Top 10 faults to look out for in your parts

Injection moulding is a tricky process, which means there can sometimes be faults in injection-moulded parts if they’re not made by a quality manufacturer.

To help you spot any damage in your parts, we, Essentra Components, put together a list of 10 of the most common faults, what they look like and how we avoid them.

1. Warping

Once a part has been moulded into shape, it is left to cool. Depending on the shape of the final part, different parts of it will cool at different rates. If certain parts of the product cool more quickly than others, it can cause the product to bend and twist, otherwise known as warpage. This is both an aesthetic problem and a structural one, as warpage also causes weaknesses in the product.

To avoid these problems, we make sure that the part is cooled gradually so there is no opportunity for warping or stresses to develop. We also make sure that our moulds have a uniform wall thickness and that the plastic flows in one direction through the mould. This means the distribution of material through the mould is as equal as possible and won’t cool at different rates in different areas of the part.

2. Sink marks

When certain areas of the part are too thick, they can become depressions or small recesses in the final product, otherwise known as sink marks. Opposite to warpage, they are caused by the plastic cooling too slowly or if there isn’t enough pressure to push the material into the mould.
If there’s a problem with sink marks in a prototype, we work to modify the parts of the mould where the wall is thickest so we can then speed up cooling in that area and make depressions less likely. We also give the plastic more time to cool while it’s being held in the mould, meaning that the structure of the part is more solidified and stronger by the time it is ejected, making depressions less likely.

3. Flash

Occasionally, extra material can end up solidifying on the edge of a part. This is usually caused by plastic leaking out from between the cavity and clamp of a mould while the part is being created. The excess caused by this material is commonly known as flash and is often removed by grinding or cutting processes after the part has been injection moulded.

We make sure our moulds are of the highest quality and tightest tolerance so the likelihood of flash is reduced significantly. We also make sure that the clamp force, injection pressure and ventilation are at the optimum level to avoid material leakage from the mould through the parting line.

4. Flow lines

Although flow lines are mainly an aesthetic error, they can ruin the look of the whole product if they’re not addressed. Often appearing as streaks, waves or lines on the final component, they are caused when the material that is injected into the mould flows at different speeds and changes directions. This not only means that the distribution of the material is uneven but that as the material dries, it leaves flow marks on the surface.

By increasing the injection speed, pressure and material temperature so that the material fills the mould before it cools, we avoid uneven distribution and different directional flow, reducing the chances of flow lines. Rounding the mould’s corners where wall thickness changes will also help to keep the flow rate consistent.

5. Vacuum voids

Sometimes, if there is inadequate holding pressure in the mould, the material closest to the wall of the mould cools too quickly, meaning that the outside surface of the part cools more quickly than its inner layers. Bubbles of air become trapped between the layers, leaving vacuum voids in the part’s material and undermining its structure.

By increasing injection pressure, air bubbles are forced out of the material, meaning that no vacuum voids can form. Taking controls to avoid premature cooling and choosing a material with lower viscosity will also help avoid bubbles forming.

6. Jetting

In contrast, having too much injection pressure of the material into the mould can cause ‘jetting’, which is when wavy folds appear in the final part from where the material was injected. This doesn’t just affect the part aesthetically but makes it structurally weaker too.

Getting the injection pressure level at its optimum so there’s an even distribution and gradual filling of the mould is key to keeping the part aesthetically and structurally sound. Having the expertise to adjust the injection pressure, material temperature and mould design is key to creating a consistent, high-quality part.

7. Burn marks

Overheating the molten material, or putting it under extreme friction during injection, can cause the plastic to degrade and burn, which then appears as marks when the part solidifies.

Lowering the temperature of the material and the mould helps to avoid this happening, as does shortening the mould cycle time, which prevents any air trapped in the mould from overheating and causing friction with the material. Increasing gas vents and gates can also help to reduce the temperature of the whole process.

8. Weld lines

When the molten material splits in the mould before coming back together and meeting, it can cause weld lines. This means that the bonds in the plastic can’t be made when the material comes back together, making them become partially solidified and unstable.

By designing a mould that creates a single-source flow pattern when the material is injected, along with raising the material temperature, helps to prevent this from happening. Increasing the injection speed will also help to limit cooling before the material fills the mould, helping the material to bond more effectively.

9. Delamination

If thin surface layers easily separate or peel off from the underlying material and start flaking from the part, the product has delamination. This impacts both on the part’s appearance and its strength and is often caused by foreign materials contaminating the plastic, stopping it from bonding effectively.

Water is one of the materials that can cause delamination if it enters the plastic before or during moulding. Making sure plastic pellets are dried properly before moulding, raising the mould temperature to reduce the likelihood of moisture and having proper storing and handling procedures for the material ensures that there’s no, or very little, material contamination.

10. Short Shot

When the machine fails to ‘shoot’ enough material into the mould to create the full part, it’s known as a short shot. This can be caused by an overly viscous material blocking the machine, or material that hasn’t been heated enough not being able to reach and cover the full cavity because it’s too viscous.

We make sure that our injection moulding machines are always maintained to avoid any gates or runners in the machine being blocked and stopping the flow of material. Checking and adapting vents for better air flow also helps to reduce the chance of trapped air blocking the machine and causing damage to the final product.

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Questions?

Email us at sales@essentracomponents.co.uk or speak to one of our experts for further information on the ideal solution for your application 0345 528 0474.