Injection mold is installed on injection machine. When designing an injection mold, in addition to mastering injection molding process, you should also have a comprehensive understanding of relevant technical parameters of selected injection machine to ensure that designed mold is compatible with injection machine used.

Injection machine is main equipment for production of thermoplastic parts. In recent years, it has also been used in molding thermosetting plastic parts. Although various injection machines have different shapes, they are basically composed of a mold clamping system and an injection system. When working, injection mold is installed on movable mold plate and fixed mold plate of injection machine. Mold clamping system closes mold and locks mold. Injection system sends plastic raw materials to barrel, heats it to plasticizing temperature, injects molten plastic into cavity of mold. Injection machine is equipped with electric heating and water cooling systems to adjust temperature of mold. After plastic is formed in mold, mold is opened when it cools to a certain temperature, and plastic part is pushed out by ejection mechanism. The more advanced injection machines are controlled by computers and can realize automatic operation.
There are many ways to classify injection machines. According to their appearance characteristics, they can be divided into three types: horizontal injection machines, vertical injection machines and angle injection machines. Among them, horizontal injection machine is the most used, as shown in Figure 4-12 .

 

Figure 4-12 Horizontal injection machine
1-Clamping hydraulic cylinder; 2-Clamping mechanism; 3-Moving mold plate; 4-Ejector; 5-Fixed mold plate; 6-control console; 7-barrel and heater; 8-hopper; 9-quantitative feeding device; 10-injection hydraulic cylinder

Horizontal injection machine is the most widely used injection machine. Axes of its injection system and mold clamping system are arranged horizontally. Its injection system has two structures: plunger type and screw type. Injection volume is 60cm3 and above is all screw type. Advantages of horizontal injection machine are that machine has a low center of gravity, is relatively stable, and is easy to operate and maintain. After molded plastic part is pushed out, it can automatically fall by its own weight, which facilitates automated production and is suitable for large, medium and small molds; its disadvantage is that mold installation is difficult.

Axis of injection system of vertical injection machine is consistent with mold clamping system and is perpendicular to ground. Injection system is mostly a plunger structure, and injection volume is generally less than 60cm3. Advantages of vertical injection machine are that it occupies a small area, is easy to load and unload the mold, and is convenient for placing inserts on the side of movable mold. Its disadvantages are that machine has a high center of gravity, is unstable, and is difficult to feed. Pushed out plastic parts need to be taken out manually or by other methods, making it difficult to achieve automated production.

Axes of injection system and mold clamping system of an angle injection machine are perpendicular to each other. A common angle injection machine clamps mold in horizontal direction and injects in vertical direction. Its injection system is generally a plunger type structure, using rack and pinion transmission or hydraulic transmission. Injection volume is small, generally less than 45cm3. Advantage of angle injection machine is between horizontal and vertical injection machines. Structure is relatively simple. It can use screw rotation when opening mold to automatically disengage threaded plastic parts; its disadvantage is that mechanical transmission cannot accurately and reliably inject, maintain pressure and clamping force, mold is subject to greater impact and vibration.

Model specifications of injection machine mainly include three expression methods: injection volume, mold clamping force, injection volume and mold clamping force expressed simultaneously. National standard adopts injection volume expression method:
Commonly used horizontal injection machine models include: XS-ZY-30, XS-ZY-60, XS-ZY-125, XS-ZY-500, XS-ZY-1000, etc.

When designing an injection mold, designer must determine structure of mold based on structural characteristics and technical requirements of plastic part. Mold structure is inevitably related to injection machine. Size of positioning ring of mold, the overall size of mold plate, size of injection volume, setting of ejection mechanism and size of clamping force must be designed with reference to relevant technical parameters of injection machine. Otherwise, mold cannot be properly matched with injection machine, and injection process cannot be carried out.

