In plastic injection molding mold design, when product has extremely high appearance requirements, cannot have gate marks on the surface, or needs to achieve automated production, three-plate mold (also known as a double parting surface mold) becomes preferred solution. Compared with a two-plate mold, three-plate mold adds a movable intermediate plate (runner plate), and uses a precise tie rod mechanism to control opening sequence of parting surface, achieving automatic separation of runner sprue from product.

As name suggests, a three-plate mold refers to a mold that opens along two parting surfaces during mold opening, forming three main parts: 1. Top Clamping Plate: Fixed to fixed mold platen of injection molding machine, it houses sprue bushing. 2. Runner Stripper Plate (also known as a stripper plate or runner plate): Separates from fixed mold platen under control of tie rods or pull plates during mold opening, used to strip runner sprue. 3. Mold Plates (A Plate, B Plate, and Mold Feet, etc.): These support cavity and core, ultimately ejecting product.

 

Core of a three-plate mold lies in the fact that gating system (runner) and product are located on different parting surfaces. During mold opening, runner automatically separates from product and falls off, while product remains on moving mold side and is ejected by ejector pins, thus achieving fully automated production.

To better understand characteristics of two-plate molds, we compare them with three-plate molds:

To ensure mold opening sequence, three-plate molds add auxiliary mechanisms such as openers/closers, small tie rods, guide pillars are often replaced with large tie rods that also guide and bear cantilever forces.
To ensure that two parting surfaces open in correct sequence, three-plate molds must be equipped with sequence control elements:

 

1. Leader Pin: The most common control method. Leader pin is fixed to panel, passes through middle plate, and is limited by a step. During mold opening, intermediate plate and front plate open first (first parting surface, ejecting runner). After tie rod completes its stroke, moving mold and intermediate plate open (second parting surface, ejecting product).
Functions: 1. Control first and second mold opening strokes; 2. Push stripper plate.
Quantity Selection: Usually four pieces are used; two pieces can be used if mold is too small. When arranging pieces, pay attention to whether it will affect removal of sprue.
As shown in diagram below, escape holes are made in female mold plate to avoid interference between small tie rod and ejector plate. However, if female mold plate is not thick enough, small tie rod can be placed above mold feet to avoid interference.
Additionally, for ease of assembly, heads of small tie rods should be trimmed for clamping.
Diameter Selection

 

 

2. Large Tie Rod (LP)
Function: A: Supports weight of mold platen and stripper plate. B: Provides guidance.
To save costs, when mold base is small, a large tie rod is usually used instead of a guide pillar. Its position is same as guide pillar position in a standard mold base, as shown in Figure 1. When designing molds for larger products, guide pillar must be retained, as shown in Figure 2.

 

3. Nylon Puller: Installed between intermediate platen and moving mold, it uses friction to ensure that the first parting surface is pulled open first.
Function: Transfers the opening force of male mold to female mold platen.
Plastic type Applications: a. Temperature: Used for mold temperatures below 100 degrees Celsius. b. Force is not too large.
Mechanical opening and closing device Advantages: a. Can withstand high temperatures. b. Can withstand considerable force

 

4. Mechanical opening and closing device
Application scope: a. Suitable for molds with high mold temperatures; b. Larger molds requiring high tensile force

 

5. Three-plate mold sprue bushing design
Front end of sprue bushing should be tapered. Front end of sprue bushing should have barbs to facilitate pulling out sprue head. Length of sprue head inside sprue bushing should be minimized. Please refer to molding machine model comparison table for ΦG size selection. If depth (H) exceeds 35mm, then ΦD should preferably be greater than 85mm. Because mobile phone molds are often molded using smaller injection molding machines, ΦK is usually taken as 4.5mm, and SR is usually 11mm.

 

6. Pull Pin Design
Pull Pin Design Method
Function of Pull Pin
A. To detach sprue from female mold plate.
B. During the first mold opening, barbs of pull pin create resistance to separate upper fixed plate and stripper plate.
Design Points
A. Arrange pull pins above injection point.
B. For long sprues or those with curves, add pull pins at bends at intervals.
C. Front end of stripper plate should have a 5° slope.
D. Head of pull pin should be embedded within stripper plate.

 

7. Runner Layout In a three-plate mold, runners are usually arranged between middle plate and moving platen.
·Runner Shape: Usually circular or trapezoidal cross-section.
·Runner Balance: For multi-cavity molds, runner length from main runner to each gate must be equal (geometric balance); otherwise, uneven filling will occur.

 

 

 

 

 

 

Advantages:
1. Aesthetically pleasing appearance: Gate mark is located at the top of product, is small and flat, and has no impact on product's appearance.
2. Automated production: Runner and product automatically separate, eliminating need for manual gate trimming, suitable for fully automated injection molding production, improving efficiency.
3. Flexible injection: Glue can be injected from center of product, which is beneficial for improving weld lines and balancing cavity filling.
4. Low stress: Gate automatically breaks off during mold opening, reducing stress on product caused by gate removal.
Disadvantages:
1. Complex structure: More mold parts, higher machining precision requirements, manufacturing cost is 30% - 50% higher than a two-plate mold.
2. Larger mold opening stroke: Requires a larger injection molding machine opening space (mold capacity), which may be limited by small injection molding machines.
3. Higher waste: Runner system (especially main runner + branch runners) is relatively long, resulting in lower plastic utilization (unless a hot runner system combined with a three-plate mold is used).
4. High maintenance requirements: Wear parts such as nylon plugs and tie rods need to be replaced regularly, requiring high-level mold maintenance.

Three-plate mold is a classic structure in plastic mold design. It solves appearance defects caused by side gates and efficiency bottleneck of manual gate trimming. Through ingenious parting line sequence control and point gate technology, high-quality molding and automated production are achieved. For mold designers, mastering design of a three-plate mold not only means understanding structure of an extra plate, but more importantly, understanding core concept of mold opening sequence control—how to precisely control movement sequence of mold through mechanical structures (tib rods, latches, nylon plugs). This concept has universal guiding significance in design of complex molds (such as stacked molds and three-plate molds with sliders). In practical applications, whether to choose a three-plate mold requires comprehensive consideration: if product has high appearance requirements, large production volume, and high automation requirements, a three-plate mold is the first choice; if product has a simple structure, is cost-sensitive, or injection molding machine's mold opening stroke is limited, then a two-plate mold or hot runner solution should be given priority.