Properly improve injection and dwell pressure as well as the injection rate to increase melt’s compression tightness; prolong time of injection and dwell( the freezing of gate should not occur too early) to realize sufficient shrinkage compensation;
properly reduce melt temperature and properly raise mold temperature( whereas cooling should be reinforce at parts with thick walls) on the premises that the ejection and dimension precision of products after ejection are ensured; properly increase melt plastication quantity to ensure sufficient material supply and effective transmission of pressure; when depression occurs around the insert, the pre-warming temperature of insert should be increased to a certain degree. In addition, depression and shrinkage porosity which are unavoidable due to the structural property of products can be removed by adopting gas-assisted injection molding.
A sink mark is a local depression on the surface of a plastic injection molded part, typically found in a thicker area of a component. Sink marks can originate from processing methods, tooling design, part geometry, and material selection.
There are varying standards regarding whether sink marks can be present on plastic injection molded components, depending on the product and its end use. For example, parts for consumer electronics must be nearly free of sink marks, while toys often have visible sink marks on their surfaces.
WHAT CAUSES SINK MARKS
In plastic injection molding, a plastic resin material is melted and then injected into a mold, where it cools and solidifies into the final part or parts before being ejected. Sink marks can occur when there is a part thickness that is too large for the type of resin being injected into the mold. They are typically found in thicker areas due to varying cooling rates across the part or insufficient cooling while the part is in the mold. Sink marks can also be caused by low pressure in the mold cavity or an excessive temperature at the gate through which material is injected into the mold.
5 TECHNIQUES FOR RESOLVING INJECTION MOLDING SINK MARKS
Here are some of the most common changes that can be made to resolve plastic injection molding sink marks:
1) Lower the mold temperature: By pulling heat away more quickly from a thick area where sink is likely to occur, the chance of differential cooling between thick and thin areas can be reduced. The coolant temperature can be altered from core to cavity side, moving the sink mark in one direction or another, however, caution must be taken, for this can cause problems such as distortion.
2) Increase the holding pressure: When sink marks are caused by low pressure in the cavity, this may be addressed by increasing the holding or packing pressure. The pressure of molten plastic being injected into the mold can have a strong effect on thermal and mechanical stresses. Modifying this pressure can be helpful, but changes in holding pressure must be carefully controlled.
3) Increase the holding time: Increasing the holding time encourages more adequate and even cooling. The ability to control sink marks in this way depends on the location of the gates in relation to the thicker wall sections, and whether there is enough time to fill all areas of the cavity before the gate freezes off. To avoid molded-in stress, pressure, injection speed, and holding time must all be taken into account and balanced.
4) Reducing wall thickness: The part design can be adjusted to reduce the thickness of thick wall sections. This will promote quicker cooling and reduce the probability of sink marks.
5) Moving the gate location: If the gate is sealed too early, sink marks can result. Relocating gates during design can change the timing of gate sealing, eliminating the sink marks.
SINK MARKS AND DFM
Injection molding contract manufacturers (CM) use design for manufacturability to prevent sink marks during the design phase, which is the least expensive time to address them. The CM engineers use specialized mold flow simulation software to simulate the injection molding processes, creating 3D simulations of flow, heat flux, and warpage for injection molding, including the complete mold with all details. During the simulation, they inspect the project for sink marks and other problems using a checklist for DFM in plastic injection molding. They make changes to the design and plans for the mold, materials, etc., rerunning the simulation and revising until sink marks and other problems are eliminated.
Producing parts through injection molding involves the injection of melted material into a mold. Within the mold, the material solidifies into the desired parts. Subsequently, its ejection occurs.
Injection molding sink marks often occur when the thickness of the part injected into the mold is too large for the resin used. However, there are other causes of sink marks. Now, let’s examine what causes them and how to reduce sink marks in injection molding.
Sink Marks Cause 1: Incorrect Melt Temperature
Using an incorrect temperature when melting a resin could lead to sink mark on the surface of the finished plastic. Often, the sink marks are because of the low temperature of the insert, and if this is the case, increasing the temperature can resolve these issues.
