In injection mold industry, there is a "hidden key link" that directly determines appearance, precision, demolding efficiency, even manufacturing cost and lifespan of mold—this is parting surface (PL).
Many novice designers often cause problems such as inclusions, flash, and step differences in molded parts due to improper parting surface design, or difficulties in demolding and complex structures that increase costs. Today, we will combine professional design standards and use simple language to break down core logic of PL parting surface design, compiling this comprehensive guide from basics to practical application. Whether you are a beginner or an experienced designer looking to fill in gaps in your knowledge, this is a valuable resource!

 

In simple terms, to remove molded plastic part or solidified runner from mold cavity, mold must be divided into two or more main parts. Contact surface of these separable parts is what we commonly call parting surface (PL surface).
It's like mold's "dividing line." While seemingly just a simple contact surface, it actually runs through the entire process of mold design, processing, and production, serving as core hub connecting mold structure and quality of plastic part. Even a tiny design deviation can trigger a series of production problems; therefore, mastering scientific design standards is crucial.

Parting surface design is not about "drawing lines based on experience," but rather follows clear principles. These 7 core principles are prerequisite for ensuring design rationality and key to avoiding common problems.
1. Prioritizing Appearance: Maintaining Bottom Line of Plastic Part Aesthetics
Appearance of plastic parts directly affects product competitiveness; therefore, parting line should be prioritized in areas that do not affect appearance, such as product's edges or rounded corners. Direction and size of any creases, steps, or flash caused by parting line must be confirmed with customer in advance to avoid rework. Particular attention should be paid to ensuring that appearance surface (usually front mold surface) strictly avoids interference from parting line.
2. Facilitates Demolding: Reduces Production Operation Difficulty
Smooth demolding is foundation of efficient production. Core principle is to default to having plastic part remain on rear mold side after mold opening, as rear mold side is easier to install ejection mechanisms. During design, side of plastic part with greater clamping force on mold should be placed in rear mold; for plastic parts with simple external shapes and complex internal shapes (such as buttons, shells), external shape should be placed in the front mold as much as possible.
If plastic part has metal inserts, since inserts have no shrinkage, glue area must be designed in the rear mold; at the same time, to facilitate installation of inserts, inserts should be placed as close as possible to PL surface and on moving mold side.
3. Ensure Precision: Strictly Control Part Dimensions and Geometric Tolerances
Precision is a core performance indicator for plastic parts. Parting surface design requires careful attention to two key points: First, structures requiring coaxiality must be molded on same side of mold. If parted on opposite sides of front and rear molds, manufacturing and assembly errors will make it difficult to guarantee coaxiality. Second, for box-shaped plastic parts with wall thickness requirements, a locking mechanism must be used between front and rear mold cores or between front mold core and B-plate to ensure uniform glue thickness.
4. Simplify Structure: Reduce Mold Manufacturing Costs
The simpler mold structure, the lower manufacturing difficulty and cost, and the better production stability. Side core-pulling mechanisms should be avoided as much as possible during design. If unavoidable, core-pulling distance should be minimized, and slide should be designed primarily in rear mold section. Simultaneously, parting surface should not obstruct gating system. For multi-cavity molds with large sprues, parting surface at runner should not have excessive undulations. Furthermore, parting surface should coincide with end of material flow as much as possible to facilitate venting, prevent problems such as air bubbles and material shortages in plastic parts.
5. Facilitates Machining: Improves Mold Making Efficiency
While meeting product requirements, number of parting surfaces should be minimized, following principle of "flat surface > inclined surface > curved surface"—use a flat surface (perpendicular to mold opening direction) instead of an inclined surface, use an inclined surface instead of a curved surface to reduce machining difficulty, improve machining accuracy and efficiency.
6. Avoids Sharp Corners: Ensures Mold Lifespan
Sharp corners and edges should be avoided at parting surfaces as much as possible, otherwise it will reduce mold strength and easily lead to chipping, wear, and other problems. If plastic part has rounded edges, parting surface should be designed along arc normal direction to avoid sharp corners in mold and extend mold's lifespan.
7. Compatible with Equipment: Meets Molding Machine Specifications
Design must be combined with molding machine parameters to ensure that mold opening stroke meets requirements. If plastic part is deep and molding machine's mold opening stroke is insufficient, a hydraulic core-pulling method can be used to solve problem, but it should be noted that this will increase molding costs, and cost-effectiveness must be evaluated in advance.

