China plastic injection molders experience of transparent part
Today , let us talk about plastic injection molders transparent parts, with our 20 years experince, following article that hope can give you more help
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The plastic has the advantages of light weight, good toughness, easy to injection molding, low cost, in modern industrial and consumer products, more and more use of plastic instead of glass, especially used in optical instruments and packaging industry developing rapidly. However, due to the transparency and better wear resistance higher impact firmware is better, and therefore the composition of the china plastic injection molders whole process, equipment, molds, have to do a lot of work, to ensure that these used in place of glass plastic (hereinafter referred to as transparent plastic), the surface quality is good, so as to achieve the requirements of use.
Generally on the market use a transparent plastic or polymethyl methacrylate (PMMA, i.e., commonly known as acrylic or perspex,), polycarbonate (PC), polyethylene terephthalate (PET), transparent nylon, acrylonitrile-styrene copolymer (AS), polysulfone (PSF), wherein contact is PMMA, PC and PET three plastic, the following three kinds of plastic, for example, discussion of a transparent plastic material characteristics and the china plastic injection molder process.
The performance of a transparent plastic
Transparent plastic must first have a high transparency, to have a certain strength and abrasion resistance, high impact, heat resistance, better chemical resistance are superior absorb water small, the only way able to meet the transparency requirements over the long term, are listed in Table 1 below, compare the performance of PMMA, PC, and PET.
Table 1: The transparent plastic Performance Comparison
(G/am2) tensile strength
(MPa) Notched Impact
(%) Distortion temperature
(° C) allows moisture shrinkage abrasion chemical resistance
PMMA 1.18 75 1200 92 95 0.04 0.5 difference benign
PC 1.20 66 1900 90 137 0.02 0.6 in benign
PET 1.37 165 1030 86 120 0.03 2 Good Excellent
Note: (1) because of the variety, this is only averaging, actual different varieties of data vary.
(2) PET data (mechanical) for the data after stretching.
From the data in Table 1 shows that the PC is the ideal choice, but the master of its expensive price of raw materials and injection molding process is difficult, so is still used PMMA-based products, (general requirements), and RET to go through the stretch to get good mechanical properties, multi-use in packaging, containers.
Second,china plastic injection molders build transparent process should be noted the common problem.
Higher transmittance, transparent plastic inevitable requirement plastic products strict quality surface, can not have any markings, porosity, white, fog halo, black spots, discoloration, poor gloss and other defects, thus the entire plastic injection molder process of raw materials, equipment, molds, and even the design of the product, should pay great attention to strict and even special demands. Both full Second, because of the transparent plastic, mostly high melting point, poor mobility, so as to ensure the surface quality of the product, often at higher temperature, injection pressure, injection speed and other parameters for minor adjustments, the plastic injection molders will produce internal stress caused deformation and cracking.
Here in the preparation of raw materials, equipment and tooling requirements, injection molding process and raw materials processing aspects, should pay attention to talk about the matter.
(A) the preparation and drying of raw plastics containing any point impurities may affect the transparency of the product, and storage, transportation, feeding process, you must pay attention to sealing to ensure a clean raw materials. Raw materials containing water, heating cause material deterioration, so be sure to dry injection, feeding must use the drying hopper. Also note that the drying process, the input air should preferably by filtration, dehumidified, in order to ensure that no contamination of raw materials. The drying process is as shown in Table 2.
Table 2: The transparent plastic material drying process:
The the process drying temperature (℃) Drying time (h) the material layer thickness (mm) Remarks
PMMA 70 ~ 80 2 ~ 4 30 ~ 40 hot air circulation drying
PC 120 ~ 130 greater than 6 less than 30 hot air circulation drying
PET 140 ~ 180 3 ~ 4 by continuous drying feeding device preferably
(B) of the barrel, screw and its attachments clean
To prevent the contamination of raw materials and the depression in the screw and accessories there are old materials or impurities, particularly thermal stability of the resin is present before use, after the shutdown applications screw cleaning agent to clean, it may not stick with impurities cleaning screw screw cleaning agent, available PE, PS and other resins. Temporary shutdown to prevent raw stay a long time at a high temperature, causing the lower solution should reduce the temperature of dryer and barrel, such as PC, PMMA cylinder temperature should be reduced to 160 ℃. For PC (hopper temperature should be reduced to less than 100 ℃)
(C) should pay attention to the problem of mold design (including product design)
In order to prevent back flow of poor or uneven cooling caused by bad plastic molding, surface defects and deterioration, generally in the design of the mold, you should be aware of the following points.
a) wall thickness should be uniform, and the the stripping slope be large enough;
b) the transition section should gradually. Smooth transition and prevent any sharp corners. Sharp edges, especially the PC product must not have a gap;
c) gate. Flow channel as much as possible large, chunky, and the location of the gate should be set according to the contraction of the condensation process, necessary, add cold slug well;
d) the mold surface should be smooth, low roughness (preferably less than 0.8);
e) vent. The slots must be sufficient, and when the exhaust air and the gas in the melt;
f) In addition to PET, the wall thickness of not too thin, generally not less than lmm.
