9 questions and answers about injection molding process, do you know answer?

Time:2022-08-15 08:35:56 / Popularity: / Source:

1. Briefly describe working state of check valve in process of plasticization, injection and pressure holding;

Plasticization: On
Injection molding: off
Holding pressure: off

2. Briefly describe relationship between injection speed and injection pressure, pressure holding speed and holding pressure;

Relationship between injection pressure and injection speed is as follows:
In injection molding process parameters, speed is dominant between speed and pressure, and there is no direct relationship between the two.
In production, for example, injection speed of 100mm/s is set. When actual injection pressure is less than set pressure value, screw will be molded at injection speed of 100mm/s. When actual injection pressure exceeds set pressure value, screw cannot reach injection speed of 100mm/s. Relationship between actual injection pressure and injection speed is that the faster speed, the greater actual pressure value. The higher injection speed, the more pressure you are required to set.
A high set pressure is also required at very slow speeds. That is to say, point with the lowest injection pressure will not be at the fastest speed, nor will it be at the slowest speed.
injection molding process 

3. If product has focal spots, how do you usually solve it?

Clean die surface
Find point position of focal spot and reduce injection speed
Lower mold temperature
Reduce clamping force
Reduce material temperature
Die surface sticker, increase mold exhaust, generally not recommended

4. Briefly describe influence of mold temperature on product deformation?

In terms of molds, main factors affecting deformation of plastic parts are gating system, cooling system and ejection system.
For control of mold temperature, temperature difference between movable mold and fixed mold, mold core and mold wall, mold wall and insert should be determined according to structural characteristics of product, so as to control difference in cooling and shrinkage speed of each part of product.
After product is demolded, it tends to bend in the direction of traction on the side with higher temperature to offset difference in orientation shrinkage and prevent product from deforming according to orientation law. For products with exactly same product shape, mold temperature should be kept consistent accordingly, so that cooling of all parts of product is balanced.

5. If a product has both incomplete fill and flash during production, what are possible causes?

There is a gap between movable and fixed die cores by broken surface
Poor mold exhaust
Insufficient clamping force of machine
There is a foreign body on mold surface

6. Why is process of retreating after plasticization is necessary? How to optimize?

Whether back suction is large or small will directly affect quality of product. If it is large, it is easy to cause silver threads on the surface of product. If it is small, it is easy to have unstable residual quantity and unstable product quality. General experience value shall not be less than 5mm.
How to optimize: According to experience value, look at bad phenomenon of product in actual production, fine-tune it up and down until there is no overflow and a qualified product is produced.

7. How does back pressure affect product? How is back pressure set in general?

High back pressure:
Melt pressure at the front end of barrel is too high, temperature of material is high, viscosity decreases, and it is easy to fill, but it will increase flash of product outline
Storage time is long, and heating time of melt in the barrel is long, resulting in thermal decomposition, and color difference on the surface of product is poor.
Screw retreats slowly, storage time is long, and molding cycle is long, which affects production efficiency.
When melt pressure is high, mold of cold runner (needle valve is not affected) is prone to drooling, causing cold material in runner to block glue inlet or cold material spots on the surface of product when the lower mold is injected.
Mechanical wear of screw and barrel is large, which affects service life.
Low back pressure:
Screw retreats quickly, melt density flowing into front end of barrel is small, air is trapped, and surface of product is silvery.
Poor plasticization, unstable material volume, large changes in product weight and size
Product surface shrinkage, cold grain, uneven gloss and other undesirable phenomena
Bubbles are prone to appear inside product
Low melt temperature, high viscosity, poor melt flow, etc.
Back pressure setting: principle is from small to large (for details, please refer to value given by material supplier and structural characteristics of product)

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

Check whether three-piece screw set is in good condition
Check whether injection pressure is stable and whether inside of nozzle is clean
Whether nozzle is leaking
Whether screw temperature and temperature of mold temperature machine are stable or not within allowable deviation value
Whether loosening value of screw is set too small, causing cold material

9. How to quickly find a reasonable clamping force?

Calculation method: clamping force = projected area of all products (including runners) X pressure in cavity
Practical method: first set a clamping force, set an empirical value according to size of product and tonnage of machine (not too small), when product has no burrs, gradually reduce clamping force until product has burrs, take pressure value of the last (without burrs) with burrs.

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