Mobile phone products have huge market development space. At the same time, market is also in a stage of rapid development. This is an extremely important development stage for manufacturers, distributors and retailers, and is a stage where market structure is gradually taking shape. Therefore, in next few decades, design and processing of mobile phone casings and corresponding mold design will create huge business opportunities.
Design appearance of mobile phone case is shown in Figure 1, which is a three-dimensional modeling of mobile phone case based on Solidworks software.

 

Figure 1 Three-dimensional design drawing of mobile phone case
Figure 2, Figure 3 and Figure 4 are engineering dimensional drawings of mobile phone case.

 

Figure 2 Mobile phone case dimensions

 

Figure 3 Mobile phone case dimensions

 

Figure 4 Mobile phone case dimensions
Material selection:
We selected ABS+PC (SABIC Innovation Plastics US company's Cycoloy MC7000 brand) material as mobile phone case injection molding material, which has comprehensive characteristics of both ABS and PC. For example, easy processability of ABS and excellent mechanical properties and thermal stability of PC. Composition ratio of the two will affect thermal stability of ABS+PC materials. ABS+PC hybrid material also shows excellent flow characteristics. Commonly used in computer and business machine casings, electrical equipment, lawn and garden machines, automotive parts, and more.

Optimal location of gate can be determined based on Autodesk's Moldnow software. As shown in figure, optimal gate position is at the arrow, close to grid node 3489. Gate location shown in Figure 5 is selected based on production experience.

 

Figure 5 Optimal gate location
Moldflow software also provides analysis cloud diagrams of flow resistance and gate matching. Flow resistance indicator indicates resistance of melt at different gate positions at flow front. The higher resistance value, the more difficult melt flow is (Figure 6). Gate matching represents factor distribution diagram of rationality of gate position, in which blue position indicates the best gate matching, red position indicates the worst gate matching, and matching of other positions is in transition area (Figure 7).

 

Figure 6 Flow resistance

 

Figure 7 Gate matching
Moldflow software can be used to analyze molding window to find optimal mold temperature, melt temperature and injection time. Molding window analysis is used to define range of molding process conditions that can produce qualified products. If it is within this range, plastic parts with better quality can be produced. After analysis, software recommended mold temperature is 85.56℃, recommended melt temperature is 283.69℃, and recommended injection time is 0.3938s. Based on experience, we select injection pressure as 150MPa.

Material used for mobile phone case is ABS+PC. Reference values of injection molding process parameters of ABS+PC materials are as follows:

1) Barrel temperature: The most suitable temperature range of barrel should be between viscous flow temperature or melting point temperature θf (or θm) and thermal decomposition temperature θd. Temperature distribution of barrel generally adopts principle of high in the front and low in the rear, that is, temperature at feeding port (rear section) of barrel is the lowest and temperature at nozzle is the highest. Temperature of rear section of barrel should be 5~10℃ lower than temperature of middle section and front section. Nozzle temperature is generally slightly lower than maximum temperature of barrel, which can prevent "salivation" phenomenon that may occur on straight-through nozzle.

2) Melt temperature: According to molding window analysis using Moldow software, recommended melt temperature is 283.69℃.
3) Barrel constant temperature: 200℃.
4) Mold temperature: Under premise of meeting use conditions, a lower mold temperature is used, generally 70~90℃. According to molding window analysis of Moldflow software, recommended mold temperature is 85.56℃.
5) Injection pressure: Size of injection pressure depends on type of plastic, type of injection machine, mold structure, wall thickness and process of plastic products and other process conditions, especially structure and size of pouring system. For general thermoplastic engineering plastics, pressure is 70~150MPa.
6) Holding pressure: Holding pressure is generally less than injection pressure, about 40% to 50% of injection pressure, to avoid wall shrinkage of part. In order to minimize internal stress of product, holding pressure should be set as low as possible.
7) Plasticizing pressure (back pressure): Size of plasticizing pressure has an impact on actual melt temperature, plasticizing efficiency and molding cycle. Generally it is about 6~20MPa. According to actual situation of this injection machine, it only needs 2~5MPa.
8) Residual material amount: 2~5mm, depending on metering stroke and screw diameter
9) Pre-drying: 4 hours at 80℃
10) Recovery rate: 20% recycled material can be added, as long as material does not undergo thermal degradation and is properly pre-dried.
Based on above reference and combined with company's actual production experience, process parameters are determined as follows: add 20% recycled material, use multi-stage injection: injection time 0.3938s; cooling time 10s: holding time 4s: holding pressure 60MPa.

Based on company's actual situation and past experience, EH80 plastic injection molding machine was selected. Technical parameters of molding machine are shown in Table 1:
Table 1 Parameter table of EH80 plastic injection molding machine

1) Checking maximum injection volume
Maximum injection volume of injection molding machine is calculated according to following formula:
In the formula: gB - injection volume specified by injection machine (g)
dB--specific gravity of polystyrene at room temperature (1.06g/mm3)
D--Specific gravity of injection plastic at room temperature (g/mm3)
According to Solidworks, volume of mobile phone cover and aggregate can be calculated to be 8720.856mm3, and density of ABS + PC is 1.5g/cm3, so weight of plastic part is W=13.08g.
According to above formula, maximum injection volume of this injection molding machine:
 
