Product designers and manufacturers can avoid some of the most common Injection moulding defects by carefully considering the part design, mould design, material and processing parameters.
Engineering product designers use Injection moulding to produce plastic parts with high accuracy, speed, and efficiency. However, like any manufacturing process, injection moulding can also have defects in the final product, affecting its quality and functionality. Here are some common Injection moulding defects and how to avoid them. Injection moulding defect causes are tabulated and grouped into Machine, Mould, Material and design.
Shrinkage
What is shrinkage in injection moulding?
Injection moulding shrinkage is an unavoidable side effect of plastic contracting when it cools inside the mould cavity, similar to the metal casting process. Since thermoplastics and thermosets have a higher thermal expansion coefficient than metal, plastic contracts more than metal; hence parts shrink during the cooling cycle of the injection moulding process.
Shrinking can reduce the finished part’s size and dimensions, which can cause problems with form, fit and function.
What are the common causes of shrinkage?
Several factors can contribute to shrinkage in injection moulding, including material, part design, mould design and processing parameters.
| Defect categories | Possible defect causes |
|---|---|
| Machine | The barrel temperature is too high Insufficient injection pressure or time Inadequate cooling time Low cushion and hold time Faulty check valve |
| Mould | Mould temperature is too high Gates or runners are too small Unsuitable gate location Excessive gate land length |
| Material | Incorrect flow rate |
| Design | Non-uniform wall thickness Thick walls, sections and intersections |
Different materials have different rates of shrinkage. For example, some materials, such as thermoplastics, have a high shrinkage rate, while others, such as thermosetting plastics, have a lower shrinkage rate.
The design contributes to shrinkage as thicker parts shrink more than the thin, uniformed walled parts.
The mould design can also have an impact on shrinkage. If the mould cavity walls are too thick, for example, it can take longer for the material to cool and solidify, leading to more significant shrinkage.
Processing parameters such as temperature, pressure, and cooling time during injection moulding can also affect shrinkage. For example, higher temperatures and pressures can reduce shrinkage, while longer cooling times can increase it.
How to prevent shrinkage?
Removing shrinkage is nearly impossible. To minimise shrinkage in injection moulding, it is essential to carefully consider material selection, part design, mould design, and processing parameters. It may also be necessary to adjust the injection moulding process based on the specific requirements of the part.
Short shots
What are short shots in Injection moulding?
A short shot is when the injected molten plastic solidifies inside the mould before filling the mould cavity. Injection moulding defect short shots occur when the moulded part is not filled during the injection moulding process, resulting in incomplete or missing features.
More detailed article of how to avoid short shots?
What are the common causes of Short shots?
Various factors, including insufficient injection pressure, resin flow, or holding pressure, can cause short shots. The table below lists the possible causes of shorts in injection moulding.
| Defect categories | Possible defect causes |
|---|---|
| Machine | Insufficient material feed The barrel temperature is too low Faulty check ring Inadequate back pressure Nozzle is too small Injection pressure or speed too low Insufficient injection time Excessive feed cushion Excessive non-return valve clearance Bridging in the feed throat Insufficient press capacity |
| Mould | Mould temperature too low Gates or runners too small Improper gate location Insufficient venting Sprue bushing too long The runner diameter is too small The sprue diameter is too small |
| Material | Improper flow rate Excessive regrind Use of regrind that is too coarse Excessive moisture content Non-uniform granule size |
| Design | The wall section is too thin Thin walls Complex and intricate features |
How to prevent Short shots?
Optimising the moulding process parameters, such as the injection pressure, temperature, and holding pressure, is essential to prevent short shots.
Design the mould gating system and vent to promote even filling and good resin flow.
Flashing
What are flashings in Injection moulding?
Flashes are when the molten plastic squeezes through the gap between the two mould halves. It typically occurs near the ejection pins.
What are the common causes of flashing?
Injection moulding defect flashing occurs when molten plastic leaks from the mould cavity and solidifies outside the moulded part. Flashing can happen when there is excessive pressure or material in the mould or when the mould is not closed correctly or clamped.
| Defect categories | Possible defect causes |
|---|---|
| Machine | Excessive injection pressure Excessive residence time The barrel temperature is too high Excessive cycle time Inadequate clamp pressure |
| Mould | Improper split line seal Inadequate mould supports Inadequate venting Sprue bushing too long |
| Material | Excessive mould lubricant |
| Design | Parting lines placed at intricate joints |
Flashing can be a common defect in injection moulding and can affect the appearance and functionality of the finished product. It can also result in increased production costs and material waste.
