Following on from a previous 2-part blog - 10 Impacts of Non-Uniform Wall Sections in Plastic Injection Moulded Parts (see here).
Being only a brief over view of a complex topic, it seemed the next appropriate step was a deeper dive into each of the outlined Negative Impacts in more detail to highlight how a seemingly minor design detail can have a significant impact on the productivity, efficiency and therefore competitive advantage of an injection moulded part and its success out in the market.
First a little revision of Impact no.4 from Part 1 of the blog.
“4. Sink Marks and Voids
Non-uniform wall sections are prone to sink marks and voids. Sink marks are depressions or indentations that appear on the surface of the part, typically above thicker sections where the material has contracted more during cooling. These marks are not just visually unappealing but can also indicate underlying structural weaknesses.
Voids are empty spaces within the part caused by insufficient material flow or air trap seen as bubbles when a part is cross sectioned.
As well as being visually unsightly, possibly failing aesthetic criteria of a specification, sink marks and voids compromise the mechanical integrity of the part, making it less durable and more susceptible to failure under load.
In certain markets like the medical and consumer industries where a high investment in and therefore a high emphasis placed on the aesthetic quality of a part, a seemingly structurally viable part may be scrapped as it fails a visual criterion.”
Understanding and Avoiding Sink Marks in Plastic Injection Moulding
To understand how and why sink marks occur and how to avoid them, it is good to understand what happens when the plastic melt flow fills a cavity.
Plastic Flow in a cavity
As the melt front moves a along the cavity it forms a skin where ever it comes into contact with the cooler metal of the cavity surface of the tool.
As can be seen in the image below the central core of the melt is still molten and fluid.
Once the gate has frozen off, the pack and hold phase is complete and no more material can be injected into the cavity and the parts enters the cooling phase of the cycle.
This is where sinking will occur in parts that have non-uniform wall sections or changes in geometry, like ribs connected to exterior walls of the part.
What Causes Sink Marks in Plastic Injection Moulding?
Sink marks primarily occur due to differential shrinkage in the moulded part. Several factors contribute to this phenomenon:
Material Shrinkage:
Different plastics have varying shrinkage rates. Materials with high shrinkage rates, such as polycarbonate (0.6% to 0.8%), polypropylene (1.5% to 2%) and polyethylene (1.5% to 3.0), are more prone to sink marks.
Part Design:
Thick sections of a part cool and solidify more slowly than thin sections, leading to differential shrinkage. This causes now cooled surface skin to collapse inward, forming a sink mark.
Moulding Conditions:
Cooling Rate: Rapid cooling can cause the outer surface to solidify while the inner core is still molten, leading to shrinkage and sink marks.
Injection Pressure: Insufficient injection pressure can result in inadequate packing of the molten plastic, causing voids and subsequent sink marks as the material cools and shrinks.
Mould Temperature: Uneven or incorrect mould temperatures can lead to non-uniform cooling and shrinkage.
Gate Design and Location: The position and size of the gate (where the molten plastic enters the mould) can impact the material flow and packing, influencing the occurrence of sink marks.
How can a designer Prevent Sink Marks
Preventing sink marks involves addressing the factors that cause them through careful design, material selection, and process optimization. Here are some strategies to avoid sink marks:
Optimize Part Design:
Uniform Wall Thickness: Design parts with uniform wall thickness to ensure even cooling and shrinkage. Avoid thick sections that can lead to differential shrinkage.
On the right, here is greater thermal mass shown by the red circle with greater cooling differential when compared to the cooler skin, pulls a large sink.
On the right the thermal mass is smaller, the differential reduced, and the sink is smaller or with carefully chosen moulding parameters, eliminated.
Now a part design has been optimised for injection moulding to reduce sinks, the designer will need to be aware of the other pieces to part quality puzzle.
Careful material selection and liaising with the toolmaker and moulder will ensure that the intended quality part is achieved.
Material Selection:
Low Shrinkage Materials:
Choose materials with lower shrinkage rates to reduce the likelihood of sink marks. Consult with material suppliers to select the most suitable resin for your application for fit, form and function.
Tool Design - Gate Design and Placement:
Proper Gate Size:
Use appropriately sized gates to ensure sufficient material flow and packing. Small gates can restrict flow and lead to sink marks.
Strategic Gate Placement:
Position gates in areas that facilitate even material distribution and reduce the risk of differential shrinkage.
Adjust Moulding Conditions:
Optimize Cooling Time:
Ensure adequate cooling time to allow for even solidification of the part. Longer cooling times can help reduce differential shrinkage.
Increase Packing Pressure: Use higher packing pressure during the injection process to compensate for material shrinkage and fill any potential voids.
Control Mould Temperature: Maintain uniform mould temperatures to ensure consistent cooling. Use mould temperature controllers to precisely manage temperature profiles.
Sink marks in plastic injection moulding are a common challenge, but with careful attention to part design, material selection, tool design, and process parameters, they can be effectively minimized or eliminated.
By implementing the strategies outlined above, manufacturers can produce high-quality, aesthetically pleasing, and structurally sound plastic parts.
Understanding the root causes and adopting best practices in mould design and processing can lead to more efficient production cycles, reduced waste, and ultimately, higher customer satisfaction.
---
équipe design & consulting , with 20 years experience in design and manufacture of Medical grade moulded parts and product, including 5 years at the coal face as Operations Manager at a world class medical grade moulding facility; we are specialist in Design for Manufacture (DFM).
Please reach out if you feel you need assistance with your part design for plastic injection moulding, from Design Coaching and Guidance to Full Service Design Consulting.
www.equipedesign.au #Designforplasticinjectionmoulding #DesignForManufacturing #DesignThinking #DesignforManufacture #DesignConsulting #DesignCoaching #equipedesign #DesignforAssembly #IndustrialDesign #CREODesign
Comments