PVC material because of cheap, born with anti-inflammatory properties, but also tough and strong, good chemical resistance, shrinkage rate of 0.2-0.6%, products in electrical appliances, machinery, construction, daily necessities, toys, packaging applications increasingly widespread, According to the characteristics of PVC material, analysis of product injection molding process is as follows:
First, the characteristics of PVC material
Poor PVC thermal stability, molding temperature and decomposition temperature are close, poor fluidity, easy to form the appearance of poor defects, PVC material poor heat resistance, the most easy to scorch, resulting in acid gas and then corrosion mold, processing can add plastic Agents to increase its liquidity, generally need to add additives to use, its strength, electrical insulation, good chemical resistance.
Second, mold and gate design
In order to shorten the injection molding cycle, the shorter the better the nozzle, cross-section to the park-shaped, the diameter of the nozzle Tsui minimum of 6 mm, into the park cone, the angle of 5 degrees, it is best to add cold material wells, cold material Wells prevent poorly melted semi-solid material from entering the mold cavity, which can affect surface modification and product strength.
Draft taper should be between 0.50 and 10 to ensure adequate venting in the mold cavity. Common vent sizes are 0.03-0.05mm deep, 6mm wide, or 0.03-0.05 per tip thimble mm. The mold is made of stainless steel or hard chrome plated.
Third, PVC molding process
PVC is a thermosensitive plastic. Overheating or over-shearing can cause decomposition and spread rapidly as one of the decomposition products (such as acid or HCI) catalyzes the reaction and causes the process to further decompose. The acidic material can also attack the metal and make it more volatile Into depression, it will make the protective layer of metal spalling, causing rust, more harmful to the human body.
Common screw aspect ratio of 18 to 24: 1, the ratio of three sections of 3: 5: 2, the compression ratio of 1.8 to 2, the feed section of the groove depth recommended as follows:
Screw diameter (mm) 35 50 60 75 100
Groove depth (mm) 2 3 4 5 6
The tip of the screw should have an internal angle of 25 to 30 degrees. When the screw is ejected in place, the distance between the tip and the nozzle should be 0.7 to 1.8 mm. The screw must be made of stainless steel or chrome-plated.
1) Screw litter: Screw litter between 2 ~ 3mm, larger opportunities for larger.
2) injection volume: the actual cylinder retention time can not exceed 3 minutes.
3) barrel temperature setting:
The temperature provided is for reference only. It may be out of the recommended range according to the actual conditions of the machine and raw materials.
Location Front of the mold nozzle feed section of the middle
Temperature (0C) 30-60 170-190 160-180 150-170 140-160 The injection volume is 20 ~ 85% of the theoretical injection volume of the machine, the actual injection volume is smaller, the longer the residence time of the material, The risk of degradation after heating is also greater.
4) barrel retention time:
In 2000C under the temperature control (rubber) barrel retention time of more than 5 minutes
5) Injection speed:
Injection speed is slower, or excessive shear will make the material degradation, the use of UPVC to produce extremely smooth thick-walled products, should be used multi-stage injection speed, if there are light brown stripes emitted from the gate, which means that the injection speed too fast.
6) back pressure: the commonly used surface value is 5bar, increase the back pressure is conducive to mixing and exhaust, but the back pressure should be as low as possible.
7) Shutdown: Due to the overheating stability of PVC is very low, it is very important to stop the process, it is necessary to clean the barrel, without leaving the slightest PVC, available other overheating stability and high resistance to PVC thermoplastic (such as PMMA, PP, LDPE, or GPPS). Do not mix POMT UPVC in the same barrel. Failure to do so can result in excessive chemical reaction and serious damage to the machine.
8) Mixing ratio: The maximum mixing ratio of the material in the outlet is 20%, otherwise the quality of the product will be affected.
9) Typical Products: The main purpose of UPVC is to make drainage pipes, or the housing of computers and TV sets, filter press slots, photocopier housings, meters and gas housings, transparent covers of printing machines, ventilation and various electronic components Wait.