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 to 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.
PVC pipe
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 three-stage ratio of 3: 5: 2, the compression ratio of 1.8 to 2, the screw shot 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.
Screw litter: Screw litter between 2 ~ 3mm, larger opportunities for larger.
Injection volume: the actual barrel retention time can not exceed 3 minutes.
Barrel temperature setting: the temperature provided is only for reference, according to the actual situation of the machine, raw material to make the appropriate adjustments, may also be outside the recommended range.
Barrel retention time: under the temperature of 2000C (rubber) barrel retention time of more than 5 minutes
Injection speed: injection speed is slow, or excessive shear will make the material degradation, the use of UPVC to produce extremely smooth thick-walled products, the injection speed should be multi-stage, if there are light brown streaks emitted from the gate that shoot Feed speed too fast.
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.
Downtime: Due to the overheating stability of PVC is very low, so it is very important to stop the process, it is necessary to clean the barrel, do not leave the slightest PVC, available other high thermal stability and non-resistance to PVC thermoplastics (such as PMMA, PP, LDPE, or GPPS). Do not mix POMTUPVC in the same barrel. Otherwise it will cause excessive chemical reaction and cause serious damage to the machine.
Mixing ratio: The maximum mixing ratio of the material is 20%, otherwise it will affect the product quality.