Determining number of cavities is the first step in mold design. For multi-cavity injection molds, number of cavities is related to parameters such as plasticizing rate, maximum injection volume and clamping force of injection machine, also directly affects accuracy of plastic parts and economy of production. Following introduces several methods for determining number of cavities based on performance parameters of injection machine. These methods can also be used to check whether number of cavities initially selected matches specifications of injection machine.

 
where n——number of cavities
m——Weight or volume of a single plastic part, g or cm3;
K——Utilization coefficient of maximum injection volume of injection machine, generally 0.8;
M——Rated plasticizing capacity of injection machine, g/h or cm3/h;
t——molding cycle, s;
m1——Weight or volume of plastic required by pouring system, g or cm3.

n p A≤Fp－p A1 (4-2)
Where n——number of cavities;
p——Molding pressure of plastic melt on cavity, MPa, its size is generally 80% of injection pressure. Size of injection pressure is shown in Table 4-2;
A——Projected area of a single plastic part on mold parting surface, mm2;
Fp——Rated clamping force of injection machine, N;
A1——Projected area of pouring system on mold parting surface, mm2.
When determining or checking number of mold cavities according to above method, dimensional accuracy of molded plastic part, economical production and size of injection machine installation template must also be considered. The greater number of cavities, the lower accuracy of plastic parts (generally speaking, for each additional cavity, dimensional accuracy of plastic parts will be reduced by 4% to 8%), and manufacturing cost of mold will be higher, but production efficiency will increase significantly. .

Maximum injection volume refers to maximum capacity of injection machine that can inject plastic at one time. Since density of polystyrene plastic is 1.05g/cm3, which is approximately 1g/cm3, maximum injection volume of plunger injection machine is based on maximum number of grams of polystyrene plastic injected at one time; while screw injection machine uses volume to express maximum injection volume, regardless of type of plastic.
When designing mold, it should be ensured that the total injection volume required to form plastic part is less than maximum injection volume of selected injection machine, that is:
n m+m1≤K mp (4-3)
Where n——number of cavities;
m——Weight or volume of a single plastic part, g or cm3;
m1——Weight or volume of plastic required by pouring system, g or cm3;
K——Utilization coefficient of maximum injection volume of injection machine, generally 0.8;
mp - Maximum injection volume of injection machine, g or cm3.

When high-pressure plastic melt fills mold cavity, a force will be generated to expand mold parting surface. Magnitude of this force is equal to sum of projected areas of plastic part and gating system on parting surface multiplied by pressure of cavity. , it should be less than rated clamping force Fp of injection machine to ensure that no overflow occurs during injection, that is:
Fz＝p（n A +A1）＜Fp (4-4)
In formula, Fz——expansion force of molten plastic on parting surface, N;
p——Molding pressure of plastic melt on cavity, MPa, its size is generally 80% of injection pressure. Size of injection pressure is shown in Table 4-2;
n——number of cavities;
A——Projected area of a single plastic part on mold parting surface, mm2;
A1——Projected area of gating system on mold parting surface, mm2;
Fp——Rated clamping force of injection machine, N.
Pressure in cavity is about 80% of injection pressure of injection machine, usually 20 to 40MPa. Cavity pressure values used in commonly used plastic injection molding are shown in Table 4-3.
Table 4-3 Cavity pressure MPa selected for commonly used plastic injection molding

Check of injection pressure is to verify whether rated injection pressure of injection machine is greater than injection pressure required during molding. Injection pressure required for plastic molding is determined by factors such as plastic type, injection machine type, nozzle form, plastic part shape and pressure loss of gating system. For plastics with higher viscosity and plastic parts with thin shapes and long processes, injection pressure should be higher. Since pressure loss of plunger injection machine is larger than that of screw injection machine, injection pressure should also be larger.

In order to ensure that injection mold can be successfully installed on injection machine and injection molded to produce qualified plastic parts, dimensions related to injection machine and mold installation must be checked when designing mold. Dimensions related to mold installation include nozzle size, positioning ring size, maximum and minimum thickness of mold, and mounting screw hole size on mold plate.

When designing mold, spherical radius at the beginning of main runner must be slightly larger than spherical radius at injection machine nozzle head, as shown in Figure 4-13, that is, R is 1 to 2 mm larger than r. Diameter of small end of main runner is slightly larger than diameter of nozzle, that is, D is 0.5~1mm larger than d, to prevent accumulation of condensate at mouth of main runner and affect demoulding.