To avoid this, adjust the melting resin within the manufacturer’s recommended temperature range. Before injecting the resin into the mold, verify melt temperature.
Sink Marks Cause 2: Low Pack and Hold time
When pack and hold time is too low, it can also cause sinks on the surfaces of products. Ideally, the pack and hold time should be long. This ensures proper sealing of the part gate, thereby preventing the entry and exit of plastic resin from the mold cavity. If gate sealing does not occur properly, the plastic resin will exit the cavity when laid to rest.
To avoid sink marks, hold it long enough to ensure the gate solidifies after filling the mold. This would help prevent melted material from exiting the mold, preventing sinks. In other words, where a mark or depression occurs near the gate, prolonging the pack and hold time helps eradicate it.
Sink Marks Cause 3: Improper Pack or Hold Pressure
Improper holding pressure distribution is another reason why sinks appear on molded parts. That is to say, applying proper packing pressure to different parts of the product would help keep it in shape.
To prevent sink marks or shrinkage, a typical mold cavity pressure ranges from 8000–15,000 psi. The rule to this is that the pack or hold pressure should be 50-70% of injection pressure.
The best way to apply pressure is by gauging thickness. Consequently, thick mold sections require more plastic and pressure than less thick sections to maintain shape.
Since this defect results from shrinkage when material like the plastic resin in thick areas contracts during drying, getting enough plastic into thick areas with the right pressure could help eradicate it.
Sink Marks Cause 4: High Mold Temperature
Not using the recommended mold temperatures, (which are between 80-120o Celsius) could result in this depression. Setting the temperature too high could deter the gate from sealing properly and on time. To fix this, use the correct mold temperature range while ensuring correct plumbing.
Sink Marks Cause 5 : Improper Part Geometry
Improper part geometries and mold design could also cause these depressions to form. It is important to note that the ribs and bosses are the most common areas where sink marks occur. Below are a few ways to remedy injection molding defects caused by geometry.
Balance Rib And Wall Thickness
Creating a balance between rib and wall thickness would help reduce the tendency of these marks to form. Avoid increasing the height of the rib to the thickness of the wall. Because doing so might lead to an increment in the number of distortions or defects in the plastic.
Also, note that molten plastic, like other liquids, flows along the path with little or no resistance. Therefore less thick areas would fill up first during the plastic injection. After this section is full will the molten plastic start filling the rib section.
Use a Gradual 7-degree Slope at the Base of the Rib
To minimize distortion and blemishes, incorporating a gradual 7-degree slope at the base of the rib is necessary. Using this slope method only works when the gate is near the area. This slope helps ensure a gradual rise in the molten material injected into the mold, preventing blemishes and distortions on its surface.
Boss Design
When designing a boss where the hoop would go through minimal stress, the outside diameter should be double the inside diameter. This type of boss design would help reduce sink marks. On the other hand, if the boss hoop would go through a lot of stress if there is an application of heavier loads, the outside diameter should be two and a half times the inner diameter.
The Thinner Base Area Around The Boss
It is best to avoid thinning the base area around the boss. This practice, which is common these days, makes the construction weak. As a result, the boss would require strong ribs for support which consequently interrupts the flow of material during injection, resulting in sink marks in the plastic.
Small Nozzle Hole
If the nozzle gets blocked or the nozzle hole of the molding machine is small, lack of pressure could cause a sink mark on the plastic. Hence, the best way to prevent this type of defect on plastic is by cleaning or replacing the injection machine nozzle to get the ideal injection speed.
Sink marks show as depressions on a moulded part’s surface. These depressions are usually quite little, but since they reflect light in diverse angles, they are often extremely obvious. Sink marks are visible depending on the part’s colour and surface texture, thus depth is only one factor to consider. Sink marks are seen as significant quality faults, despite the fact that they have no effect on part durability or performance.
Whenever the substance in the vicinity of thick characteristics such as ribs or bosses compresses more than the substance in the abutting wall, sink marks appear in injection moulded plastic segments. The existence of such characteristics results in a thicker zone that cools much slower than the surrounding areas. Distinctive cooling rates create a sink mark, which is a dip on the neighbouring surface.
