Due to its good toughness, ease of molding, and low cost, more and more plastics replace glass in modern industries and everyday products, especially in the optical instrument and packaging industries.
However, due to the requirement for transparency, the wear parts are high and the impact resistant parts are better. Therefore, a lot of work must be done on the composition of the plastic, the process of the injection molding process, the equipment, the mold, etc., to ensure that the glass is replaced by these materials.
Plastic (hereinafter referred to as transparent plastic) has a good surface quality to meet the requirements of use.
The transparent plastics currently on the market are polymethyl methacrylate (commonly known as acrylic or plexiglass), codenamed PMMA), polycarbonate (codenamed PC) and polyethylene terephthalate (codenamed PET). , transparent nylon and hard transparent PVC.
AS (acrylonitrile-styrene copolymer), polysulfone (codename PSF), etc., we are most often in contact with PMMA, PC and PET plastics. Due to limited space, the following three plastics are used as examples to discuss the characteristics of transparent plastics and injection molding process. .
First of all, the performance of transparent plastic
Transparent plastic must first have high transparency. Second, it must have a certain strength and wear resistance. It is resistant to impact, heat, chemical and water absorption. Only in this way can transparency be met.
The requirements are long lasting. Table 1 below lists the performance of PMMA, PC and PET.
Table 1: Comparison of performance of transparent plastics
Performance density
(g / am2) tensile strength
(MPa) notched impact
(J / m2) transparency
(%) deformation temperature
(°C) Permissible water content, shrinkage resistance, abrasion resistance, chemical resistance
PMMA 1.18 75 1200 92 95 0.04 0.5 bad quality
PC 1.20 66 1900 90 137 0.02 0.6 Zhongliang
PET 1.37 165 1030 86 120 0.03 2 good
Note: (1) Due to the wide variety, this is only an average value, and the actual data of different varieties is different.
(2) PET data (mechanical aspect) is tensile data.
As can be seen from the data in Table 1, PC is an ideal choice, but it is mainly because of the high raw material price, injection molding process, PMMA is still the main choice (general requirements), RET can be obtained by stretching.
Good mechanical properties, so it is mainly used for packaging and containers.
Second, the common problems that should be paid attention to during the transparent injection molding process
Due to the high light transmittance of transparent plastics, the surface quality of plastic products must be strict, and there must be no defects such as streaks, pores, whitening, halos, dark spots, discoloration, and poor gloss.
Equipment, molds, and even product design must be highly valued and imposed with strict or even special requirements.
Second, because of transparent plastic
High melting point and poor fluidity. Therefore, in order to ensure the surface quality of the product, process parameters such as higher temperature, injection pressure and injection speed are usually finely adjusted so that the plastic can fill the mold without internal stress.
Causes product deformation and cracking.
The following are things to be aware of in terms of raw material preparation, equipment and mold requirements, injection molding processes and raw material handling.
(1) Preparation and drying of raw materials Since impurities contained in plastics may affect the transparency of the product, it is necessary to pay attention to sealing during storage, transportation and feeding to ensure that the raw materials are clean.
In particular, since the raw material contains moisture and the raw material is deteriorated after heating, it is necessary to dry, and when it is injected, it is necessary to use a dry hopper for feeding.
It should also be noted that during the drying process, the input air should preferably be filtered and dehumidified to ensure that the material is not contaminated.
The drying process is shown in Table 2.
Table 2, drying process of transparent plastic:
Process drying temperature (°C) drying time (h) thickness layer (mm) remarks
PMMA 70~80 2~4 30~40 hot air circulation drying
PC 120~130 u003e6c uses hot air circulation drying
PET 140~180 3~4 is best to use continuous drying and feeding device.
(2) Cleaning the barrel, screws and accessories
In order to prevent contamination of raw materials and the presence of old materials or impurities in the grooves of the screws and accessories, there are resins with poor thermal stability. Therefore, clean the screws before use and after shutdown to prevent impurities from sticking.
When there is no screw cleaner, the screws can be cleaned with a resin such as PE or PS.
When suspending, in order to prevent the raw material from staying at a high temperature for a long time, the solution is lowered, and the temperature of the dryer and the barrel should be lowered. For example, the temperature of the barrel such as PC and PMMA should be reduced to below 160 °C.
(PC hopper temperature should be lower than 100 ° C)
(3) Problems that should be paid attention to in mold design (including product design)
The order is to prevent poor reflow or uneven cooling, resulting in poor plastic molding, surface defects and deterioration. Generally, in the mold design, the following points should be noted.
a) the wall thickness should be as uniform as possible, and the draft angle should be large enough;
b) The transition should be gradual.
Smooth transitions to prevent sharp corners.
Produce sharp edges, especially PC products must have no gaps;
c) Door.
The flow path should be as wide and thick as possible, and the gate position should be set according to the shrinkage condensation process. If necessary, a cold well should be added;
d) the surface mold should be smooth and the roughness is low (preferably less than 0.8);
e) Ventilation holes.
The tank must be sufficient to discharge air and gas from the melt;
f) In addition to PET, the wall thickness should not be too thin, generally not less than 1mm.
