Common problems during the transparent plastic injection molding
Due to the high light transmittance demand of transparent plastic parts, the surface quality of plastic products should be very high, and there must be no markings, whitening,fog, black spots, discoloration, poor gloss and other defects.
We must pay great attention in the entire injection molding process from raw materials, equipment, molds, to product design. Secondly, because transparent plastics have high melting point and poor fluidity, in order to ensure the surface quality of the products, the process parameters such as barrel temperature, injection pressure and injection speed are often adjusted so that the plastic can be filled full with mold without Internal stress to cause deformation and cracking of the product.
In the process , we should pay special attention to the following points：
(1) plastic raw materials
Containing any impurities in the plastic may affect the transparency of the product, and therefore, in the process of storage, transportation and feeding, it must ensure that the raw materials are clean.
In particular, the raw material contains moisture, which causes deterioration of the raw material after heating, so it must be dried.
And in the injection molding, the hopper must be used for the feeding. It should also be noted that during the drying process, the input air should preferably be filtered and dehumidified to ensure that the raw materials are not contaminated.
(2) the barrel, screw and its accessories
In order to prevent contamination of raw materials and existing materials or impurities in the recesses of screws and accessories, specially the resins with poor thermal stability,therefore, the screws should be cleaned before use and after shutdown to prevent impurities remained. When there is no screw cleaning agent, the screw can be cleaned with a resin such as PE or PS.
When temporarily shut off, in order to prevent the raw material from staying at high temperature for a long time, causing the degradation, 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 Below 160 °C. (The hopper temperature should be reduced to below 100 °C for PC)
(3) mold and product design
In order to prevent the poor flow, or uneven cooling, resulting in poor plastic molding, surface defects and deterioration, in the mold design, it should pay attention to the following points.
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 transition could prevent sharp corners. Sharp edges are produced, especially PC products must not have gaps;
c) Gate. The runner should be as wide and thick as possible, and the gate position should be set according to the shrinkage condensation process, and a cold well should be added if necessary;
d) the surface of the mold should be smooth and the roughness is low (preferably less than 0.8);
e) Venting. The slot must be sufficient to vent the air and the gas in the melt;
f) Except for PET, the wall thickness should not be too thin, generally not less than l mm.
(4) the injection molding process
In order to reduce internal stress and surface quality defects, the following should be noted in the injection molding process.
a) Special screw and injection molding machine with separate temperature control nozzle should be used;
b) The injection temperature should be higher as possible , but below decompose degree.
c) Injection pressure should be higher to overcome the high viscosity of the melt, but if the pressure is too high, internal stress will cause internal deformation and difficult ejection.
d) injection speed, in the case of full filling the mold, generally should be low, it is best to use slow-fast-slow multi-stage injection;
e) holding time and molding cycle: in the case of full filling, no dents or bubbles; holding time should be as short as possible to minimize the residence time of the melt in the barrel;
f) Screw speed and back pressure: it should be as low as possible to prevent the possibility of depreciation;
g) Mold temperature: The cooling of the mold has great influence on the quality. Therefore, the mold temperature must be accurately controlled. If possible, the mold temperature should be higher.
In order to prevent the deterioration of the upper surface quality, the mold release agent should be used as little as possible during injection molding; when it is used, it should not be greater than 20%.
For products other than PET, the product should be treated to eliminate internal stress after injection molding. PMMA should be dried in hot air at 70-80 °C for 4 hours; PC should be cleaned with air and glycerin. Liquid paraffin and other heating 110-135 ° C, the time depends on the product, the most need more than 10 hours. PET must undergo a two-way stretching process to achieve good mechanical properties.
Transparent plastic injection molding：
PMMA has high viscosity and poor fluidity. Therefore, it must be filled with high temperature and high injection pressure. The influence of injection temperature is greater than the injection pressure, but the high 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 material temperature adjustment range is wide and the processability is good.
Therefore, improving fluidity can be started from the injection temperature. Poor impact, poor wear resistance, easy to scratch, easy to brittle are common defects, it should improve the mold temperature and the condensation process to overcome these defects.
Compared with PMMA injection molding, the PC injection temperature adjustment range is narrower and the process is not as good as PMMA. The injection pressure has little effect on the fluidity, but due to the large viscosity, the injection pressure is still large, and the pressure holding time should be as short as possible in order to prevent the internal stress.
The shrinkage is small and the size is stable, but the internal stress of the product is large and easy to crack. Therefore, it is better to increase the temperature instead of the pressure to improve the fluidity, and to raise the mold temperature , better the mold structure and take post-treatment to reduce the possibility of cracking.
When the injection speed is low, the gate is prone to defects such as ripples, so the nozzle temperature should be controlled separately, the mold temperature should be high, and the runner and gate resistance should be small.
PET molding temperature is high, and the temperature adjustment range is narrow (260-300 °C), but after melting, the fluidity is good, so the processability is poor, and the anti-ducting device is often added in the nozzle. Mechanical strength and performance are not high after injection, and performance must be improved by stretching and modification.
The mold temperature is accurately controlled to prevent warpage. An important factor in deformation is therefore recommended for hot runner molds. The mold temperature should be high, otherwise it will cause poor surface gloss and difficulty in demolding. Difficulties in demolding.