Troubleshooting 16 Types of Thin-Wall Molding Defects: A Complete Solution Guide

3 Main Definitions of Thin-Wall Products

1. Flow Length Ratio (L/T)
A part is thin-wall when the ratio of flow length to wall thickness L/T ≥ 100.
2. Wall Thickness
A part is thin-wall if its wall thickness is less than 1.0 mm.
3. Thickness-to-Diameter Ratio (t/d)
For round parts, it is thin-wall when t/d < 0.05.

Material Selection for Thin-Wall Products

1. Good melt flowability and low viscosity for easy filling.
2. Wide processing temperature range, stable and not easy to degrade.
3. High toughness and good crack resistance to avoid damage during ejection.
4. Low and stable shrinkage rate to reduce warpage and deformation.
5. Good demolding performance for smooth ejection.
6. Good thermal resistance to meet application requirements.

– Suitable materials：

1. High-flow PP (most commonly used: lunch boxes, thin-walled shells)

2. High-flow ABS (electronic shells, thin-walled structural components)

3. HIPS (thin-walled appliance shells, inexpensive and stable)

4. PC/ABS (high toughness, thin-walled precision parts)

5. PET, PETG (transparent thin-walled parts)

6. PE (low density, ultra-thin walls)

Raw materials unsuitable for thin-walled applications:

– Poor flowability: ordinary PC, ordinary PA, ordinary POM

 

While thinner plastic parts offer many advantages, they also reduce their moldability, leading to the following common problems when using conventional injection molding methods:

01. Short Shot (Incomplete Filling)

Causes and Solutions for Short Shots in Thin-Walled Products

Thin-walled products, due to their thin walls and high flow length ratio, are highly susceptible to short shot defects. These defects are mainly caused by four factors: raw materials, molds, processes, and equipment.

Regarding raw materials: Poor melt flowability, high viscosity, or moisture content in the raw material can lead to high filling resistance and incomplete filling at the far end. High-flow grades should be used, and the material should be thoroughly dried.

Regarding molds: Small gate size, long and narrow runners, or improper gate placement can result in significant pressure loss; poor venting can obstruct airflow within the cavity; and excessively low mold temperature can cause rapid cooling and solidification of the melt. Solutions include enlarging the gate and runner, optimizing the gate location, adding venting channels, and increasing the mold temperature.

Regarding processes: Slow injection speed, insufficient pressure, low material and mold temperatures, insufficient injection volume, and insufficient holding pressure can all lead to incomplete filling. Solutions include increasing injection speed and pressure, raising material and mold temperatures, increasing injection volume, and extending holding pressure time.

Regarding equipment, insufficient injection molding machine tonnage, worn check rings leading to leakage, and aging screws can all cause insufficient injection pressure. Appropriate machine models should be selected, and worn parts should be replaced promptly to ensure equipment stability.

To resolve short injection pressure issues with thin-walled injection molding, prioritize process adjustment, then optimize the mold and raw materials, and finally check the equipment, controlling flowability, venting, speed, and temperature. This will effectively improve the situation.

02 .Warpage Deformation

Causes and Solutions for Warpage Deformation of Thin-Walled Products

Thin-walled products, due to their thin walls and poor rigidity, are highly susceptible to warpage deformation. This is mainly caused by uneven shrinkage, uneven stress, improper cooling, and mold and process issues.

Regarding raw materials, high shrinkage rates, uneven crystallization, and excessive moisture content can lead to significant differences in shrinkage across different parts of the product, causing deformation. Low-shrinkage, high-rigidity materials should be selected, and thorough drying and stable melt properties should be ensured.

Regarding molds, improper gate placement or insufficient number of gates can lead to unbalanced filling; an unreasonable ejector layout can result in uneven stress; uneven cooling channels can cause large temperature differences; and insufficient draft angle in the mold cavity can also contribute. Optimizing gate placement, increasing the number of gates, properly arranging ejector pins, balancing the cooling channels, and increasing the draft angle are necessary.

Regarding processes, insufficient cooling time, excessively high mold temperature, and insufficient holding pressure can lead to high internal stress in the product; unreasonable injection speed and pressure can cause stress concentration; and excessively high material temperature can exacerbate shrinkage. To mitigate internal stress, cooling time should be extended, mold temperature lowered, holding pressure increased, and speed, pressure, and material temperature rationally controlled.

