Q &A About the Injection Molding Process

9 questions about the Injection Molding Process

Do you think yourself know a lot about the injection molding process? Let’s have a test to check if that’s true.

1. The working status of the check valve during the process of plasticization, injection and pressure holding

  • Plasticizing: open
  • Injection: close
  • Holding pressure: close

2. The relationship between injection speed and injection pressure, holding pressure speed and holding pressure.

The relationship between injection pressure and injection speed is as follows:

Among the injection molding process parameters, speed is dominant, and there is no direct relationship between the two.

In production, for example, an injection speed of 100 mm/s is set, when the actual injection pressure is less than the set pressure value, the screw is formed at an injection speed of 100 mm/s. When the actual injection pressure exceeds the set pressure value, the screw cannot reach the injection speed of 100 mm/s. The relationship between the actual injection pressure and the injection speed is that the faster the speed, the greater the actual pressure value. The higher the injection speed, the more pressure you need to set.

When the speed is very slow, pressure should be also set high. That is to say, the point with the lowest injection pressure is generally neither at the fastest speed, nor at the slowest speed.

3. How do you usually solve the focal spots?

  • Clean the mold surface
  • Find the position of the focal spot and reduce the injection speed
  • Lower the mold temperature
  • Reduce clamping force
  • Lower material temperature
  • Die surface stickeror increase the mold exhaust ( generally not recommended )

4. The effect of mold temperature on product deformation?

In terms of mold, the main factors affecting the deformation of plastic parts are pouring system, cooling system and ejection system.

For the temperature control of the mold, the temperature difference between the moving mold and the fixed mold, the core and the mold wall, the mold wall and the insert should be determined according to the structural characteristics of the product, so as to control the difference in the cooling shrinkage speed of each part of the product.

After demoulding, the product tends to bend in the direction of higher temperature to compensate for the difference in orientation shrinkage and prevent the product from deforming according to the orientation law. For products with exactly the same shape, the mold temperature should be kept the same, so that the cooling of each part of the product will be balanced.

5. What are the possible causes for the incomplete filling and flash?

  • There is a gap between the core of moving die and the fixeddie by the broken surface
  • Poor mold exhaust
  • Insufficientclamping force
  • There are foreign objects on the mold surface

6. Why is it necessary to retreat after plasticization? How to optimize?

Resorption, large or small, will directly affect the quality of the product. If it is large, it will easily cause silver wire on the surface of the product. If it is small, the product The product is prone to unstable residual quantity, unstable product quality, mold runner overflow and nozzle threading, etc.. The general empirical value should not be less than 5mm.

How to optimize:

According to the empirical value, fine-tune the process in actual production until no overflow occurs and qualified products are produced.

7. What is the effect of back pressure on the product? How to set the back pressure?

Large back pressure:

  • The overly high melt pressure at the front of the barrel, high temperature of the materialand the reduced viscosity will make it easy to fill, but it will increase the flash on contour of product.
  • Long storage timeand long heating time for the melt in the barrel causes thermal decomposition, and the color difference of the product surface.
  • The screw retreats slowly, the storage time is long, and the molding cycle is long, which will affect the production efficiency
  • The melt pressure is high, and the mold of the cold runner (the needle valve does not affect)) is prone to drooling. When the lower mold is injected, the cold material in the runner blocks the glue inlet or cold spotsmay occur on the product surface
  • Screw and barrel mechanical wear is large, affecting service life

Low back pressure:

  • The screw retreats fast, the density of the melt flowing into the frontof the barrel is small, the air is caught, and the silver wire occur on the surface of the product
  • Poor plasticization, unstable material volume, large product weight and dimensional changes
  • Product surface shrinkage, cold grain, uneven gloss and other undesirable phenomena
  • Bubbles easily appear inside the product
  • Low melt temperature, high viscosity, poor melt fluidity, etc.

Back pressure setting: the principle is from small to large (refer to the value given by the material supplier and the structural characteristics of the product for details)

8. What should you do when you find that the remaining material amount is unstable?

  • Check whether the three-piece screw is intact
  • Check if the injection pressure is stable and the inside of the nozzle is clean
  • Whether the nozzle has leakage
  • If the temperature of the screw and the mold temperature controller are stable within the allowable deviation
  • Whether the loosening value of the screw is set too small, causing cold material

9. How to quickly find a reasonable clamping force?

  • Calculation method: clamping force = projection area of all products (including flow channel) X pressure in the mold cavity
  • Practical method: first set a clamping force, and set an empirical value according to the product size and machine tonnage (not too small). When the product has no burrs, gradually reduce the clamping force until the product has burrs. Take the pressure value of the last one (without burrs) where burrs appear.

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