Analysis on Injection Molding Defects

1. The plastic product dimension is out of tolerance.

Plastic part dimension exceeds the tolerance range (especially the appearance dimension), the reasons for dimension out of tolerance are as follows:

A. Wrong materials are used.

B. Size of injection mold cavity NG (wrong shrinkage rate, wrong processing)

C. The dimension of the injection mold cavity is OK, however the mold structure is unreasonable (number or location of gates is wrong, mold without heating system or heating failure, uneven cooling).

D. The product structure is unreasonable (cavity is too deep, wall is too thin, uneven wall thickness).

E. The process parameters are unreasonable (hold pressure, sequence valve), and the injection molding machine (tonnage is too small, injection molding pressure is not big enough)

2. The plastic molding part is deformed include bending, twisting, warping and unevenness.

Plastic part deformation reasons are as following:

A. The holding pressure is too large, as the injection molding part internal stress too big, the part will be deformed in 24 hours after part is taken out from the injection mould.

B. Uneven cooling will also cause warping.

C. The gate position is unreasonable will cause uneven flow plastic material.

D. Cavity sticking leads to pulling deformation during the mold ejection.

E. Mold cavity deformation (polishing deformation).

F. The ejection structure of injection mould is unreasonable. The position of the ejector pin does not reach the place with the maximum wrapping force, cause part deformation during ejection process.

3.  Surface peeling

A. Poor quality raw materials.

B. There is more than one material, the resin cannot be melted.

C. The recycled materials are too much.

D. The recycled materials are reused and degraded for many times.

4. The plastic part is burning, and reasons are below:

A. The heating temperature is too high (especially at the hot nozzle and gate).

B. The heating time is too long.

C. The venting of the mold is poor.

5. The plastic molding part color is uneven, and reasons are as following:

A. Too little master batch ratio is used.

B. The remaining materials in the injection machine barrel are not used up.

C. The color master batch is mixed unevenly.

6. Splay mark, small pieces with different luster on the surface or side of the product

A. Poor drying of raw materials.

B. The injection molding parameter to be optimized.

7. Color mixing: the product surface is mixed with other colors.

A. The hot runner was not cleaned.

B. The barrel is not cleaned.

8. The plastic injection molding product is cracked.

A. Too much nozzle material affects the strength.

B. Mould cavity sticking and pulling crack.

C. Ejection is unbalanced cause crack.

9. Bubbles form molding parts inside or on the surface of plastic parts

There has two reasons, air in the melt cause poor plasticization and overheating cause raw material decomposition.

10. The residual material residue after the previous product is in the mold cavity, which causes adhesion to the surface of the next molding product

There are residues, material powder at the gate breaking point.

11. The part is incomplete filling.

A. Poor liquidity of raw materials, especially when the material is added fiber glass.

The mold gate is small, and the runner is too long.

B. Poor exhaust and gas trapping lead to short of material.

C. The plastic injection molding part is too thin, in such condition it will be difficult to fill.

Also the injection pressure is not enough or heating temperature is too low.

12. Flash, that means edge of product or other area has over flow plastic, there are many different reasons cause flash.

A. The plastic material flu is poor.

B. The mold fits poorly, and the exhaust and assembly clearance are too large.

C. There is foreign matter on the parting surface.

D. The parting surface is deformed due to too high pressure and fast clamping speed.

E. The injection pressure is too large, and the locking force is not enough.

13. Shrinkage, which means local depression caused by shrinkage of plastic parts.

A. The main wall thickness of the product is too thin and the bone or boss position is too thick.

B. The injection molding pressure is small, and the overall size is small.

C. Uneven and insufficient cooling, some area is overheating, and large shrinkage.

14. The plastic part is forced to make concave.

The plastic injection mold cavity or core is partially polished, the part stick to the mold.

Ejection is unbalanced.

15. Stress mark: there are shiny areas on the surface of the plastic product corresponding to the back area, rib area etc.

A. The injection molding part surface is too smooth, the texture is not deep enough or the product thickness is not even. This may lead to a sudden increase in the flow rate of the plastic melt, resulting in stress imbalance.

B. The fitting of the injection mold insert is not well, and there is a small looseness when adding high pressure during filling, which will produce stress (such as banana gate insert often appears sunken).

C. Mold deformation or assembly problems, such as deformation of ejector pin plate during injection molding process, slight back off of ejector pin and ejector block, etc.

D. The injection molding holding pressure is too high, the mold temperature is too low, and the material temperature is too low.

E. Some areas of the mold are overheated (large ejector pin, large ejector block and large lifter without cooling), the cooling is uneven, and the water pipe is too far away from the stress mark.

16. There are white area in the plastic product, such as "edge area, buckle area, boss area, and drag mark area"

A. Buckle area is pulled during the injection molding part ejection process.

B. Poor polishing, part sticking, slight deformation, the sticking area will turns white.

C. The ejection position is unreasonable.

D. The ejector pin plate is deformed or the ejector rod length of the injection molding machine is not the same, resulting in uneven ejection.