Besides core principles, these 3 details are easily overlooked and directly affect quality of parting surface design:
Interception Draft: Insertion draft of PL surface should ideally be designed to be above 3°, with a minimum of 1.5°. If customer's product cannot meet this requirement, it must be raised and confirmed in advance.
Sealing Area: To prevent PL surface from chipping due to impact, sealing area must have sufficient area (recommended 5mm or more). Sharp corners or lines for sealing are strictly prohibited.
Selection Order: Strictly follow order of "flat surface → inclined surface → curved surface," prioritizing parting surface types that are easy to process and have high stability.

 

Different types of plastic parts require different parting surface processing methods. Below are practical solutions for 12 common parting surface types, covering most application scenarios:
1. Flat PL Surface
If flat surface is perpendicular to mold opening direction, it can be directly extended into a PL surface. Note: When using UG automatic mold parting, slight undulations (a few micrometers of change in height) may appear at corners, affecting mold closing accuracy. These should be replaced with flat surfaces for easier grinding.
2. Stepped PL Surface
This design must be approved by customer. Slope of pillow position should be ≥3°, with a minimum of 1.5°. Three options are available: Option A (PL lines follow product outline, minimal impact on appearance but higher processing difficulty); Option B (reduces number of steps, some visible lines but lower processing difficulty); Option C (PL is made flat, simplest processing but significant impact on appearance). Option C is preferred.
3. Angled PL Surface
Three requirements must be met: First, sufficient sealing distance must be provided (5-12mm for molding machines of 100 tons and below; 12-20mm for molding machines below 350 tons; 20-30mm for molding machines of 350 tons and above); second, two flat positioning points must be reserved for easy processing and mold closing; third, a stop must be provided in original body or inserted into mold plate (5°~10°) to prevent mold closing slippage and injection offset.
4. Curved PL Surface: Processing method is same as for inclined PL surfaces. Core is to ensure sealing distance, reliable positioning, avoid burrs and misalignment.
5. Composite Curved PL Surface: Principle is "simple and smooth." Corners are smoothly connected using scanned surfaces or mesh surfaces to minimize steps and sharp corners, ensuring direct NC machining and avoiding EDM to reduce costs.
6. PL Surface for Shaft-like Parts: Both sides need to be cut 0.1mm to create a flat surface to prevent burrs from affecting shaft's use. Front and rear molds need to be positioned and machined together to avoid misalignment affecting accuracy.
7. PL Surface with Hole on the Front: There are three processing options: Fully retain front mold (no surface marks but prone to sticking to front mold); Fully retain rear mold (can be used for inserts but ejection is difficult; 0.05mm step difference around circumference needs customer confirmation); Half front and rear molds (needs to balance appearance and machining difficulty). Choice can be made according to product requirements.
8. Side Insertion Hole PL Surface
In principle, insertion angle should be ≥3°, and at least 1.5° if limited, to ensure smooth demolding and avoid burrs.
9. Top and Bottom Insertion Surfaces PL Surface
Use side insertion holes instead of main insertion holes whenever possible. Insertion angle should be ≥3°, and at least 1.5° (height difference ≥0.2mm). Prioritize reserving a flat surface of at least 0.5mm; even if burrs occur, it will not affect product use and reduce correction costs.
10. Snap-in Surface PL Surface
PL position should avoid snap-in functional area: If PL position is unreasonable, burrs are likely to occur, dimensions are difficult to guarantee, and use will be affected; a properly designed PL position will not affect function even if burrs occur, and no additional correction is required.
11. PL Surface Steps and Direction Treatment: Amount and direction of step changes affecting appearance, function, and dimensions must be confirmed by customer. Design corrections should be 0.02-0.1mm based on product size. When selecting a step method, visual orientation of product, paper feed direction, rotation and sliding direction, contact surface, and PL surface position must be considered (avoid sharp corners on large surfaces).
12. Pillow Position Treatment: Slope, treatment method, and PL position of pillow position must be confirmed by customer before implementation. Slope should be ≥3°, at least 1.5° (height difference ≥0.2mm). Sealing distance should be selected according to molding machine tonnage (same as sloped PL surface standard). There are three treatment options: reduce glue to create steps (standard solution); create a sliding position (suitable for scenarios with high appearance requirements); partially cancel rounded corners (not recommended, only used in special cases).

Essence of PL parting surface design is finding a balance between "product quality," "mold cost," and "production efficiency"—meeting appearance, precision, and functional requirements of plastic part while simplifying mold structure, reducing processing difficulty, and adapting to production equipment to ensure smooth production.
For mold designers, mastering 7 core principles is fundamental, memorizing practical solutions for various parting surfaces is crucial, paying attention to details such as insertion draft angles and sealing areas is essential for avoiding problems. Hopefully, this guide will help you clarify your design thinking and avoid detours!