In Pursuit Of The Perfect Injection Moulded Part 1 – Flow Lines

It is very easy to make expensive errors when it comes to injection moulding. Quality issues in injection-moulded products can range from minor surface defects to more serious problems that can affect the safety, performance and function of the product. They can be caused by problems related to the moulding process, material use, tooling design or a combination of all three.

In this series of posts, we will look at a number of issues that can affect the quality of the end product – and some of the common remedies.

Moulding defects

Whatever part we are trying to design we usually have to make compromises along the way. Weather that be a Toolmaking one or an Injection Moulding or Design. They are generally brought on by budget costs. Commercially am I going to sell enough of these parts to warrant the cost of the tooling to produce these parts at X cost. Will I ever recoup my costs? What can I do to cheapen my cost of entry yet maximise my return. We all work to a budget (unless you are JLR) and if that’s the case we must all (or nearly all of us) then have to consider these potential moulding defects as we make design compromises.

Flow lines

In Pursuit Of The Perfect Injection Moulded Part - Flow LinesFlow lines appear as a wavy pattern often of a slightly different colour than the surrounding area and generally on narrower sections of the moulded component. They may also appear as ring-shaped bands on a product’s surface near the entry points of the mould, or “gates”, which the molten material flows through. Flow marks won’t typically impact the integrity of the component. But they can be unsightly and may be unacceptable if found in certain consumer products, such as high-end sunglasses.

Causes and remedies for flow lines

Flow lines are most often the result of variations in the cooling speed of the material as it flows in different directions throughout the mould. Differences in wall thickness can also cause the material to cool at different rates, leaving behind flow lines.  For example, molten plastic, cools very quickly during the injection process and flow marks are evident when the injection speed is too slow. The plastic becomes partially solid and gummy while still filling the mould, causing the wave pattern to appear.

Here are some common remedies for flow lines in injection-moulded products:

  • Increase the injection speed, pressure and material temperature to ensure the material fills the mould before cooling
  • Round the corners of the mould where wall thickness increases to help keep flow rate consistent and prevent flow lines
  • Relocate mould gates to create more distance between them and the mold coolant to help prevent the material from cooling too early during flow
  • Increase the nozzle diameter to raise flow speed and prevent early cooling

Patterson & Rothwell are a Plastic Injection Moulding Company based in the North West of England, UK producing goods for many business sectors. Our ethos is to offer our customers a professional yet flexible approach and of course provide attention to detail at every stage. Please contact us if you wish to discuss any Plastic Injection Moulding Project.