Transparent plastic due to higher light transmission rate, which inevitably calls demanding surface quality of plastic products such like PC injection molding, PMMA injection molding, etc, can not have any markings, holes, white, fog halo, black spots, color, luster is not so good a result, the entire injection molding process on raw materials, equipment, molds, and even product design, we must pay great attention to a strict and even special requirements. Second, because of transparent plastic, mostly high melting point, poor mobility, so as to ensure the surface quality of products that often require a higher temperature, injection pressure, injection speed and other process parameters have to make minor adjustments to make when both full of plastic injection mold, Also does not produce the internal stress caused by deformation and cracking products.
Therefore, from raw materials preparation, equipment and tooling requirements, raw material for injection molding processes and products must be strictly deal with several aspects of the operation.
(A) the preparation and drying of raw materials
Since any point in the plastic contains impurities, may affect the product transparency, and storage, transport, feeding the process must pay attention sealed to ensure a clean raw material. In particular, raw materials contain moisture, heat, might cause deterioration of raw materials, they must be dry. In the injection, the feeding must be used a dry hopper. Would also like to note that the drying process, the type of air should preferably be through a filter, dehumidification, in order to ensure no contamination of raw materials. The drying process the following table, transparent plastic drying process: material drying temperature (℃) drying time (h) Material thickness (mm) Remarks PMMA 70 ~ 80 2 ~ 4 30 ~ 40 PC 120 ~ 130> 6 <30 with hot air circulating dry PET 140 ~ 180 3 ~ 4 with continuous drying is better feeder
(B) the barrel, screw and its annexes clean
In order to prevent contamination of raw materials and in the screw and accessories depression there is old stuff, or impurities, in particular, the existence of thermal stability of the resin, so before use, stopping applications after they screw cleaning agent to clean the parts, so that it can not stick with impurities, when there is no screw when the cleaning agent can be used PE, PS and other resins cleaning screw. When the temporary shutdown, in order to prevent the raw materials at high temperatures to stay a long time, causing lower solution should be drying machines and barrel temperature decreases, such as PC injection molding, PMMA injection molding, etc. barrel temperature should be reduced to 160 ℃ below (hopper temperature for the PC should be down to 100 ℃ below).
(C) It should be noted on the mold design issues (including product design)
To prevent backflow of poor or uneven cooling caused by plastic molding bad, resulting in surface defects and deterioration in general in the mold design, should pay attention to the following points.
a) wall thickness should be uniform, stripping slope be large enough;
b) the transition part should be a smooth and gradual transition to prevent any sharp corners, sharp edges produce, especially in PC products must not have gaps;
c) gate, flow channel as large, chunky, and the condensation process of contraction set in accordance with gate location, if necessary, add cold material should be well;
d) die surface should be smooth, roughness low (preferably less than 0.8);
e) Vent. Trough must be sufficient, as well as air and melt when the exhaust gas;
f) In addition to PET, the thickness should not too thin, generally not less than 1 mm.
(D) injection molding process should pay attention to the issues (including injection molding machine requirements)
In order to reduce internal stress and surface quality defects in the injection molding process should pay attention to the following aspects.
a) should use special screws, with a separate temperature-controlled radio Tsui injection molding machine;
b) injection of the temperature in the plastic resin is not decomposed under the premise of injecting suitably high humidity;
c) Injection pressure: a general high melt viscosity in order to overcome the shortcomings, but the pressure is too high will produce stripping difficulties caused by internal stress and deformation;
d) Injection speed: In the case of filling to meet in general should low, it is best to adopt a slow-slow a fast multi-level injection;
e) holding pressure time and molding cycle: to meet the product filling and do not produce depression, the case of air bubbles; should as short as possible to minimize the melt residence time in the barrel;
f) screw speed and backpressure: in meeting the quality of plastics under the premise should be as low as possible and prevent the possibility of the lower solution;
g) Mold temperature: cooling products, good or bad, a great impact on quality, so mold temperature must be able to precisely control the process, if possible, die Wenyi higher good.
(V) other aspects of the
In order to prevent the upper surface due to deterioration in the quality, generally when you minimize the use of injection mold release agent; When we go back to the materials used shall not be greater than 20%.
For addition to PET, the product should be carried out post-processing to eliminate stress, PMMA injection moulding should be 70-80t hot air circulation drying 4 hours; PC should be clean air, glycerin, liquid paraffin, and other heating 110-135 ℃, time as a product may be the highest need for more than 10 hours. The PET must be bi-oriented processes in order to get good mechanical properties.