What is the Parting Line in Injection Molding?
The injection mold comprises a cavity and a core. Parting refers to the division of the mould’s core section into several modules, with the contact interface between these modules forming the parting line (PL).
The parting line also marks the position where the cavity and core separate upon completion of the injection cycle.
Parting lines are unavoidable; all plastic parts will have parting lines.
Due to the impossibility of achieving perfectly seamless alignment between mold modules, the mold gap is replicated onto the plastic part after injection molding, forming a slight, flash-like mark that appears as a line.
The parting surface primarily describes the mold structure, whereas the parting line represents its corresponding manifestation on the plastic component.
The thickness and visibility of the parting line can not only affect the product’s appearance but may also impact its functionality.
In the ASME Y14.8M Standard Castings, Forgings, and Molded Parts, the parting line is defined as the separating interface between molds or mold sections.
When developing 3D models for injection molding, you may not pay special attention to the location of parting lines, but it is very important because parting lines influence the product in multiple ways. So, the layout of parting lines must be reviewed strictly during the structural design phase.
Multiple kinds of parting lines will appear on plastic parts, such as the main parting line between the core and cavity, lifter parting lines, slider (side-action) parting lines, angled-ejector parting lines, and insert parting lines.
If these parting lines lie along the plastic part edge and are coincident with it, their visual impact is slight and can be ignored, but there may still be slight flash.
These parting lines are not directly shown in the product structure drawings. However, experienced injection mold engineers will proactively consider the positioning of parting lines during the design phase, as well as their potential impact on product functionality or aesthetics. So that we can identify potential aesthetic risks at an early stage, particularly when the product’s appearance is formed, and apply recommendations to optimize it.
If potential functional or aesthetic impacts arising from parting lines, design decisions, or adjustments can be made more promptly. This will help prevent delays to the overall development schedule.
Parting Lines Design Principles
The objective of the mold designer is to select a parting line position that ensures both robust and reliable mold construction and minimizes parting line visibility on the finished product at the same time.
When selecting the parting surface, we should minimize dimensional deviations at the parting line edge caused by draft angles. The layout of the parting surface should avoid affecting the product’s appearance, the product must meet the specified aesthetic quality requirements.
Taking a cup as an example, the parting line is generally positioned along the outer rim of the cup body; however, this mark is often barely recognized by consumers on the finished product.
Areas with aesthetic requirements should avoid parting surfaces wherever possible. If the parting surface is unavoidable, it should be placed in the less visible location or at least treated with texture to minimise any marks caused by the parting line.
The overall structure of injection moulds should be simplified wherever possible, minimising the number of parting lines and using straight parting surfaces as the first choice. Meanwhile, we need to take into consideration on mold’s machinability, manufacturability, and the ease of parting. Ideally, parting surfaces should be positioned along the part’s edges or natural transitions.
Mould plates distance: When determining the parting line,we should minimise the opening stroke of both cavity and core to enhance structural stability and reduce mould moving stress load.
Because of the low clamping force of inclined slides during mold closure, for molded parts with large projected areas, the primary parting surface should be positioned on the main parting plane between the cavity and core, while smaller projected areas should be placed on the lateral parting surface. Otherwise, the clamping mechanism of the inclined slide would require large reinforcement; insufficient clamping force may lead to flash or burrs.
The location of parting surfaces must not impede the normal operation of the injection gate system, this is especially important for two-plate moulds with multi-cavity structures.
When molten plastic is injected into a mold, the mold’s shape will influence its flow and distribution within the cavity. Meanwhile, it affects the plastic’s cooling rate and the number of solidification layers formed. Friction between these layers generates the shear rate. If the shear rate is too large, it will lead to stresses within the molded part, we should take careful consideration when evaluating the part’s functionality and strength.
When designing plastic parts, it is essential to remember that the melt typically flows towards the parting line, as this location facilitates the most efficient venting of air and provides the most efficient venting conditions.
To ensure effective mold venting, the parting surface should be positioned as far as possible towards the end of the plastic flow path.
Upon finishing the molding process and separating the mold, the injected part often has marks from the separation of the front and rear mold sections, as well as ejection. Therefore, these factors must be considered in both the mould structure design and the ejection mechanism design to minimise potential adverse effects on the product’s appearance.
Sharp corners should be avoided on the parting line, as they reduce the mold’s service life. Mold designers must thoroughly examine these areas. If these sharp corners can’t be avoided, it is necessary to confirm whether the product structure can be adjusted.
At Yongliang, we provide a DFM (Design for Manufacturability) report recommending suitable parting line locations for your product. You don’t need to conduct your own parting line analysis, confirming these positions before mold production is very important.
Parting Line Design Challenges Caused by Complex Shapes
When a product has complex structures, determining the location of the parting line becomes more challenging.
The position of the parting line determines the mold’s demolding direction, therefore also determining the draft angles directions to ensure smooth ejection.
Parting line position can not only affect the cost of mold but also influence the secondary processes required for parts machining and associated expenses. Parts with complex structure typically incur higher moulding costs.
For injection molding, an excellent product design should feature a clearly defined and well-designed parting line position, and maintain the simplest possible parting line structure.
For parts with complex structures, the parting line can be hidden beneath obvious features such as edges, buttons, or beneath covers. This is an effective method to reduce the visibility of parting line marks. However, areas such as handles still require precise molding to ensure that any protrusions caused by the parting line do not affect tactile feedback or user experience.
Determining front and rear molds: Complex structural areas (such as ribs, boss pillars, etc.) are assigned to the rear mold side in most cases, because these structures remain more securely attached to the rear mold during opening. The product’s main visible surfaces are designed on the front mould side generally.
During product structural design, pay attention to the impact of parting lines on part appearance. Like the lifter parting line, the slider parting line, and the main parting line between the front and rear molds. Taking a charger plug as an example, its case will have an obvious slider parting line. If this parting line is too obvious, it not only affects aesthetics but also shows poor workmanship to the user.
For a charger plug, its case is a straight cylindrical structure with no draft angle on the outer surface. Therefore, a four-sided slider (slide) mechanism is typically required. In this case, the slider parting line is almost unavoidable. The following measures can be taken to reduce its impact:
- Optimize mold design and manufacturing processes to make the parting line less obvious.
- Arrange the slider layout in the mold design to minimise the number of slider parting lines.
- Cover the parting lines through post-processing methods such as spray painting or coating.
- Add decorative grooves in the product design to hide the parting line.
Get Support for Parting Lines
If you use CAD software such as SolidWorks, you may design draft angles as required, and the software can automatically generate corresponding parting lines.
But if you are uncertain about the optimal placement of parting lines, simply upload your 3D CAD model to us. We will recommend the most suitable parting line position based on your product’s molding orientation. Rest assured, we stand ready to provide support if you require it.
When collaborating with us, our DFM (Design for Manufacturability) engineers will determine the optimal parting line position in conjunction with you, based on the structural characteristics of the mold core and cavity.
Project Evidence: Parting-Line Approval
This de-identified DFM view marks the main cavity/core separation so the buyer can review where a witness line may appear before steel is cut.
Review the Parting Line on Your 3D Model
Send the 3D model, resin, appearance zones, critical dimensions and expected volume. We will identify the inputs needed for DFM, tooling and trial planning.