Characteristics of plastic commonly used in transparent materials (b)

(IV) injection molding process should pay attention to the issues (including the requirements of the injection molding machine)

In order to reduce internal stress and surface quality defects in the injection molding process should pay attention to the following aspects of the problem.

a) should be used as the special screw with a separate temperature control nozzle of the injection molding machine;

b) injection temperature plastic resin does not decompose under the premise, to use the high humidity injection;

c) Injection pressure: generally higher in order to overcome the melt viscosity of the defect, but the pressure is too high will produce stripping difficulties caused by internal stress and deformation;

d) Injection speed: In the case of filling meet, generally should be low, it is best to use a slow-fast and slow multi-stage injection;

e) dwell time and molding cycle: to meet product filling, no depression, the case of the bubble; should be kept short to minimize melt residence time in the barrel;

f) screw speed and back pressure: Under the premise of quality to meet the plastics should be as low as possible to prevent degradation of the possible;

g) Mold temperature: cooling products good or bad, a great impact on the quality, so the mold temperature must be able to precisely control the process, if possible, some good mold temperature should be high.

(5) other aspects 

Because you want to preventing the deterioration of the surface quality, minimize the use of general injection molding release agent; when the use of regrind shall not exceed 20%. Except for the outside pet, after the products should be treated to eliminate stress, pmma should at 70-80t hot air circulation drying 4 hours; pc should be in clean air, glycerol. Liquid paraffin and other heating 110-135 ℃, time according to the product, the highest need more than 10 hours. The pet must biaxial stretching step, in order to obtain good mechanical properties.

Third, transparent plastic injection molding process

(A) process characteristic transparent plastic: In addition to these common problems, also have some processes transparent plastic properties, are summarized as follows:

1, process characteristics pmma the pm-ma viscosity, somewhat less liquidity, and therefore must be high temperature materials, high injection pressure injection job, which affect the injection temperature is greater than the injection pressure, but the injection pressure is increased, it will help to improve shrink products rate. Injection wide temperature range, the melting temperature of 160 ℃, decomposition temperature of 270 ℃, material temperature and therefore wide adjustment range, the process better. Therefore, to improve mobility, we can proceed from the injection temperature. Poor impact resistance, good abrasion resistance, easy to draw flowers, easy to rattle, it should raise the mold temperature, improve the condensation process, to overcome these deficiencies.

2, the technical characteristics of pc pc viscosity, high melting temperature materials, poor mobility, must therefore be at a higher temperature injection (between 270-320 ℃), the material temperature adjustment range is relatively narrow, technology is not as good as pmma. Injection pressure has little effect on liquidity, but the viscosity, still larger injection pressure, corresponding to prevent stress, dwell time should be as short as possible. Shrinkage, dimensional stability, but a large internal stress, easy to crack, it is appropriate to raise the temperature rather than pressure to improve fluidity, and improving the mold temperature, mold structure and to improve the post-processing to reduce the possibility of cracking. When the low injection speed, the gate prone to defects such as corrugations, radiation nozzle temperature to be controlled separately, mold temperature is higher, the flow channel, gate resistance is smaller.

3, the technical characteristics of pet pet molding temperature is high, and the material temperature adjustment range is narrow (260-300 ℃), but after melting, good liquidity, so the process is poor, and often at a nozzle extension to be added to the anti-flow devices. Mechanical strength and performance after the injection is not high, we must in order to improve performance and modified by stretching step. Precise mold temperature control, to prevent warping, distortion of the important elements back to back this proposal hot runner mold. Official high mold temperature, otherwise it will cause poor surface gloss and stripping difficulties.

Fourth, transparent plastic parts defects and solutions

 (A) silver streaks: from filling and condensation process, the internal stress anisotropy effects of stress generated by the vertical direction, so that the resin flow occurs on orientation, while non-current orientation and produce different refractive index and raw silk flash pattern, when After its expansion, may make the product cracks. In addition to the injection molding process and mold note, the best products for annealing. As pc material can be heated to above 160 ℃ holding 3-5 minutes, then natural cooling can be.

(B) bubble: Master in moisture and other gases within the resin did not break out, (in the mold condensation process) or because of insufficient filling, excessive condensation condensing surface and form a "vacuum bubble."

(C) the surface gloss difference: mold roughness, on the other hand the condensation too early, so that the resin can not copy the state of the mold surface, all of which make the surface uneven produce small concave strike, leaving the products lose their luster.

(d) shock pattern: refers to intensive ripple from the center sprue formed, the reason because the melt viscosity is too large, front-end material has been condensed in the cavity, then break through this material and condensing surface, leaving the surface shock pattern ʱ??

(E) white, fog halo: mainly due to the fall in the dust in the air into raw material or feedstock caused by too much moisture.

(f) white smoke, black spots: mainly due to the plastic barrel, due to local overheating of the machine and produce resin cylinder formed by the decomposition or deterioration.

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