10 Most Common Injection Moulding Defects: Causes and Solutions

Investing in injection moulding can be very costly. It would be a waste to invest in the process only to encounter defects and unwanted outcomes, which in some cases not only affect the aesthetics but also the structural integrity of the part. Some defects are less severe, while others can be very costly to remedy. In this article, we will go through the top 10 most common injection moulding defects, their severity, root causes, and solutions to prevent them.

10 most common plastic injection moulding defects: causes and solutions

1. Sink marks
2. Flow lines
3. Short shots
4. Flash
5. Jetting
6. Voids
7. Blister
8. Burn marks
9. Warpage
10. Weld lines

1. Sink Marks

As the name suggests, sink marks are depressions that form on the surface of an injection-moulded part during the cooling process. Sink marks occur when the outer layer solidifies before the inner layer, causing a depression as the inner material continues to cool and contract. Although considered less serious since they do not affect the strength or function of the part, sink marks are a common defect and are visible on the surface.

Causes:

• Thick wall designs that are typically found at the bases of ribs and bosses
• Excessive injection pressure
• Inadequate venting in the mould, leading to trapped air pockets

Solution:

• Design parts with uniform wall thickness, minimize abrupt changes, and add ribs for additional support
• Improve venting system to allow for proper air escape during injection
• Optimize injection moulding process parameters like mould temperature, injection speed, and pressure

2. Flow Lines or Flow Marks

Flow lines are visual defects—dents, divots, depressions, or craters—on the surface of a moulded part. Manifesting as wavy lines, patterns, or streaks on the surface of the moulding, flow lines generally occur when molten material flows unevenly or when some areas cool faster than others. While they are commonly considered less serious injection moulding defects because they typically don’t affect tolerance or the structural integrity of a moulded part, they can be unsightly and may be unacceptable for visually critical parts.

Causes:

• Uneven flow of the plastic material during injection moulding that result in some areas cooling faster
• Parts with varying wall thicknesses making achieving uniform cooling challenging
• Variations in material temperatures or inconsistent injection speed or pressure

Solution:

• Increase nozzle diameter to increase flow speed and prevent early cooling
• Round the corners of mould where wall thickness increases to keep flow rate consistent
• Increase injection moulding process parameters like mould temperature, injection speed, and pressure, to prevent quick cooling

3. Short Shots

Short shot is considered a major defect in injection moulding. Short shots defect occurs when the material does not completely fill the mould. The most common cause of short shots is flow restriction resulting from narrow or blocked gates. Sometimes the material is too viscous or the mould is too cold to allow the molten material to completely fill the mould before cooling. And other times trapped air pockets may be hindering proper flow or injection pressure may be inadequate.

Causes:

• Parts with insufficient ventilation, which may cause air to get trapped in the mould cavity
• Parts with very thin walls, which may prevent the mould cavity from filling completely
• Excessively high injection rates, which can prevent the material from properly filling the mould

Solution:

• Add additional vents or enlarge existing vents in the mould to allow trapped air to escape
• Add additional vents or enlarge existing vents in the mould to allow trapped air to escape
• Optimize injection moulding process parameters like mould temperature, injection speed, and pressure

4. Flash or Spew

Attempting to “fix” short shots can result in flash defects — the opposite of short shots. Flash, in injection moulding, refers to the excess material that leaks out of the mould cavity during the injection process. This defect most often results from using a poorly-design or degraded mould, while other causes include improper clamping or excessive injection pressure. Flash is usually subtle but might be considered a major defect if particularly obvious on a product.

Causes:

• Poorly-designed or worn and degraded mould
• Improper alignment or clamping force of the mould halves
• Excessively high mould temperature or injection pressure

Solution:

• Increase plate clamping force to keep the mould halves tightly sealed during injection moulding
• Monitor the fill pattern and adjust accordingly to avoid over-packing
• Retool or redesign mould if plates don’t fit together properly

5. Jetting

Jetting occurs when the melted plastic is injected too quickly through a restive area (such as a gate, nozzle, or runner) into an open cavity in the mould. This rapid injection can cause the plastic to cool and set before the cavity is fully filled, leading to visible, squiggly flow patterns on the surface of the part. Jetting is considered a major injection moulding defect because it affects the visual aesthetics and can decrease the strength of the part.

