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ISSUE: 33
 Publication date: 2021-07-22
Why the samples comes shrink & bubble?
Why the samples comes shrink & bubble?

Q: Customer often ask why the mechanical strength of my product is not enough? Why product has shrink and bubble? Why the fluidity of plastic materials is getting worse and worse?



In response to these problems, first eliminate the mold and plastic factors,Summarize the core main reason “Back Pressure” function is not fully understood or not rigorously managed .But what is the “Back Pressure” and what causality will be caused by its setting. Here is a brief description of the hope that it will be helpful to the need to adjust the crew.


Simply explain that the back pressure is the resistance of the injection cylinder to backward when the material charging.

The greater the resistance: When the material charging, the injection cylinder retreats more slowly due to the large resistance

The lower the resistance: When the material charging, the injection cylinder retreats faster due to the lower resistance


Under the condition that Nozzle without leakage, The volume of plastic accumulated between the nozzle and the shooting tip of the screw rotating storage is positively correlated with the distance of the shooting cylinder axis retreat. That is mean the more material that is piled up, the greater the retreat distance of the shooting axis. If the shaft of injection cylinder backward slowly ( backpressure high) , that is meaning the input volume is greater than the retreated volume. The greater the compression of the plastic material in the barrel, and the material density more bigger. On the contrary, if the back pressure is smaller, the density of the material is smaller.


Because of the high density, the product is solid. High mechanical strength, low shrinkage. The size is relatively consistent and there will be no bubbles. If under the same charging speed and the shaft of injection cylinder backward slowly. Them the material might be Stir more times inner the screw. The greater the shear friction heat, the higher the plasticizing capacity of the barrel. Plastic can maintain good fluidity in a short period of time.



Q: 「Backpressure」Whether the setting is bigger the better?


Based on our explanation, is it better to set the back pressure as large as possible? In fact, it is not the case. Excessive back pressure also has the following disadvantages:

(1) Too high frictional heat will cause the plastic to overheat and discolor or even carbonize, resulting in color difference or poor mechanical strength of the product

(2) The internal pressure of the material pipe is too high, and the material nozzle is easy to salivate or explode.


Therefore, the back pressure adjustment should be appropriate due to different product requirements. But it should be noted that the back pressure needs to be adjusted and set together with the loosening distance after storage. If the back pressure increases, but the release distance is also increased, the effect of the back pressure will be discounted




Q: Is the back pressure adjustment method the same for vertical and horizontal injection machines?



The answer is not exactly the same. The injection unit of the small vertical injection machine is a vertical mechanism that shoots from top to bottom. Therefore, the accumulation pressure in the screw and the barrel must be greater than the weight of the injection mechanism above to climb up. Therefore, the basic back pressure resistance of the vertical injection machine is greater than that of the general horizontal machine. When the screw diameter is small, the nozzle diameter is large, and the injection mechanism is heavy, even if zero back pressure is set, there is a chance to store the material but the screw does not climb. At this time, it can be matched with a non-return nozzle or "negative back pressure" ( Please check the below note ① to achieve precise back pressure setting)


Note ① : TKC can be based on the difference of the injection mechanism of each model to design a special circuit to achieve precise control of extremely low back pressure