A comprehensive introduction to PVC injection molding

Time:2021-02-21 15:43:30 / Popularity: / Source:

PVC injection molding 
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%. Products are increasingly used in electrical appliances, machinery, construction, daily necessities, toys, and packaging. According to characteristics of PVC material, injection molding process of products is analyzed as follows:

1. Characteristics of PVC material

Thermal stability of PVC is poor, molding temperature is close to decomposition temperature, fluidity is not good, and appearance is easy to form defects. PVC material has poor heat resistance and is most likely to scorch, generate acid gas and corrode mold. Plasticizer can be added to increase its fluidity during processing. Generally, additives must be used. Its strength, electrical insulation and chemical resistance are good.

2. Mold and gate design

In order to shorten injection molding cycle, the shorter injection port, the better, cross-section should be circular, minimum diameter of nozzle opening is 6 mm, it is a circular cone, and inner angle is 5 degrees. It is best to add a cold slug well, which can prevent poorly melted semi-solid materials from entering mold cavity, and these materials will affect surface modification and product strength.
Draft angle should be between 0.50 and 10 to ensure that there is enough venting equipment in cavity. Commonly used vent size is 0.03-0.05mm deep and 6mm wide, or clearance around each thimble is 0.03-0.05 mm. Mold should be made of stainless steel or plated with hard chrome.

3. PVC molding process

PVC is a heat-sensitive plastic. Overheating or excessive shearing will cause decomposition and spread rapidly, because one of decomposition products (such as acid or HCI) will have a catalytic effect, causing process to further decompose. Acid will erode metal and turn it into dents. It will also peel off protective layer of metal and cause rust, which is more harmful to human body.
1. Screw gasket: Screw gasket is between 2mm and 3mm. larger machines will be larger.
2. Injection volume: Actual retention time of cylinder should not exceed 3 minutes.
3. Barrel temperature setting:
Temperature provided can only be used as a reference. Make appropriate adjustments according to actual conditions of machine and raw materials, or it may exceed recommended range.
4. Residence time of barrel: under temperature control of 2000C (compound), residence time of barrel can exceed 5 minutes at most.
5. Injection speed: Injection speed should be slow, otherwise excessive shearing will degrade material. When using UPVC to produce extremely smooth thick-walled products, multi-stage injection speeds should be used. If there are light brown stripes radiating from gate, it means that injection speed is too fast. 
6. Back pressure: Commonly used surface value is 5bar. Increasing back pressure is beneficial to color mixing and exhaust, but back pressure should be as low as possible.
7. Shutdown: Due to low overheating stability of PVC, shutdown part is very important. Clean barrel without leaving any PVC. Other thermoplastics with high overheating stability and non-resistance to PVC can be used, such as PMMA, PP, LDPE, or GPPS), do not mix POMT UPVC in same barrel, otherwise it will cause excessive chemical reaction and cause serious damage to machine.
8. Mixing ratio: Maximum mixing ratio of nozzle material is 20%, otherwise it will affect product quality.
9. Typical products: Main purpose of UPVC is to make drainage pipes, shells of computers and televisions, water filter tanks, photocopier shells, electric meters and gas shells, transparent covers of printing machines, communication styles and various electronic parts.

Solution to overflow of injection molding machine head

PVC molding process 
Flash, also known as flashing, burrs, piercing, etc., mostly occurs at opening and closing positions of mold. Such as: parting surface of mold, sliding part of slider, gap of insert, hole of ejector, etc. If flash is not resolved in time, it will be further enlarged, resulting in a partial collapse of imprinting mold, causing permanent damage. Flash of insert gap and ejector pin pores will also cause product to be stuck on mold and affect demolding.

1. Equipment 

1. Real clamping force of machine is insufficient. When selecting an injection molding machine, rated clamping force of machine must be higher than tension formed by longitudinal projection area of injection molded product during injection, otherwise it will cause mold expansion and flash.
2. Mold clamping device is not well adjusted, toggle mechanism is not straight, resulting in uneven clamping of left and right or top and bottom molds, phenomenon that parallelism of mold cannot be achieved, causing one side of mold to be tightly closed while the other side is not closely attached, flash will appear during injection.
3. Mold itself is not parallel well, or installation is not parallel, or mold plate is not parallel, or tension rod is unevenly distributed and deformed. These will cause mold to be not tightly closed and flash.
3. Check ring is severely worn, spring nozzle spring fails, barrel or screw is worn too much, inlet cooling system fails to cause "bridging" phenomenon, barrel set injection volume is insufficient, cushion is too small, etc., which may cause flashes to appear repeatedly, and must be repaired or replaced in time .

2. Mold

1. Poor precision of mold parting surface. Movable mold plate (such as middle plate) is deformed and warped; foreign objects are stained on parting surface or there are protruding skid print burrs on the periphery of mold frame; old mold is fatigued and collapsed due to previous flash extrusion.
2. Mold design is unreasonable. Opening position of mold cavity is too off, which will cause tension on one side of mold during injection, causing flash.
Fluidity of plastics is too good, such as polyethylene, polypropylene, nylon, etc., in molten state, viscosity is very low, and it is easy to enter movable or fixed gaps, which requires high mold manufacturing accuracy.
On the premise of not affecting integrity of product, it should be placed on the center of mass symmetry as much as possible, material should be fed into thick part of product to prevent situation of flashing while lacking material. When there is a molding hole in or near center of product, it is customary to open a side gate on the hole. Because under high injection pressure, if clamping force is insufficient, supporting force of mold in this part will be insufficient and slight warping will occur, resulting in flash. For example, when there are movable components on the side of mold, projected area of side is also affected by molding pressure. If supporting force is not enough, it will cause flash.
Sliding core has poor matching accuracy, offset of fixed core and cavity installation position will also cause flash. Poor venting of cavity, no venting groove on parting surface of mold, or venting groove is too shallow, too deep or too large, or blocked by foreign objects will cause flash. For multi-cavity molds, attention should be paid to reasonable design of each runner and gate, otherwise it will cause uneven filling force and flash.

3. Process

1. Injection pressure is too high or injection speed is too fast. Due to high pressure and high speed, opening force to mold increases and causes flash. Injection speed and injection time should be adjusted according to thickness of product. Thin products should be filled quickly at high speed, no injection after full; thick products should be filled at low speed, skin should be generally fixed before reaching final pressure.
2. Excessive feeding volume causes flash. It is worth noting that do not inject too much melt in order to prevent depression, so that depression may not be "filled", but flash will appear. This situation should be resolved by extending injection time or holding time.
3. Too high temperature of barrel and nozzle or too high mold temperature will reduce viscosity of plastic and increase fluidity, which will cause flashing under condition of smooth injection.

4. Raw materials 

1. If plastic viscosity is too high or too low, flash may appear. Plastics with low viscosity, such as nylon, polyethylene, polypropylene, etc., should increase clamping force; plastics with strong water absorption or water-sensitive plastics will greatly reduce flow viscosity at high temperatures and increase possibility of flashing. These plastics must be thoroughly dried; viscosity of plastics mixed with too much recycled materials will also decrease, and if necessary, replenish retained components. If viscosity of plastic is too high, flow resistance will increase, and a large back pressure will increase pressure of cavity, resulting in insufficient clamping force and flash.
2. When size of plastic raw materials is not uniform, feeding amount will vary, parts may not be full or flash.

Go To Top