How to prevent flashing?
Ensuring that the mould is adequately closed and clamped and that the injection pressure and material volume are set correctly is vital to avoid flashing. This may require adjustments to the moulding process parameters, such as the injection speed and pressure.
If flashing does occur, it can often be removed by trimming or sanding the affected area. However, in some cases, the part may need to be remoulded if the flashing has significantly affected the appearance or function of the part.
Sink marks
What are Sink marks in Injection moulding?
Sink marks in injection mouldings are shallow depressions or indentations on a moulded part’s surface. Sink marks occur when the part’s surface cools and solidifies at different rates, causing a variation in the volume of the material.
What are the common causes of sink marks?
Sink marks can be caused by several factors, including insufficient packing pressure, cooling time that is too short, wrong gate location or size, or improper mould design. These factors can result in areas of the moulded part that cool and solidify at a slower rate than others, leading to the formation of sink marks.
| Defect categories | Possible defect causes |
|---|---|
| Machine | The barrel temperature is too high Insufficient injection pressure or time Inadequate cooling time Short cushion and hold time Faulty check valve |
| Mould | Mould temperature too high opposite ribs and intersections Gates or runners are too small Improper gate location Excessive gate land length Trapped air or gasses Unbalanced flow pattern |
| Material | Incorrect flow rate Excessive regrind use Use of regrind that is too coarse |
| Design | Excessive rib thickness Thick bosses Thick walls |
How to prevent sink marks?
To prevent sink marks, optimising the moulding process parameters, such as the injection speed, pressure, and temperature, is critical to ensure that the molten plastic flows evenly and that the part is packed uniformly. Additionally, increasing the cooling time can help ensure that the material solidifies evenly throughout the part.
If sink marks occur, they can often be improved or eliminated through post-moulding processes such as sanding or filling. First, however, it is crucial to address the root cause of the issue to prevent it from occurring in future production runs.
Voids (bubbles)
What are Voids in Injection moulding?
Injection moulding defect voids, also known as air pockets, vacuum voids, or bubbles, are trapped pockets of air or gas in the moulded part. Voids can affect the moulded part’s strength and appearance, increasing material waste and production costs. In some cases, voids can also cause the part to fail during use.
What are the common causes of Voids?
These voids occur due to poor venting, low injection pressure, or moisture-contaminated resin. Injection moulding Air pockets are one of the most common injection moulding defects in thick-walled parts.
| Defect categories | Possible defect causes |
|---|---|
| Machine | Injection temperature too high Injection pressure too low Injection forward time too low Insufficient material feed Improper injection temperature profile Excessive injection speed Insufficient back pressure |
| Mould | Improper venting Improper runners or gates Mould temperature too low |
| Material | Excessive moisture |
| Design | Section thickness is too great Thick walls |
How to prevent Voids?
The manufacturer should ensure that the mould has good venting, uses the correct injection pressure, and the resin is free of moisture and other contaminants. Using vacuum-assisted resin transfer moulding (VARTM) can also help reduce the occurrence of voids.
If voids occur, they can sometimes be filled with additional resin or filler material. In other cases, the part may need to be remoulded. In either case, it is essential to identify and address the root cause of the voids to prevent them from occurring in future production runs.
Weld lines ( Knit lines)
What are weld lines in Injection moulding?
Injection moulding defect weld lines, also known as knit lines or meld lines, occur when two or more flow fronts of molten plastic meet and then solidify. This creates a visible line or seam on the surface of the moulded part.
What are the common causes of weld lines?
Weld lines can occur for various reasons, including the part’s design, the material used, and the processing conditions. Common causes include insufficient injection pressure, low mould temperature, improper gating, and inadequate venting.
| Defect categories | Possible defect causes |
| Machine | The barrel temperature is too low Inadequate back pressure Injection pressure or speed too low |
| Mould | Mould temperature too low Gates or runners are too small Unsuitable gate location Excessive gate land length |
| Material | Incorrect flow rate |
| Design | Features like holes and cutouts block flow from the gate location |
How to prevent weld lines?
