What are the negative effects of excessive temperature on injection molded parts

Views: 5     Author: Site Editor     Publish Time: 2021-07-25      Origin: Site

During the processing of plastic injection molded parts, the temperature is too high or low, which will cause adverse effects on the molded injection parts, which will directly cause the molded injection parts to be scrapped. 

Therefore, the temperature control in the injection molding process is very important. When the temperature of injection molding is too high, the viscosity of the oil will decrease, and leakage may increase. At this time, the volumetric efficiency of the pump and the efficiency of the entire system will also be significantly reduced.

If the viscosity of the oil decreases, the oil film of the moving parts such as the slide valve will become thin or the phenomenon of being cut slope will appear. At this time, the frictional resistance will also increase, which will increase the phenomenon of wear. Excessive heating of the injection molding process will accelerate the oxidation and deterioration of the oil, resolve the bituminous substance, and reduce the service life of the hydraulic oil. The resolved substance will block the damping orifice and the gap-type valve port, which will cause the pressure valve to become stuck and not work. The metal pipe stretches and bends, and even breaks.

Excessive heating of the injection molding process will result in poor working quality of the injection molded parts, and the dissolved air in the oil will escape and air pockets will appear, which will reduce the working performance of the hydraulic system. The parts with different coefficients of thermal expansion in hydraulic components will become smaller and get stuck in the training, which will cause work failure, seriously affect the transmission accuracy of the hydraulic system, and cause the work quality of injection-molded parts to deteriorate.

Excessive temperature in injection molding is likely to be unfavorable for injection molded parts. Therefore, for injection molding staff, during the processing process, they must always pay attention to the temperature of the injection molding machine and strictly follow the originally set temperature for production and processing.

What is  the influencing factors of plastic injection molding process?

According to the analysis of injection molded parts manufacturers, there are many factors that affect the molding shrinkage of thermoplastics. Due to the volume change caused by crystallization during the molding process, the internal stress is strong, the residual stress frozen in the plastic part is large, and the molecular orientation is strong. 

Therefore, compared with thermoset plastics, the shrinkage rate is larger, the range of shrinkage rate is wide, and the directionality is obvious. In addition, the shrinkage after molding, annealing or humidity treatment is generally greater than that of thermoset plastics.

When the injection molded part is formed, the outer layer of the molten material in contact with the surface of the cavity is immediately cooled to form a low-density solid shell. 

Due to the poor thermal conductivity of the plastic, the inner layer of the plastic part is slowly cooled to form a high-density solid layer with large shrinkage. Therefore, the wall thickness, slow cooling, and high-density layer thickness will shrink more. 

In addition, the presence or absence of inserts and the layout and quantity of inserts directly affect the direction of material flow, density distribution and shrinkage resistance. Therefore, the characteristics of plastic parts have a greater impact on shrinkage and directionality.

When injection molded parts manufacturers produce injection molded parts, factors such as the form, size, and distribution of the feed inlet directly affect the direction of material flow, density distribution, pressure maintaining and shrinking effect, and molding time. Direct feed ports and feed ports with large cross-sections (especially thicker cross-sections) have less shrinkage but greater directivity, and shorter feed ports with shorter width and length have less directivity. The ones that are close to the feed inlet or parallel to the direction of the material flow will shrink more.

Molding conditions:

 High mold temperature, slow cooling of molten material, high density and large shrinkage, especially for crystalline materials, due to high crystallinity and large volume changes, so the shrinkage is greater. The mold temperature distribution is also related to the internal and external cooling and density uniformity of the plastic part, which directly affects the size and direction of the shrinkage of each part.  In addition, holding pressure and time also have a greater impact on shrinkage. 

Therefore, adjusting mold temperature, pressure, injection speed and cooling time during molding can also appropriately change the shrinkage of plastic parts.

According to the shrinkage range of various plastics, the wall thickness and shape of the plastic part, the size and distribution of the inlet form, the mold design of the injection molding manufacturer determines the shrinkage rate of each part of the plastic part based on experience, and then calculates the cavity size.

Accuracy inspection during the processing and use of injection molded parts

Sometimes we find that the reason for the so-called unqualified processing quality of injection molded parts may not be the quality of the mold, but the improper installation and adjustment of the mold. Improper mold installation and adjustment are also important reasons that aggravate mold wear and cause mold safety accidents.

Therefore, when starting the production of injection molded parts, try to manufacture and inspect several initial parts, and compare the results of the inspection with the results of the accuracy inspection before the mold is put into the warehouse or the final part inspection results at the last use. , To confirm whether the mold installation and adjustment are correct. And before the mass production of the parts, the first part must be inspected before it can start.

In the process of mold production, after a certain interval of injection molding or after a certain number of parts are produced, several parts should be selected for sampling inspection, that is, an intermediate inspection. The purpose of the intermediate inspection is to understand the rate of wear of each part of the mold during use, and to evaluate the impact of the rate of mold wear on the accuracy of the mold and the quality of the parts, so as to prevent the occurrence of various unqualified plastic parts in batches.

After the completion of the production of plastic molds, the processing quality of the injection molded parts should be checked again, combined with the inspection of the appearance of the mold, to determine the degree of wear of the various parts of the mold and whether the mold needs to be repaired or reworked.

In addition, by comparing the results of the first part inspection and the last part inspection, the wear rate of the mold can be measured, so as to facilitate and rationally arrange the production batch of the next job, avoiding the need to regrind or regrind the mold during the next useage. Loss caused by interruption of repairs.

When repairing injection molding parts, replacing parts and disassembling, assembling, and adjusting the mold may cause great changes in the accuracy of each part of the mold. Therefore, the accuracy of the mold must be checked again after the mold is repaired. The main inspection methods and requirements are the same as the accuracy inspection before the new mold is put into storage.

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