(4) Problems that should be paid attention to during injection molding (including the requirements of injection molding machines)
In order to reduce internal stress and surface quality defects, the following aspects should be observed in the injection molding process.
a
) A special screw should be used, and the injection molding machine has a separate temperature-controlled nozzle;
b) the injection temperature should be higher than the plastic resin without decomposition;
c) Injection pressure: usually higher to overcome the high viscosity of the melt, but if the pressure is too high, internal stress will be generated, resulting in demoulding and deformation;
d) Injection speed: the mold is generally low in the case of filling, preferably slow - slow - slow multi-stage injection molding;
e) holding time and molding cycle: no dent or bubble in the case of filling the product; it should be as short as possible to minimize the residence time of the melt in the barrel;
f) screw speed and back pressure: under the premise of satisfying the plasticizing quality, the possibility of depreciation should be prevented as low as possible;
g) Mold temperature: The quality of the product is cooled, which has a great impact on the quality. Therefore, the mold temperature must be able to precisely control the process. If possible, the mold temperature should be higher.
(5) Other issues
In order to prevent surface quality degradation, the release agent should be used as little as possible during the injection molding process; it should not exceed 20% when used.
For products other than PET, the product should be post-treated to eliminate internal stress. PMMA should be dried in 70-80T hot air for 4 hours; PC should be cleaned with air and glycerin.
Liquid paraffin and other heating 110-135 ° C, depending on the product, most need more than 10 hours.
PET must undergo a biaxial stretching process to achieve good mechanical properties.
Third, transparent plastic injection molding process
(I) Process characteristics of transparent plastics: In addition to the above-mentioned common problems, transparent plastics also have some process characteristics, which are described as follows:
1. The process characteristics of PMMA PMMA have high viscosity and poor flow. Therefore, high material temperatures and high injection pressures must be filled. The injection temperature has a greater influence than the injection pressure, but the injection pressure is increased, which is advantageous for increasing the shrinkage rate of the product.
The injection temperature range is wide, the melting temperature is 160 ° C, and the decomposition temperature is 270 ° C, so the temperature adjustment range is wide and the processability is good.
Therefore, the fluidity can be improved from the injection temperature.
Poor impact, poor wear resistance, easy to scratch, brittle, should increase mold temperature, improve condensation process, overcome these defects.
2, PC process characteristics
The PC has a high viscosity, a high melting temperature, and poor fluidity. It must be injected at a higher temperature (between 270 and 320 T). Relatively speaking, the temperature adjustment range is narrower and the processing performance is not as good as PMMA.
Injection pressure has little effect on fluidity, but because of the high viscosity, the injection pressure is still large. To prevent internal stress, the holding time should be as short as possible.
The shrinkage rate is large and the size is stable, but the internal stress of the product is large and it is easy to crack. Therefore, it is preferable to increase the temperature rather than the pressure to improve the fluidity, increase the mold temperature, and improve the mold structure and post-treatment to reduce the possibility of cracking.
When the injection speed is low, the gate is prone to defects such as ripples, the temperature of the radiation nozzle should be controlled separately, the mold temperature should be high, and the runner and gate resistance should be small.
3, PET process characteristics
The PET molding temperature is high, and the material temperature adjustment range is narrow (260-300 ° C), but the fluidity after melting is good, the workability is poor, and an anti-catheter device is usually added to the nozzle.
Mechanical strength and performance after injection are not high and must be improved by stretching and modification.
Precise control of mold temperature to prevent warpage.
Therefore, for hot runner molds, it is recommended to use important factors of deformation.
The mold temperature is high, otherwise the surface gloss will be poor and demolding is difficult.
Fourth, defects and solutions for transparent plastic parts
Due to the length of the relationship, only the defects affecting product transparency are discussed here. For other defects, please refer to the product manual or other materials.
The defects may be as follows:
(1) Silver streaks: During filling and condensation, internal stress anisotropy affects the stress generated in the vertical direction, causing the resin to flow upward, and the non-flow orientation produces different refractive indices and produces flashing filaments.
After expansion, it may cause cracks in the product.
In addition to the injection molding process and mold attention (see Table 4, the best product for annealing treatment. If the PC material can be heated to 160 ° C or longer for 3-5 minutes, then naturally cooled.
(2) Bubbles: Water vapor and other gases in the resin are not discharged (during mold condensation) or due to insufficient filling, the condensation surface is too fast to condense to form 'vacuum bubbles'.
The method of its overcoming is shown in Table 4.
(3) Poor surface gloss: mainly due to the large roughness of the mold, on the other hand, the condensation is too early, therefore, the resin cannot replicate the surface of the mold, all of which will result in slight unevenness of the surface, which will make the product tarnish.
The method of its overcoming is shown in Table 4.
(4) Anti-seismic mode: refers to the dense corrugation formed by the center of the gate, because the melt viscosity is too large, the front end material has been condensed in the cavity, and then the material is broken through the condensation surface, and the surface is destroyed.
.
The method of its overcoming is shown in Table 4.
(5) Whitening and halo: mainly due to the excessive moisture content of raw materials or raw materials that fall into the air.
The method of its overcoming is shown in Table 4.
(6) White smoke and black spots: mainly due to the plastic in the barrel, they are formed by the decomposition or deterioration of the barrel resin due to local overheating.
The method of its overcoming is shown in Table 4.
Overcome method defects Silver pattern Bubble surface gloss difference Impact pattern white, halo white smoke, black spots Raw materials have impurities or pollution, remove impurities, pollution, remove impurities, pollution, remove impurities, pollution
Drying of the resin material, drying should be sufficient, drying should be sufficient, and drying should be sufficient
The melting temperature is reduced, the control is precise, the plasticization is reduced, the amount of increase is increased, the special nozzle is lowered, the control is precise, and the material temperature is minimized.
Increase injection pressure, increase, increase, increase, increase, adjust and not change
Increase injection speed
Increase injection time
Guaranteed pressure

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