Even minor errors can cause physical defects in a part during the injection molding process. These defects may damage the part’s structural integrity or make it less physically appealing, with both problems likely leading to the part being unsaleable. Injection molding sink marks are an example of such a defect. This article examines what these marks are and how manufacturers can prevent them.
What Are Injection Molding Sink Marks?
Injection molding sink marks are small depressions or craters that tend to develop in the thicker areas of an injection molded part. They typically occur due to shrinkage, which affects the inner portions of the product. These marks have several potential causes, meaning solving them is rarely a simple task.
Possible Causes of Injection Molding Sink Marks
The causes of injection molding sink marks can be broken down into five main categories:
Poor Part Design Optimization
Issues with Machines or Processes
Mold Design Problems
Poor Part Design Optimization
In an ideal world, parts would be designed with uniform wall thickness. When this isn’t the case, the designer must account for varying wall thickness. Failure to do so by building walls that are too thick in the part can lead to the development of sink marks. The melt can’t flow as consistently into these areas as it can to other areas of the part, leading to visual defects.
Issues With Machines or Processes
Several issues with the machines used or the injection molding process implemented can cause sink marks:
Sink marks may occur if the second stage of the process, often called the pack and hold time, is too short. Though pressure may be correct in this scenario, the short time means the part’s gate doesn’t get sealed. Without the seal, the melt may exit the part, leading to the creation of sink marks.
Cooling of the melt must be sufficient to prevent sink marks. If the barrel temperature is too high, the melt enters the mold at a higher temperature, leading to cooling inconsistencies in thicker areas of the part.
If a part takes too long to cool, the possibility of sink marks increases. Adjusting the part’s wall thickness can aid in faster cooling. So too can proper regulation of cooling by ensuring runners, nozzles, and barrels are maintained at sufficient temperatures.
If the injection speed is too fast, manufacturers may find they end up with inadequate pressure levels. Increasing packing pressure can solve this issue, as can slowing down the melt flow.
An incorrect flow rate is usually to blame if the materials used in the melt result in the creation of injection molding sink marks. This is particularly the case if the marks appear far away from the gate. The problem here is that an incorrect flow rate hinders pressure transmission. Solving the problem usually requires the manufacturer to expand the gate, thus eliminating bottlenecking issues that hinder flow.
The operator may be the cause of sink marks if they allow inconsistencies in the process cycle. Such inconsistencies can include failing to maintain appropriate machine and mold temperatures or allowing for variances in the time taken for the injection molding process. Ideally, the molding process should run automatically, with the operator only intervening in cases of error.
Mold Design Problems
Several mold issues may lead to sink marks developing:
Improper gate placement is a common cause of sink marks. Typically, this is the issue if the mold has an insufficient number of gates or if the main gate is placed away from the part’s thickest wall.
If a junction forms between a mating wall and another wall, the secondary wall should be between 60% and 70% of the mating wall’s thickness. If it isn’t, shrinkage can lead to sink marks developing.
Material suppliers often provide data for gate and runner dimensioning. Failure to follow these directions can lead to the manufacturer having smaller gates or runners than required.
Failing to maintain a balance between rib and wall thickness can be a problem. If the rib is too high relative to the thickness of the wall, sink marks may occur.
How to Prevent Injection Molding Sink Lines
Thankfully, manufacturers can use several techniques to prevent the formation of injection molding sink marks.
Adjust Pack and Hold Times
Pack and hold pressures are one of the most common causes of sink marks. Increasing the pack and hold pressure in the cycle may help to increase the amount of material sent to the thicker sections of the mold. Getting enough material into these areas with enough pressure ensures the material’s molecules don’t pull on themselves and get rid of the sink lines.
Change Melt Temperature
Ensure the melt temperature is within the material manufacturer’s recommended range. If the temperature is too high, the melt takes longer to cool, and sink marks can form.
Adapt the Mold Design
Ideally, all areas of the mold should allow for the creation of a part with nominal wall thickness. However, this isn’t always possible if a part needs thicker walls. In these cases, creating multiple thinner sections in the thick area or coring out the thick wall can solve sink mark issues.
Corners are also a problem with injection molding. Rounding the inner and outer corners prevents increased thickness from occurring due to the joining of two walls.
Avoid Excessive Mold Temperatures
If a mold’s temperature is too high, the gate may take longer to seal. As mentioned, a gate that doesn’t seal presents an opportunity for melt to exit the mold, increasing the possibility of sink marks. Manufacturers can contact the material manufacturer to discover the ideal
mold temperature range they should abide by.
Balance Rib and Wall Thickness
Melt follows the path of least resistance, meaning it will fill the thicker wall sections before moving to thinner rib sections. The thicker wall sections cool faster, causing sink marks to form if the rib sections are too high. Avoid making a rib height that is more than three times the thickness of a part’s wall. The following diagram demonstrates how this can occur:
Use the Seven Degree Rule
Implement a seven-degree slope at the base of any ribs built into the part’s design. This allows the mold to pack the melt more uniformly, thus preventing surface blemishes link sink marks.
Adjust the Boss Design
Bosses are reinforced posts that hold screws or inserts. Incorrect boss design can lead to sink marks because each boss represents additional mass. Following these steps ensures proper boss design:
Make the boss’s wall thickness equal to the hole’s inner diameter
Use the seven-degree rule at the base of the boss
Don’t place bosses directly against outer wall sections
Conduct a Thickness Analysis
In their study Visualization of potential sink marks using thickness analysis of finely tessellated solid model, Masatomo, I, et al, proposed an interesting method of extracting sink marks from a part’s surface.
Using finely tessellated polyhedral models of the part, the researchers found that the amount of shrinkage likely to occur is proportional to the part’s thickness. Using the sphere method, they calculated the thickness of the polygons in the tessellated model, allowing them to extract sink marks. The study is found at the above link and may represent a novel way to counter sink marks.
Use External Gas-Assisted Injection Molding
External gas-assisted injection molding (EGAIM) is often used to reduce sink marks in amorphous polymer parts. However, Sheofei, J, et al, discovered that it can also be used when working with crystalline polymers in their study Reducing the Sink Marks of a Crystalline Polymer Using External Gas-Assisted Injection Molding.
Prevent Sink Marks
Injection molding sink marks are visual defects that affect a part’s aesthetic and structural qualities. By taking steps to remedy the issues causing these marks, manufacturers make products that offer greater commercial viability and increased quality.