Sink marks can be caused by a variety of reasons, including processing procedures, part shape, material alternative, and tooling design. Part shape and choice of materials are often determined by the OEM and are difficult to modify.
Nevertheless, the toolmaker has control over various variables of tooling design that might affect the sink. Gate type, gate size, and cooling-channel design can all have a range of consequences. A tiny gate, such as a tunnel gate, for instance, freezes off considerably faster than an edge gate. Untimely gate freeze-off reduces the amount of packing time obtainable in the cavity, increasing the risk of sink marks.
Sink mark is a defect on the surface of injection molded parts with variations in wall thickness. In other words, sink marks injection molding occurs on thicker sections of molded parts and this defect looks more like a dimple or groove on its surface. For a section of a product to be thicker, it means it contains more parts. Furthermore, these thicker regions extend their cooling time. In contrast, the outer portion of the plastic part in contact with the mold steel often cools very fast.
Consequently, this difference in cooling time between the molded part’s outer portion and its interior causes the defect. How? As the molecules in the thicker part of the prototype begin to cool, they contract, resulting in the outer part pulling in, causing a sink mark. However, if the outer part is strong enough, this contraction causes a void instead.
The degree of visibility of a sink is dependent on its depth, the color of the product, and its texture. However, it is important to note that small sink marks are often visible because of their ability to reflect light in different directions.
How Does Shrinkage Occur
Most of the shrinkage happens in the mold while the molten plastic is cooling. However, a small amount of shrinkage occurs after ejection as the molded part continues to cool. In fact, the plastic part may continue to shrink slightly after that till the temperature and moisture content becomes stabilized.
How to Avoid the Shrinkage in Plastic Moulding?
While shrinkage tends to happen often, it must be avoided when dealing with some delicate and sterile products, such as medical device injection molding. There must not be any form of distortion in the medical parts to ensure safety in healthcare procedures.
Optimize the Cooling Effects
One of the most effective ways to avoid shrinkage problems in injection molding is to optimize the cooling effects. Cooling channels or plates in the mold design should be used to ensure that the cooling process is uniform throughout the mold cavity. This will help regulate the temperature of the mold and prevent the formation of hot spots, which can cause uneven cooling and shrinkage.
Mold temperature controllers can also help with the cooling effects and maintain a consistent temperature throughout the mold. It is essential to maintain a balance as excessive cooling can cause warpage, which can lead to a defective molded part.
Reduce Wall Thickness Reasonably
Careful design and engineering of the part should be done to ensure that the wall thickness is consistent and optimized for the specific material being used. Thinner walls will result in a shorter cooling time, which will help minimize shrinkage.
However, it is important to note that reducing wall thickness can also cause the part to become more fragile and prone to breakage. Thus, it is essential to strike a balance between wall thickness and overall part strength. Furthermore, other factors such as the mold design and processing parameters can also affect the wall thickness of the molded part.
Reduce the Plasticizing Temperature
Lowering the plasticizing temperature can be done by adjusting the temperature of the plasticizing unit or by using a material with a lower melting point. A reduced plasticizing temperature will decrease the viscosity of the molten plastic, which will reduce the shrinkage caused by uneven cooling.
However, the plasticizing temperature should not be too low as this can cause other defects such as flash or incomplete filling of the mold cavity. It is essential to optimize the plasticizing temperature for the specific material being used to achieve the desired results.
Enhance Injection Pressure Speed
Increasing the injection pressure speed will force the material into the mold cavity more quickly and with greater force, resulting in a more uniform and consistent molded part. However, increasing the injection pressure speed can also cause other defects such as flow lines or burn marks on the molded part.
Therefore, it is important to optimize the injection pressure speed according to the customized plastic injection molding specifications to achieve the desired results.
Improve Back Pressure
Back pressure is the resistance of the molten plastic as it flows through the mold cavity. By increasing the back pressure, the material is forced to fill the mold cavity more completely and with greater force, resulting in a more uniform and consistent molded part.
However, excessive back pressure can cause other defects such as sink marks or voids on the molded part. It is essential to optimize the back pressure for the specific material and mold design to achieve the desired results.