Why Injection Molding Backpressure is the Key to Prevent Bubbles & Shrinkage?

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Are You Experiencing Shrinkage and Bubbles during Injection Molding ?

Many of you may ask why the mechanical strength of my product is not enough? Why are molded parts with defects of shrinkage and bubbles? Why is the fluidity of plastic materials getting worse and worse?In response to these problems,first we should exclude the factors of molds and plastic resins. The main reason would be “Injection Molding Back Pressure.

What is Injection Molding Back Pressure ?

Do you know what “Back Pressure” is and what are the effects of “Back Pressure” in injection molding? Briefly speaking, back pressure is the first stage of pressure in the injection molding process. The material is pushing back against the screw as the material is mixed and pushed forward. Then the back pressure is then generated.

  1. Higher Injection Molding Backpressure:

    When the material is charging, the injection cylinder retreats more slowly due to the higher pressure.
  2. Lower Injection Molding Backpressure:

    When the material is charging, the injection cylinder retreats faster due to the lower pressure.

If there is no nozzle leaking, 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. It means there will be more material piled up and the retreat distance of the shooting axis will be greater too. If the shaft of the injection cylinder backward slowly (back pressure high) , that means the input volume is greater than the retreat volume. If the compression inside of the barrel is provided greater, the material density will be higher. On the contrary, if the back pressure is lower, the density of the material is lower.

The product is solid with high density, high mechanical strength, and low shrinkage. The size is relatively consistent and there will be no bubbles. If under the same charging speed, the shaft of the injection cylinder backward slowly. Then the material might stir more times inside 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.

Is the Backpressure Set up at a High Level Better for Production?

In fact, it is not the case. Too high back pressure will cause some problems as follows.
  1. Too high frictional heat will cause the melting material to be overheated and color-changed or even carbonized, resulting in color difference or poor mechanical strength.
  2. If the pipe pressure is too high, the melting plastic will drool in the nozzle and the plastic will decompose.

Therefore, the back pressure setting should be adjusted properly against different product requirements. And you also need to set the back pressure together with the loosening distance after storage. The back pressure is increased, the release distance is also increased, the bubble defect will still exist and cannot be solved.

Is the Setting of Back Pressure Same for Both 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. 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)

Vertical Injection Molding(Moulding) Machines and Manufacturer

TKC offers competitively priced, high quality Vertical Injection Molding Machines! Our machines are recommended by our customers throughout the plastic injection industry, and around the world. We hope that you choose Taiwan Kinki Machinery to manufacture your plastic products! Feel free to contact us.

And be sure to check out our web site to learn more about our other products at: https://www.kinki.tw/en/products.html

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Note ① : TKC can design a special circuit to achieve precise control of extremely low back pressure basepressurebe based on the different injection differentence of the injection mechanism. of each model to design a special circuit to achieve precise control of extremely low back pressure
How to Select a Right Vertical Injection Molding Machine?