Sink marks are some kinds of depression on the surface of the plastic part due to the shrinkage of the material underneath. Sink marks most commonly occur on a surface that has ribs, bosses, and other voluminous features on the rear side of the moulding. All plastics shrink as they freeze off, some more than others. One of the things you can do during the design phase is to identify where these injection molding sink marks are going to be and choose a material that doesn’t shrink as much. Or you can even add something to the plastic material like glass fibers or you can put talcum powder in and it will mitigate the shrinkage quite a bit.
How to prevent sink marks in injection molding?
Sinks are common injection molding defects that occur in thicker sections of moulded parts. Since thicker sections contain more plastic, they will naturally take longer time to cool. The outer portion of the part that is in contact with the mold steel cools much faster than the internal part. As the molecules in thicker sections begin to contract, they pull in on themselves, leaving a defect known as a sink. Although they may be unavoidable in some cases, we think we can offer useful suggestions based on our expertise in mold manufacturing.
Ensure Proper Part and Mold Design
Part geometry and mold design are like the blueprint for your injection molding project. Similar to how architects need to consider the purpose and function of a building when drawing up plans, injection molders also must consider the ultimate purpose and functionality of the plastic parts. But here’s the catch — failing to design the part and the mold correctly could lead straight to a sink mark disaster. For instance, incorporating thicker wall sections without appropriate considerations for cooling can result in localized shrinkage. Consequently, it may lead to sink marks in the injection molded part. Complex part geometries can also compound the potential for shrinkage and the formation of sink marks. It’s like attempting a multi-level architectural marvel without considering the weight distribution — can lead to structural instability.
Sink marks can also occur due to part and mold design. Because sink marks often occur in thicker sections of a part, the best approach in part design is to create nominal wall thickness, but this is not always possible. If the part is in the design phase, the thickness can be reduced by coring out those sections or by creating multiple thinner sections.
Corners can be problem areas too, since the joining of two walls can lead to increased thickness. Rounding outer and inner corners can help maintain constant wall thickness and eliminate sink.
One of the critical aspects of mold design in relation to sinks is water line placement. The thicker sections usually get adequate cooling at the part surface, however the inner portions of the thick area are often neglected. Getting water to inner portions of the thick section may be challenging but can improve sinks, part quality, and possibly cycle time.
There are 6 methods available to prevent sinks in your injection molded part. Remember, the goal is to provide uniform cooling to the area of the part where the sink occurs.
1. Check Melt Temperature
The melt temperature can affect the viscosity of the plastic and the final product’s molecular weight, and if the melt temperature is inconsistent or is not uniform during injection, it can result in part defects. In processing, one of the first things we want to verify to correct sinks is melt temperature. The melt temperature should be within the resin manufacturer’s recommended range, and can be adjusted to affect the properties of a part. For example, if it is too high it will take longer to cool the parts, leading to sink marks. If you slightly decrease the melt temperature of certain resins, such as polypropylene, the finished product will have higher impact resistance, a shorter mold cycle time, and the molding process will consume less energy.
For plastic injection molders, calculating the ideal melt and mold temperatures is pivotal to achieving optimal part performance. For most resins, the melt is lower than the mold temperature, which acts together to streamline the cycle time, reduce costs, and generate a sturdy and reliable product. Inexperienced molders may see higher melt temperatures as a means to reducing resin viscosity and increasing production. If these two elements are not working in conjunction with one another, they can lead to resin degradation, increased energy consumption, and prolonged cooling times.
2. Avoid Excess Mold Temperature
Mold temperature is another factor that can influence the formation of sink marks. Excessive temperature can cause the plastic material to cool and solidify slowly, leading to differential shrinkage and sink marks. It’s like trying to cool a hot pie in a warm room; it’s just going to take longer.
On the other hand, a low temperature of the insert can lead to a rapid cooling rate, which might not give the molten plastic enough time to fill the thicker sections of the mold, leading to sink marks. The goal is to maintain an optimal mold temperature that allows the plastic to cool at a rate that minimizes the risk of sink marks.
3. Mold defects
If sink marks occur far away from the gate, it is generally due to poor flow of melt in a certain part of the mold structure, which hinders pressure transmission. In this regard, the structural size of the mold gating system should be appropriately expanded, especially for the “bottleneck” that hinders the flow of the melt, the sprue section must be increased, and it is best to extend the sprue to the recessed part.
For thick-walled plastic parts, wing type sprue should be preferred. In this way, for the scene that plastic parts are not suitable for setting the sprue on the plastic part and the plastic parts that are prone to residual deformation at the gate after molding, it is a method that attach a wing-shaped sprue on the plastic part and set the sprue on the wing. Thereby transferring the concave defects of the plastic parts to the winglet, and the winglet will be cut off after the plastic parts are formed.
4. Use unmatched Raw material
If the plastic raw material does not meet the molding requirements, the shrinkage rate of the molding material is too large or the flow performance is too poor, and the lubricant in the raw material is insufficient or the raw material is wet, the surface of the plastic injection molding part will produce depression and shrinkage marks. Therefore, for the plastic parts with high surface requirements, low shrinkage resin grade should be selected as far as possible.
5. Ribs and Bosses
The areas of the part design that are close to ribs and bosses are the most common places where sink marks occur. Bosses are important features, of course, because they’re used as support structures of inserts or self-tapping screws that hold assemblies together. Use these design tips to help optimize your use of bosses.
6. Adjust Pack and Hold Time
In the world of injection molding, pack and hold time and pressure are like the secret ingredients in a recipe. They play a critical role in the process and can significantly impact the formation of sink marks. Pack and hold time is the period during which the molten plastic is packed into the mold cavity under pressure to compensate for shrinkage as the material cools. It’s like the time you leave your cake in the tin after it comes out of the oven to let it set. If the hold time is too short, the molten plastic might not have enough time to fill the thicker sections of the mold, leading to sink marks. On the other hand, if the hold time is too long, it can lead to over-packing and excessive internal stresses, which can also result in sink marks. Now, let’s talk about holding pressure. It’s like the pressure you apply when you’re icing a cake. Too little pressure, and you might not cover the whole cake. Too much pressure, and you might end up with a mess. In injection molding, insufficient hold pressure might not compensate for the material shrinkage, leading to voids and sink marks. Conversely, excessive hold pressure can cause over-packing, leading to war-page and, again, sink marks.
So, it’s all about finding the right balance. The key is to apply the proper packing pressure and maintain the optimal hold time to ensure the mold cavity is filled completely and uniformly, thereby minimizing the risk of sink marks.
Conclusion
Let’s wrap things up. Sink marks in plastic injection molding can be problematic, but as we’ve learned, they’re not inevitable. A little bit of attention to materials, process conditions, and most importantly, part and mold design, can prevent these little buggers from showing up in your plastic parts.
Whether you are making sure that your molten plastic flows smoothly into the mold cavity or adjusting rib and wall thickness, these factors can significantly impact the outcome. By adopting best practices like incorporating a gradual slope at rib bases, optimizing boss design, and applying Design for Manufacturability (DFM) principles, you can avoid sink marks.
The effort to avoid sink marks is not merely for cosmetic reasons; it’s important for preserving the structural integrity of parts, decreasing production costs, and protecting the reputation of your brand. Armed with the insights we've discussed, you'll be set to achieve high-quality injection molded parts that are free from sink marks.