 

Figure 4-13 Matching of spherical surface of gate sleeve and nozzle of injection machine
1-Injection machine nozzle; 2-Gate sleeve

In order to ensure that injection mold is installed accurately and reliably on injection machine, design of positioning ring is very critical. Outer diameter of mold positioning ring must match positioning hole size of injection machine. Clearance fit is usually used to ensure that center line of main channel of mold coincides with center line of injection machine nozzle. Generally, outer diameter of protruding positioning ring on fixed mold plate of mold should be less than 0.2mm smaller than size of positioning hole on fixed mold plate of injection machine.

When designing mold, the total thickness of mold should be between maximum and minimum thickness of mold that can be installed on injection machine. At the same time, the overall dimensions of mold should be checked so that mold can be loaded from between tie rods of injection machine.

There are two ways to install mold on injection machine: one is to fix it directly with screws; the other is to fix it with screws and pressure plates. When fixing directly with screws, screw holes on mold fixing plate and injection machine mold plate should be completely consistent; when fixing with a pressure plate, it can be tightened as long as there are screw holes near outside of mold fixing plate where pressure plate needs to be placed, so it has greater flexibility. For large molds with heavy weight, it is safer to fix them directly with screws.

Mold opening stroke of injection machine is limited. Mold opening distance required when plastic part is taken out from mold must be less than maximum mold opening distance of injection machine, otherwise plastic part cannot be taken out from mold. Due to different mold clamping mechanisms of injection machines, mold opening stroke can be checked according to following three situations:

When injection machine adopts a hydraulic and mechanical clamping mechanism, maximum mold opening degree is determined by maximum stroke of linkage mechanism and is not affected by thickness of mold.
For single parting surface injection mold as shown in Figure 4-14, mold opening stroke can be checked as follows:
s≥H1+ H2+ (5～10)mm (4-5)
In formula, s——maximum mold opening stroke of injection machine, mm;
H1 - push-out distance (demoulding distance), mm;
H2——Height of plastic part including gating system, mm.

 

Figure 4-14 Mold opening stroke of single parting surface injection mold
1-moving mold; 2-fixed mold base plate
For double parting surface injection mold as shown in Figure 4-15, in order to ensure that both plastic part and pouring system condensate can be taken out after mold is opened, it is necessary to increase separation distance a between fixed mold plate and intermediate plate in mold opening distance. Size of a should ensure that pouring system condensate can be easily taken out. At this time, mold opening stroke can be checked as follows:
s≥H1+ H2+ a + (5～10)mm (4-6)

 

Figure 4-15 Mold opening stroke of double parting surface injection mold
1-moving mold; 2-middle plate; 3-fixed mold base plate

For injection machines with fully hydraulic clamping mechanisms and right-angle injection machines with screw mold opening and clamping mechanisms, maximum mold opening stroke is affected by thickness of mold. At this time, maximum mold opening stroke is equal to maximum distance s between injection motorized mold plate and fixed mold plate minus mold thickness Hm.
For single parting surface injection molds, calibration formula is:
s≥Hm+ H1+ H2+ (5～10)mm (4-7)
For double parting surface injection molds, calibration formula is:
s≥Hm+ H1+ H2+ a + (5～10)mm (4-8)

When mold needs to use mold opening action to complete lateral core pulling, mold opening stroke required for lateral core pulling should be considered when checking mold opening stroke. As shown in Figure 4-16, assume that mold opening stroke required to complete lateral core pulling is HC. When HC ≤ H1 + H2, HC has no effect on mold opening stroke, and above formulas are still used for verification; when HC> When H1+H2, HC can be used to replace H1+H2 in aforementioned calibration formula for calibration.

 

Figure 4-16 Mold opening stroke with lateral core pulling

Ejection devices and maximum ejection distances of various types of injection machines are different. When designing, ejection mechanism of mold should be adapted to injection machine. It is usually based on ejection form of ejection device of mold opening and closing system (center ejection or two-side ejection), ejector diameter of injection machine, ejector spacing and ejection distance, etc. to check whether ejection mechanism of mold is reasonable and whether push-rod push-out distance can meet requirements for ejection of plastic parts.