Causes

• Moisture in materials due to poor storing or storage condition
• Excessive injection speed causing the plastic to cool too quickly
• Improper gate placement causing certain areas to fill faster or lead to irregular flow patterns

Solution

• Reduce injection speed, specifically at the gate, to minimize occurrence of jetting
• Increase the mould and resin temperature to prevent any jets from pre-emptively setting
• Change gate location, if necessary

6. Voids or Air Pockets

Vacuum voids, bubbles or air pockets are trapped air bubbles that appear on the inside the finished moulded part. Quality control professional typically consider voids as a more minor defect, although, in some cases if a large number of voids form, it can be considered a serious defect, as it can weaken the moulded part.

Causes:

• Inadequate moulding pressure to force trapped air out of the mould cavity
• The material closest to the mould wall cools too quickly
• If the material density changes significantly from molten to solidified state

Solution:

• Raise the injection pressure to force out trapped air pockets
• Place gates close to the thickest parts of the mould to prevent premature cooling
• Choose a grade of material with lower viscosity to limit the risk of air bubbles forming

7. Blister

Blisters are pockets of trapped air that can look like a bubble on parts. However, unliked trapped air bubbles under the surface of the part, a blister is a raised or layered bubble on or near the surface of a moulded part. Blistering is often caused by tools or melted resin that is too hot and does not have adequate time to cool.

Causes:

• Insufficient cooling time
• Excessively high injection rates
• Moisture in pellets

Solution:

• Ensure the part has sufficient cooling time in the mould
• Ensure proper storage of materials and test resins for moisture content before processing
• Optimize injection parameters such as injection rate and temperature to reduce heat buildup and minimize blister formation.

8. Burn Marks

As the name suggests, burn marks appear as black or brown discoloration or streaks on the moulded part. Burn marks are a common injection moulding defect that can be caused by multiple factors, not just high temperatures. Other causes include excessive moisture in pellets or poor mould tool conditions. Although burn mark defects do not affect the structural integrity of the part, they can make the part look worn out and highly unappealing.

Causes:

• Excessively high injection speed, pressure or temperature
• Excessive moisture content in pellets
• Poor mould conditions can cause problems with venting and resin flow causing the resin to burn at the edges

Solution:

• Reduce the injection speed and pressure
• Ensure proper storage of materials and test resins for moisture content before processing
• Enlarge gas vents and gates to allow trapped air to escape the mould

9. Warpage

Warps are one of the most common injection moulding defects. Warping is a defect that results in the distortion—bends, twists, or other deformations—of the shape of a plastic part, which becomes apparent after it has been ejected from the mould. Such defect typically appears when there are different cooling rate across the part. If different sections of a part cool and solidify at different rates or times, internal stress, known as “stress creep,” can build up and lead to distortion.

Causes:

• Choice of material — different plastics have different shrinkage rates
• Parts with uneven wall thickness
• Improper adjustment of injection process parameters (like injection speed, pressure, and temperature)

Solution:

• Ensure consistent wall thickness across the part during design and manufacturing
• Proper gate positioning and appropriate gate type to encourage even filling
• Optimize cooling rate by adjusting mould temperature and cooling time to ensure uniform cooling throughout the part

10. Weld Lines

Weld lines, which are sometimes mistaken for flow lines, are injection melding defects that result in a undesired line or seam on the surface of the plastic part. Weld lines form when two or more flow fronts meet and do not fuse properly, often because they are cooler than desired. This commonly occurs around holes or inserts in the mould, or where the flow splits and rejoins. In some cases, weld lines can also create structural weaknesses in the moulded part.

Causes:

• Poor fusion at the weld line when temperatures are too low
• Premature cooling caused by low injection speeds
• Complex part geometries with thin walls or sharp angles, leading to multiple flow fronts converging and increased likelihood of weld lines

Solution:

• Increase the mould temperatures
• Increase the injection speeds
• Optimize part design to reduce complexity, such as minimizing thin walls and sharp angles