In injection mold design, parting line is a crucial concept. It defines key interface where moving mold and fixed mold contact, seal, then injection molded product and runner are removed. However, part of parting line that bears the most crucial mission cannot be ignored; it is "sealing surface." Understanding and scientifically designing sealing surface is also foundation for ensuring mold quality, production efficiency, and product precision.

 

Sealing surface specifically refers to those precision mating surfaces on mold parting line (or mating surfaces of moving parts such as sliders and ejectors) specifically designed to prevent high-pressure molten plastic from overflowing cavity. Its core functions can be summarized in following two points:
First, sealing and pressure holding to prevent flash (burrs): During injection and holding stages, molten plastic fills cavity at extremely high pressure (typically tens to hundreds of megapascals). Sealing surface must fit tightly under clamping force provided by injection molding machine, forming a closed "pressure vessel" to ensure that melt does not seep out from parting line and form flash. Flash not only affects product's appearance and increases post-processing costs, but may also affect assembly functionality.
Second, precisely defining product contour: Position of sealing surface directly determines shape of parting line (clamp line) on product. A well-designed sealing surface can leave a fine, uniform, and reasonably positioned parting line on product; conversely, it may lead to obvious step differences, sharp edges, or marks that affect appearance.
In short, parting surface solves problem of how mold "separates," while sealing surface solves problem of how mold "seals" high-pressure plastic when it closes. All mating interfaces where plastic leakage may occur, such as main parting surface, contact surface between slider and mold core, and mating surface of inserts, have sealing surfaces.

 

Design of sealing surfaces requires a systematic balance of appearance, function, precision, lifespan, and manufacturing cost. Following are its key design principles:
Principle 1: Prioritize Appearance and Functional Requirements of Product
Appearance Surface Avoidance: Parting line generated by sealing surface should avoid main appearance surfaces of product as much as possible. If this is unavoidable, its location, step direction, and size must be confirmed with customer.
Functional Surface Avoidance: Sealing lines should not be placed on functional surfaces that require sliding, sealing, or frequent human contact, to avoid affecting feel or function.

 

Principle 2: Ensure Smooth Demolding and Mold Strength
Facilitate Demolding: Design should ensure that product remains on moving mold side with ejection system after mold opening. This is usually achieved by adjusting clamping force of product on moving and fixed molds.
Guarantee Strength: Sealing area must have sufficient steel support to avoid weak, sharp "steel sheets," especially at the base of "pillow" (sealing block extended to handle undercuts). Its size should generally not be less than 3mm to prevent breakage under repeated impacts.
Principle Three: Ensure Product Dimensions and Fit Accuracy
Coaxiality Guarantee: For holes or pillars with strict coaxiality requirements, they should be designed to be formed on same side of mold (either entirely in moving or fixed mold). If they are separated by parting surface and formed on both sides, coaxiality cannot be guaranteed due to mold closing errors.

 

Uniform Wall Thickness and Positioning: For box-shaped products and other products requiring uniform wall thickness, precisely positioned "stops" or wedging should be designed around sealing surface to prevent uneven wall thickness caused by misalignment between moving and fixed molds.

 

Principle Four: Facilitate Mold Processing, Assembly, and Maintenance
Simplify Shape: While meeting product requirements, prioritize flat sealing surfaces, followed by inclined surfaces, and avoid complex curved surfaces as much as possible. A flat surface is most conducive to precision grinding and fitting, effectively ensuring sealing performance and reducing manufacturing costs.
Avoid sharp corners: Edges of sealing surface must be chamfered (usually R0.5 or higher), and sharp corners are strictly prohibited. Sharp corners are easily chipped under high pressure, becoming leakage points that are difficult to repair.
Optimize sealing distance: Not the entire parting surface needs to be in close contact. Sealing surface needs a reasonable sealing distance (or sealing width) to ensure sufficient sealing pressure, while remaining areas should be left untreated. This reduces machining area, simplifies mold fitting, and allows space for mold thermal expansion, preventing damage to sealing surface due to excessive pressure caused by thermal expansion.
Typically, sealing distance increases with mold size: 25mm for molds under 600mm, 30mm for 600-1000mm, 40mm for 1000-1500mm, and 50mm for molds over 1500mm.

Intercepting and Sealing Design:
Intercepting: This refers to one mold part obliquely cutting into another part to form a seal. Angle here is crucial. Design specifications require an insertion angle of at least 5°, and at least 7° for precision molds. Absolute minimum safe angle should not be less than 1.5°. An angle that is too small will lead to severe wear on the bevel, rapid failure, and flash.
Sealing: This refers to two parts being sealed together with flat or stepped surfaces. The key is to ensure sufficient sealing distance and flatness accuracy. For large-area sealing, a 10-15mm sealing area is usually sufficient, with remaining area left unused for processing and venting.

 

Pillow Design: Pillow is a special sealing structure formed by partially undercutting a product. Its design must pay attention to following:
Bevel of pillow must also adhere to a slope of at least 3°.
Base of pillow should have sufficient strength.
Sealing distance for pillow is usually around 15mm, with remaining area left unused.

 

Thermal Expansion of Sealing Surface and Mold: Molds expand due to heat during operation. For large molds or molds operating at high temperatures, direction of thermal expansion must be considered when designing sealing surface. Sufficient clearance should be reserved in non-critical areas to prevent "thermal expansion jamming," which could lead to seal damage or mold malfunction.
Special Characteristics of Sealing in Two-Color Molds: In two-color injection molding, part of contour of product body formed in the first injection may need to serve as "sealing surface" for second injection mold. This requires early consideration during product design phase, designing a precise, sealable structure at boundary where two materials meet, placing higher demands on dimensional stability and strength of the first injection product.

A rigorous design process can effectively mitigate risks.
Conceptual Design Phase: Define parting line, assess direction of sealing surface, prioritize using extended surfaces to construct sealing area, and avoid complex curved surfaces.
Detailed Design Phase: Determine width of sealing surface based on mold dimensions and design a reasonable clearance area. For through-hole structures, ensure angle is greater than 7 degrees.
Preparation Stage: Define high-precision, high-surface-finish machining requirements for sealing surface in CNC programming and select appropriate precision machining tools.
Trial Molding and Acceptance Stage: Focus on checking for flash on parting line, signs of collapse or wear in sealing area, and verify sealing effect.
Sealing surface design is also a crucial process in injection mold design, adhering to core logic of "effective sealing, smooth flow, long lifespan, and easy machining." A successful sealing surface design ensures mold consistently produces high-quality, flash-free plastic products throughout its entire lifespan with minimal machining and maintenance costs.