17. Weld line: the fluid from different directions forms linear traces during hot melting.

A. The temperature is low during welding.

B. The sleeve pin is too high, resulting in some area too thin.

C. Multi-point gate produces weld line.

18. Miss match: at the edge of the insert (or ejector pin, lifter, etc.) on the plastic part is uneven

A. Poor assembly of inserts.

B. The ejector pin and lifter are not fully returned to position.

C. The inserts in the same plane are processed separately, and the dimensions are not completely consistent. (For example, two inserts at the rib position are respectively processed by electric discharge machining)

19. Drag mark: caused the friction between the side wall of the plastic injection molding product and the mold.

A. Poor polishing of rib and column positions.

B. The mold is partially deformed, or the mold steel is scratched with burrs.

C. The draft angle of mold is not enough.

D. The side wall of the plastic part is rough (spark lines, texture, etc.).

20. Flow marks and air marks caused by poor exhaust of the mold cavity when the melt material flow enters the injection mold cavity, which are similar to finger marks. (It usually appears around the gate, in the form of fog, mottle and grain)

A. The injection gate is too thin and small, and the melt fluidity is poor.

B. During initial filling, the melt passes through the gate too fast and directly enters the mold cavity.

C. The gate temperature is too low.

21. Air trapped: at the end of melt filling (parting surface, rib position, mesh position, near the weld line, etc.), the cavity venting is poor, and the air in the melt cannot be discharged, forming filamentous, foggy and even scorched mark.

A. There is no venting, or there is too little venting, or the exhaust groove is too shallow.

B. Multi-point gate, trapped air near the fusion line (venting insert, venting steel, etc.).

C. The filling is not balanced.

D. The material is easy to decompose.

E. There are too few ejector forces, the venting of insert and ejector structure is limited.

22. Poor texture: texture thickness, texture depth, texture density are inconsistent, skin texture gloss is different (bright, matt), skin texture scratch, some texture omission, non-skin texture area is texture.

A. The mold is not polished well before texture process.

B. The steel material is different (uneven material, or mold is laser welding, etc.)

C. Corrosion time is not enough, and the film doesn't match or sand blasting is different.

D. The mold is poorly protected during etching.

23. Poor polishing

A. When polishing, the diamond paste is not enough, the tool is directly grinded to the surface.

B. The deep cavity could not be polished or missed.

C. The oilstone and sandpaper are not fine enough.

24. After the high gloss polish, the mirror surface has orange peel and wavy texture.

A. The polishing performance of steel is poor (too soft, easy to be damaged).

B. Polishing direction problem (polishing in the same area for a long time).

C. The polishing power is too large.

25. There are pits on the surface of high-gloss plastic products.

A. The material is a problem.

B. Polishing problem: impurity particles are worn off, and small holes are left on the mold surface.

C. There are many impurities in the steel, and small particles can be seen on the mold surface after polishing.

D. Rust spots on the mold surface.

26. Poor gate cutting: the gate is uneven and protruding after being pulled off.

A. The gate is too thick to cut, or the gate is not flat, and the arc surface is difficult to cut flat.

B. The size of the part gate is too large, and the part cut surface is uneven.

C. The plastic thickness at the gate is thin and easy to crack.

27. It is difficult to take out the product from the injection mold.

A. The ejection stroke is not enough to fully release.

B. Insufficient polishing, sticking and breaking.

C. Recycled material, easy to break.

D. There are undercuts structure.

E. Due to poor polishing, the mold of the product deviates to one side, so that the other side cannot be released.

F. The texture is too deep, or the rib and boss position are too deep, and the draft angle of the plastic injection mold is too small.

28. Water leakage /water shortage

A. The ejector pin, the sleeve, the lifter hole, etc., break the water pile.

B. The seal ring groove is deformed and collapsed.

C. The sealing ring is aged and cracked due to too long use.

D. The sealing ring that is not resistant to high temperature is used under high temperature, resulting in failure.

E. The plug is not blocked (the tape is too little, the adhesive is dry and cracked, the thread size is not correct, and the plug specification does not match, resulting in thread damage).

F. The water channels are connected with each other, and there is a gap at the bottom of the copper hole, which leads to water leakage.

G. The high temperature water pipe connected to the mold end leaks into the mold cavity.

H. The water test pressure is too small to detect water leakage.

I. There is residue in the cooling channel.

29. Welding mark

A. When use different materials to weld, there are welding marks (there are obvious differences in shade and depth after polishing or texturing).

B. When use the same material, due to the local heating in the welding area, the steel material in the welding area will also have slight marks.

30. Steel mark

The steel material in the mold texture area is uneven and contains a lot of impurities (most low cost steel are easy to appear). HanKing Mould use good quality steel can avoid this issue.

For the mold factory, the steel specified by the customer must be used. If the customer does not specify, the steel of good quality must also be used to avoid product quality problems.