Characteristics of transparent plastic materials and injection molding process

Because plastic has the advantages of a lightweight, good toughness, easy molding. Low cost and other advantages, so in modern industry and daily-use 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 parts to be high, impact resistance tough parts 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 in place of glass (hereinafter referred to as transparent plastic), good surface quality, to meet the requirements of use.

At present, the market generally used transparent plastics are polymethyl methacrylate (commonly known as Acrylic or Plexiglas, code PMMA), polycarbonate (code PC), polyethylene terephthalate (code PET), transparent nylon AS (acrylonitrile a benzene ethylene copolymer), polysulfone (code PSF), etc., of which we are most exposed to PMMA, PC and PET three kinds of plastic Due to the limited space, the following three plastics as an example, to discuss the characteristics of transparent plastics and injection molding process.
 

I. Performance of transparent plastics

Transparent plastics must first have high transparency, followed by a certain degree of strength and abrasion resistance, impact resistance, heat resistance, chemical resistance to be excellent, water absorption should be small, only in this way can meet the requirements of transparency in the use and long term, the following table l, compare the performance of PMMA, PC and PET.

Table 1: Comparison of the performance of transparent plastics

Note: (1) Because of the wide variety, this is only the average value, the actual data of different varieties vary.

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

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

II, the transparent plastic injection molding process should pay attention to the common problems

Transparent plastics, because of the high light transmission rate, must require strict surface quality requirements of plastic products, there can not be any mottling, porosity, white, haze, black spots, discoloration, poor luster, and other defects, so in the entire injection molding process of raw materials, equipment, molds, and even product design should be very careful and put forward strict and even special requirements.

Secondly, because of the high melting point and poor fluidity of transparent plastic, to ensure the surface quality of the product, it is often necessary to make fine adjustments in the higher temperature, injection pressure, injection speed, and other process parameters, so that the injection plastic can be filled with the mold, and will not produce internal stress and cause product deformation and cracking.

The following is the preparation of raw materials, equipment and mold requirements, injection molding process, and product processing of raw materials in several aspects, to talk about the matters that should be noted.

(A) Preparation and drying of raw materials

As in the plastic contains any little impurities, may affect the transparency of the product, and therefore and storage, transportation, filling process, must pay attention to sealing, to ensure that the raw materials are clean. In particular, the raw materials contain moisture, heating will cause deterioration of raw materials, so it must be dry, and in the injection molding, adding material must use a dry hopper.

Also note that in the drying process, the input air should preferably be filtered, dehumidified, to ensure that no pollution of raw materials. The drying process as Table 2 transparent plastic drying process

(B) barrel, screw, and its accessories cleaning

To prevent contamination of raw materials and in the screw and accessories depressions with old materials or impurities, especially poor thermal stability of the resin, so before use, after the shutdown should be cleaned with the screw cleaning agent so that it shall not stick impurities, when there is no screw cleaning agent, available PE, PS and other resin cleaning screw.

When the temporary stop, to prevent the raw materials in the high temperature staying for a long time, causing the solution to fall, the dryer and barrel temperature should be reduced, such as PC, PMMA and other barrel temperatures should be reduced to 160 ℃ or less. (Hopper temperature for PC should be reduced to below 100 ℃)

(C) in the mold design should pay attention to the problem (including the design of the product)

To prevent poor return flow or uneven cooling of the plastic resulting in poor molding, surface defects, and deterioration, the following points should be noted when designing the mold in general.

a) The wall thickness should be as uniform as possible, and the slope of the mold release should be large enough;

b) The transition part should be gradual. Smooth transition, to prevent sharp corners. Sharp edges are produced, especially PC products must not have gaps;

c) sprue. The runner should be as wide and short as possible, and the location of the gate should be set according to the shrinkage condensation process, and the cold material well should be added if necessary.

d) The surface of the mold should be bright and clean with low roughness (preferably below 0.8).

e) Ventilation holes. (e) Venting holes must be sufficient to vent air and gas from the melt promptly.

(f) In addition to PET, the wall thickness should not be too thin, generally not less than mm.

(d) injection molding process should be noted (including the requirements of the injection molding machine)

To reduce the internal stress and surface quality defects, the following aspects of the injection molding process should be noted.

a) Special screw, injection molding machine with separate temperature-controlled nozzle should be used;

b) injection temperature is appropriate for higher injection humidity provided that the plastic resin does not decompose.

(c) injection pressure: generally higher to overcome the defects of high viscosity of the melt, but the pressure is too high will produce internal stress caused by the difficult release and deformation.

(d) injection speed: in the case of mold filling, it is generally advisable to use low, preferably slow-fast-slow multi-stage injection.

(e) pressure-holding time and molding cycle: in the case of satisfying the product filling and not producing depressions and bubbles; it is advisable to keep it as short as possible to minimize the residence time of the melt in the barrel.

(f) screw speed and back pressure: in the premise of meeting the quality of plasticization, should be as low as possible, to prevent the possibility of decompression.

(g) mold temperature: good or bad cooling of the product, the quality of great impact, so the mold temperature must be able to accurately control the process, if possible, the mold temperature should be the higher good.

(E) other aspects of the problem

To prevent the deterioration of the surface quality, the release agent should be used as little as possible when injection molding; when using reused material, it should not be more than 20%.

For products other than PET, should be post-treated to eliminate internal stress, PMMA should be dried in 70-80T hot air cycle for 4 hours; PC should be in clean air, glycerin. Liquid paraffin, etc. is heated at 110-135℃, and the time depends on the product, the highest need is more than 10 hours. And PET must go through the process of bi-directional stretching to get good mechanical properties.
 

III, the injection molding process of transparent plastics

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

1、Process characteristics of PMMA

PMMA has a high viscosity and slightly poor flowability, so it must be injected at high material temperature and high injection pressure. 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 processability is good.

Therefore, to improve the fluidity, we can start from the injection temperature. Poor impact, 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、The process characteristics of PC

PC viscosity, high melt temperature, poor liquidity, so must be a higher temperature injection (between 270-320T), relatively narrow range of material temperature adjustment, the process is not as good as PMMA. injection pressure on the liquidity of less impact, but because of the viscosity, still need a larger injection pressure, the corresponding to prevent internal stress, holding time to be as short as possible.

The shrinkage rate 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 temperature of the radiation nozzle 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, the fluidity is good, so the process is poor, and often in the injection nozzle to add anti-delay flow device. Mechanical strength and performance after injection are not high, must be done through the stretching process and modification to improve performance.

Accurate control of mold temperature is an important factor in preventing warpage and deformation, so hot runner molds are recommended. The mold temperature is high, otherwise, it will cause poor surface gloss and difficulty in demolding.