2) Injection pressure check
Check of injection pressure is to verify whether rated injection pressure of injection machine is greater than injection pressure required for molding. Injection pressure of selected injection machine must be greater than injection pressure required for molded product. Maximum injection pressure of this injection machine is 1900kg/cm2=186MPa, and injection pressure is 150MPa. Therefore, injection pressure of injection molding machine meets requirements.
3) Check of clamping force
Clamping force is equal to sum of projections of part and gate runner system on parting surface multiplied by plastic pressure in cavity. It should be smaller than rated clamping force of injection machine so that no flashing and mold expansion will occur during injection. According to Solidworks calculation, projected area of part and gating system on parting surface is 4257mm2. Pressure in mold cavity is 4kg/mm2. Required clamping force P=4257x4=17t is smaller than clamping force of injection molding machine of 80t. Therefore, clamping force of injection molding machine meets requirements.
4) Check mold opening stroke
Maximum stroke of injection molding machine is related to thickness of mold, so mold opening stroke of injection molding machine should satisfy following formula:
 
In formula: S--maximum mold opening stroke of injection molding machine; S=470mm
H mold--mold thickness; H mold =375mm
H small - minimum mold thickness specified by injection molding machine; H small =180mm
H1--ejector stroke;H1=30mm
H2--Height of plastic part including gating system. H2=130mm
From above formula: left side = 470-(375-180) = 275mm; right side = 30+130+5=165mm; left side > right side, so mold opening stroke of injection molding machine meets requirements.

According to structural characteristics and appearance requirements of mobile phone case, a single-cavity mold is used. In addition, in order to enable automated production, a three-plate injection mold with single-point gate is used. Depending on specific situation, cold runners are used. It is necessary to design a sequential demoulding mechanism. The overall structure of mold is shown in Figure 8:

 

Figure 8 Overall structural design drawing of mold

Gating system refers to channel through which molten plastic flows from injection machine nozzle to injection mold cavity. Its function is to smoothly fill all parts of mold cavity with plastic melt, transfer injection pressure to all parts of mold cavity during filling, compaction and pressure-holding processes to obtain plastic products with clear appearance and high quality. Gating system is divided into ordinary pouring system and hot runner pouring system.
Mold adopts an ordinary runner gating system, which generally consists of four parts: main channel, branch channel, gate and cooling well.

Main channel refers to flow channel of plastic melt in pouring system from contact point between injection machine nozzle and mold sprue sleeve to runner. It is the first part of melt that flows through mold. Its function is to introduce plastic melt injected from nozzle of injection molding machine into runner or cavity. Its shape and size have a great influence on flow rate and mold filling time of plastic melt. Therefore, temperature drop and pressure loss of melt must be minimized. In mold used on horizontal injection machine, main flow channel is perpendicular to parting surface. In order to allow condensate to be smoothly pulled out from it, it needs to be designed into a conical shape. Main channel bushing is shown in Figure 9. Design process of main channel size is as follows.

 

Figure 9 Schematic diagram of main flow bushing
1) Dimensional design of main channel
(1) Main channel spherical pit radius
Radius of spherical pit of main channel R2 =R1+(1-2)mm. Since radius of nozzle head of injection molding machine EH80 is R1=10mm, R2=11mm is taken.
(2) Diameter of small end of main flow channel
Diameter of small end of main flow channel is D=d+(0.5-1)mm. Since nozzle diameter of injection molding machine is d=1.7mm, D=2.5mm is taken.
(3) Main channel taper
Cone angle of main flow channel is generally 2°-4°, so cone angle a=4° is taken.
(4) Main channel length
Take length L=60mm according to thickness of mold plate.
(5) Large end diameter of main flow channel
Diameter of large end of main flow channel is D =D+Lxtan(a/2)=2.5+60xtan 2° ≈ 4.6mm.
2) Form of main flow bushing
During molding process, small end entrance of main channel part is in repeated contact with injection machine nozzle and plastic melt of a certain temperature and pressure, which requires cold and heat exchange. It is a wearing part and has high material requirements. Therefore, main channel part of mold It is often designed as a removable and replaceable bushing type (commonly known as a sprue sleeve), so that high-quality steel can be effectively selected for separate processing and heat treatment. Generally, carbon tool steels T8A, T10A, etc. are used, and heat treatment requires quenching 53-57HRC. Main flow bushing should be set at symmetrical center of mold, and try to ensure that it is on same axis as connected injection machine nozzle.
There are two types of sprue bushings: one is that sprue bushing and positioning ring are designed as one piece, generally used for small molds; the other is that sprue bushing and positioning ring are designed as two parts, then fit on a fixed mold plate . Due to small size of mold, main sprue bushing can be selected as an integral type.
Based on above requirements, we selected M-SBA type sprue bushing produced by FUTABA Company (Figure 10). Sprue bushing is connected to fixed mold bottom plate through two M5 hexagon socket bolts (Figure 11).

 

Figure 10 Three-dimensional model diagram of the main channel

 

