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Characteristics of common transparent plastics and injection molding process

2019-10-08Click:949

Transparent plastic must have high transparency, certain strength and abrasion resistance, impact resistance, good heat-resistant parts, excellent chemical resistance, and low water absorption. Only in this way can it meet the requirements of transparency and remain unchanged in use

Transparent plastic must have high transparency, certain strength and abrasion resistance, impact resistance, good heat-resistant parts, excellent chemical resistance, and low water absorption. Only in this way can it meet the requirements of transparency and remain unchanged in use. The commonly used transparent plastics are:

1. Polymethyl methacrylate (commonly known as acrylic or plexiglass, code PMMA),

2.Polycarbonate (code PC),

3.Polyethylene terephthalate (code PET),

4.Transparent nylon, ABS

5.AS (acrylonitrile-styrene copolymer),

6. Polysulfone (codename PSF)


1)Performance comparison

Material \ performance

transparencyJ/m2 

Hot shape temperature℃

Shrinkage

PMMA 

92

95

0.5

PC 

90

137

0.6

PET

86

120

2

Generally required products are still mainly PMMA, and PET has to be stretched to obtain good mechanical properties, so it is mostly used in packaging and containers.



2) Process characteristics during injection molding

i. Process characteristics of PMMA

PMMA has high viscosity and slightly poor fluidity, so high material temperature and high injection pressure must be used for injection. The impact of injection temperature is greater than the injection pressure, but the increase in injection pressure is beneficial to improve the shrinkage of the product. The injection temperature range is wide, the melting temperature is 160 ° C, and the decomposition temperature is 270 ° C, so the range of material temperature adjustment is wide, and the processability is good. Therefore, to improve fluidity, you can start with the injection temperature. Poor impact, poor wear resistance, easy scratches, and brittle cracks, so the mold temperature should be increased, the condensation process should be improved to overcome these defects.

ii. Process characteristics of PC

PC has high viscosity, high melt temperature and poor fluidity. Therefore, it must be injected at a higher temperature (between 270-320T). Relatively speaking, the range of material temperature adjustment is narrow, and the processability is not as good as PMMA. The injection pressure has a small effect on fluidity, but due to the large viscosity, the injection pressure is still large. Correspondingly, to prevent internal stress, the holding time should be as short as possible. Large shrinkage and stable dimensions, but the product has large internal stress and is easy to crack, so it is better to increase the temperature instead of pressure to improve the fluidity, and to increase the mold temperature, improve the mold structure and post-processing to reduce the possibility of cracking. When the injection speed is low, the gate is prone to ripples and other defects. The radiation nozzle temperature must be controlled separately, the mold temperature must be high, and the runner and gate resistance must be small.

iii. Process characteristics of PET

PET molding temperature is high, and the range of material temperature adjustment is narrow (260-300 ° C), but after melting, the fluidity is good, so the processability is poor, and an anti-stretching device is often added in the injection nozzle. The mechanical strength and performance are not high after injection, and the performance must be improved through the stretching process and modification. The mold temperature is accurately controlled to prevent warping. Deformation is an important factor. It is recommended to use a hot runner mold. The mold temperature is high, otherwise it will cause poor surface gloss and difficulty in demolding.



3) Defects and solutions of transparent plastic parts

i. Silver streaks: due to the anisotropy of internal stress during the filling and condensation process, the stress generated in the vertical direction causes the resin to flow on the orientation, and the non-flow orientation has a different refractive index, resulting in glittering silk lines. After expansion, the product may crack. In addition to the attention to the injection molding process and mold, the product is best annealed. For example, the PC material can be heated to above 160 ° C for 3-5 minutes, and then naturally cooled.

ii. Bubbles: "Vacuum bubbles" are formed because the water and other gases in the resin cannot be discharged, or due to insufficient filling, and the condensation surface condenses too quickly.

iii. Poor surface gloss: Mainly due to the large mold roughness, on the other hand, the premature condensation makes the resin unable to copy the state of the mold surface, all of which make the surface produce small unevenness and make the product lose its gloss.

iv. Vibration: refers to the dense corrugations formed from the sprue as the center. The reason is that because the melt viscosity is too large, the front end material has condensed in the cavity, and then the material breaks through this condensation surface, causing the surface to vibrate.

v. Whitening, halo: Mainly caused by dust falling into the raw material in the air or too much water content in the raw material.

vi.White smoke, black spots: Mainly due to plastic in the barrel, due to local overheating, the barrel resin is decomposed or deteriorated.

Specific selection of transparent plastic



I. Transparent materials for daily use:

1.Transparent film: PE PP PS PVC and PET for packaging, agricultural PE PVC and PET, etc.

2.Transparent sheet Board: PP PVC PET PMMA and PC;

3.Transparent pipes: PVC PA, etc.

4.Transparent bottles: PVC PET PP PS and PC etc.



2. Lighting equipment materials:

Mainly used as lampshade, commonly used PS modified PS AS PMMA and PC, etc.



Three. Optical instrument materials

1.The hard lens body mainly uses CR-39 and J.D.

2.HEMA commonly used in contact lenses



4. Glass materials

1. There are two kinds of PMMA and PC commonly used in traffic glass.

2.PVF and PET are commonly used in architectural glass.

V. Solar materials: commonly used PMMA PC GF-UP FEP PVF and SI, etc.

Optical fiber material: PMMA or PC for the core layer, and the cladding layer is a fluorine-containing olefin polymer, fluorine-containing methyl methacrylates

Seven. CD material: commonly used PC PMMA

Eight. Transparent packaging materials

Surface hardened PMMA FEP EVA EMA PVB, etc.

 

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