12 usual defects of plastic molded products

Writer: Selena Wong                Updated: 2022-10-09

When Suntime mould do mould trails or plastic injection moulding production for customers, defects of plastic products can’t be 100% avoided.There are 12 usual defects of plastic molded products including silver lines, welding line, air bubble, deformation, flow marks, short shot, flash, sink mark, drag mark, cracks, ejection mark, runner draw wire. 

1. Silver lines:  This is caused by not drying enough for plastic material before injection moulding. Normally, it can happen in T0 and after the first trial in supplier’s factory, it won’t happenin normal production stage.

2. Welding line/ joint line : This is a small line in plastic molded parts. It appears in product made by injection mold having several injection points. When the melting material meet, the welding line/ joint line comes out. It is normally caused by different mould temperature or material temperature too low. It is easily to be found in big plastic molded parts and can’t be solved completely, only can do the best to eliminate it. 

3. Air bubble: Air bubble is the void created inside the wall of the finished molded product. It cannot be seen from the outside for non-transparent parts if don’t cut it. The center of the thick wall is the place with slowest cooling, so fast cooling and shrinking will pull the raw material to create voids and form air bubbles. Air bubbles are very obvious on transparent parts. Transparent lenses and transparent guide lighting most likely to occur. So, when we find the wall thickness is more than 4~5mm, it would be better to change the design of the plastic parts.

4. Deformation/ bending: During injection, the resin inside the mold generate internal stress due to high pressure. After demolding, deformation and bending appear on both sides of the finished product. thin-shell long molded product is very easy to have deformation/bending.  So, when do part design, designers should thicken the wall thickness. When Suntime designers do DFM analysis, we will find the issue and give suggestions to customers to change the wall thickness or making reinforcing ribs.

5. Flow marks: When the plastic material flows in the cavity of mold, a small ring-shaped wrinkle appears around gate on part surface. It spreads out around the injection point and  matte product is most obvious. This problem is one of the most difficulties to overcome for the appearance issues. Therefore, most mold factories will make the injection point be placed on the appearance surface in order to reduce this problem.

6. Short shot: It means that the molded product is not fully filled, and there are some missing areas in the part. This problem can be improved unless the mold design is not qualified.

7. Flash/ Burrs: The flash is normally happened around area of parting line, ejector pins, sliders/lifters and other joint places of inserts. The problem is caused by mould fitting issue or by too high pressure or too high mould temperature in plastic injection molding. Such problems can be solved finally.

8.Sink Mark: Due to the resin shrinkage, the surface has hollow marks in the thick wall area of molded product.This problem is easy to be found. Generally, if the pressure drops, the probability of shrinkage will be greater. Such problem should be discussed and resolved based on combination of checking for mould design, mold manufacturing and injection molding.

9. Drag mark: This problem is normally caused bydraft angle not enough or the force of the core side to drag the product is not as strong as that of cavity side and drag mark is made by cavity.

 Regular solution:

1. Add more draft angle.

2. Do more polishing in cavity/core.

3. Check whether the injection pressure is too big, adjust the moulding parameter appropriately.

4. Good cavity/core steel for less shrinkage

10. Cracks: Cracking is a common defect in plastic products, which is mainly caused by stress deformation which is normally from residual stress, external stressand stress deformation caused by the external environment.

11. Ejection mark: The main reasons for the ejector marks are: improper design for ejection position, holding pressure too big, holding pressure time too long, insufficient polishing, too deep ribs, insufficient draft angle, uneven ejection, uneven stress area and so on.

12. Plastic drawn wire in runner: The reasonfor happening of plastic drawn wire is high temperature in nozzle or hot tips.