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パーティングラインとは何か?

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パーティングライン

When introducing ドラフト角度, I mentioned that to remove an injection-molded part from the mold, we must design a draft angle. However, the prerequisite is that the parting line must first be determined. It’s similar to needing to cut a watermelon into at least two halves to eat the flesh inside.

In product design, details determine success or failure, and the design of the parting line is one of the crucial details.

A carefully designed parting line can not only enhance the aesthetic value of the product but also, to some extent, determine the product’s lifespan and production cost.

The Meaning of a Parting Line

Parting refers to dividing the mold core of the molding part into several modules. The contact surface between these modules is called the parting line, also known as the parting surface. In a narrow sense, the parting line refers to the parting surface at the largest contour of the plastic part, which separates the cavity and core or the front and back molds. In a broad sense, the parting line also includes the local parting lines of the plastic part (such as the parting lines of holes) and the parting lines of all modules involved in molding (such as sliders, angled lifters, インサート, ejector pins, etc.).

The boundary where the parting line intersects the surface of the plastic part is called the parting line (PL). Because it is impossible to achieve a completely gapless contact between the forming modules, this gap is transferred to the plastic part after molding, forming a flash-like residual mark. Since it looks like a line, it is called a parting line, also known as a mold line.

The thickness of the parting line affects the appearance and function of the plastic part, so pay special attention to it during structural design.

Types of Parting Lines

We generally refer to the parting line at the largest contour of the plastic part as the main parting line, while others are auxiliary parting lines.

Verticle Parting Line

This is a common and simple type, which is a plane perpendicular to the mold opening direction.

Vertical Parting Line
Vertical Parting Line

Stepped Parting Line:

According to the specific situation of some plastic parts, we make the parting line into a stepped shape.

For stepped parting lines, since one side of the cavity bears a greater force, an injection eccentric force is generated on both sides of the cavity, causing relative sliding between the core and the cavity. The sliding direction is shown below. Therefore, first, choose the parting line on the shallow step.

Choose the parting line on the side with the shallower step
Choose the parting line on the side with the shallower step

Arrange two cavities symmetrically to balance the injection force on both sides of the mold, making the mold structure compact.

Arrange the two cavities symmetrically
Arrange the two cavities symmetrically

If the step difference of the stepped parting line is too large, consider designing shut-offs and locally making a stepped parting line.

Design shut-off, localized stepped parting surface
Design shut-off, localized stepped parting surface
Shut-off design on the mold surface
Shut-off design on the mold surface

Some parts have many stepped surfaces. If we design the parting line to follow the stepped shape, it becomes complicated. To simplify the parting line, consider making it a plane, but this results in a mold line on the part’s surface. Therefore, this method is mostly used for internal parts.

PL design for parts with a lot of steps

Beveled Parting Line

The parting line of the molded part is beveled. First, make a sealing along the beveled surface for easier mold polishing, and then level it for easier processing and positioning.

Beveled Parting Line
Beveled Parting Line
Side view of beveled parting line

Curved Parting Line

Similar to the beveled parting line, first make a sealing and then level it.

Curved parting line
Curved parting line

However, not every parting line must extend along the curve. For example, the part shown below, if extended, would form a sharp corner. In this case, the cavity is sharp like a steel knife, and the parting line can be directly extended.

The product extended out, creating a sharp corner

Comprehensive Parting Line

This type combines vertical and curved parting lines or beveled and curved parting lines to form a comprehensive parting line.

Comprehensive parting line
Comprehensive parting line

For comprehensive parting lines, we should pay special attention to the smoothing of the sealing surface at the corners of the two parting lines to avoid forming sharp points. Choose a surface for sealing, which increases mold strength and prevents flash on the plastic part. Smoothing also reduces the appearance of steps and sharp corners, improving the processability of the parting line and allowing for direct CNC加工 instead of EDM machining.

Design Principles of Parting Lines

1. Parting Line Selection Should Meet Demolding Requirements:

The main parting line should be at the largest contour of the part’s projection in the demolding direction. This simplifies the mold structure. Otherwise, it complicates the mold structure, requiring additional mechanisms like sliders.

