Injection molds, also known as plastic injection molds, are production tools used for repetitive, high-volume production of plastic parts or products. These molds rely on one or more cavities formed after molded parts are assembled to create desired part shape. Injection molds are the most complex of all plastic molds, possessing the highest precision in design, manufacturing, and processing, and are the most widely used.

 

Classification of Injection Molds
Injection molds can be classified in many ways. Based on basic structure of injection mold's gating system, they can be divided into three categories: two-plate molds, also known as large-gate molds; three-plate molds, also known as fine-gate molds; and hot runner molds, also known as runnerless molds.

Two-plate molds are commonly known as large-gate molds or single-parting-surface molds. A typical structure is shown in Figure 1-1. Two-plate molds are the simplest and most commonly used type of injection mold. They are divided into a movable mold and a fixed mold, separated by a parting surface.

 

Notes on design of two-plate molds:
(1) K.O hole cannot be smaller than diameter of ejector pin of injection molding machine;
(2) Ejection stroke L must ensure that plastic part can be completely ejected;
(3) When automatic injection molding is required, it must be ensured that plastic part and condensate of pouring system can be completely and safely ejected from mold cavity; when semi-automatic or manual, it must be ensured that plastic part can be easily removed;
(4) Spherical radius SR of gate sleeve must be greater than nozzle radius of injection molding machine.
Working process of two-plate molds:
(1) Plastic melt enters mold cavity from injection molding machine barrel through gate sleeve 3.
(2) After melt fills cavity, it is kept under pressure, cooled, and solidified.
(3) When plastic part is solidified to sufficient rigidity, injection molding machine pulls movable mold of mold to open mold from parting surface PL1.
(4) This mold is a single parting surface mold. Mold opening distance must ensure that plastic part and casting system condensate (i.e., sprue material) can fall off mold smoothly. Mold opening distance of this mold shall not be less than 100mm. Mold opening distance of parting surface mold is controlled by injection molding machine.
(5) After completing mold opening stroke, injection molding machine ejector pushes ejector base plate 14 through K.O. hole of mold base plate 12, and then pushes push rod (or ejector) 10 to push plastic part away from mold. Mold completes one injection molding.
(6) Mold is closed and next injection molding is carried out.

 

1-Fixed mold plate; 2-Fixed mold core; 3-Gate bushing; 4-Locate ring; 5-Guide pillar; 6-Guide sleeve; 7-Moving mold core; 8-Moving mold plate; 9-Support pillar; 10-Push rod; 11-Square iron; 12-Base plate; 13-Garbage nail; 14-Push rod base plate; 15-Push rod fixing plate; 16-Reset rod; 17-Spring.

Three-plate mold is commonly known as fine sprue mold, or double parting surface mold. Three-plate mold is divided into three parts after mold is opened. Compared with two-plate mold, a stripper plate (commonly known as sprue plate) is added. It is suitable for occasions where there should be no gate marks around plastic part or projection area is large and multiple feeding points are required. This mold uses point gates and has a more complex structure. It is necessary to design a fixed-distance parting mechanism.

 

• Things to note when designing a standard three-plate mold:
• ①B≥S1+S2+20～30;
• ②B≥100;
• ③L≥A+B;
• ④A is 8～12mm
• ⑤ Other precautions are same as for a two-plate mold.
• Among them: A—mold opening distance of parting surface 2;
• B—mold opening distance of parting surface 1;
• S1—length of main runner;
• S2—length of branch runner;

 

1-Faceplate; 2-Spring; 3-Puller; 4-Gate Bushing; 5-Limiting Pin; 6-Guide Pillar; 7-Guide Bushing; 8-Buckling Base; 9-Square Iron; 10-Push Rod; 11-Push Rod Fixing Plate; 12-Push Rod Base Plate; 13-Base Plate; 14-Reset Rod; 15-Moving Plate; 16-Moving Plate Insert; 17-Small Pull Rod; 18-Fixed Plate Insert; 19-Fixed Plate; 20-Stripping Plate
Mold working process:
(1) Plastic melt flows from injection molding machine barrel, through main runner and branch runner of three-plate mold, and finally into mold cavity through gate.
(2) After melt fills the mold cavity, it is kept under pressure, cooled, and solidified.
(3) When plastic part is solidified to sufficient rigidity, injection molding machine pulls movable mold of mold to open mold.
(4) Under action of mold opening spring 2, mold first opens from parting surface PL1, and mold opening distance of parting surface PL1 is B. At this time, casting system slurry (or sprue material) and plastic part are separated and ejected from fixed mold plate 19 and fixed mold insert 18.
(5) After mold opening distance of parting surface PL1 is completed, under action of buckle base 8, mold is opened from parting surface PL2 again, and mold opening distance is A. At this time, stripper plate 21 forcibly pushes casting system slurry (or sprue material) away from pull rod 3.
(6) Mold continues to open, and buckle base 8 is detached under pulling force of injection molding machine. Mold opens from parting surface PL3, and opening distance must not be less than 2S+10mm.
(7) After completing mold opening stroke, injection molding machine ejector pushes ejector base 12 through K.O. hole of mold bottom plate 13, then pushes push rod (or ejector) 10 to push plastic part away from mold. Mold completes one injection molding.
(8) Mold is closed and next injection molding is carried out.
• Simplified three-plate mold:
• Since simplified three-plate mold has four fewer movable mold guide pins than standard three-plate mold, fixed mold guide pins must guide de-casting plate, fixed mold A plate and movable mold B plate at the same time, L≥A+B+20. Other precautions are same as those of standard three-plate mold.
• Precision and rigidity of the simplified three-plate mold are worse than those of standard three-plate mold, and its lifespan is also shorter than that of standard three-plate mold.

 

1-panel; 2-limit screw; 3-stripper plate; 4-gate sleeve; 5-bushing; 6-guide pin; 7-guide sleeve; 8-fixed plate; 9-guide sleeve; 10-fixed mold insert; 11-movable mold insert; 12-locking machine; 13-square iron; 14-push rod fixing plate; 15-push rod bottom plate; 16-bottom plate; 17-trash nail; 18-reset rod; 19-spring; 20-pull rod.