As a designer working with injection molded parts, you’ll come across many mold designs, seams, and parting lines. While some of these design anomalies are avoidable, many require careful understanding to place them carefully around the design.
In injection molding, the parting line is the remnant of the small gap where two mold halves meet. Parting line is not a design flaw, as it facilitates functions like mold opening, ejection time, and getting rid of air pockets.
In this article, you’ll learn how to design the parting line in injection molding, the key functions it serves, and a small tip at the end when selecting the right type of parting line.
What Is a Parting Line?
A parting line is made when two mold halves meet. This is often called mold parting line and is the result of a small gap where material flows and solidifies leaving behind a line along the length of the gap.
Almost all injection molded parts have a parting line and many times designers have to accomodate it into their designs because there’s no other way to manufacture injection molded parts.
The parting line injection molding process can be explained using the mold halves. In injection molding, a mold consists of a fixed half and a moving half. The moving half closes during the injection molding process and air expands the plastic inside to fill the mold.
Between the two mating halves, there’s a small surface gap which creates the parting line along the length of the part where mating halves meet.
Difference Between Parting Line and Seam
Plastic parts are made by different processes like blow molding, extrusion, and 3D printing. All of these processes leave behind some sort of imperfection.
Seam: Forms from parting line, but also forms by mold misalignment and material leak
Parting Line: Designed as part of the mold design
Seams can result in injection molded products from parting lines and have no fixed location. Any improper material flow, minute misalignment or temperature change can result in a seam.
Injection molding parting line is carefully designed with the product. The parting line determines the mold opening and part release. This affects the efficiency of the injection molding process and part production.
Designing parting lines without addressing optimal locations can lead to early part failure, crack upon impact, and a lower strength in a particular direction.
What Are Parting Line Types?
You’d expect parting lines to be straight and always along the middle of the part. However, parting lines depend on the shape of the part.
1. Vertical Parting Line
Vertical parting lines are also called straight parting lines and the parting line location runs perpendicular to the mold opening direction. This means the two halves of mold open left to right and the parting line is vertical.
Vertical parting line is great for cylindrical parts, parts with tall heights, and symmetrical parts.
2. Beveled Parting Line
Designers use a beveled parting line when the part has complex slanting features and slopes. A beveled parting line is nothing more than a parting line that follows an inclined slope.
Beveled slopes in mold design reduce sticking chances and ease ejection. The parting line is also less visible and reduces flash. You’ll find beveled injection molded products in car interiors like a sloped dashboard.
3. Curved Parting Line
Many times a part design has curves and whenever the two halves of a mold body meet along a curved path, a curved parting line is formed. This type of line in injection molding is reserved for designs that require flexibility.
Curved parting line improves design aesthetics by following contours and resulting in a more natural finish. Curved parting lines can be placed in non-critical areas allowing for mold design flexibility.
4. Stepped Parting Line
In injection molding, the mold doesn’t need to be symmetric or open at the same depth. The mold structure can have steps, which results in a stepped edge at the parting line.
When working non-symmetric molds, there may be more material flow on one half of the mold resulting in a heavier side. A wedge-shaped insert on this side can counter the weight
5. Comprehensive Parting Line
When you combine different parting lines in injection molding into your mold body and mold making strategy, you get a comprehensive parting line. Such parting lines are generally used in complex designs to eliminate extra parts and combine the functions of multiple parts into a single part.
Designing a Paring Line In Injection Molding
Parting line location and more importantly designing the parting line in the product influences the molded part’s quality. This section provides a detailed technical breakdown of the design considerations for parting line injection molding.
Product Requirement
Injection molded product shape and features will identify the parting line. The geometry and part complexity can tell you the best parting line to use. Some considerations to pay attention to are:
B-side (ejector half) contain part features like ribs and bosses, so the parting line location is largely influenced by the B-side of the mold.
Protruding features including raised surfaces and undercuts require careful placement that avoids core pulling (when the core is removed during injection molding).
Draft angles: In most cases, a draft angle of maximum 3° is ideal for ejection.
Parting Line Placement
| Line Type | Ideal Use | In Manufacturing |
| Vertical Parting Line | Symmetrical parts | Easier mold design, lesser misalignments |
| Stepped Parting Line | Complex parts with different depth | Exact mating surfaces requirement |
| Curved Parting Line | Flexible shapes with contours | Reduces parting line visibility |
| Beveled Parting Line | For sloped designs | Reduces parting line flash |
Mold Components
The internal features of a part are often made using cores that need to be removed during the injection molding process or after it. Mold components like core plates and cavities also determine the suitable parting line design.
Core: the mold core and cavity should align perfectly to avoid flash formation.
Mating surfaces: The parting surface should align shut and be tight enough to form a sealing surface between the mold halves
Surface Finish
Surface finish matters in injection molding because often injection molded parts are assembled. Take the example of Legos, if there are protruding features and uneven surfaces, the Lego parts don’t fit!
Surface finish ensures visual quality and functionality. A well-placed parting line reduces secondary operations like hand trimming.
Ejector pins can leave marks on the part’s surface, so you should avoid placing them on the front for aesthetic designs. If your part needs additional texture, you can use media blasting for a matte finish and at the same time reduce any seams.
Core pulls and mold moves can result in surface imperfections. You can easily fix that with tight tolerances and proper venting.
Why Does Parting Line Location Matter?
A bad parting line placement can lead to many failures and more costs. Not to mention the part defects during manufacturing. Plus, it also affects the performance of the injection molding machine, like changing the clamping force and cycle time.
Most importantly, placing the parting line ensures structural integrity. Adding parting lines to stress areas can lead to breakage and part failure.
Incorrect parting line placement can create undercuts and excess material flow through gaps, adding to trimming costs. Poor alignment can cause warpage leading to shrinkage issues (a common issue in the injection molding process).
It also affects the ejection of parts and poor parting lines can lead to sticking of parts and incomplete part removal, which can damage the next part.
Conclusion
Parting lines are an unavoidable feature of injection molding design. When addressing parting line issues, designers also pay close attention to mold material, tolerance, and core placement. The most common issue with using the wrong parting line is flash runoff, which is extra material flowing out of the mating surface gaps.
You can use this guide to design injection molding parts with correct parting lines and understand the different types of parting lines.
FAQs
What is meant by parting line?
A parting line is the small line that runs along the length of a part and is formed when two mating parts join.
What are the three types of parting lines?
The three common parting lines are vertical, beveled, and curved parting lines. If you use a stepped parting line and other parting lines in combination you get comprehensive parting line
How to determine parting line?
Choosing the right parting line depends on the part geometry, shape, core features and mold design.
What is a parting line in a mold?
A parting line separates the two mold halves.
Why is the position of parting line so important?
A well placed parting line removes the risk of part failure, warpage, and flash in injection molding.