Figure 11 Fixing form of sprue sleeve

Runner refers to a section of plastic melt flow channel between end of main channel and gate. Function of runner is to change flow direction of melt so that it can be evenly distributed to each cavity in a smooth flow pattern. In a single mold cavity, there is generally no need to open a runner, but this mold uses a three-plate spot pouring mode, so a runner is also required. When plastic flows along runner, it is required to fill mold cavity as quickly as possible, temperature drop and resistance during flow should be as small as possible.
Design principles of runner are: melt should be quickly injected into mold cavity through the shortest path, with minimal heat and pressure loss; temperature and pressure of melt entering the mold cavity from each gate should be same to ensure same shrinkage rate of products in each cavity; turning point of runner should be an arc transition, and connection with gate should be processed into a slope to facilitate flow of molten material; when ensuring sufficient injection pressure, cross-section and length of runner should be as small as possible.
1) Cross-sectional shape of shunt channel
Commonly used cross-sectional shapes include circular, trapezoidal, U-shaped and hexagonal. To reduce pressure loss in flow channel, it is hoped that cross-sectional area of flow channel is large and surface area of flow channel is small to reduce heat transfer losses. Therefore, efficiency of flow channel can be expressed by ratio of cross-sectional area and circumference of flow channel. Circular cross-section has the highest efficiency (that is, the smallest specific area). Since it is difficult to demold condensate from square runner, trapezoidal runner with a side slope of 5°-10° is actually used. Rectangular and semicircular cross-section flow channels are usually not used due to their low efficiency (large specific surface area). Generally, when parting surface is flat, a circular cross-section runner is often used; when parting surface is not flat, considering difficulty of processing, a trapezoidal or semicircular cross-section runner is often used.
When molten material flows in flow channel, a solidified layer will be formed on the surface of flow channel wall due to cooling. Due to poor thermal conductivity of plastic, solidified layer plays a thermal insulation role, allowing melt to flow smoothly in the center of flow channel. Therefore, center of runner should be on same straight line as center of gate. Circular cross-section flow channels can achieve this, while trapezoidal cross-section flow channels are difficult to achieve. Although there is a slot at the bottom of mobile phone case, which causes parting surface to be non-flat, it is only a small part. Considering above reasons, we use a circular cross-section instead of a trapezoidal cross-section. Moreover, use of circular cross-section can reduce heat loss of runner and improve efficiency of runner. Cross-section of runner of this mold is circular.
2) Cross-sectional dimensions of shunt
Cross-sectional size of runner should be determined based on factors such as molding volume of plastic part, wall thickness of plastic part, shape of plastic part, process performance of plastic used, injection rate, and length of runner.
(1) For plastic parts with a wall thickness less than 3mm and a weight less than 200g, following empirical formula can also be used:
 
In formula: D--diameter of shunt channel, mm;
m--weight of plastic parts, 8;
L--Length of shunt channel, mm.
Diameter of shunt calculated by this formula is limited to 3.2-9.5mm.
As can be seen from above,weight of plastic part is m=13.08g, length of runner is L=74.32mm, so diameter of runner is , so it is not within scope of this application.

(2) Based on plastic shape of mobile phone casing and Table 2, we can determine that diameter of shunt is 5mm.

Table 2 Split channel diameters of commonly used plastics

3) Length of shunt

Length of shunt should be as short as possible and there should be as few bends as possible. Based on comprehensive consideration of design size of the overall mold and location of gate, length of runner is L=6.52+67.8=74.32mm.
4) Surface roughness of shunt channel
Since outer layer of plastic in contact with mold in runner is cooled rapidly, only flow state of plastic melt in the center is ideal. Therefore, inner surface roughness Ra of runner is not required to be very low, generally 0.63um-1.6um, so that surface is slightly rough, which helps to increase flow resistance of outer layer of plastic melt, avoids surface slippage of melt flow, and enables central layer to have a higher shear rate.

Gate is a short, thin channel that connects runner and cavity. Shape, quantity, size and location of gates have a great influence on quality of plastic parts. Main functions are as follows: Since cross-section at gate is the smallest, flow rate is fast and frictional heat is easily generated, which heats up melt and facilitates mold filling; after melt is filled, it first solidifies at gate to prevent backflow: it is easy to remove gate tailings and facilitate secondary processing.
There are many types of gates. In order to automate production and improve production efficiency, gates should automatically fall off during demoulding to reduce subsequent processing, so point gates are selected. Point gate, also called pin point gate or diamond gate, is a gate with very small cross-section, commonly known as small gate. Due to large pressure difference between front and rear ends of this type of gate, it can greatly increase shear rate of plastic melt and generate large shear heat, resulting in a decrease in the apparent viscosity of melt and an increase in fluidity, which is beneficial to filling mold cavity. Gate diameter d of this form cannot be too small, and gate length I cannot be too long, otherwise gate aggregate will break during demoulding and block gate, affecting normal progress of injection.
1) Determination of gate size
(1) Diameter of point gate can be calculated according to empirical formula as follows:
 
In formula: d--point gate diameter (mm);
A--Surface area of cavity, that is, outer surface area of plastic part (mm2);
c--gate wall thickness coefficient, which is a function of wall thickness of plastic part.
n--Plastic material coefficient.
However, this formula is only applicable to products with a wall thickness of 0.7-0.55m. Thickness of mobile phone shell is 1.8m, which is not within applicable range and cannot be calculated by this formula.
(2) Table 3 shows empirical dimensions of side gates and point gates. We take diameter d=1mm and length 1=1mm of point gate according to actual situation.
Table 3 Recommended dimensions for side gates and point gates

2) Determination of number and location of gates
Selection of gate number and location has a great impact on quality of plastic parts. According to mold, we choose a point gate. Following principles should be followed when selecting gate location: size and location of gate should be selected to avoid spraying and creeping: gate should be opened at the thickest section of plastic part; selection of gate position should make plastic process the shortest and material flow change direction the least; selection of gate position should be conducive to discharge of gas in cavity; welding marks of plastic parts should be reduced or avoided to increase welding fastness; gate position should be selected to prevent material flow from squeezing and deforming cavity, core, and inserts; consider influence of molecular orientation.