It is essential to optimise the mould design and processing conditions to minimise the occurrence of weld lines. This may include adjusting the injection speed and pressure, increasing the mould temperature, using higher-quality materials, and ensuring proper gate and vent placement.
If weld lines occur, they can often be reduced or eliminated through post-moulding processes such as heat treatment or surface finishing. In some cases, however, the part may need to be remoulded to achieve the desired quality.
Burn marks
What are burn marks in Injection moulding?
Injection moulding defect burn marks are dark discolourations or charred areas on the surface of the moulded part. Burn marks occur when the plastic resin is overheated, causing it to degrade and discolour.
What are the common causes of burn marks?
The main causes of burn marks are typically related to the moulding process parameters, such as excessive melt temperature, prolonged residence time in the barrel, or a slow injection speed. Poor venting can also lead to the accumulation of trapped air, which can cause local overheating and result in burn marks.
How to prevent burn marks?
Optimising the moulding process parameters is essential to prevent burn marks, such as the melt temperature, injection speed, and cooling time. In addition, the mould should be appropriately vented to avoid the accumulation of trapped air, and any trapped air should be vented to the outside of the mould.
| Defect categories | Possible defect causes |
| Machine | Excessive injection speed or pressure Excessive back pressure Screw speed too high An improper compression ratio of screw Faulty temperature controller Nozzle is too hot Excessive barrel temperature Nozzle diameter to small Inadequate or inconsistent cycle time |
| Mould | Improper venting – size and location Vents plugged or peened shut Improper gating – size and location |
| Material | Excessive regrind use Flow too low Excessive lubricant Use of regrind that is too coarse |
| Design | N/A |
Discolouration
What is Discolouration in Injection moulding?
Discolouration in injection moulding occurs when the colour of the moulded part appears different from the intended colour. This can happen when the resin is contaminated or the moulding process parameters, such as temperature or pressure, are not correctly set.
What are the common causes of discolouration?
Discolouration can be caused by various factors, including contamination of the resin with moisture or other foreign materials, exposure to excessive heat, or exposure to ultraviolet (UV) light. Additionally, incompatible colourants or pigments can also cause discolouration.
| Defect categories | Possible defect causes |
|---|---|
| Machine | Excessive shot size ratio Excessive residence time The barrel temperature is too high The nozzle temperature is too high Excessive cycle time Unsuitable screw design Excessive or inconsistent cycle time |
| Mould | Improper mould temperature Inefficient cooling Inadequate venting |
| Material | Contaminated material |
| Design | N/A |
How to prevent discolouration?
It is essential to use high-quality resins and colourants to prevent discolouration and properly store and handle them to prevent contamination. OptimisingOptimising the moulding process parameters, such as the temperature and pressure, is also essential to ensure the material is appropriately processed.
Removing or reducing it through post-moulding processes such as sanding or polishing may be possible if discolouration occurs. However, in most cases, the discolouration is permanent, and the manufacturer may need to remould the part.
Flow marks
What are flow marks?
Injection moulding defect flow marks, also known as Injection moulding flow lines, occur when the surface of the moulded part appears to have visible lines or streaks. Flow lines can happen when the molten plastic flows unevenly through the mould or when the material cools and solidifies at different rates.
What are the common causes of flow lines?
Various factors cause flow lines, including improper mould design, inadequate injection pressure or speed, or a mould that is too cold. These factors can result in areas of the moulded part that cool and solidify slower than others, forming flow marks.
| Defect categories | Possible defect causes |
|---|---|
| Machine | Inadequate injection pressure Inadequate residence time The barrel temperature is too low The nozzle temperature is too low Inconsistent cycle time |
| Mould | N/A |
| Material | N/A |
| Design | N/A |
How to prevent Flow marks?
To prevent flow marks, optimising the injection moulding process parameters, such as the injection pressure, speed, and temperature, is essential to ensure that the molten plastic flows evenly through the mould. Additionally, adjusting the mould design or adding gating can promote even flow and reduce the formation of flow marks.
Delamination
What is Delamination?
