Moulding Issues & Solutions
The main considerations when trying to cure or prevent common moulding issues are:
- Part Design
- Tool Design
- Process Set Up
- Material Quality
These all have an impact on the quality of a moulded part, either individually or in combination.
Below are some common moulding issues, their most common causes and some possible solutions. At LVS Small Plastic Parts, we have the experience to ‘design out’ these issues before a tool even goes into trial production.
Definition: This is where a tool is not completely filled, the extremities of a moulding being incomplete.
Root cause: This is usually a combination of material flow length, speed of filling and material temperature. Molten plastic can also release gasses which become trapped within the tools cavity.
Modify Part Design
- Aid material flow by increasing the thickness of thin sections, adding ribs etc.
Modify Tool Design
- Gas may be being trapped within the tool, the material in effect both releasing the gas and compressing it into a ‘dead end’ within the tool. Adding ‘vents’ can release the build-up of pressure.
- Use techniques such as Mould Flow Analysis to see how adding or changing the position/size of material injection points (gates) can help the mould to fill correctly.
- Increase polymer melt temperature, making it less viscous. This must be done with consideration to material degradation and increases to the ‘cooling time’.
- Increase mould temperature. It is common for at least one half of the tool to be heated using water channels. This prevents polymer from setting too quickly as it flows into the cavity.
- Increase shot size or ‘pack out’ the mould by applying secondary injection pressure during the cooling phase.
- Increase primary injection pressure so that the mould is filled more rapidly.
Sink Marks & Voids
Definition: A Sink Mark is a depression caused by excessive localized shrinking of the material after the part has cooled. A void is often not visible, as it is a ‘bubble’ of missing material within a wall section or feature.
Root Cause: Not enough material has been injected to counter the natural shrinkage that occurs during cooling, or the area is being ‘starved’ of plastic by another underlying feature such as a thick rib section.
- Ensure that wall sections are sufficiently thick in the area effected
- Reduce thickness of features such as ribs in the area where the sink occurs
- Increase the size of the runner system (if hot-runner, increase the size of the nozzles)
- Check the number and position of the injection points
- Employ simulation techniques such as Mould Flow Analysis
- Make sure that the moulding is ‘packed’ during the initial stages of the cooling phase. This introduces additional material to compensate for shrinkage.
- Reduce the melt temperature slightly
- Increase the injection pressure and or the injection time
- Increase the length of the cooling phase
- In the case of voids, this can be due to the material being insufficiently dried prior to processing. (Some materials are hydroscopic, i.e. they absorb moisture from the atmosphere).
Definition: A thin layer of material is forced out of the mould cavity generally appearing around tool sealing faces and ejection points. This excess material remains attached to moulded product and must be manually removed.
Root Cause: The internal pressure generated by the material being injected is higher than that keeping the two halves of the tool in contact, or the mating faces of the tool are imperfect.
- Check ejector pins and bores are within tolerances, replacing any worn components
- Check shut-off faces of mould are aligned and identify any points of non-contact, e.g. by using a surface dye or bluing agent.
- Reduce injection pressure
- Increase the clamping force of the moulding machine
- Reduce mould temperature so that the material is more viscous
Jetting or Silvering
Definition: A swirling effect emanates from the injection point (gate), sometimes appearing to be lighter in colour than the surrounding plastic. (If discolouration is present at the furthest point from injection point, then this is called ‘burning’ and is caused by trapped gasses. Venting of the area is the usual solution)
Root Cause: Turbulence at the injection point causing flow lamination, or possible moisture within the material.
- Increase the thickness of thin sections to aid material flow
- Avoid sudden changes in direction of flow, for example by adding radii to internal corners.
- The injection point may be in the wrong position, e.g. at 90 degrees to a thin wall section. Changing the position or angle of the gate can reduce turbulence levels.
- Increase the size of the material injection gate, therefore reducing speed/pressure of material entry
- Reduce the injection speed
- Adjust the melt temperature (the temperature at which the material is heated to before being injected)
If moisture in the material is suspected, make sure that a suitable material pre-drying takes place, i.e. at the correct temperature and for a long enough period of time.
Definition: This is a dimensional distortion such as twisting or bending that occurs once a component is removed from the mould (tool).
Root Causes: Un-uniform wall sections that cool and shrink at varying rates, lack of structural integrity or damage during part ejection.
- Add or thicken ribs in areas that are large in surface area, but poorly supported
- Keep wall thicknesses uniform
- Check location and size of injection gates
- Check the tools ejector system to see if pressure is applied evenly to mouldings, or to well supported areas.
- Address improperly balanced multiple gates (if relevant).
- Check that core & cavity temperatures are correctly balanced, i.e. adjust the temperature of each half of the tool.
- Increase cooling time to provide a longer period for material to solidify prior to part ejection.
Weld Lines & Flow Marks
Definition: These are an indication of where material has made contact with the cooler surface of the tool, or where material has converged. They are cosmetically unacceptable and can also provide an area of structural weakness.
Root Causes: Flow marks are caused by turbulence or too rapid cooling when material hits an internal surface of the tool. Weld lines are caused when two material flow paths converge but do not fully mesh together.
- Check the location of injection gates, e.g. to see if flow lengths are too long.
- Increase the size of the runner system to improve flow characteristics
- Check tool venting and increase if necessary
- Increase injection pressure and/or speed
- Increase melt temperature
- Increase mould temperature