During interval between completing an injection cycle, considering that melt at nozzle of injection machine and entrance of main flow channel is lower than required temperature of plastic melt due to radiation heat dissipation, there is an area where temperature gradually increases from the end of nozzle to depth of about 10-25m inside barrel of injection machine, then normal plastic melt temperature is reached. Flow performance and molding performance of plastic located in this area are poor. If relatively low cold material here enters cavity, defective products will be produced. In order to overcome influence of this phenomenon, a well cavity is used to extend main channel to receive cold material, prevent cold material from entering flow channel and cavity of pouring system. This well hole used to accommodate cold material generated by injection interval is called a cold material well.
When main channel and branch channel are long, ends of main channel and branch channel can be extended along forward direction of material flow as branch channel cold material well to store forward cold material. Its length is 1.5-2 times diameter of main channel or shunt channel.
The entire pouring system (aggregate) is shown in Figure 12:

 

After mold is closed, certain parts between movable mold plate and fixed mold plate form a mold cavity that can be filled with plastic melt. Shape and size of mold cavity determine shape and size of plastic part. All parts that make up mold cavity are called molded parts. Molded parts are in direct contact with plastic melt during operation, must withstand high-pressure erosion and demoulding friction of molten plastic flow. Therefore, formed parts not only require correct geometry, high dimensional accuracy and low surface roughness values, but also require reasonable structure, high strength, stiffness and good wear resistance.
When designing molded parts of injection mold, the overall structure of cavity should be determined based on plastic properties, usage requirements, and geometric structure of molded plastic parts, combined with selection of parting surfaces and gate locations, demoulding methods, and exhaust position considerations; calculate size of cavity of molded part based on size of parts; determine combination of cavities; determine requirements for machining, heat treatment, assembly, etc. of molded parts; check strength and stiffness of key parts.
Formed parts are parts that determine geometry and size of plastic parts. It is main part of mold, mainly including concave mold, punch and inserts, forming rods and forming rings. Due to particularity of plastic molding, design of plastic molded parts is different from convex and concave mold designs of cold stamping dies.

For selection of mold steel, following requirements must be met:
1) Good machining performance. Choose a steel type that is easy to cut and can produce high-precision parts after processing.
2) Excellent polishing performance. Working surface of injection molded parts often needs to be polished to a mirror surface, Ra≤0.05um. Hardness of steel is required to be between HRC35-HRC40. A surface that is too hard can make polishing difficult. Microstructure of steel should be uniform and dense, with very few impurities and no defects or pinpoints.
3) Good wear resistance and fatigue resistance. Injection mold cavity is not only washed by high-pressure plastic melt, but also affected by alternating hot and cold temperature stresses. General high-carbon alloy steel can obtain high hardness through heat treatment, but it has poor toughness and is prone to surface cracks, so it is not used. Selected steel type should enable injection mold to reduce number of polishing repairs, maintain dimensional accuracy of cavity for a long time, and achieve service life of planned mass production.
4) Corrosion resistant. For some plastic types, such as polyvinyl chloride and flame-retardant plastics, steel types with corrosion resistance must be considered.
Based on fact that the higher surface quality of plastic part determines the higher surface quality of mold, and based on above standards, mirror steel PMS was selected in design of molded part (concave mold).
PMS (10Ni3CuAIVS) is supplied with a hardness of HRC30 and is easy to cut. Then it is aged at 500℃-550℃ in a vacuum environment for 5h-10. Composite alloy chemical has just been dispersed and precipitated to harden steel, which has HRC40-HRC45, good wear resistance and small deformation during processing. Because material is pure, it can be mirror polished, has good electrical processing and corrosion resistance properties.

A mold that can only produce one plastic product in one injection is called a single-cavity mold. If a mold can produce two or more plastic products in one injection, such a mold is called a multi-cavity mold. Although in the case of mass production, multi-cavity molds should be a more suitable form, it can improve production efficiency and reduce the overall cost of plastic parts. However, single-cavity mold plastic parts have good shape and size consistency, molding process conditions are easy to control, mold structure is simple and compact, mold manufacturing cost is low, and manufacturing cycle is short. Therefore, after comprehensive consideration, we use a single-cavity mold.

Parting surface is an important factor in determining structural form of mold. It is closely related to the overall structure of mold and manufacturing process of mold, directly affects flow characteristics of plastic melt and demoulding of plastic.
Since parting surface is affected by many factors such as molding position of plastic part in mold, design of gating system, structural craftsmanship and dimensional accuracy of plastic part, position of insert, push-out of plastic part, and exhaust, etc., a comprehensive analysis and comparison should be made when selecting parting surface to select a more reasonable solution. When selecting parting surfaces, following basic principles should be followed:
1) Parting surface should be selected at largest cross-section of plastic part.
2) Keep plastic parts on movable mold side as much as possible, because demoulding mechanism on movable mold side is simple and easy to operate.
3) Helps ensure dimensional accuracy of plastic parts:
4) Helps ensure appearance quality of plastic parts.
5) Minimize projected area of plastic part on mold clamping plane to reduce clamping force.
6) Place long core in mold opening direction
7) Conducive to exhaust
In actual design, it is impossible to satisfy all above principles. Generally, main contradictions should be grasped and a reasonable parting surface should be determined under this premise. Based on above principles, parting surface of mold can be determined as shown in Figures 13 and 14.

