and 6mm wide, or the clearance around each ejector pin is 0.03-0.05 mm. The mold should be made of stainless steel or hard chromium plated.
Third, the PVC molding process
PVC is a heat-sensitive plastic. Overheating or excessive shearing will cause decomposition and spread quickly, because one of the decomposition products (such as acid or HCI) will catalyze, which will cause the process to further decompose, and acidic materials will erode the metal and change The formation of a depression will cause the protective layer of the metal to peel off, causing rust, which is more harmful to the human body.
The common screw length-diameter ratio is 18 ～ 24: 1, the third section ratio is 3: 5: 2, and the compression ratio is 1.8 ～ 2. The depth of the screw groove in the feed section is recommended as follows:
Screw diameter (mm) 35 50 60 75 100
Screw groove depth (mm) 2 3 4 5 6
The screw should have an internal angle of 25 to 30 degrees. When the screw is in place, the distance between the screw and the nozzle should be 0.7 to 1.8 mm. The screw must be made of stainless steel or chrome-plated.
1) Screw dunnage: The screw dunnage is between 2 and 3mm, and the large-scale opportunity is greater.
2) Injection volume: the actual cylinder residence time cannot exceed 3 minutes.
3) barrel temperature setting:
The temperature provided can only be used as a reference. It can be adjusted appropriately according to the actual situation of the machine and raw materials, and it may also exceed the recommended range.
Position Mold Nozzle Front section Middle feed section Temperature (0C) is 30-60 170-190 160-180 150-170 140-160
The injection volume used is 20 to 85% of the theoretical injection volume of the machine. The smaller the actual injection volume, the longer the residence time of the material, and the greater the risk of degradation due to heat.
4) Retention time of barrel:
Under 2000C temperature control (rubber), the cylinder residence time cannot exceed 5 minutes at most
5) Injection speed:
The injection speed should be slow, otherwise excessive shearing will degrade the material. When using UPVC to produce extremely smooth thick-walled products, multi-stage injection speed should be used. If light brown stripes radiate from the gate, the injection speed is too high. fast.
6) Back pressure: The commonly used surface value is 5 bar. Increasing the back pressure is beneficial for color mixing and exhaust, but the lower the back pressure, the better.
7) Shutdown: Due to the low overheating stability of PVC, the shutdown section is very important. The barrel must be cleaned without leaving a little bit of PVC. Other thermoplastics that have high overheating stability and do not resist PVC, such as PMMA, PP, LDPE, or GPPS), do not mix POMT UPVC in the same barrel, otherwise it will cause excessive chemical reaction and cause serious damage to the machine.
8) Mixing ratio: The maximum mixing ratio of nozzle material is 20%, otherwise it will affect the product quality.
9) Typical products: The main use of UPVC is to make drainage pipes, or casings for computers and televisions, water pressure tanks, photocopier casings, electricity meters and gas casings, transparent covers for printing machines, general styles, and various electronic parts. Wait.
Heat-sensitive materials are generally divided into hard and soft materials. The difference lies in the amount of plasticizer added to the raw materials. Less than 10% is hard and more than 30% is soft.
characteristic:
1. No obvious melting point, softened at 60 ℃, viscoelastic state at 100 ℃ -150 ℃, narrow melting temperature range at 140 ℃, poor thermal stability, easy to decompose, rapid decomposition at 170 ℃, softening point close to decomposition point, decomposition release HC1 gas.
2. UPVC has high viscosity, poor thermal stability, and temperature and time will cause decomposition, so it has a corrosive effect on general steel.
3. The temperature control is strict, the screw design should be as low as possible to prevent overheating, the flow channel must be smooth, there must be no dead corners, and the accumulation of materials to prevent decomposition.
4. The screw barrel must be protected from corrosion.
5. The injection molding process needs to be strictly controlled.
Preferences:
1.L / D = 18-21, h3 = 0.067-0.08D, ε-1.6-2, L1 = 40-45%, L2 = 35-45%, in order to prevent the storage material from having no rubber ring, the head is 20 ℃ ～ 30 ℃, using hard chromium-plated screw;
2. Choice of temperature control; ① Multi-stage air-cooling device for melted plastic barrel; ② Low shear and high mixing UPVC special screw barrel device, strict temperature control, screw design should be as low as possible to prevent overheating.
3． The screw and barrel must be protected against corrosion.
4． The injection molding process needs to be strictly controlled.
5. In order to prevent the storage of materials, there is no non-return ring, the taper of the head is 20 ° ～ 30 °, and it is more suitable for soft rubber. It is more suitable, and in order to cooperate with temperature control, cooling screw or oil hole is added inside the screw of the feeding section, and cold water or oil tank is added outside the barrel.
