To share with you: transparent injection molding shell injection molding should pay attention to the method

At present, the transparent plastics commonly used on the market are polymethyl methacrylate, commonly known as the adrite code PMMA and the polycarbonate code PC. Polyethylene terephthalate glycol grease code PET, transparent nylon. AS acrylic-styrene copolymer, polysulfone code PSF, etc., among which we are most exposed to PMMA, PC and PET three kinds of plastics, now take these three kinds of plastic raw materials as an example, to discuss the characteristics of transparent plastic raw materials and injection molding process.

First, the common problems that should be paid attention to in the injection molding process of transparent plastic raw materials

Transparent plastic raw materials due to high light transmittance, it is necessary to require strict quality requirements of the surface of the plastic shell, and there can be no markings, pores, and whiteness. Fog halo, black spots, discoloration, poor luster and other defects, so in the entire injection process of plastic raw materials, injection molding equipment, plastic molds, and even the design of plastic shells, should pay great attention to and put forward strict and even special requirements.

Secondly, because the transparent plastic raw materials are mostly high melting point and poor fluidity, in order to ensure the surface quality of the plastic shell, it is often necessary to make subtle adjustments in the process parameters such as the temperature of the barrel, the injection pressure, and the injection speed, so that the injection molding can be filled with the mold, and the internal stress will not cause product deformation and cracking.

Here, we talk about the matters that should be paid attention to in terms of injection molding equipment and plastic mold requirements, injection molding process and raw material treatment of plastic shell.

1. The preparation and drying of plastic raw materials, due to any impurities contained in plastic raw materials, may affect the transparency of the product, so in the process of storage, transportation and feeding, attention must be paid to sealing to ensure that plastic raw materials are clean. In particular, the plastic raw materials contain water, which will cause the deterioration of the raw materials after heating, so it must be dry, and in the injection molding, the feeding must use the drying hopper. It should also be noted that during the drying process, the input air should preferably be filtered and dehumidified to ensure that it will not pollute the raw materials.

2. Cleaning of injection molding machine barrel, screw and accessories

In order to prevent the pollution of plastic raw materials and old materials or impurities in the depression of the screw and accessories, especially the plastic raw materials with poor thermal stability exist, so the screw cleaning agent is used before use and after shutdown to clean each part, so that it is not stuck with impurities, when there is no screw cleaning agent, PE, PS and other resins can be used to clean the screw.

When the temporary shutdown, in order to prevent plastic raw materials from staying at high temperatures for a long time, causing degradation, the temperature of the dryer and the barrel should be reduced, such as PC, PMMA and other barrel temperatures should be reduced to below 160 ° C, and the hopper temperature should be reduced to below 100 ° C for PC.

3. The problems that should be paid attention to in plastic mold design include product design,

In order to prevent poor back flow, or uneven cooling caused by poor molding of plastic raw materials, resulting in surface defects and deterioration, generally in the design of plastic molds, the following points should be paid attention to:

① The wall thickness should be as uniform as possible, and the demoulding slope should be large enough;

② The transition part should be gradual and smooth transition to prevent sharp corners. Sharp edge generation, especially PC products must not have gaps;

③ Gate, the runner as wide and short as possible, and the gate position should be set according to the shrinkage condensation process, and the cooling well should be added if necessary;

④ The surface of the mold should be smooth and clean, with low roughness (preferably less than 0.8);

The exhaust holes and slots must be sufficient to discharge the air and the gas in the melt in a timely manner;

In addition to PET, the wall thickness should not be too thin, generally not less than lmm.

4. Problems that should be paid attention to in the injection molding process include the requirements of the injection molding machine

In order to reduce internal stress and surface quality defects, the following aspects should be paid attention to in the injection molding process:

① Special screw and injection molding machine with separate temperature control nozzle should be selected;

② The injection temperature should be higher if the plastic raw material resin is not decomposed;

(3) Injection molding pressure: generally higher to overcome the defects of large viscosity of hot melt adhesive material, but too high pressure will produce internal stress resulting in difficult demording and deformation;

④ Injection speed: In the case of mold filling, it is generally low, and it is best to use slow-fast-slow multi-stage injection;

⑤ Pressure holding time and molding cycle: in order to meet the product mold filling, no depression, bubbles; It should be as short as possible to minimize the residence time of hot melt adhesive in the barrel;

⑥ Screw speed and back pressure: under the premise of meeting the plasticizing quality, it should be as low as possible to prevent the possibility of degradation;

7 Plastic mold temperature: the cooling of the plastic shell has a great impact on the quality, so the mold temperature must be able to accurately control the process, if possible, the mold temperature should be higher.

