Nine Questions and Answers on Injection Molding processes

Novice engineers will inevitably encounter some problems in the process of injection molding. How to solve these difficult and complicated problems?

FCS provides 9 injection molding questions to help you master the molding points and improve production efficiency and product yield.

1. Briefly describe the working state of check valve in the process of plasticizing, injecting and maintaining pressure:

• Plasticizing: On
• Injecting: Off
• Maintaining pressure: Off

2. Briefly describe the relationship between injection speed and injection pressure, pressure maintaining speed and pressure:
The relationship between injection pressure and injection speed is as follows:
In the injection process parameters, the velocity is dominant between the velocity and the pressure, and there is no direct relationship between the two.

In production, for example, the injection speed of 100mm/s is set. When the actual injection pressure is less than the set pressure value, the screw is molded at the injection speed of 100mm/s; when the actual injection pressure exceeds the set pressure value, the screw cannot reach the injection speed of 100mm/s.

The relationship between the actual injection pressure and the injection speed is that the faster the speed, the greater the actual pressure value, and the higher the injection speed, the greater the pressure you are required to set. High pressure is also required when the speed is very slow. That is to say, the point with the lowest injection pressure will not be at the fastest speed or the slowest speed.

3. If the product has burnt marks, you need to take the following steps:

• Clean the mold surface
• Locate the burnt and reduce the injecting speed.
• Reduce the temperature of the mold
• Reduce clamping force
• Reduce material temperature
• Using mold surface sticker to increase mold exhaust is generally not recommended.

4. Briefly describe the influence of mold temperature on product deformation?
In terms of molds, the main factors that affect the deformation of plastic parts include the gating system, cooling system and ejection system.

For the control of mold temperature, in addition to the design of the mold cooling system, different cooling rates of each component should be set according to the characteristics of the product structure. More attention should be paid to the different shrinkage rates of plastics parallel to and perpendicular to the flow direction of molten glue. The solidification of molten glue will also deform due to different cooling rates, different crystallinity and shrinkage differences.

With a sound mold temperature and cooling system, after the product is injected, demolded and ejected, the temperature of all parts of the product is balanced, so that a good product with the lowest deformation amount can be obtained, otherwise the deformation of the product will be out of control.

5. If a product is not completely filled and has burrs during production, what are the possible reasons?

• Certain gap between the mold core of the moving fixed mold and the broken surface
• Poor exhaust of the mold
• Insufficient clamping force of the machine
• Foreign matter on mold surface

6. Why is it necessary to retract after plasticizing? How to optimize it?
Large or small suction will directly affect the quality of the product. Large suction will easily cause stress marks on the surface of the product, while small suction will easily cause unstable residual quantity, unstable product quality, overflow of mold runner, or silky leakage from runner, etc. The general experience value shall not be less than 5mm.
How to optimize: According to the empirical value, it is necessary to fine-tune the adverse phenomena of the products in actual production until no overflow is generated and qualified products are produced.

7. How does the back pressure affect the product? How is the setting of back pressure generally carried out?

High back pressure:

• The melt pressure at the front end of the barrels is too high, the material temperature is high, the viscosity decreases, and it is easy to fill, but it will increase the rough edge of the product profile.
• The long storage time and the long heating time of the melt in the barrels cause thermal decomposition and poor color difference on the surface of the product.
• Slow screw retreat, long storage time and long molding period affect production efficiency.
• The melt pressure is high, and the mold of the hot runner (needle valve is not affected) is easy to produce salivation, causing the cold material in the runner to block the inlet gate or the cold material spot on the surface of the product when the mold is injected out of the lower mold.
• The mechanical wear of screw and barrels is large, which affects the service life.

Low back pressure:

• The screw retracts quickly, the melt density flowing into the front end of the barrel is small, the air is clamped in, and silver marks are seen on the product surface.
• Poor plasticization, unstable material quantity, large changes in product weight and size.
• Product surface shrinkage, cold material lines, uneven luster and other undesirable phenomena.
• Bubbles are easy to appear inside the product.
• Low melt temperature, high viscosity, poor melt fluidity, etc.

Back pressure setting:
From small to large (for details, please refer to the values given by the material supplier and the structural characteristics of the product)

8. What should you do when you find that the surplus is unstable?

• Check whether the screw three-piece set is in good condition
• Check whether the injecting pressure is stable and whether the inside of the injecting nozzle is clean
• Is there any material leakage in the nozzle
• Whether the screw temperature and the mold temperature are stable or not and whether they are within the allowable deviation value
• Is the suck back value of the screw set too small to cause cold material.

9. How to quickly find out the reasonable clamping force?

• Calculation method: clamping force = projected area of all products (including runners) × pressure in mold cavity
• Practical method: First, you need to set a clamping force, and set an empirical value (not too small) according to the size of the product and the tonnage of the machine. When the product has no burrs, you need to gradually reduce the clamping force until the product has burrs, and take the pressure value of the previous (no burrs) with burrs.

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