 

Function of exhaust system is to discharge the air in mold cavity and gas generated by melt out of mold to ensure normal flow of melt during mold filling process. Poor exhaust will have many harmful effects, such as insufficient filling; affecting surface quality; generating high temperatures, causing plastic melt to decompose and even carbonize and burn; formation of flow marks and weld marks, which will reduce mechanical properties of plastic parts; reduce mold filling speed, affect molding cycle, reduce production efficiency, etc.
For large and medium-sized molds, vent slots are provided. It is usually opened on one side of die, at the end of melt flow. Under normal circumstances, size of exhaust groove is based on principle that gas can be discharged smoothly without overflow. Width of exhaust groove can be 1.5-6m, depth is less than 0.05mm, and length can be 0.8-1.5m. Depth of exhaust groove varies due to different materials, as shown in Table 4.
Table 4 Depth of commonly used plastic exhaust grooves

For small molds, use parting surface to vent, which needs to be located at the end of melt flow.
According to actual situation of this plastic part, injection bubbles are mainly concentrated near parting surface, so exhaust groove of mold is also opened on parting surface. According to material of mobile phone casing, we set depth of exhaust groove to 0.03, and based on experience, open an exhaust groove with a width of 2mm on die (Figure 15).

 

Figure 15 Exhaust diagram

Working dimensions of molded parts refer to dimensions directly used to form surface of plastic part, such as: radial dimensions, depth and height dimensions of cavity and core, distance dimensions between holes, distance dimensions from holes or bosses to a molding surface, radial dimensions and pitch dimensions of threaded molding parts, etc.
There are many factors that affect dimensional accuracy of plastic parts. In summary, there are plastic raw materials; plastic part structure and molding process, mold structure, mold manufacturing and assembly, wear and tear during mold use and other factors. Factors related to plastic raw materials mainly refer to impact of shrinkage. Since there are many factors that affect size of plastic parts, especially impact of plastic shrinkage, calculation process is more complicated than that of cold stamping dies.
1) Structural design of concave mold and punch mold
Concave mold, also known as cavity, is main part of outer surface of molded plastic part. Punch, also known as core, is part that shapes inner surface of plastic part. Part that shapes inner surface of main part is called main core or punch, and core that shapes other small holes is called small core or forming rod. According to different structures, concave and convex molds can be mainly divided into two structural forms: integral type and combined type.
This mold is a small complex mold. Convex and concave molds are generally embedded as a whole, and processing methods such as EDM and wire cutting are used. Advantage of integral type is that it is firm, not easily deformed, and has high precision. Finally, they are connected to fixed mold plate and the movable mold plate respectively through bolts.
(1) Calculation of die size
Calibration is based on "into-body" principle: dimensions of containing surface (cavity and inner surface of plastic part) are marked with a one-way positive deviation; dimensions of contained surface (core and outer surface of plastic part) are marked with a one-way negative deviation. Dimensions of working parts of plastic parts and molded parts must comply with principle of "convex and negative, concave and positive, center alignment".
a. Calculation of radial dimensions of die
Calculation of radial size of die adopts average size method, and formula is as follows:
 
In formula:
Lm--radial dimension of die (mm);
S--Average shrinkage rate of plastic parts (shrinkage rate of ABS/PC is 0.3%-0.8%, average shrinkage rate is 0.55%);
Ls--radial nominal size of plastic parts (mm);
Δ--Plastic part tolerance value (mm) (3Δ/4 coefficient changes with accuracy and size of plastic part, generally between 0.5-0.8, take 3Δ/4 as 0.6, then Δ is 0.8);
δz--Concave mold manufacturing tolerance (mm) (when size is less than 50mm, δz, = Δ/4; when plastic part size is greater than 50mm, δz= Δ/5);
Length of die is calculated as:
 
Die width dimension is calculated as:
 

b. Calculation of depth dimension of concave mold:

Calculation of depth dimension of concave mold adopts average size method, and formula is as follows:
 
In formula: Hm--die depth dimension (mm);
S--Average shrinkage rate of plastic parts (shrinkage rate of ABS/PC is 0.3%-0.8%, average shrinkage rate is 0.55%);
Hs - nominal height dimension of plastic parts (mm);
Δ--Tolerance value of plastic parts (mm) (take 2Δ/3 as 0.5);
δz--Concave mold manufacturing tolerance (mm) (when size is less than 50mm, δz = Δ/4; when plastic part size is greater than 50mm, δz = Δ/5);
Depth dimension of die is calculated as:

 

(2) Calculation of punch size
Calibration is based on "into-body" principle: dimensions of contained surface (cavity and inner surface of plastic part) are marked with a one-way positive deviation; dimensions of contained surface (core and outer surface of plastic part) are marked with a one-way negative deviation. Dimensions of working parts of plastic parts and molded parts must comply with principle of "convex and negative, concave and positive, center alignment".
a. Calculation of punch radial dimensions
Calculation of radial size of punch adopts average size method, and formula is as follows:
 
In formula: Lm - radial dimension of punch (mm);
S--Average shrinkage rate of plastic parts (shrinkage rate of ABS/PC is 0.3%-0.8%, average shrinkage rate is 0.55%);
Ls--radial nominal size of plastic parts (mm);
Δ--Plastic part tolerance value (mm) (3Δ/4 coefficient changes with accuracy and size of plastic part, generally between 0.5-0.8, take 3Δ/4 as 0.6, then Δ is 0.8);
δz--Punch manufacturing tolerance (mm) (when size is less than 50mm, δz = A/4; when plastic part size is greater than 50mm, δz = Δ/5);
Length of punch is calculated as:

 

Punch width dimension is calculated as:

 

c. Calculation of punch depth dimensions:
Depth dimension of punch is calculated using average size method, and formula is as follows:

 

In formula: Hm--punch depth dimension (mm);
S--Average shrinkage rate of plastic parts (shrinkage rate of ABS/PC is 0.3%-0.8%, average shrinkage rate is 0.55%);
Hs - nominal height dimension of plastic parts (mm);
Δ--Tolerance value of plastic parts (mm) (take 2Δ/3 as 0.5);
δz--Punch manufacturing tolerance (mm) (when size is less than 50mm, δz= Δ/4; when plastic part size is greater than 50mm, δz= Δ/5);
Punch depth dimension is calculated as:
 
(3) Calculation of cavity wall thickness
Generally, mold wall thickness size is obtained by looking up Table 5 or calculating relevant wall thickness formula.
Table 5 Empirical data of cavity wall thickness

Note: Cavity is integral, L>100mm, value in table needs to be multiplied by 0.85~0.9.
Length of mobile phone case is L=105mm, so cavity wall thickness S=(0.2x105+17)x0.9=34.2mm, we take S=35mm. Length of cavity mold plate is M=35+105+35=175mm, and width of cavity mold plate is N=35+45+35=115mm. Figures 16 and 17 are cavity core diagrams.

