jackymechanical

PVC injection molding process
Because of its low price, PVC material has inherent anti-inflammatory properties, is strong and strong, has good chemical resistance, and has a shrinkage rate of 0.2-0.6%. The products are increasingly used in electrical appliances, machinery, construction, daily necessities, toys, and packaging. According to the characteristics of PVC material, the injection molding process of the product is analyzed as follows:

1. Characteristics of PVC material

The thermal stability of PVC is poor, the molding temperature is close to the decomposition temperature, the fluidity is not good, and the appearance is easy to form defects. The PVC material has poor heat resistance and is most likely to scorch, generate acid gas and corrode the mold. It can be plasticized during processing. Additives are generally required to increase its fluidity. Its strength, electrical insulation and chemical resistance are good.

2. Mold and gate design

In order to shorten the injection molding cycle, the shorter the injection port, the better, the cross-section should be circular, the minimum diameter of the nozzle opening is 6 mm, it is a circular cone, and the inner angle is 5 degrees. It is best to add a cold well and cold material. The well can prevent 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 enough venting equipment in the cavity. The commonly used vent size is 0.03-0.05mm deep and 6mm wide, or the clearance around each thimble is 0.03-0.05 mm. The mold should be made of stainless steel or plated with hard chrome.

Three, PVC molding process

PVC is a heat-sensitive plastic. Overheating or excessive shearing will cause decomposition and spread rapidly, because one of the decomposition products (such as acid or HCI) will produce a catalytic effect, causing the process to further decompose, and the acid will attack the metal and change it If it becomes dent, it will peel off the protective layer of the metal and cause rust, which is more harmful to the human body.

The common screw length-to-diameter ratio is 18-24:1, the three-stage ratio is 3:5:2, and the compression ratio is 1.8-2. The screw groove depth of 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 tip of the screw should have an internal angle of 25-30 degrees. When the screw is in place, the distance between the tip and the nozzle should be 0.7-1.8mm. The screw must be made of stainless steel or chrome-plated.

1) Screw gasket: The screw gasket is between 2mm and 3mm. Large opportunities are greater.

2) Injection volume: The actual retention time of the cylinder should not exceed 3 minutes.

3) Barrel temperature setting:

The temperature provided can only be used as a reference. Make appropriate adjustments according to the actual conditions of the machine and raw materials, or it may exceed the recommended range.

Position Mould Nozzle Front section Middle feeding section
Temperature (0C) 30-60 170-190 160-180 150-170 140-160 The injection volume used is 20-85% of the theoretical injection volume of the machine. The smaller the injection volume actually used, the longer the residence time of the material. The greater the risk of degradation after heating.

4) 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-level injection speeds should be used. If light brown stripes radiate from the gate, the injection speed is too high. fast.

5) Back pressure: The commonly used surface value is 5bar. Increasing the back pressure is beneficial to color mixing and exhaust, but the back pressure should be as low as possible.

6) Shutdown: Due to the low overheating stability of PVC, the shutdown part is very important. Clean the barrel without leaving any PVC. Other thermoplastics with high overheating stability but not resistant to PVC can be used, 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.

7) Mixing ratio: The maximum mixing ratio of the nozzle material is 20%, otherwise it will affect the product quality.

8) Typical products: The main purpose of UPVC is to make drainage pipes, or shells of computers and televisions, water filter tanks, shells of photocopiers, electric meters and gas shells, transparent covers of printing machines, general styles and various electronic parts Wait.