How To Improve The Sink Mark In Injection Molding

how to improve the sink mark

Table of Contents

Many factors affect plastic part design and functionality. A sink mark appears on the surface where a plastic part has variations in wall thickness. Large variations mean a large number of sink marks on a plastic molded part. If cooling time and the mechanism is not enough for the cooling process. Then the sink mark creates as thick parts need much time for cooling. By reducing the thickness of the thickest wall, you can faster the cooling process. In this way, you can reduce the sink mark on the plastic molded parts. They occur at the localization above ribs, internal fillets, and bosses.

A void and sink create because of shrinkage of the material at a thick section. Shorthold time with high melt temperature are also factors for the sink mark. Sink formation also involves many other factors. Such as tool design, material, part geometry, and process method.

Some Steps For The Improvement Of The Sink Mark

  • Change in the molten plastic parts’ pressure can reduce sink marks. But it should control and check. In some cases, the sink mark reduces with the increase of packing pressure and hold time. You have to slow the injection speed to maintain it.
  • Unequal cooling induced is a cause of sink mark. Altering the cooling temperature from the core to the cavity may prove effective. It can also lead to another defect as distortion.
  • We can avoid sink marks by the addition of up to 0.5 of a blowing agent. It will depend on the product design and end-use of the product. It can decrease the mechanical durability of the components.
  • Sink marks appear on the areas of ribs and bosses. By following industry guidelines, you can avoid sink marks. A professional designer uses these guidelines to reduce sink marks. Such as the seven-degree rule, thinning the wall’s thickness around the boss. Proportioning the thickness of the rib base to the wall thickness.
  • If the sink appears in a hidden place and it doesn’t impact the structural integrity or strength. You don’t need to resolve it. Leave it, and it will cover with other parts.
  • Labels also use for covering a sink mark. A texture is also proving effective for hiding a sink mark. It can provide camouflage, but it can’t cover a sink mark. You can make hiding possible by changing the topology of your model part.
  • 3D modeling provides a powerful capability for plastic part designing. In Solidworks, we can use thickness analysis for the assessment of our part. The plastic software makes possible feature modification and packing size. It will be helpful for the reduction of the sink mark.

Plastic part designing produces many products. With consistency and hard work, you can provide your customers with high-quality products.

We can reduce sink marks by changing the parameters of molded parts and geometry. Some steps are:

What Are Shrinkage And Sink Marks In Plastic Injection Molding?

Sink marks and void create from the shrinkage of the material. They appear as depression on the surface of plastic molded parts. They are tiny in size and reflect the light in a different direction. Sink mark is a quality defect and can’t affect product strength and functionality. Voids are gaps and they may be a single group or group of smaller holes.

Sink mark becomes because of thermal contraction (shrinkage) during the cooling procedure. When materials have cooled and solidified, the core material starts to cool.  Its shrinkage pulls the surface of the main wall in and then makes a sink mark. In the rigid skin, deformation of the skin may replace it with the formation of a void in the core. High volumetric shrinkage causes sink marks.Short packing and cooling time also result in the void and sink mark. Material having more shrinkage creates sink marks on the molded part. You can change contraction at low for the elimination of the sink mark. On HIPs material, you can lower the mold temperature and decrease the roughness of the part. This method also proves effective in curing this issue. But it’s challenging to remove it from the plastic surface if a sink mark occurs.

A sink mark that creates in packing can reduce by controlling packing pressure. You can use a simulation package like Moldflow insight. You can alter the part geometry to reduce the thickness of any features. Using material and optimizing the runner system design also use as a remedy of void and sink mark. Some further remedies for avoiding sink mark are:

  • You can reduce barrel and plastic injection mold temperature to avoid sink marks. If you found a small place around the material. You can thicken this place to cure the sink mark.
  • During designing, be careful about making differences. And elongated shapes must be as short as possible.
  • Material having more shrinkage is more prone to create a sink mark. The difference in the material proves effective for the improvement of sink marks.

How Can You Avoid Sink Marks With SolidWorks Plastics?

A sink mark appears on the area of the plastic molded part where the surface changes in depression. The depression causes by uneven cooling of the material and in the area of thick geometry. Some areas are where the boss and rib meet the wall, a large solid region, and an area where the thickness is changing. All areas are not starting cooling and hardening at the same rate. As the outer region has solidified, we can find a molten region in the center of the wall. With the start of cooling of the inner material, it also starts to shrink. This shrinkage starts to pull the outer surface in and deform the part. This type of defect leads from simple blemishes to structural deformity. You can find many different ways to solve these issues and problems.

