Plastic Material PVC

PVC is a non-toxic and odorless white powder. The density of PVC plastic is 1.40g/cm3, and the density of pvc plastic with plasticizers and fillers is usually 1.15-2.00g/cm3.

The water absorption of PVC is usually below 0.1%. For products with low requirements, it is not necessary to dry before molding. In order to reduce the corrosion of equipment and molds caused by HC1 gas decomposed during pvc processing, equipment and molds must choose anti-corrosion metal materials and do the corresponding anti-corrosion work.

PVC has excellent chemical stability. It is stable to most inorganic acids and alkalis, but it is explained under the action of concentrated sulfuric acid, nitric acid and complex acid.

In rigid PVC (PVC-U), in order to improve thermal stability, lubricity, toughness and appearance quality, various additives should be added. The main way to improve the performance of PVC plastics is to find suitable stabilizers, plasticizers, fillers and other additives for reasonable configuration. It is an effective method to modify PVC through copolymerization and blending.

Factors affecting part characteristics

1. There is no obvious melting point, it will become soft above 60 degrees Celsius, and it will be viscous at 100-150 degrees Celsius. The decomposition temperature of PVC-U mixture rarely exceeds 200 degrees Celsius.

2. Poor thermal stability, whether it is time or temperature, can cause the decomposition of PVC-U, so the temperature and the residence time of the material in the barrel must be strictly controlled.

3. In PVC-U, despite the addition of additives, its fluidity is still poor. Therefore, it is difficult to fill the mold cavity with melt in a very short time. Therefore, it is necessary to use a higher injection pressure and Lower injection rate.

4. As the process of processing inevitably produces decomposition and releases HC1 gas, the anti-corrosion work of the equipment should be done well

5. The product is brittle

6. The melting temperature of PVC during processing is a very important process parameter. If this parameter is not appropriate, it will cause the problem of material decomposition.

7. The flow characteristics of PVC are quite poor, and its process range is very narrow. Especially PVC materials with large molecular weights are more difficult to process (this material usually needs to be added with lubricants to improve the flow characteristics), so PVC materials with small molecular weights are usually used .

8. The shrinkage rate of pVC is quite low, generally 0.6-1.0%

9. When the temperature is too high during molding, it is easy to decompose chlorine gas. The chlorine gas combines with water vapor in the air to form hydrochloric acid, which is easy to corrode the surface of the mold cavity.

Product and mold design requirements

1. The wall thickness of the product is as uniform as possible, and the wall thickness is not too thin, generally not less than 1.2mm, about 1.5-5mm

2. The molding shrinkage rate of PVC-U is between 0.6%-1.5%, and the product is required to have a certain demolding slope, generally 1-1.4. For porous products or products with large demolding resistance feet, the demolding slope is still Bigger

3. All conventional gates can be used, and should be larger to prevent dead ends from stagnating, generally not less than 1mm in diameter or 1mm in thickness.

4. The flow channel should be large, and there should be enough cold slug holes in the reverse direction of the flow channel. Generally, each turning position should be smoothly transitioned, and sharp corners should be avoided as much as possible.

5. Due to the low molding temperature, in order to prevent the cold material from clogging the gate or flowing into the mold cavity, a sufficient cold material cavity should be opened at the end of the main runner, and a cold material cavity should also be opened for a longer runner.

6. In molding, most molds are cooled by water to control mold temperature, but at the same time, in order to improve product performance, sometimes heating/cooling methods are used to control mold temperature. The range of mold control is between 30-60 degrees Celsius.

7. In order to prevent the corrosive effect of HC1 gas, the runner, cavity and other parts of the mold should be chrome-plated, and the anti-corrosion treatment should be done. The goods are made of wear-resistant and corrosion-resistant materials.

Precautions

1. Master the technology of cleaning the barrel. For example, when there are heat-sensitive plastics such as POM in the barrel, the indirect refueling method should be used to clean the barrel.

2. In the process of heating up the barrel, you should pay close attention to the heating situation. When the temperature reaches the process requirements, the injection molding machine must be started in time for air injection, and the barrel should not be kept at a constant temperature for a period of time.

3. The material injected into the air is bright and clean, indicating that the plasticization is good, and it can be put into normal production.

4. When molding, the molding cycle before and after should be as consistent as possible, and the temperature of the oil circuit system of the injection molding machine should be kept constant

5. When shutting down, the material in the barrel should be exhausted first, and the barrel should be cleaned in time with PE or PS, etc.

6. During the molding process, the room should be well ventilated to ensure personal safety.