5. Other questions

In order to prevent the deterioration of the quality of the upper surface, the release agent is generally used as little as possible during injection molding. Not more than 20% when using recycled materials.

In addition to PET, the plastic shell should be post-processed to eliminate internal stress, PMMA should be dried at 70-80℃ hot air circulation for 4 hours; PC should be glycerin in clean air. Liquid paraffin is heated at 110-135 ° C, the time depends on the product, and the maximum need is more than 10 hours. PET must undergo a bidirectional drawing process in order to obtain good mechanical properties.

Second, the performance of transparent plastic raw materials

Transparent plastic raw materials must first have high transparency, followed by a certain strength and wear resistance, impact resistance, heat resistance, chemical resistance to be excellent, water absorption to be small, only in this way in use, can meet the requirements of transparency and long-term unchanged.

Comparison found that PC is the ideal choice, but mainly due to the price of its plastic raw materials and injection molding process is difficult, so it is still based on the selection of PMMA, and PPT because it needs to be stretched to get good mechanical properties, so it is used in packaging and containers.

Third, injection molding process of transparent plastic raw materials

Process characteristics of transparent plastic raw materials: In addition to the above common problems, transparent plastic raw materials also have some process characteristics, which are described as follows:

1.PMMA process characteristics, PMMA viscosity, slightly poor flow, so it must be high material temperature, high injection pressure injection molding, the injection temperature influence is greater than the injection pressure, but the injection pressure increase, is conducive to improving the shrinkage of the product.

The injection temperature range is wide, the melting temperature is 160℃, and the decomposition temperature is 270℃, so the material temperature adjustment range is wide, and the process is good. Therefore, the fluidity can be improved from the injection temperature. The impact is poor, the wear resistance is not good, the scratching is easy, and the cracking is easy, so the mold temperature should be increased and the condensation process improved to overcome these defects.

2.PC viscosity, high melt temperature, poor fluidity, so it must be injected at a higher temperature (between 270 and 320℃), relatively speaking, the material temperature adjustment range is narrow, the process is not as good as PMMA. The injection pressure has little effect on the fluidity, but because of the large viscosity, the injection pressure is still larger, and the corresponding pressure holding time should be as short as possible in order to prevent the internal stress.

The shrinkage rate is large, the size is stable, but the internal stress of the product is large and easy to crack, so it is appropriate to increase the temperature rather than pressure to improve the fluidity, and to reduce the possibility of cracking by increasing the temperature of the plastic mold, improving the structure of the plastic mold and post-processing. When the injection speed is low, the gate is prone to corrugation and other defects, the nozzle temperature should be controlled separately, the plastic mold temperature should be high, and the flow path and gate resistance should be small.

3.PET molding temperature is high, and the material temperature adjustment range is narrow (260-300 ° C), but after melting, the flow is good, so the process is poor, and often in the nozzle to add anti-casting device. The mechanical strength and properties are not high after injection, and must be improved by stretching process and modification.

Accurate control of plastic mold temperature is to prevent warping. Deformation is an important factor, so it is recommended to use hot runner plastic mold. The temperature of the plastic mold should be high, otherwise it will cause poor surface luster and difficulties in stripping.

4. Defects and solutions of transparent plastic raw materials

Here we only discuss the defects that affect the transparency of the plastic shell, and there are probably the following defects:

1. Silver pattern: caused by the influence of internal stress anisotropy in the process of mold filling and condensation, the stress generated in the vertical direction makes the resin flow upward orientation, and the refraction rate is different from that generated by non-flow orientation, resulting in flash silk pattern, which may crack the plastic shell when it expands.

2. Bubbles: the water and other gases in the resin can not be discharged, and the "vacuum bubble" is formed during the condensation process of the plastic mold or due to insufficient filling of the mold and too fast condensation of the condensing surface.

3. Poor surface luster: mainly due to the large roughness of the plastic mold, on the other hand, premature condensation, so that the resin can not copy the state of the mold surface, all of which make its surface produce small uneven, and make the plastic shell lose luster.

4. Whitening. Fog halo: mainly due to dust in the air into the plastic raw materials or plastic raw materials caused by too much water content.

5. White smoke. Black spot: mainly due to the plastic plastic in the barrel, due to local overheating and the formation of the barrel resin decomposition or deterioration.

6. Jarring: refers to the dense ripples formed from the center of the straight gate, the reason is that because the viscosity of the hot melt adhesive is too large, the front end of the hot melt adhesive has been condensed in the cavity, and later the hot melt adhesive has broken through the condensing surface, and the surface of the jarring.