Quality control 6 injection moulding defects and how to prevent them

Author / Editor: Peter Jacobs* / Ahlam Rais

Creating an accurate injection moulding prototype requires a great amount of skill and practice. And while it is generally held that injection moulding defects are a reflection of poor craftsmanship, even the most experienced CNC machine operators may face this issue at some point.

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Flow line defect caused by slow injection speed leading the plastic to solidify while injecting at different speeds.
Flow line defect caused by slow injection speed leading the plastic to solidify while injecting at different speeds.
(Source: www.creativemechanisms.com)

Injection moulding defects could arise due to various reasons other than expertise. And these factors range from material differences in batches to machine calibration changes post-repair or maintenance.

Since injection moulding defects can sometimes prove expensive, operators must take extra care to prevent these at any cost. Here's a list of the commonest injection moulding defects and how you can avoid them:

1. Flow lines and flow marks

Flow lines or marks appear as banded discolouration on the surface.
Flow lines or marks appear as banded discolouration on the surface.
(Source: www.enginewheel.com)

Flow lines or marks appear as banded discolouration on the surface. These are typically found near the narrow sections of the mould gate and can even be wavy or ring-shaped in some instances.

They are caused due to the non-uniform cooling time and flow rates of different materials as they flow through the mould. While flow lines do not negatively affect the part or component’s integrity, they damage the aesthetic appeal and final finishing.

How to prevent these?

• Increase the distance between mould gates and coolants to slow down the mould flow.

• Round the mould corners, especially where the walls are thick, to make the flow consistent.

• Modify the injection speed, material temperature, and pressure to prevent premature cooling.

• Increase the injection nozzle diameter to increase the flow rate.

2. Burn marks

Burn marks result from the overheating of materials as they interact with high temperatures of mould, injection, nozzle, and even trapped air.
Burn marks result from the overheating of materials as they interact with high temperatures of mould, injection, nozzle, and even trapped air.
(Source: www.nanomoldcoating.com)

As the name indicates, burn marks result from the overheating of materials as they interact with high temperatures of mould, injection, nozzle, and even trapped air. The resultant injection moulding defect appears rusty or blackened, especially at the edges.

In cases where the burn marks cause material degradation, it could compromise the quality of the component. That being said, burn marks generally do not affect the part’s integrity.

How to prevent these?

• Decrease the melt and mould temperatures to avoid overheating of elements.

• Bring down the injection speed to allow uniform cooling and to prevent the risk of trapped air inside the mould.

• Add, enlarge, or clean gates and gas vents to facilitate the escape of trapped air before it overheats.

• Reduce the mould cycle times to allow faster cooling of trapped air and resins to prevent burning.

3. Air pockets or vacuum voids

Air pockets or vacuum voids: These injection moulding defects are characterized by tiny air bubbles or vacuum pockets.
Air pockets or vacuum voids: These injection moulding defects are characterized by tiny air bubbles or vacuum pockets.
(Source: www.boyanmfg.com)

These injection moulding defects are characterized by tiny air bubbles or vacuum pockets. Air pockets could arise due to two primary reasons, namely:

1. Insufficient injection pressure that traps air in the mould cavity, and

2. Irregular cooling of the material, causing shrinkage and formation of abscesses.

It is also likely that the material, by its inherent nature, is prone to air pocket formation.

Typically, these defects are often present on the surface of the component. While quality control experts may classify these as ‘minor’ defects, the size, density, location, and volume of the air pockets could be detrimental to the structural integrity of the finished product.

How to prevent these?

• Increase the moulding pressure and hold times to eject trapped air pockets.

• Maintain a wall thickness of less than 6 mm to limit the risk of abscesses.

• If the above is not possible, one can position mould gates at the thickest portions to avoid premature or irregular cooling.

• Work with materials with lower viscosity to prevent air bubble formation.

4. Warping

Warpage is a result of uneven shrinkage during the cooling process.
Warpage is a result of uneven shrinkage during the cooling process.
(Source: www.stratasysdirect.com)

Warpage is a result of uneven shrinkage during the cooling process. This shrinkage gives rise to sections that experience undue stress, which, in turn, births distortion in the component’s shape.

As such, it causes the part to bend, fold, or twist, in part or as a whole. Even an untrained eye, like that of a customer, can detect warping as parts that are meant to lie flat against a plain surface leave a gap at the spot.

How to prevent these?

• Increase the cooling time to allow for gradual and even cooling of the moulding.

• Decrease the temperature of the mould or material.

• Use materials that are less prone to shrinkage during the cooling process.

• Apply new designing techniques to incorporate uniform wall thickness and component symmetry to grant higher stability.

5. Delamination

Delamination takes place when the finished product has thin, flaky layers that peel or separate easily.
Delamination takes place when the finished product has thin, flaky layers that peel or separate easily.
(Source: www.midstatemold.com)

Delamination takes place when the finished product has thin, flaky layers that peel or separate easily. Quality assurance professionals tag such defects as ‘serious’ as it compromises the overall strength of the part. Furthermore, it also reduces the aesthetic appearance of the finished goods.

Delamination is caused by the cross-contamination of the melted resin with foreign agents or dissimilar resin, which affects the bonding process. Plus, improper storage may lead to moisture or release agents coating the pellets, thereby decreasing their ability to bond.

How to prevent these?

• Source virgin plastic resins to mitigate contamination at the manufacturing stage.

• Ensure that the resin pellets are stored in a manner to avoid cross-contamination.

Pre-dry the resin pellets and increase the mould temperature to limit moisture intervention.

• Redesign the mould to optimise the injection nozzle and to minimise dependence on release agents.

6. Short shot and short moulding

Short shot and short moulding: the resin shot falls short of filling the mould.
Short shot and short moulding: the resin shot falls short of filling the mould.
(Source: www.improve-your-injection-molding.com)

The short shot injection moulding defect is rather self-explanatory - the resin shot falls short of filling the mould. In simpler words, the flow of the molten resin does not fully occupy the cavities present. Resultantly, the moulded component, after cooling, remains incomplete.

Short moulding defects are major defects, which can inhibit the functionality or appearance of the component. Missing tines in plastic forks or incomplete mesh compartments in plastic shelves are a few classic examples of this defect.

How to prevent these?

• Redesign the mould with wider gates and channels or switch to low viscosity material for improved flow.

• Increase mould temperature to prevent it from rapid cooling.

• Increase injection speed or pressure to ensure the molten material reaches all cavities.

• Add air vents or enlarge the existing ones to allow trapped air to escape.

Final thoughts

As we move towards the age of sustainable moulding, preventing injection moulding defects could be one of the fastest ways to reduce wastage and cut costs. Additionally, limiting these moulding defects at the initial stages can prevent any serious damage in the future.

With the above knowledge, you are in an advantageous position to meet the highest quality standards and keep your customers happy!

You can view CNC Masters' website here.

* *Peter Jacobs is the Senior Director of Marketing at CNC Masters. He is actively involved in manufacturing processes and regularly contributes his insights for various blogs in CNC machining, 3D printing, rapid tooling, injection molding, metal casting, and manufacturing in general.

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