 

Figure 16 Cavity Core Figure 1

 

Figure 17 Cavity Core Figure 2

Guide mechanism is an essential part of mold. Because mold requires a certain direction and position when closing, a guide mechanism must be provided. Guiding mechanism mainly has functions of positioning, guiding and withstanding certain lateral pressure. Mold clamping guide mechanism has two methods: guide pillar guide and cone surface positioning. Functions of guide mechanism: positioning function; guiding function; withstand a certain lateral pressure.
1) Guide pillar guide mechanism
Guide post guide mechanism is the most commonly used, its main parts are guide posts and guide sleeves. Guide pillar can be set on either movable mold side or fixed mold side, and should be determined according to mold structure. Guide posts of standard mold base are generally located in movable mold part. Under condition that it does not hinder demoulding, guide pillar is usually set on the side of core that is higher than parting core surface.
Guide posts and bushings of this mold are all standard parts selected when using a standard mold base. Guide post (Figure 18) adopts a stepped guide post with a diameter of 30mm and a length of 290mm. Material is made of 20 steel, which has been surface carbon quenched and has a hardness of 50-55HRC. Matching accuracy between fixed end of guide post and mold plate adopts transition fit of H7/m6, and guide part of guide post adopts clearance fit of H7/f7.

 

Figure 18 Guide post
Guide bush (19) adopts a stepped guide bush, which is easy to repair and replace, can ensure guiding accuracy. It is mainly used for large molds with high precision requirements. Guide bushing material can be made of same material as guide post or a resistant material such as copper alloy, but its hardness should be lower than guide post hardness, which can improve friction and prevent guide post or guide bushing from burring. Fitting accuracy of straight guide bushing adopts H7/r6 interference fit and is inserted into mold plate. In order to increase firmness of guide bushing insertion and prevent guide bushing from being pulled out during mold opening, stop screws can be used to tighten it. Lead guide sleeve is embedded into mold plate with H7/m6 transition fit.

 

Figure 19 Guide bushing
2) Precision positioning device
Although alignment guide of guide post and guide bush has good centering, after all, due to matching gap between guide post and guide bush, guiding accuracy cannot be very high. When high alignment accuracy or large lateral pressure is required, a cone, bevel or guide horizontal pin precision positioning device is usually designed on mold. Mold uses a tapered guide post positioning device with high positioning accuracy, but it is only suitable for small molds with small lateral force. Increasing taper positioning can withstand greater lateral pressure, reduce lateral pressure on guide pillar, increase life of guide pillar, and ensure mold closing accuracy. There are two forms of cone surface fit. One is that there is a gap between two cone surfaces, and quenched parts are mounted on mold to match cone surface to prevent offset; the other is that two cone surfaces are directly matched. In this case, both cone surfaces must be quenched.
Mold adopts form of direct matching between two conical surfaces (Fig. 20).

 

Figure 20 Taper positioning

In each cycle of injection molding, plastic part must be ejected from mold cavity. This mechanism for ejecting plastic part from cavity is called an ejection mechanism, which may also be called an ejection mechanism or a push-out mechanism.
1) Push pin pushing mechanism
Mold adopts a simple ejector pin ejection mechanism. Ejector pins are all standard parts, and ejection surfaces are all flat, so there is no need to process special-shaped surfaces. Because plastic parts are small, ejector pins used are thinner and there are more ejector pins (25), including a total of 4 circular ejector pins with a diameter of 3.5mm (Figure 21). Its structure is simple and it is widely used; a total of 21 stepped ejector pins with a diameter of 1.8mm (Figure 22) are generally used when diameter is less than 2.5-3mm to improve rigidity.

 

Figure 21 Round ejector pin

 

Figure 22 Stepped ejector pin
After demoulding mechanism completes demoulding of plastic part, it must be returned to its original position, and a reset part must be provided to reset it. And an ejection limit ring is set on guide pillar to limit ejection stroke (ejection stroke is 30mm). As shown in Figure 23.

 

Figure 23 Eject limit ring
2) Sequential demoulding mechanism
Sequential demoulding mechanism is also called sequential parting mechanism or fixed distance parting mechanism. Due to needs of plastic parts and mold structure, when mold is parted, fixed mold must be parted first, then moving and fixed molds must be parted, that is, it has two parting surfaces. Sequential demoulding mechanism usually completes more than two parting actions. Common forms of sequential demoulding mechanisms include spring screw type, swing hook type, slide plate type and guide pillar type. Mold adopts a swing hook type sequential demoulding mechanism.
As shown in Figure 24, swing hook type sequential demoulding mechanism is mainly composed of a hook, a baffle, a pressure block and a spring. Function of spring is to keep hook in position of tightening baffle. Due to function of retractor and baffle, mold is first separated from A-A surface when opening mold. Main channel condensate is pulled out from the sprue sleeve. After opening to a certain distance, under action of pressing block, hook swings, compression spring at the right end of hook moves downward, left end of hook moves upward, fixed mold stops moving under action of limit screw, and is separated from B-B surface; condensate is pulled off from point gate. Due to shrinkage of mobile phone case mold, mobile phone case wraps around core and moves with movable mold. After moving a certain distance, movable mold stops moving, ejector pin pushes out mobile phone case mold, and finally condensate is removed manually.