Sliders and parting lines

2. Parting Line Selection Should Facilitate Smooth Demolding

Since the ejector system of the injection molding machine is on the core side, the parting line should ideally leave the part on the core side after the mold opening. This aids the ejection mechanism on the core side. Setting an ejector mechanism in the cavity side complicates the mold structure.

3. Parting Line Selection Should Ensure Dimensional Accuracy and Surface Quality:

1. For parts with high coaxiality requirements, place the parts with coaxiality requirements on the same side of the mold when selecting the parting line.

For example, the stepped hole in the middle of the image below requires high coaxiality. In the original design, the stepped hole is formed by two cores from the core and cavity. Ensuring the coaxial accuracy of these cores after mold assembly is difficult. In the optimized design, the stepped hole is formed by a single core, ensuring machining accuracy.

The stepped hole is formed using a single core

Similarly, for parts with coaxiality requirements for the outer circle, place the parts with coaxiality requirements on the same side of the mold.

The parts with coaxiality requirements should be placed on the same side of the mold

2. The dimensional accuracy of sizes related to the parting line in the mold closing direction is affected by the parting line’s tendency to expand during injection. In the original design, the size L is difficult to ensure. In the optimized design, the size is unrelated to the parting line, making accuracy easy to ensure.

Design of the dimensions related to the parting line in the mold closing direction
Design of the dimensions related to the parting line in the mold closing direction

3. For parts requiring surface quality, consider the impact of the parting line on the appearance when selecting the parting line.

The choice of parting line should consider the impact of the parting line on the appearance
parting line sample
The parting line is on the edge and become less noticeable or even imperceptible
The parting line is on the edge and becomes less noticeable or even imperceptible

4. Parting Line Selection Should Facilitate Mold Processing

In mold design, the simpler the parting line, the easier the processing, and the more guaranteed the processing accuracy. In the original design, the parting line is at a small fillet, making processing and subsequent mold saving difficult. The optimized design removes the fillet from the parting line.

Keep the design of the parting lines as simple as possible
Keep the design of the parting lines as simple as possible

For multiple shut-offs on the same side, consider combining them into one large shut-off to simplify the parting line structure and facilitate processing.

For multiple shut-offs on the same side, consider combining them into one large shut-off
For multiple shut-offs on the same side, consider combining them into one large shut-off

5. Parting Line Selection Should Facilitate Venting

To ensure good venting of the cavity, set the parting line at the end of the plastic melt flow direction as much as possible. In the original design, the plastic melt fills the cavity and seals the parting line, making it difficult to vent the gas at the cavity’s deep end. The optimized parting line design at the end of the melt flow forms good venting conditions.

Selection of parting lines should be conducive to air venting
The selection of parting lines should be conducive to air venting

6. Parting Line Selection Should Consider Mold Size

The parting line selection affects the mold size. In the original design, the mold height is higher, with a deeper cavity and core, requiring longer processing time and possibly needing a larger injection machine for small parts. The optimized design lowers the mold height, with a shallower cavity and core, making processing easier and reducing the impact of demolding on part size. The downside is that some parts may need a slider, increasing the mold’s width and length.

Selection of parting line should consider the size of the mold
The selection of parting line should consider the size of the mold

7. Parting Line Selection Should Simplify Mold Structure:

Avoid core-pulling (slider) mechanisms for side structures with low precision requirements to simplify the mold structure, reduce mold size, and lower mold costs.

Selection of parting lines should be considered to simplify the mold structure
Take the selection of parting lines into consideration to simplify the mold structure

If core-pulling (slider) mechanisms are unavoidable, choose the shortest core-pulling distance to reduce mold size. Ensure the large core is in the mold opening direction due to high clamping force. Placing it in the side mechanism is unfavorable for demolding.

The choice of parting line should aim to leave the plastic part on the core side as much as possible after the mold is opened
The choice of parting line should aim to leave the plastic part on the core side as much as possible after the mold opening

概要

Although parting line selection and design mainly occur in the mold design stage, structural engineers must consider the overall direction during the structural design stage, especially for parts with easily determined parting lines. If the parting line’s impact on structure (dimensional accuracy, fit clearance, appearance quality, etc.) is considered during structural design, frequent modifications after DFM and 金型トライアル can be avoided.

Other factors may also affect parting line selection, and each parting line scheme has pros and cons. Choose the parting line based on the actual part requirements to meet the current part needs.

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