Composition of injection mold: movable mold and fixed mold. Movable mold is installed on movable mold plate of injection molding machine, and fixed mold is installed on fixed mold plate of injection molding machine.
During injection molding, movable mold and fixed mold are closed to form a pouring system and a cavity. When mold is opened, movable mold and fixed mold are separated to take out plastic product. According to role of each component in mold, injection mold can be divided into following basic components.

Composition: core and die.
Function: Core forms inner surface shape of product, and die forms outer surface shape of product. After mold is closed, core and die form cavity of mold (Figure 5-1, cavity of mold is composed of pieces 13 and 14).
Structure: According to manufacturing process requirements, sometimes core or die is composed of several pieces, and sometimes it is made into a whole, inserts are used in parts that are easy to damage and difficult to process.

Also known as runner system.
Function: A set of feed channels that guide plastic melt from injection machine nozzle to cavity.
Composition: Main runner, branch runner, gate and cold material hole. Design of casting system is very important, which is directly related to molding quality and production efficiency of plastic parts.

Function:
① Ensure that movable mold and fixed mold can be accurately aligned when they are molded;
② Avoid skew of push plate during product ejection process;
③ Support weight of moving parts.
Composition: Guide pins and guide sleeves are often used, sometimes it is necessary to set inner and outer conical surfaces that match each other on movable mold and fixed mold to assist positioning.

 

Figure 5-1 Typical single parting surface injection mold
1--locating ring; 2-main channel bushing; 3-fixed mold base plate; 4-fixed mold plate; 5-movable mold plate; 6-movable mold pad; 7-movable mold base; 8-ejection fixed plate; 9-push plate; 10-pull rod; 11-push rod; 12-guide column; 13-core; 14-die; 15-cooling water channel

Function: During mold opening process, plastic parts and condensed materials in runner are pushed out or pulled out.
Composition: Figure 5-1, push rod 11, ejection fixed plate 8, push plate 9 and pull rod 10 of main runner. Among them, function of ejection fixed plate and push plate is to clamp push rod. A reset rod is generally fixed in push plate. Function of reset rod is to reset ejection mechanism when movable mold and fixed mold are closed.

Function: to meet requirements of injection process for mold temperature.
Common methods:
① For injection molds for thermoplastic plastics, cooling water channels are mainly opened in mold to use circulating cooling water to take away heat of mold;
② In addition to using hot water or steam through cooling water channel to heat mold, electric heating elements can also be installed inside and around mold.

Function: to fully exhaust gas during molding process.
Common methods:
① Open an exhaust groove at parting surface;
② There is a small gap between parting surfaces. For smaller plastic parts, because exhaust volume is not large, parting surface can be used directly for exhaust, and there is no need to open an exhaust groove;
③ Clearance between ejector or core of some molds and mold can play a role in exhaust, and sometimes there is no need to open an exhaust groove.

Some plastic parts with side concave or side hole must be parted sideways before being pushed out. Only after side core is pulled out can they be demolded smoothly. At this time, a side core pulling mechanism needs to be set in mold.

In order to reduce heavy workload of mold design and manufacturing, most injection molds adopt a standard mold base structure. Positioning ring 1, fixed mold base plate 3, fixed mold plate 4, movable mold plate 5, movable mold pad 6, movable mold base 7, ejection fixed plate 8, push plate 9, push rod 11, guide column 12, etc. in Figure 5-1 are all parts in standard mold base, and they can all be ordered from relevant manufacturers.

From perspective of mold design, injection molds are divided into following categories according to their overall structural characteristics.

Also known as two-plate mold, it is the simplest and most commonly used type of injection mold, accounting for about 70% of all injection molds.
Structure: (Figure 5-1) Part of cavity (core) is on movable mold plate, and the other part (concave mold) is on fixed mold plate. Main runner is set on one side of fixed mold, and branch runner is set on parting surface. After mold is opened, due to pulling action of pull rod on movable mold and plastic parts are wrapped tightly on core due to shrinkage, product and condensate in runner are left on the side of movable mold. Movable mold is provided with an ejection mechanism to eject product and condensate in runner.
Single-parting surface injection mold has a simple structure and is easy to operate, but except for direct gate, gate position of cavity can only be selected on the side of product.

 

Figure 5-2 Double parting surface injection mold
1- fixed distance pull plate; 2- spring; 3- limit pin; 4- guide column; 5- push plate; 6- movable mold plate; 7- movable mold pad; 8- mold base; 9- push plate; 10- push plate; 11- push rod; 12- guide column; 13- middle plate; 14- fixed mold plate; 15- main channel bushing
Injection mold takes out runner condensate and plastic parts from two different parting surfaces. Compared with two-plate single-parting surface injection mold, double-parting surface injection mold adds a movable middle plate (also known as gate plate) between movable mold plate and fixed mold plate, so it is also called a three-plate mold.
Structure: Figure 5-2 A flow channel is set between fixed mold plate and middle plate, a cavity is set between middle plate and movable mold plate. Middle plate is suitable for single-cavity or multi-cavity molds that use point gates for feeding. When opening mold, due to limitation of fixed-distance pull plate 1, middle plate 13 and fixed mold plate 14 are separated by a fixed distance to remove condensate in flow channel between two plates. After middle plate and movable mold plate are separated, push plate 5 is used to remove plastic part wrapped tightly on core.
Double-parting surface injection molds can set point gates in the center of plastic part. Manufacturing cost is high, structure is complex, and a large mold opening stroke is required.