(D)china plastic injection molders process should pay attention to the issues (including the requirements of the plastic injection molders machine)
In order to reduce the internal stress and surface quality defects, in the injection molding process should pay attention to the following aspects of the problem.
a) should use special screws, with separate thermostat nozzle injection molding machine;
b) injection temperature of the plastic resin is not decomposed under the premise, suitably higher injection humidity;
c) injection pressure: generally higher, in order to overcome the defects of melt viscosity, but the pressure is too high internal stresses stripping difficulties caused and deformation;
d) the injection speed: the meet filling the case, generally should be low, much better to use slow – fast – slow multi-stage injection;
e) dwell time and the molding cycle: meet the product to fill the mold does not produce depression bubble; should be as short as possible, to minimize the melt in the barrel residence time;
f) screw speed and back pressure: under the premise that meet the quality of plastics should be as low as possible, to prevent the generation of the solution down the possible;
g) mold temperature: cooling products, good or bad, on the quality of a great impact, mold temperature must accurately control its process may mold temperature should be high.
(E) other aspects of the problem
General injection should guard against the deterioration of the surface quality, as little as possible with a release agent; nor greater than 20% when using regrind.
In addition to PET, the products should be post-processed to eliminate the internal stress, PMMA 70-80T hot air circulation drying for 4 hours; PC should be clean air, glycerol. Such as liquid paraffin is heated to 110-135 ℃, time products, the highest need more than 10 hours. While the PET must go through the step of biaxial stretching, in order to obtain good mechanical properties.
3, transparent plastic injection molders in china process
(A) transparent plastic process characteristics: In addition to the above common problem, clear plastic and have some process characteristics are described as follows:
1, PMMA process characteristics
PMMA viscosity, mobility somewhat less, and therefore must be high material temperature, the job of the high injection pressure injection molding, injection temperature is greater than the injection pressure, injection pressure increases, will help to improve the shrinkage of the product. The wider range of injection temperature and a melting temperature of 160 ° C, while the decomposition temperature of 270 ° C, therefore the wide adjustment range of the material temperature, the process is better. Therefore, to improve the liquidity, proceed from the injection temperature. Poor impact resistance, abrasion resistance and bad, easy to draw flowers, easy to rattle, it should raise the mold temperature, improve the condensation process, to overcome these deficiencies.
2, PC process characteristics
PC viscosity, high temperature of the melt material, poor mobility, back to this must be the injection of the higher temperature (270-320T), is relatively narrow adjustment range of the material temperature, process is better of PMMA. The injection pressure on liquidity, but the viscosity still larger injection pressure accordingly in order to prevent the internal stress generated, dwell time should be as short as possible. Shrinkage, dimensional stability, but the stress within the product, easy to crack, so it is appropriate to raise the temperature rather than the pressure to improve liquidity and increasing the mold temperature, to improve mold structure and post-processing to reduce the possibility of cracking. When the injection speed is low, the gate at prone corrugated and other defects alone control the temperature of the radiation Tsui, mold temperature is higher, the runner, gate resistance is smaller.
3 the PET A process characteristics
PET molding temperature and material temperature adjustment range is narrow (260-300 ° C), but after melting, good liquidity, so the process is poor, and often in the nozzle to increase the anti-extension of flow control devices. Mechanical strength and performance are not high, after injection must be through the stretching step and modified in order to improve performance. Mold temperature is accurately controlled, is to prevent warping. The deformation of the important factors, so we recommend using hot runner mold. High mold temperature, otherwise it will cause the surface luster poor and stripping difficulties.
4, transparent plastic defects
Due to space constraints, only discuss defects affecting product transparency, and other defects, please refer to the product manual or other information. Its defects about the following:
Transparent product defects and overcome:
(A) crazes: filling and condensation processes, internal stress anisotropy affects the stress generated by the vertical direction, so that the resin flow orientation, refractive index and non-current orientation of different raw flash silk pattern when After its expansion, may make the product cracks. In addition to the attention on the injection molding process and the mold (see Table, the best products for the annealing treatment. Pc material can be heated to more than 160 ℃ maintained for 3 to 5 minutes, and then can be cooled naturally.
(B) the bubble: the main resin, water vapor and other gases row does not go out, mold condensation process or inadequate due to filling the condensing surface excessive condensation and the formation of a “vacuum bubble”. Them overcome the method are shown in Table.
(C) poor surface gloss: Master injection mold large roughness, on the other hand condensate prematurely, so that the resin can not copying the state of the surface of the mold, all the tiny irregularities of its surface cows, leaving the products lose luster. Then overcome the method are shown in Table.
(D) shock pattern: from the sprue intensive corrugated reason for the melt viscosity is too large, front-end material has been condensed in the cavity, and later material and break through this condensation surface, leaving the surface of the shock pattern . Then overcome the method are shown in Table.
(E) whitened. The fog Halo: mainly due to fall into the raw material or feedstock moisture content is too large due to dust in the air. Then overcome the method are shown in Table.
(Vi) white smoke. Black spots: mainly due to decomposition or deterioration of resin plastic drum machine, leaving the barrel due to local overheating