  • With SolidWorks plastic’s help, you can identify the area where this type of issue can find. You can change your part before the manufacturing of the physical mold.
  • You can solve this problem by adjusting the material and mold temperature setting.
  • The change in packing and injection pressure also helps in avoiding sink marks.
  • Modifying the geometry is beneficial in improving voids and sink marks in the parts.

All these steps can prove helpful for cost and time limit and can reduce the design cost. Some guidelines for solving sink mark issues are:

  • You should increase the gate size that can make it possible to fill and pack mold.
  • You can increase the pressure that is essential to force more material into the mold. It can cut the shrinkage.
  • You should reduce the number and size of the mold region. After hollowing the features, you can add a pattern of ribs if you need extra rigidity.
  • The Wall thickness of ribs should be no more than 50% to 80%.
  • Part wall thickness should be uniform for making fewer sink marks.

All these will help solve these issues and reduce the cost and time. It needs for the plastic part production.

What Are Injection Molding Defects And How To Fix These Defects?

Plastic Injection molding is one of the standard processes for plastic part production. We can make components with various shapes and textures with the molding procedure. In this process, plastic material melts with the hot runners and pours into the mold. Three main components need for this production. A molding machine, a mold, and raw plastic material. The plastic injection molding process requires attention and skill expertise to complete it. If you don’t give it proper attention, you have to face defects that can damage the quality of molded parts. We’re discussing some common defects that causes during plastic injection molding. What are their reasons, and how can you avoid them? These defects may be minor or severe. They can affect the quality, performance, and function of the product. It’s not easy to address these defects. You can solve these issues by adjusting the temperature, pressure, and flow rate.

1: Burn Marks

They are black or rust color discoloration that can appear on the molded part’s edge and surface. The usual cause of the burn mark trapped air or the resin itself. Excess heating or injection speed becomes a reason for overheating. It leads to the burn mark.

By following these prevention measures, you can avoid sink marks. Such as enlarge the gas vents and gates that release the trapped air and short the mold cycle time. This step eliminates the chance of overheating. By lowering the mold and melt temperature, you can prevent overheating, and a burn mark can’t occur.

2: Vacuum Void/Air Pockets

A vacuum void is a trapped air bubble that you can see on a finished molded part. It’s a minor defect, but large vacuum voids can weaken the plastic part to some extent. So it’s necessary to prevent vacuum void. One primary reason for creating an air pocket is insufficient molding pressure. The material cools and hardens very much and pulls the material outside.

Molded parts thicker than 6mm are more prone to create vacuum void, and you can’t avoid them. You can choose material of low viscosity and can increase the injection pressure. It forces out trapped air pockets and can prevent a vacuum void from occurring. You can locate the gate at the thick part of the injection molding to prevent air pockets.

3: Weld Lines

Weld lines appear on the molded part where molten plastic meets. It’s the result of weak material bonding, and it lowers the strength of the plastic part. For maintaining a specific temperature during colliding. You need two or more in front of the polymer. If you don’t have it, they become solidified and can’t bond.

This method finally leads to creating weld lines. You can solve this issue by increasing material temperature to control half solidification. For the elimination of the partition, you should redesign the mold. Except for these, an increase in the injection pressure and speed can also prove useful.

4: Flow Lines

Flow lines are a wavy pattern that occurs on a narrow section of the molded plastic components. It has a different color than the other surrounding areas. They don’t have a severe impact on the product, but it’s unacceptable. When flow marks appear on consumer products such as sunglasses. When we find variation in the thick walls, the material also cools at different rates. It becomes a cause of flow lines. For avoiding these issues, make sure that the material fills the mold before cooling. You can increase the injection speed, temperature, and pressure for solving flow lines. With the increase of the nozzle diameter, you can raise the flow rate. It prevents early cooling, leading to flow marks.

Bottom Line

You should ensure the quality of finished products. Your mold part should finish and design. It should be great for leaving no rooms for extra cost and efficiency. Defects such as sink marks, vacuum void, and flow lines can affect your product. So you should take steps for improving these issues. So your products can meet your customer standards of quality.


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