Defects of thin-wall injection molding technology and solutions

Thin-wall injection molding technology is also called thin-wall plastic parts injection molding technology. The requirements for raw materials 

for thin-wall injection molding technology include: large flow length, high impact strength, high thermal deformation temperature, high thermal 

stability, low directionality and good dimensional stability; the low-temperature impact rigidity and flame retardancy of the plastic raw materials 

must also be considered , mechanical assembly and appearance quality, etc.

Currently commonly used thin-wall injection molding raw materials include: polypropylene (PP), polycarbonate (PC), acrylonitrile-butadiene-styrene (ABS), 

PC/ABS blends and PA6, etc. As wall thickness decreases, plastics with better physical properties are needed to maintain part strength.

Although thin-walled plastic parts have many advantages, it reduces the formability of the plastic parts, making it impossible to form these thin-walled 

plastic parts using conventional injection molding methods. When molding thin-walled plastic parts, there are the following common problems:

1. Short shot

Short shot refers to the quality defect of incomplete plastic parts caused by incomplete filling of the mold cavity, that is, the melt has solidified 

before filling is completed.

The filling and cooling processes of conventional injection molding are intertwined. When the polymer melt flows, the melt front encounters the 

relatively low-temperature core surface or cavity wall, and a condensation layer is formed on its surface. The melt continues to flow forward in the 

condensation layer. As the thickness of the condensation layer increases, the actual cavity flow channel becomes narrower. The thickness of the 

condensation layer has a significant impact on the flow of the polymer.

Because the thickness of the plastic part is relatively thick during conventional injection molding, the impact of the condensation layer on injection 

molding is not great at this time. However, in thin-wall injection molding, when the ratio of the thickness of the condensation layer to the thickness 

of the plastic part gradually increases as the thickness of the plastic part becomes thinner, this effect will be great. Especially when the sizes of the 

two can be compared with each other.

Improvement method: 

When the thickness of the plastic part decreases, the influence of the condensation layer on the flow will increase exponentially, 

which also illustrates the great influence of the condensation layer in thin-wall injection molding. If we only consider injection molding, the injection 

molding machine needs to have a high injection rate so that the rate at which the plastic melt fills the cavity exceeds the growth rate of the condensation 

layer (or the growth rate of the condensation layer is slowed down), so that the flow section can be Complete the filling action before closing and 

perform injection molding of thin-walled plastic parts.

When the flow length is 300mm and the wall thickness of the plastic part is 3.0 mm, L/T is 100, which is easily achieved using conventional injection 

molding technology; but when the wall thickness of the plastic part drops below 1.0mm, this was once easy Achieving a flow length/thickness ratio (100) 

becomes very difficult to achieve.

2. Warping deformation

Warpage deformation is a defect in plastic parts caused by uneven internal stress. The causes of warpage deformation are uneven shrinkage, uneven 

orientation and uneven cooling.

Improvement method: 

The warpage deformation defects of plastic parts can be improved by balancing the cooling system, adjusting cooling time, 

holding pressure and holding time.

3. Welding line

The weld line is the boundary formed when two or more melt flow fronts in the cavity fuse. Stress concentration is easy to occur at the welding line, 

which weakens the mechanical strength of the plastic part, which is particularly detrimental to the mechanical properties of the plastic part, especially the 

thin-walled plastic part. The plastic part is very easy to crack at the welding line after being subjected to external force.

Improvement method: 

During design, you can reduce or change the position of the weld line by reducing the number of gates or changing the position of 

the gates to meet the design requirements of the plastic part.

4. Shortage of materials

The small parts and corners of the finished product cannot be completely formed because the mold is not processed in place or the exhaust is not smooth, 

and the molding is due to insufficient injection dose or pressure, etc., resulting in design defects (insufficient flesh thickness).

Improvement methods: 

Correct the mold where the material is missing, take or improve exhaust measures, increase the thickness of the meat, improve 

the gate (increase the gate, increase the gate), increase the injection dose, increase the injection pressure and other measures to improve.

5. Shrink

It often occurs where the wall or flesh thickness of the molded product is uneven, due to differences in the cooling or solidification shrinkage of hot-melt 

plastics. For example, steal the meat from the back of the ribs, the edges with side walls, and the back of the BOSS pillar, but keep at least 2/3 of the meat thickness.

Improvement methods: 

Improvements can be made by thickening the flow channel, enlarging the gate, adding exhaust, increasing the material temperature, 

increasing the injection pressure, and extending the pressure holding time.