 

Figure 24 Swing hook type sequential demoulding mechanism

Cooling system not only affects molding quality of injection mold, but also determines production efficiency. Design principles of mold cooling system are:
1) Reasonably determine center distance of cooling pipe, distance between cooling pipe and cavity wall. If distance between cooling pipe and cavity wall is too large, cooling efficiency will decrease, while if distance is too small, cooling will be uneven. According to experience, distance between center line of cooling pipe and cavity wall should generally be 1-2 times diameter of cooling pipe (usually 12-15mm), and center distance of cooling pipe should be 3-5 times diameter of pipe.
2) Make distance from cooling hole to cavity surface same as possible. When wall thickness of plastic part is uniform, distance between cooling medium hole and cavity, cavity surface should be equal everywhere; when wall thickness of plastic part is uneven, cooling should be strengthened at wall thickness, water hole should be close to cavity, and distance should be small, but not less than 10mm.
3) Strengthen cooling at gate. Generally, when molten plastic fills cavity, temperature is the highest near gate, and the farther away from gate, the lower temperature. Therefore, cooling should be strengthened near gate as inlet of cooling water, and only warm water after heat exchange needs to pass through outside where temperature is lower.
4) Reduce temperature difference at cooling medium inlet and outlet. If temperature difference between incoming water and outgoing water is too large, temperature distribution of mold will be uneven. Especially for large plastic parts with a long process, material temperature will become lower as flow flows. In order to make cooling rate of the entire piece roughly same, in addition to shortening cooling circuit, arrangement of cooling channels can also be changed.
5) Reasonably consider arrangement of cooling pipes. It is necessary to combine characteristics of plastics and structure of plastic parts to rationally arrange cooling water channels.
6) Reasonably determine location of cooling water pipe joint. Location of inlet and outlet water pipe joints is as close to same side of mold as possible.
7) Cooling system should be designed to avoid interference with other parts of mold structure.
8) Inlet and outlet joints of cooling channel should not be higher than surface plane of mold. In other words, it should be buried in mold plate to prevent mold from being damaged during transportation.
9) Cooling water channel should be easy to process and clean. General aperture design is 8-12mm.
According to above principles, a U-shaped cooling water channel is opened on movable and fixed mold plates. Diameter of water channel hole is 8mm, distance between center line of cooling pipe and cavity wall should be 15mm. See Figure 25 and Figure 26.

 

Figure 25 Cooling system 1

 

Figure 26 Cooling system 2

Based on design of above components, final assembly drawing design of mold is completed in IMOLD. IMOLD is an expert mold plug-in system for Solidworks for calling standard mold bases and detailing and designing molds. Call Futaba's FC-TYPE standard mold base in IMOLD, as shown in Figure 27, with a size of 350mm*350mm*375mm.

 

Figure 27 FC-TYPE standard mold base
According to structure of each component designed above, call corresponding standard room in IMOLD to complete three-dimensional assembly drawing of the entire tool (Figure 28).

 

Figure 28 Mold design drawing
Assembly diagram of injection mold is shown on last attached page.

Create a new project and import mobile phone shell file with suffix name stl. Generally, global grid side length is 1.5-2 times minimum wall thickness of part. Default global grid side length is 3.28mm, which meets above requirements. First, divide it according to default initial grid side length. Result is shown in Figure 29:

 

Figure 29 Initial meshing
After division is completed, we need to evaluate quality of grid and first perform grid statistics (Figure 30).

 

Figure 30 Initial grid evaluation
It can be seen from statistical information of grid that there are no major grid defects in grid, but aspect ratio needs further modification. It should be noted that mesh matching percentage does not reach more than 90% of warpage analysis accuracy requirements, so mesh needs to be re-divided. Modify global mesh side length to 1.5mm. Mesh statistics show that matching percentage of mesh has increased, meeting requirements of warpage analysis. Of course, aspect ratio has also increased accordingly. See Figure 31 and Figure 32.

 

Figure 31 Corrected meshing

 

Figure 32 Corrected Grid Evaluation

Grid connectivity diagnosis: From diagnosis results, it can be easily judged that grid is all connected and there are no disconnected areas (Figure 33).

 

Figure 33 Grid connectivity diagnosis

Mesh thickness diagnosis: Thickness attribute of mesh is very important. Generally, it is necessary to ensure that thickness attribute of mesh is consistent with wall thickness of product to ensure accuracy of analysis results obtained. Judging from diagnostic results, there is nothing wrong with its thickness (Figure 34).

 

Figure 34 Mesh thickness diagnosis

From cooling analysis log (Figure 35), we can see that temperature rise is 0.5℃, which is very small.

 

Figure 35 Cooling analysis log
Figure 36 shows analysis log of partial filling + pressure holding. From log information, we can know that weight of product after pressure holding is 0.477g heavier than weight of product at the end of filling.

 

Figure 36 Analysis log of partial filling + pressure holding
From warpage analysis log (Figure 37), we can see that warpage values along three axes are relatively large, with maximum on Y axis.

