The Characteristics and Injection Molding Process of Commonly Used Transparent Plastics

Because of the plastic has light weight, good toughness, easy molding. Low cost and other advantages, so in modern industry and daily products, more and more use of plastic instead of glass, especially in the application of optical instruments and packaging industry, the development is particularly rapid. However, because of the requirements of its transparency to be good, wear resistance to high, impact toughness to be good, so the composition of the plastic, injection molding the entire process of technology, equipment, molds, etc., to make a lot of work to ensure that these plastics used to replace the glass, good surface quality, so as to meet the requirements of use.

The transparent plastics generally used in the market today are polymethyl methacrylate (PMMA) also known as acrylic, polycarbonate (PC). Polyethylene terephthalate (PET), transparent nylon. AS (acrylonitrile a styrene copolymer), polysulfone (PSF), etc., of which we are most exposed to PMMA, PC and PET three plastics, due to space limitations, the following three plastics as an example, to discuss the characteristics of transparent plastics and injection molding process.

I. Performance of transparent plastic

Transparent plastics must first have high transparency, followed by a certain degree of strength and wear resistance, can resist impact, heat resistance should be good, chemical resistance should be excellent, water absorption should be small, only in this way can meet the requirements of transparency in use for a long time, the following list of Table 1, compare the performance of PMMA, PC and PET.

Table 1: Comparison of the performance of transparent plastics

Note: (1) Due to the wide variety of varieties, this is only the average value, the actual data varies from variety to variety.

        (2) PET data (mechanical aspect) is after stretching.

From the data in Table 1, we can see that PC is the more ideal choice, but the main raw material is expensive and injection molding process is difficult, so it is still the main choice of PMMA, (for the general requirements of the product), and PET due to stretching to get good mechanical properties, so more in the packaging, containers used.

II. Injection molding process of transparent plastic

1. Process characteristics of PMMA

PMMA viscosity is large, the liquidity is slightly poor, so it must be high material temperature, high injection pressure injection, where the influence of injection temperature is greater than the injection pressure, but the injection pressure to improve the shrinkage rate of the product. The injection temperature range is wide, the melt temperature is 160℃, and the decomposition temperature is 270℃, so the material temperature adjustment range is wide and the processability is good. Therefore, to improve the fluidity, we can start from the injection temperature. Poor impact, bad wear resistance, easy to scratch, easy to brittle crack, so the mold temperature should be increased, improve the condensation process, to overcome these defects.

2. Process characteristics of PC

   PC has high viscosity, high melt temperature and poor fluidity, so it must be injected at a higher temperature (between 270-320℃), relatively speaking, the range of material temperature adjustment is narrow, and the process is not as good as PMMA. injection pressure has less influence on fluidity, but because of the high viscosity, it still needs larger injection pressure, and accordingly, in order to prevent internal stress, the holding time should be as short as possible. The shrinkage is large and the size is stable, but the product has high internal stress and is easy to crack, so it is advisable to improve the fluidity by increasing the temperature rather than the pressure, and to reduce the possibility of cracking by increasing the mold temperature, improving the mold structure and post-treatment. When the injection speed is low, the gate is prone to ripples and other defects. The radiating nozzle temperature should be controlled separately, the mold temperature should be high, and the flow channel and gate resistance should be small.

3. Process characteristics of PET

PET molding temperature is high, and the material temperature adjustment range is narrow (260-300 ℃), but after melting, good fluidity, so the process is poor, and often in the injection nozzle to add anti-delay flow device. Mechanical strength and performance after injection is not high, must be through the stretching process and modification to improve performance. Accurate control of mold temperature is an important element to prevent warpage and deformation, back to this recommend the use of hot runner mold. The mold temperature official high, otherwise it will cause poor surface gloss and difficult to release the mold.

III. Defects of transparent plastic parts and solutions

Due to space limitations, only defects affecting product transparency are discussed here. For other defects, please refer to other information from our company. The defects are roughly as follows:

(A) silver stripe: by the mold filling and condensation process, the internal stress anisotropic influence, the vertical direction of the stress, so that the resin flow on the orientation, and non-flow orientation to produce a different refractive index and the birth of flash filament, when it expands, may make the product appear cracks. In addition to the injection molding process and mold attention, the best products for annealing treatment. Such as PC material can be heated to 160 ℃ above the hold 3-5 minutes, and then natural cooling can be.

(B) bubbles: the main in the resin of water and other gases cannot be discharged, (in the process of mold condensation) or due to insufficient mold filling, condensing surface and too fast condensation and the formation of "vacuum bubble".

(C) poor surface gloss: mold roughness, on the other hand, condensation too early, so that the resin cannot copy the state of the mold surface, all these make its surface produce tiny bumpy, and the product lost gloss.

(D) shock lines: refers to the formation of dense ripples from the straight gate as the center, the reason for the melt viscosity is too large, the front-end material has been condensed in the cavity, and later the material and break through the condensation surface, and make the surface appear shock lines.

(E) white, haze: mainly due to dust in the air into the raw materials or too much water content of raw materials and caused.

(F) white smoke, black spots: mainly due to plastic in the barrel, due to local overheating and the barrel resin decomposition or degradation and the formation of.