Delamination is an injection mould surface defect when the moulded part separates into layers or flakes. Delamination can occur due to several factors, including poor material bonding, insufficient cooling, or inadequate injection pressure.
What are the common causes of delamination?
Delamination can occur when the material does not bond well during the injection moulding. This defect can happen if the mould temperature is too low or the injection pressure is not high enough to fill the mould properly. In addition, if the cooling time is not sufficient, the surface layers of the part may solidify before the core has had a chance to harden fully, causing the layers to separate.
| Defect categories | Possible defect causes |
|---|---|
| Machine | The injection speed is too low Inadequate injection cushion The injection hold time is too short The barrel temperature is too low |
| Mould | Mould temperature too low Sharp gate and runner corners Excessive mould release |
| Material | Contaminated regrind Use of regrind that is too coarse Foreign materials and additives in material Excessive moisture |
| Design | N/A |
How to prevent delamination
OptimisingOptimising the moulding process parameters, such as the injection pressure, temperature, and cooling time, is essential to prevent delamination. In addition, the manufacturer should design the mould to promote even cooling and choose the material carefully to ensure good material bonding properties.
If delamination does occur, it may be possible to repair the part by bonding the layers back together. However, the moulded part may need to be remoulded in some cases.
Jetting
What is Jetting?
Jetting is a common defect in injection moulding, where molten plastic shoots out of the mould and causes streaks or string-like marks on the surface of the moulded part.
What are the common causes of Jetting?
Jetting occurs when the molten plastic enters the mould too quickly, causing it to push against the mould surface and create turbulence or shear forces that disrupt the plastic flow.
| Defect categories | Possible defect causes |
|---|---|
| Machine | Excessive injection speed Barrel temperature is too high or too low The nozzle opening is too small Inadequate nozzle temperature |
| Mould | The mould temperature is too low Gates or runners are too small Improper gate location Excessive gate length |
| Material | Incorrect flow rate |
| Design | N/A |
How to prevent jetting
The following measures can be taken to eliminate jetting in the injection mould. Optimising the mould design and gate location to ensure that the plastic flows smoothly into the cavity
Reduce injection speed
Increasing the packing pressure to fill the mould cavity
Ensure that the melt temperature and mould temperature are within the recommended range. Properly venting the mould and using materials with a higher melt viscosity can also help to prevent jetting.
Warping
What is Warping?
Warping is a common defect in injection moulding, where the moulded plastic part undergoes undesirable shape changes or deformations after it has cooled and solidified. Warping is characterised by the part being distorted, twisted, or bent, which may result in the part not fitting or functioning correctly.
What are the common causes of warping?
Warping in injection moulding is typically caused by uneven cooling of the part during the cooling phase, where one part of the part cools faster than another.
| Defect categories | Possible defect causes |
|---|---|
| Machine | Inadequate injection pressure or time Inadequate residence time The barrel temperature is too low The nozzle temperature is too low Inadequate cycle time Lack of cushion – underpacking Excessive stress buildup |
| Mould | Mould temperature too low Gates or runners are too small Unsuitable gate location Uneven mould temperature Non-uniform ejection |
| Material | Incorrect flow rate |
| Design | Excessive rib thickness Thick bosses Thick walls |
How to prevent warping?
To prevent warping in injection moulding, several measures can be taken. These include optimising the mould design and cooling system to ensure even cooling throughout the part, reducing injection pressures and mould temperatures, and adjusting the gate location and orientation to minimise residual stresses. Proper material selection and ensuring that the melt and mould temperatures are within the recommended range can also help prevent warping.
FAQ – Injection Moulding Defects
Injection moulding residence time refers to the amount of time that the molten plastic material spends inside the barrel of an injection moulding machine, from the moment it enters the barrel until it is injected into the mould cavity.
Regrind in injection moulding refers to the recycled plastic material that is generated during the manufacturing process. When plastic parts are moulded, there is often leftover material trimmed from the finished parts or excess material purged from the machine during colour or material changes. Instead of throwing away this material, it can be ground into small pellets and reused in future moulding processes.
In injection moulding, cushion refers to the amount of molten plastic that remains in the barrel of the Injection moulding machine after the mould cavity is filled. It is the difference between the maximum shot size of the machine and the amount of plastic required to fill the mould.