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30 Common Problems and Solutions in the Injection Molding Process

This article will tell you some common problems in the injection molding process and also the corresponding solutions.

1. The product runs off the edge (flash edge)when being turned on. The reason and solution of the product lacking glue after a period of production.

When the machine starts to work, the melt glue in the material tube of the injection molding machine has a long heated time, low viscosity and good fluidity. So the product is easy to run off the edge.

After a period of production, the melt glue constantly takes away the heat. Then, that results in insufficient melt glue, high viscosity and poor fluidity, which makes the product lacks glue. Therefore, you should gradually increase the temperature of the feed pipe to solve this problem.

2. Sometimes when the product is short of glue,  it’s no use increasing the injection pressure and speed, why?

After a period of production, the melted glue continuously takes away the heat. And that results in insufficient melt glue, high viscosity and poor fluidity, which makes the product lacks glue. Now, you should increase the material tube temperature.

3. The reason and solution for ellipse-shaped product.

The ellipse shape of the product is due to the uneven glue input, resulting in uneven pressure around the product, making the product become ellipse shape. You can inject the glue from three point to make the injected glue even.

4. The requirements of precision products on the mold.

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

5. Purpose of acid resistance test for product.

The acid resistance test of the product is to detect its internal stress and the location of the internal stress focus point to eliminate the internal stress of the product.

6. Reasons and solutions for cracking of metal inserts in the product.

When inserts are placed in the product, internal stress will be formed due to hot melt meets cold inserts during injection, which reduces the strength of the product and make it easy to crack. During production, you can preheat the inserts.

7. Rationality and selection method of the mold exhaust point.

If the exhaust point of the mold is unreasonable. Not only does it fail to exhaust, but it will cause deformation or dimensional changes of product. So the exhaust point of the mold should be reasonable.

You can set the exhaust point at the place where the product is injected with glue and burnt in the trapped gas.

8. Reasons and solutions for brittle products.

The product is brittle because too many degraded material and defective material are used in the product, or the material stays in the material tube for too long, causing the aging of rubber and making the product brittle.

You can increase the proportion of new materials and reduce the recycle of degraded materials ( generally not more than three times ). That can avoid a long stay of the rubber material in the material tube.

9. Reason and solution for the products with fiberglass prone to reveal.

The low melt temperature or low mold temperature, and insufficient injection pressure can’t help to make a good combination of glass fiber and plastic in the glue, causing revealed fiber.

You can increase the temperature of melt and mold to increase the injection pressure.

10. Effect of feed inlet temperature on the product.

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

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

Cause: the cold glue in the product or the dust in the material.

You can raise the temperature of the nozzle, add a cold feed well and keep the raw materials well to prevent dust from entering.

12. Injection ability

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

13. Injection horsepower

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

14. Injection rate

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

15. Injection thrust

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

16. What is the injection pressure of injection molding machine?

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

17. What is the plasticizing capacity of 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’s the difference with injection pressure?

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

19. Requirements of hydraulic oil for 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 foaming

(5) Low corrosion to machine parts and sealing devices

(6) Low ignition point (flash point)  and freezing point requirements

20. The influence of hydraulic oil viscosity on injection molding machine?

When the working environment temperature of the system is high, oil with higher viscosity should be used. Conversely, use oil with lower viscosity.

The oil with higher viscosity should be used. Because at high pressure under the high working pressure, sealing is difficult and leakage Is the main problem. Conversely, when the working pressure is low, oil with a lower viscosity should be used. When the working speed of the working parts of the hydraulic system is high, the flow rate of the oil is also high. At this time, the pressure loss will also increase, and the leakage will be relatively reduced. You should use oil with lower viscosity. Conversely, use oil with higher viscosity according to the low moving speed of working parts.

21. Release setting

Correct release position = moving position of the aprons + screw offside distance.

22. The importance of setting the release position

If the setting of the release position is too large, it will cause the return material to absorb oxygen, oxidize the rubber material and generate bubbles. If the position is set too small, the pressure in the barrel is high. The shear force is so high that the rubber will be decomposed and the nozzle will flood. So the position error cannot exceed 0.4 mm.

23. Setting of melt position

Melt position = product weight / ( maximum stroke / maximum melt amount )

24. The main advantages of gas-assisted injection ( GAM )

The core of thick material can be evacuated to make hollow pipe fittings, which can save materials and shorten the cycle time.

The use of gas in injection molding can evenly distribute the pressure. When the plastic cools and solidifies, the gas can compensate for the volume shrinkage of the plastic through expansion.

Reduce the internal stress of molded products, thereby improving the stability of the shape and eliminating deformation and warpage.

25. Why the outer diameter of the piston rod is small in the middle and large at both ends?

It’s caused by the overheated middle hole needle and the uneven shrinkage of product. The middle hole needle can be made of phosphor copper material with fast heat dissipation and the mold is exhausted in the middle of the product.

26. Cracking after spherical screen printing

Due to the stress on the surface of product, it causes cracking after screen printing. You can increase the mold temperature and reduce the stress. And annealing can be used to eliminate the stress.

27. The problem of fragility at the nozzle of the spectacle frame

The large injection pressure and holding pressure, and the residual internal stress at the nozzle make the product easily broken.

Solution: Try to reduce the injection pressure and holding pressure as much as possible, and increase the mold temperature appropriately.

28. The four pillars of the electrical enclosure are prone to bursting when screwing

Because of the weld line of column, the assembly column is prone to bursting.  Solution: increase the exhaust of mold, appropriately increase the mold temperature and speed up the injection rate to reduce the weld line.

29.The deformation of product

It mainly results from the imbalance of heat shrinkage, or the internal stress of the product itself.

30. Bubbles appear in the transparent PC shell

Raw materials that are not dry enough, uneven thickness of the glue, poor exhaust of the mold, and easy decomposition of the raw materials may make the product bubbles.

Solution: Fully dry the raw materials, increase the mold exhaust, minimize the uneven thickness of the glue.