In terms of product design, uneven wall thickness and unreasonable ribs lead to inconsistent shrinkage. Wall thickness should be made as uniform as possible, with a smooth transition, and reasonable rib design should be implemented.

By controlling uniform cooling and shrinkage, and reducing internal stress, the warpage of thin-walled parts can be effectively improved, ensuring dimensional stability.

03 .Weld Lines

Causes and Solutions for Weld Lines in Thin-Walled Products

Weld lines are thin lines formed when the melt fails to fully fuse after splitting and merging within the mold cavity. They are particularly common in thin-walled products due to their rapid cooling and long flow path.

Main Causes:
Low melt temperature and high viscosity result in insufficient fusion force at the fusion point; poor mold venting traps air at the fusion point, hindering welding; numerous gates in inappropriate locations cause multiple melt streams to merge; excessively low mold temperature causes rapid cooling of the melt front, preventing sufficient fusion; slow injection speed and insufficient pressure lead to excessively rapid cooling of the melt front; excessively thin wall thickness results in rapid cooling and insufficient fusion time.

Solutions: Increase melt temperature and mold temperature to reduce melt viscosity and enhance fusion; accelerate injection speed and increase injection pressure to ensure melt front temperature; optimize the number and location of gates to reduce the number of weld lines; add venting channels or pins at weld lines to promptly remove gas; select high-flow, well-blending raw materials; rationally design product wall thickness to avoid excessive thinness and abrupt changes; appropriately increase holding pressure to improve weld line density.

By increasing melt temperature, accelerating filling, improving venting, and optimizing gates, weld lines in thin-walled products can be significantly reduced or even eliminated, ensuring strength and appearance.

04. injection-molding-flash

Causes and Solutions

Burs are excess material edges formed when molten material overflows from the mold parting surface or insert gaps. Main causes: Insufficient clamping force; excessively high material or mold temperature; excessive injection pressure or speed; excessive injection volume; large mold clearance, wear, or deformation; excessively deep venting grooves. Solutions: Increase clamping force; reduce material temperature, injection pressure, and injection speed; reduce injection volume; repair the mold and reduce clearance; properly control the depth of venting grooves; optimize holding pressure and avoid overfilling.

05. Die Scratching

Causes and Solutions for Die Scratching (Scratching, Scratching)

Die scratching is often caused by a rough mold surface and insufficient draft angle, resulting in scratches on the product during demolding. It can also be caused by uneven ejection, excessive clamping force, high mold temperature, material sticking to the mold, and poor venting, leading to frictional scratches. Solutions: Increase the draft angle; polish the mold cavity and core; properly arrange ejector pins for balanced ejection; appropriately lower the mold temperature to improve material lubricity; improve venting to prevent sticking; adjust injection pressure and speed to reduce clamping force.

06. Insufficient Filling (Incomplete Filling)

1. Increase Melt Temperature and Mold Temperature
Improve melt flowability and reduce viscosity, allowing the material to more easily fill the thin-walled area.

2. Increase Injection Speed
Thin-walled molds cool extremely quickly, requiring high-speed filling to prevent premature solidification at the front end, resulting in short shots.

3. Increase Injection Pressure and Holding Pressure
Increasing injection pressure overcomes runner and cavity resistance, ensuring full filling at the far end; appropriately extending holding pressure ensures proper shrinkage compensation.

4. Increase Injection Volume
Check for sufficient metering to avoid incomplete filling due to insufficient material.

5. Improve Mold Venting
Add venting grooves at the ends, ribs, and weld lines. Poor venting is one of the most common causes of insufficient filling in thin-walled molds.

6. Optimize Gate and Runner
Increase gate size, shorten runner, and reduce bends to reduce pressure loss. 7. Adjust the gate location. Place it where the wall thickness is greater and the flow path is shorter, avoiding excessively long flow paths that may not reach the target area.

8. Change to a high-flow-rate raw material. Using a high melt index, high-flow-rate material is more suitable for thin-wall molding.

9. Check for equipment problems. Worn check rings leading to leaks, aging screws, and insufficient pressure can all cause incomplete filling; timely repair or replacement is necessary.