6. Surface image

It often occurs on the back side of BOSS pillars or ribs that have been stripped of flesh, or the stress marks are reduced due to excessively high design of 

the core or ejector pin.

Improvement method: 

It can be processed by modifying the core, ejector pin, sandblasting of the master mold surface, etc., and reducing the brightness of 

the mold surface, lowering the injection speed, reducing the injection pressure, etc.

7. Air streak

It occurs at the gate, mostly because the mold temperature is not high, the injection speed and pressure are too high, the gate is improperly set, and the plastic 

hits the spoiler structure during pouring.

Improvement methods: 

Change the inlet gate, polish the runner, enlarge the cold material area of the runner, enlarge the inlet gate, add texture to the surface 

(you can also adjust the machine or repair the mold to catch up with the joining line), raise the mold Temperature, lowering the injection speed, reducing 

the injection pressure, etc. can be solved.

PP INJECTION MOLDING GRADE HP500N

8. Joining line

It occurs at the intersection of two material flows, such as the intersection of material flows from two inlets and the intersection of material flows bypassing the core. 

It is caused by a drop in material temperature and poor exhaust.

Improvement methods: 

You can change the inlet gate, add a cold material well, open an exhaust slot or nip on the male mold surface, etc. You can also increase the 

material temperature, increase the mold temperature, etc.

9. Burrs

The joint between the male and female molds often occurs due to poor mold clamping, improper processing of mold surface corners, insufficient clamping force during 

molding, excessive material temperature and pressure, etc.

Improvement methods: 

You can modify the mold, re-close the mold, increase the clamping force, lower the material temperature, reduce the injection pressure, reduce 

the holding time, reduce the holding pressure, etc.

10. Deformation

Slender parts, thin-walled parts with large areas, or larger finished products with asymmetric structures are caused by uneven cooling stress or uneven ejection force 

during molding.

Improvement method: 

You can correct the ejector pin, set up a pull pin for tensioning, etc. If necessary, add nip to the male mold to adjust the deformation, adjust 

the mold temperature of the male and female molds to reduce the pressure holding, etc. The adjustment of small parts deformation mainly depends on the pressure 

and the The adjustment of time and deformation of large parts generally depends on the mold temperature.

11. Unclean surface

The unclean surface is caused by the rough surface of the mold.

Improvement method: 

For PC materials, sometimes due to the mold temperature being too high, there are residual glue and oil stains on the mold surface. The mold 

surface needs to be cleaned, polished, and the mold temperature reduced in time.

12. Pull white

It easily occurs at the thin-walled corners of molded products or at the roots of thin-walled RIBs. It is caused by poor stress during demoulding, improper ejection 

pin setting or insufficient draft angle.

Improvement methods:

Increase the R angle at the corner, increase the demoulding angle, increase the ejector pin or increase its cross-sectional area, polish the 

mold surface, polish the ejector pin or oblique pin, reduce the shooting speed, reduce the injection pressure, and reduce the holding pressure. and time etc.

13. Die drawing

It manifests itself as poor demolding, mold damage, or embossing. Mainly due to insufficient draft angle or rough mold surface, the molding conditions also have an impact.

Improvement methods: 

Increase the draft angle, polish the mold surface, add or change the pull pin when gluing the female mold surface, pay attention to the 

horn diameter when feeding horns, add nip to the male mold, reduce the injection pressure, reduce the holding pressure and Time etc.

14. Stomata

Transparent finished PC materials are prone to appear during molding. Since the gas is not exhausted during the injection molding process, improper mold design 

or improper molding conditions will have an impact.

Improvement methods:

Increase exhaust, change the gate (increase the inlet gate), PC material flow channel must be polished, strict baking conditions, increase 

injection pressure, reduce injection speed, etc.

15. Breakage

It occurs at the joints of male and female modules, sliders, oblique pins, etc. It is manifested as uneven levels of the joint surface, etc., due to improper mold closing 

or problems with the mold itself.

Improvement method: 

Correct the mold, or re-close the mold.

16. Dimensions out of tolerance

Problems with the mold itself, or improper molding conditions causing inappropriate molding shrinkage.

Improvement method: 

Usually changing the holding time and injection pressure (second paragraph) have the greatest impact on the size. For example: increasing the 

injection pressure, increasing the pressure-holding and shrinkage effect can significantly increase the size, and lowering the mold temperature can also improve the 

adjustment effect by enlarging the gate or adding more gates.