Questions and Answers of 30 Common Defects in Injection Molding

Time：2023-04-22 17:39:38 / Popularity： / Source：

1. Product runs out of edge (flash) when it is first started, reason and solution for lack of glue after a period of production.

Due to long heating time, melt in barrel of injection molding machine has low viscosity and good fluidity, and product is prone to burr. After a period of production, heat is continuously taken away by melt, resulting in insufficient melted glue, high viscosity, and poor fluidity, resulting in a lack of glue in product. After a period of production, gradually increase temperature of material pipe to solve problem.

2. During production process, product is short of glue, sometimes increasing injection pressure and speed is ineffective, why?

After a period of production, heat is continuously taken away by melted glue, resulting in insufficient melted glue, high viscosity, and poor fluidity, resulting in a lack of glue in product. Increase temperature of barrel to solve it.

3. Reason and solution of product ellipse.

Ellipse of product is due to uneven pressure of product, which causes uneven pressure around product, making product elliptical. Three points of glue are used to make product evenly glued.

4. Requirements for precision products on molds.

It is required that mold material has good rigidity, small elastic deformation and small coefficient of thermal expansion.

5. Purpose of product acid resistance test.

Product acid resistance test is to detect internal stress of product and position of internal stress point, so as to eliminate internal stress of product.

6. Reasons and solutions for metal inserts in products that are easily cracked under stress.

Inserts are placed in product. When hot melt meets cold inserts during injection molding, internal stress will be formed, which will reduce strength of product and make it easy to crack. During production, inserts are preheated.

7. Rationality and selection method of mold exhaust point.

Mold exhaust point is unreasonable, not only will not achieve exhaust effect, but will cause product deformation or size change, so mold exhaust point must be reasonable. When choosing exhaust point of mold, exhaust should be opened at the place where product is filled with glue at the end and where product is trapped and burnt.

8. Reasons and solutions for brittleness of product.

Brittleness of product is that product is brittle due to use of too many nozzle materials and secondary materials, or material stays in material tube for too long, resulting in aging of rubber material and making product brittle. Increase proportion of new materials and reduce number of recycling times of nozzle materials. Generally, it should not exceed three times, so as to avoid rubber materials staying in material pipe for a long time.

9. Reasons and solutions for glass fiber products prone to flooding.

It is due to low melt temperature or low mold temperature, and insufficient injection pressure, resulting in glass fiber not being well combined with plastic in glue, resulting in appearance of pan-fiber. Increase melt temperature, mold temperature, and increase injection pressure.

Defects in Injection Molding, mold exhaust point

10. Influence of inlet temperature on product.

If temperature of feed inlet is too high or too low, it will cause unstable feeding of machine and unstable feeding amount, which will affect size and appearance of product.

11. Reasons and solutions for white spots on transparent products.

Transparent products have white spots caused by cold glue entering product, or dust in material. Raise temperature of nozzle, add cooling material well, pay attention to preservation of raw materials, and prevent dust from entering.

12. What is injection capacity of injection molding machine.

Injection capacity PW=shooting pressure (kg/cm2)×injection volume (cm3)/1000

13. What is injection horsepower of injection molding machine?

Injection horsepower PW (KW) = injection pressure (kg/cm2) × injection rate (cm3/sec) × 9.8 × 100%

14. What is injection rate of injection molding machine?

Injection rate V (cc/sec)=л/4×d2×rd2: tube diameter r: material density

15. What is injection thrust of injection molding machine?

Injection thrust F (kgf) = л/4 (D12-D22) × P × 2D1: cylinder inner diameter D2: piston rod outer diameter P: system pressure

16. What is injection pressure of injection molding machine?

Injection pressure P (kg/cm2)=[л/4×(D12-D22)×P×2]/(л/4×d2)

17. What is plasticizing capacity of an injection molding machine?

Plasticizing capacity W (g/sec)=2.5×(d/2.54)2×(h/2.54)×N×S×1000/3600/2h= screw front tooth depth (cm) S= raw material density

18. What is system pressure? What is difference with injection pressure?

System pressure (kg/cm2) = the highest working pressure set in hydraulic circuit, injection pressure refers to actual pressure of injection molding machine, the two are not equal.

19. Requirements for hydraulic oil of injection molding machine:

(1) Appropriate viscosity and good viscosity performance
(2) Good lubricity and rust resistance
(3) Good chemical stability, not easy to gasify into colloid
(4) good foam performance
(5) Less corrosive to parts and sealing devices
(6) Ignition point (flash point) is required, and freezing point should be low.

20. Effect of hydraulic oil viscosity on injection molding machine?

When working environment temperature of system is high, oil with higher viscosity should be used; on the contrary, oil with lower viscosity should be used. When working pressure of system is higher, oil with higher viscosity should be used, because at high pressure, sealing is more difficult and leakage is the main problem;
On the contrary, working pressure of system is low, and oil with low viscosity should be used. When working parts of hydraulic system run at high speed, flow rate of oil will also be high. Oil with lower viscosity, on the contrary, when moving speed of working parts is low, oil with higher viscosity should be used.

21. Loose setting.

Correct position of loosening and retreating = moving position of apron + offside distance of screw.

22. Importance of loosening and retreating position setting

If loosening position is set too large, it will cause return material to absorb oxygen, oxidize rubber material and generate air bubbles. If position is set too small, pressure inside barrel will be high, and if shear force is too high, rubber material will be decomposed, and nozzle will salivate. Position error cannot exceed 0.4mm.

23. Glue position setting

Melt position = product weight / (max stroke / maximum melt volume)

24. Key Benefits of Gas Assisted Injection Molding (GAM)

It can evacuate core of thick material to make hollow pipe fittings, which can save material and shorten cycle time.
Use of gas in injection molding can make pressure evenly distributed. When plastic cools and solidifies, gas can compensate for volume shrinkage of plastic by expanding.
Reduce internal stress of molded products, thereby improving shape stability, eliminating deformation and warping.

25. Outer diameter of piston rod is small in the middle, but the two ends are big?

Due to uneven shrinkage of overheated product of middle-hole needle, outer diameter of piston rod is small in the middle and large at both ends. Middle-hole needle can be made of phosphor bronze material with fast heat dissipation, and mold is exhausted in the middle part of product.

26. Cracking problem after spherical screen printing

Due to stress on the surface of product, it will crack after silk screen printing. Increase mold temperature, reduce stress, and relieve stress by annealing.

27. Nozzle edge of spectacle frame is easy to break

Injection pressure and holding pressure are greater than residual internal stress at nozzle, causing product to be easily broken. Minimize injection pressure and holding pressure, properly increase mold temperature to solve problem.

28. Problem that four pillars of electrical appliance shell are easy to burst when screws are screwed

Due to presence of water lines in pillars, product assembly pillars are easy to burst, mold should be ventilated, mold temperature should be increased appropriately, and injection speed should be accelerated to reduce water line.

29. Product deformation problem

Deformation of product is mainly caused by imbalance of heat shrinkage, or deformation of product due to internal stress of product itself.

30. Transparent PC shell bubble problem

Insufficient drying of raw materials, uneven thickness of glue in product, poor exhaust of mold, and easy decomposition of raw materials may cause product bubbles. Fully dry, increase exhaust of mold, and minimize uneven thickness of glue.

Gud Mould Industry Co., Ltd