10. Optimize product structure. Avoid excessively thin areas, sharp corners, and deep ribs. Appropriately add glue and ensure smooth transitions to reduce flow resistance.。

07 .Sink marks

Causes and Solutions for Sink marks (100 words)
Sink marks is caused by cooling Sink marks in thicker areas of the plastic part, and failure to compensate for Sink marks in time. Common causes: Insufficient holding pressure or short holding time; excessively high material and mold temperatures; uneven cooling; gate that is too small or improperly positioned; uneven wall thickness. Solutions: Increase holding pressure and extend holding time; lower material and mold temperatures; extend cooling time; enlarge the gate and optimize its position; ensure the product has a uniform wall thickness and improve cooling channels.

08. Causes and Solutions for Dimensional Out-of-Tolerance Issues

Dimensional out-of-tolerance issues are mainly caused by uneven shrinkage, process fluctuations, insufficient mold precision, and raw material differences. Excessively high material or mold temperatures, or uneven cooling, can lead to unstable shrinkage; insufficient holding pressure and fluctuations in process parameters can cause product dimensional deviations; mold wear, cavity machining deviations, and ejection deformation can also affect dimensions; different raw material grades and shrinkage rates can also lead to dimensional out-of-tolerance issues. Solutions: Stabilize process parameters, control temperature appropriately, and strengthen holding pressure; regularly calibrate the mold; select raw materials with stable shrinkage; optimize cooling and ejection to ensure uniform product shrinkage.

09. Air Marks

Causes and Solutions

Air marks are formed on the surface by air or moisture trapped in the mold cavity during high-speed melt filling, creating a hazy or radial pattern. Causes: Excessive injection speed, poor venting, excessively high melt temperature, damp material, insufficient gate size, and low mold temperature. Solutions: Slow down the injection speed in stages, improve mold venting, lower melt temperature, thoroughly dry the material, appropriately enlarge the gate size, increase mold temperature, and optimize the runner system to reduce turbulence.

 

10. Weld Line

Occurs at the confluence of two material flows, such as the convergence of flows from two gates or flows bypassing the core. This is caused by a drop in material temperature and poor venting.

Improvement Methods: This can be addressed by changing the gate, adding a cold slug well, adding venting channels, or adding texture to the mold surface. Increasing the material temperature or mold temperature is also an option.

11. Deformation

This occurs in slender parts, large thin-walled parts, or larger finished products with asymmetrical structures due to uneven cooling stress during molding or uneven ejection force.

Improvement Methods: This can be addressed by modifying ejector pins, installing tensioning pull pins, adding texture to the mold surface to adjust deformation if necessary, and adjusting the mold temperature of both the male and female molds to reduce holding pressure. For small parts, deformation adjustment mainly relies on pressure and time; for large parts, deformation adjustment generally relies on mold temperature.

12. Whitening

This easily occurs at thin-walled corners or the root of thin-walled RIBs in molded parts. It is caused by poor force during demolding, improper ejector pin placement, or insufficient draft angle.

Improvement methods: Increase the radius (R) at corners, increase the demolding angle, add ejector pins or increase their cross-sectional area, polish the mold surface, polish ejector pins or angled pins, reduce injection speed, reduce injection pressure, and reduce holding pressure and time, etc.

13. Porosity

Causes and solutions for porosity

Porosity is mostly caused by moisture in the raw material, air trapped in the melt, excessive injection speed, or improper process. Insufficiently dried material, excessively high material temperature leading to decomposition, poor mold venting, and insufficient holding pressure can all cause porosity. Solutions: Thoroughly dry the raw material; properly control the material temperature to avoid decomposition; reduce injection speed and inject in stages; improve mold venting; increase holding pressure and extend holding time; optimize runners and gates to reduce gas retention.

14. Discontinuity

Occurs at the joints of male and female modules, sliders, angled pins, etc., manifesting as uneven layers at the joint surfaces, due to improper mold closing or problems with the mold itself.

Improvement methods: Repair the mold or re-close the mold.

15. Surface Imperfections

These often occur on the back of boss pillars or ribs where material has been skimmed away, or due to stress mark reduction caused by excessively high core or ejector pin designs.

Improvement Methods: This can be addressed by correcting the core, ejector pins, and sandblasting the mold surface. Other methods include reducing mold surface brightness, decreasing injection speed, and lowering injection pressure.

16. Surface Dirt

This is due to a rough mold surface.

Improvement Methods: For PC materials, sometimes excessive mold temperature can cause residual glue or oil stains on the mold surface. Timely cleaning, polishing, and lowering the mold temperature are necessary.

If you have any questions about thin-walled products, please feel free to contact us!

Precision Mold Machining | Plastic Injection Molding Manufacturer

https:www.goldchain-trade.com