 

Figure 5-3 Injection mold with movable inserts
1-fixed mold plate; 2-guide pillar; 3-movable insert; 4-core; 5-movable mold plate; 6-movable mold pad; 7-mold base; 8-spring; 9-push rod; 10-ejection fixed plate; 11-push plate
Shape and structure of plastic part are complex, and plastic part cannot be removed from mold by simple parting. At this time, movable inserts and movable lateral cores or half blocks (half blocks) can be set in mold.
Structure: Figure 5-3. When mold is opened, movable parts cannot be simply separated from workpiece along mold opening direction. Instead, they are moved out of mold together with product during demolding, then separated from plastic part by hand or simple tools. When these movable inserts are inserted into mold, they should also be reliably positioned. Production efficiency of this type of mold is not high, it is often used for small batches or trial production.

 

Figure 5-4 Injection mold with lateral parting and core pulling
1-Wedge block; 2-Oblique pin; 3-Oblique slider; 4-Core; 5-Fixed plate; 6-Moving mold pad; 7-Block; 8-Moving mold seat plate; 9-Push plate; 10-Push fixed plate; 11-Push rod; 12-Pulling rod; 13-Guide column; 14-Moving mold plate; 15-Main channel bushing; 16-Fixed mold plate; 17-Location ring
There are side holes or side recesses on plastic part. A lateral parting core pulling mechanism composed of an inclined pin or an inclined slider can be set in mold to make side core move laterally.
Structure: Figure 5-4. When mold is opened, inclined guide pin uses mold opening force to drive side core to move laterally, so that side core is separated from workpiece, then push rod can smoothly push product out of core. In addition to inclined pin, inclined slider and other mechanisms that use mold opening force to perform lateral core pulling, a hydraulic cylinder or a pneumatic cylinder can also be installed in mold to drive side core to perform lateral parting core pulling. This type of mold is widely used in mass production of plastic parts with side holes or side recesses.

 

Figure 5-5 Injection mold with automatic thread removal
1-thread core; 2-mold base; 3-movable mold pad; 4-fixed distance screw; 5-movable mold plate; 6-bushing; 7-fixed mold plate
When it is required to automatically remove plastic parts with internal or external threads, a rotating thread core or mold ring can be set in mold, and threaded product can be removed by rotational motion or reciprocating motion of mechanism, or a special drive and transmission mechanism can be used to drive thread core or mold ring to rotate, remove threaded product.
Injection mold with automatic thread removal Figure 5-5, this mold is used for right-angle injection molding machine, thread core is driven to rotate by screw of injection molding machine clamping mechanism to separate from product.

 

Figure 5-6 Injection mold with ejection mechanism on fixed mold side
1-Mold base; 2-Moving mold pad; 3-Molding insert; 4-Screw; 5-Moving mold; 6-Screw; 7-Ejector plate; 8-Pull plate; 9-Fixed mold plate; 10-Fixed mold base plate; 11-Core; 12-Guide column
Generally, when injection mold is opened, plastic products are left on the side of movable mold, so ejection mechanism is also set on the side of movable mold. This form is the most commonly used and convenient, because ejection mechanism of injection molding machine is on the side of movable mold. But sometimes, due to special requirements or shape restrictions of product, product must remain in fixed mold. At this time, an ejection mechanism should be set on one side of fixed mold to remove product from fixed mold. Ejection mechanism on fixed mold side is generally driven by movable mold through a pull plate or chain.
Figure 5-6 Plastic clothes brush injection mold. Due to special shape of product, a direct gate is used to facilitate molding. After mold is opened, product is retained on fixed mold. Therefore, a push plate 7 is provided on one side of fixed mold. When mold is opened, push plate 7 is driven by pull plate 8 on the side of movable mold to force product to be removed from core 11 in fixed mold.

 

Figure 5-7 Hot runner injection mold
1-movable mold base plate; 2-pad; 3-push plate; 4-push fixed plate; 5-push rod; 6-movable mold pad; 7-guide sleeve; 8-movable mold plate; 9-core; 10-guide column; 11 fixed mold plate; 12-die; 13-bracket; 14-nozzle; 15-hot runner plate; 16-heater channel; 17-fixed mold base plate; 18-insulation layer; 19-main runner bushing; 20-positioning ring; 21-injection machine nozzle
Often referred to as runnerless injection mold. Including: hot runner and insulated runner mold.
Principle: Plastic between nozzle of injection machine and gate is kept in a molten state by heating or insulating runner. There is no plastic condensate in runner after each injection molding.
Advantages: Improved productivity, saved plastics, ensured transmission of injection pressure in flow channel, helped improve quality of parts, and achieved fully automatic operation.
Disadvantages: High mold cost, high requirements for pouring system and temperature control system, certain restrictions on shape of parts and plastics.