 

Figure 37 Warp analysis log
From above analysis log, we can see some intermediate data results during analysis process and some summaries of analysis. If you want to analyze flow, cooling and warpage of product in more detail, you can view results below in solution task browsing area.

(1) Filling time
Analysis of filling time is shown in Figure 38. It takes 0.9622s for this product to complete filling. From contour map, it can be seen that contours are basically uniform, and flow is considered to be relatively stable.

 

Figure 38 filling time
(2) Pressure during speed/pressure switching
Analysis results of pressure during speed/pressure switching are shown in Figure 39. At the moment when speed and pressure control points are switched, injection pressure reaches maximum value of 69.08MPa, which is 186MPa higher than maximum injection force of injection molding machine and meets requirements. When setting holding pressure curve, initial holding pressure can be set to 80% of this value. Of course at this moment, product is not completely filled.

 

Figure 39 Pressure during speed/pressure switching
(3) Overall temperature
Analysis results of the overall temperature are shown in Figure 40. The overall temperature distribution of product is uniform, temperature difference is within 10℃, and the overall temperature should be lower than recommended ejection temperature. It is worth noting that temperature of runner is too high, indicating that cooling of gating system needs to be strengthened.

 

Figure 40 Overall temperature
(4) Pressure at injection position: XY diagram
Pressure at injection position: Analysis results of XY chart are shown in Figure 41. Maximum pressure at injection position is at speed/pressure conversion moment. It can be seen from chart that curve is relatively smooth, indicating that there is no obstruction during melt flow.

 

Figure 41 Pressure at injection location: XY diagram
Analysis results of volume shrinkage during ejection are shown in Figure 42. When product is ejected, volume shrinkage is quite different on both sides of product and is uneven. In the future, holding pressure curve needs to be optimized to reduce volume shrinkage.

 

Figure 42 Volume shrinkage rate during ejection
(5) Time to reach ejection temperature
Analysis results of time to reach ejection temperature are shown in Figure 43. For product itself, time to reach ejection temperature is basically same, but it is seen that time for runner to reach ejection temperature is about 3 times ejection time of product, so it is not completely consistent. Therefore, gating system requires enhanced cooling.

 

Figure 43 Time to reach ejection temperature
(7) Freezing layer factor
Analysis results of frozen layer factor are shown in Figure 44. After cooling, product has completely solidified and gate has been completely frozen, which will not cause backflow, so cooling at gate is appropriate.

 

Figure 44 Frozen layer factor
(8) Clamping force: XY diagram
Clamping force: Analysis results of XY diagram are shown in Figure 45. Maximum clamping force of this product is 7.976t. Rated clamping force of injection molding machine selected at 1.623s is 80t, which meets requirements.

 

Figure 45 Clamping force: XY diagram

(1) Loop cooling medium temperature
Analysis results of temperature of loop cooling medium are shown in Figure 46. Temperature difference between water inlet and water outlet is 0.3℃. Temperature difference between inlet and outlet should not exceed 2-3℃, otherwise cooling effect will be poor.

 

Figure 46 Temperature of cooling medium in loop
(2) Loop flow rate
Analysis results of loop flow rate are shown in Figure 47. Since this cooling system is connected in series, flow rate of loop remains consistent.

 

Figure 47 Loop flow rate
(3) Loop pipe wall temperature
Analysis results of loop pipe wall temperature are shown in Figure 48. Temperature difference between loop pipe wall is relatively uniform, and difference from cooling medium temperature is less than 2℃. This temperature difference is less than 5℃, which is acceptable.

 

Figure 48 Loop wall temperature
(4) Maximum temperature position, product
At the highest temperature position, analysis results of product are shown in Figure 49. Indicates temperature distribution on wall thickness of product. Most logically, maximum temperature should be in the middle of wall thickness. It is not difficult to know from analysis results that the highest temperature is mainly concentrated on the inside of product, so inside needs to be strengthened for cooling.

 

Figure 49 Maximum temperature position, product

(1) Deformation, all factors: deformation
Deformation, all factors: Analysis results of deformation are shown in Figure 50. This analysis result can determine the overall warpage value of part, with maximum value being 0.4982mm. For better observation, deformation scale factor is set to 10. It can be seen that deformation on both sides of part is larger.

 

Figure 50 Deformation, all factors: Deformation
(2) Deformation, uneven cooling: deformation
Deformation and uneven cooling: Analysis results of deformation are shown in Figure 51. Warpage value caused by uneven cooling is 0.0281mm, accounting for 5.6% of the total warpage value, which shows that uneven cooling is not main cause of deformation. For better observation, deformation scale factor is set to 100. It can be seen that deformation at parting surface is larger.

 

Figure 51 Deformation, uneven cooling: Deformation
(3) Deformation, uneven shrinkage: deformation
Deformation, uneven shrinkage: Analysis results of deformation are shown in Figure 52. Warpage value caused by uneven shrinkage is 0.4927mm, accounting for vast majority of the total warpage value. It can be seen that deformation of part is mainly caused by uneven shrinkage. For better observation, deformation scale factor is set to 10. It can be seen that deformation on both sides of part is larger. Uneven shrinkage is caused by insufficient holding pressure, so an effective measure is to optimize holding pressure curve.

 

Figure 52 Deformation, uneven shrinkage: Deformation
(4) Deformation, orientation factors: deformation
Deformation, orientation factors: Analysis results of deformation are shown in Figure 53. Deformation due to orientation is 0 and does not need to be taken into account.
Figure 53 Deformation, orientation factors: deformation

 

Attachment: Injection mold engineering drawing

 

Assembly drawing top view:

 

Assembly drawing left view:

 

Assembly drawing section view: