What, then, is a parting line in injection molding? It is a visible border or seam on a plastic part that has been processed. This line occurs at the edge of the two halves of the injection mold, where they meet. These two parts are known as the core and the cavity.
This feature is simply a standard part of the Kunststoff-Spritzguss technique. It is not regarded as a defective one. But it is crucial to determine its location and its aesthetic appearance. The skill to manage parting lines is the main thing that differentiates good from poor part design. It can have a significant impact on the appearance of the product and its production cost as well.
This handbook covers the following:
* The parting line is a critical design feature.
* The variety of parting lines.
* How to find the best position for a parting line?
* Common parting line issues and solutions.
Why the Parting Line is A Predominant Design Feature
Parting line is seen by many as just a small trace left on the part. However, the engineers and designers see it as a serious matter of placement. This choice, in turn, has a huge effect on the final product like cost, quality, and even its functionality. Shortcomings in the arrangement of parting lines can even lead to serious malfunctions.
The effect on the Part’s Appearance
The parting line directly contributes to the appearance of a product, be it good or bad. It is quite paradoxical, but a visibly seam in a well-designed smooth-aesthetic part may impair it.
It can be applied also to disrupt a textured finish. An intelligently conceived design places the parting line along a sharp edge or a less visible part. This way, the main face surfaces stay clean and aesthetic for the end-user.
The Impact on Structural Integrity
The injection molding parting line can be positioned in such a way that a weak spot is created. If the line is at a point of high stress, it could result in a part being weaker.
Also, the part is where a defect of “flash” may occur. Flash is a thin layer of excessive plastic which is released from the mold. This can, in turn, lead to the parts not fitting right or not being sealed properly.
Direct Effect on Mold Cost
Parting line complexity is a significant factor in tooling costs. A parting line that is flat, non-overlapping, is the easiest and cheapest way to make a mold. If it is more complicated, such as a parting line that is on a curve or has a step, then the making of the injection mold will involve more difficult and expensive processes.
These complex forms must be cut very accurately to close properly. Thus, a minute revision in design intended to simplify the parting line structure may lead to large financial benefits. The parting line should be visually noticeable to different extents. Managing this is definitively a quality and cost issue.
Deciding Part Removal
The parting line is a “line of draw.” It is the direction that the mold takes to eject the part.
The location of the line directly affects the facilitation of part ejection. A poor placing may bring undercuts into the play. These are parts of the mold which retain the part within the mold. This often results in additional complexities, such as the necessity of installing side-actions or lifters to the mold where they were not initially designed.
Types of Parting Lines: Ages of Complexity
Parting lines come in varying patterns. Their contour matches the part shape. Choosing the right one is a significant part of the design process. The main types of parting lines include: vertical, stepped, inclined, curved and integrated parting lines.
Flat or Straight Parting Line
This is the most simple and the most common type of parting line. The two halves of the mold are joined at a single flat plane. It is the cheapest method and the easiest to make; it works best for simple shapes like a box or flat lid.
Stepped Parting Line
A stepped parting line is made up of two or more flat planes that run parallel. It has the appearance of stairs. This design is applied when the part has any steps or offsets. It helps to avoid replacing expensive side-action.
Angled or Beveled Parting Line
In this case, the meeting of the two mold halves takes place at an angle. It is mainly used in matching an angled edge on a part. An angled parting line can help to reduce the formation of a very thin sharp edge of plastic. This thin edge would be weak and easily break.
Curved or Complex Parting Line
This kind of line is shaped like a winding creek following a curved, non-flat path. For complex organic shapes, a line of this kind is essential. Think of detailed parts like a computer mouse or a toy figure. Curved or complex parting lines are also the trickiest and the most expensive type to create. The two mold halves must match the curved surface exactly; otherwise, it is impossible to achieve the required part.
Smart Parting Line Placement: A Decision-Making Framework
Choosing parting line in injection molding is a real balancing act. You have to consider how the part will appear, how it will function, and how much it will cost to produce. Using an organized approach will lead to the best outcome.
The Core Trade-Offs
The ideal parting line is very often a trade-off. Full hiding may require a very complicated mold which increases costs. A cheap and simple solution may make it placed on a visible area, going harm it. The key is to find a reasonable compromise based on your unique project.
A 4-Step Checklist for Best Placement
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Find the Line of Draw: You need to decide on the line of draw — which direction the mold will open. This is the basis for all other decisions. The parting line should be on the part so that it is pulled out of the mold along that line.
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Look for Undercuts: Supply the features of the part that are terminal in the cavity. These are called undercuts. The parting line must be positioned to either avoid them or allow them using side-actions.
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Focus on Pretty Surfaces: Try to place the parting line on a surface that won’t be seen. If that is not possible, hide it along an existing sharp edge, a corner, or where two surfaces meet. This makes it much less noticeable.
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Make it Simple to Build: Always ask the question, “Can we simplify this?” A flat line is better than a stepped one. A stepped line is better than a curved one. Making the line simpler directly cuts the cost of the mold tool. This often depends on high-precision China CNC-Bearbeitungsdienstleistungen.
Parting Line Placement: Trade-Off Analysis
| Priority | Best Parting Line Strategy | Potential Compromise |
|---|---|---|
| Looks | Hide on a sharp edge or non-pretty surface. | May need a more complex and costly mold. |
| Funktion | Place away from sealing surfaces or mating features. | May be more visible on the part. |
| Low Cost | Keep the line flat and simple. | May hurt the part’s looks or function. |
Fixing Common Parting Line Problems
Even with good design, problems can happen during production. Knowing how to spot and fix common defects related to the parting line is a vital skill. This knowledge comes from hands-on experience in a molding facility.
Problem 1: Flash
Flash is a thin, unwanted film of plastic that escapes the mold at the parting line. It looks like a paper-thin sheet sticking off the part.
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Common Causes:
- The molding machine’s clamp force is too low to hold the mold shut.
- The parting surfaces on the mold are worn, damaged, or dirty.
- The plastic is being injected too fast or at too high a pressure.
- The mold is not vented properly. Trapped air forces the mold open.
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Solutions:
- Increase the clamp tonnage on the machine.
- Clean or repair the mold’s parting surfaces.
- Reduce the injection pressure or speed.
- Check and clean the mold vents.
Problem 2: Mismatch
Mismatch is when the two halves of the part are not lined up correctly. This creates a noticeable step along the parting line.
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Common Causes:
- The core and cavity halves of the mold are not lined up in the machine.
- Guiding parts in the mold, like leader pins, are worn out.
- The mold is heating or cooling unevenly. This causes parts of it to expand differently.
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Solutions:
- Re-align the mold halves on the press.
- Replace worn alignment pins and bushings in the mold.
- Make sure the mold temperature control system is working correctly.
- This issue is often related to how the draft—an angle that’s applied to vertical walls to aid in part removal—changes direction on either side of the line. If one side is off, it can cause a visible shift.
Conclusion: Getting High-Quality Parts Through Smart Design
The parting line is much more than just a mark. It is a basic part of the design for manufacturing process. Understanding what the parting line in injection molding is, and how to plan for it, is essential for creating high-quality, cost-effective plastic parts.
Smart planning needs balancing looks, function, and tooling cost from the very beginning. A well-placed parting line leads to a better product and a smoother production run. The best way to avoid costly mistakes is to work with an experienced partner early in your design process.
For expert guidance on your next project, explore our complete manufacturing solutions at Mekalit.
Frequently Asked Questions (FAQ)
1. Can you completely get rid of a parting line in injection molding?
No, a parting line is an unavoidable result of using a two-part mold. However, how it looks can be minimized. Through careful design and precise mold making, it can be hidden on a sharp edge or textured surface. This makes it almost impossible to see or feel.
2. What is the difference between a parting line and a parting surface?
The parting surface is the entire face of the mold where the core and cavity meet and seal. The parting line is the specific edge on the final plastic part that is formed where the parting surface touches the part’s shape. The line is on the part. The surface is on the tool.
3. How does draft angle relate to the parting line?
The parting line is usually where the draft angle changes direction. Draft is a small angle added to the walls of the part. On one side of the line, surfaces are drafted to release from the cavity. On the other side, they are drafted to release from the core. This is critical for removing the part without damage.
4. Does the type of plastic material affect the parting line’s looks?
Yes, it has a big effect. Softer materials like TPE are more likely to create flash if the mold doesn’t seal perfectly. The color and finish also matter. A parting line is often more obvious on a dark, glossy part than on a light-colored, textured part.
5. What is a “shut-off” in relation to a parting line?
A shut-off is an area where the core and cavity of the mold press tightly together to form a hole or opening inside a part. These surfaces must seal perfectly to stop plastic from flowing into the hole. While they are part of the overall parting surface, shut-offs specifically create internal features, not just the outer edge of the part.
