For previous reading, please refer to Mold Design Guidelines - Product Structure (2).
Mold Structure Design
Mold structure is determined by type of injection molding machine and characteristics of plastic parts. Due to wide variety of plastic parts and different injection molding machines used, mold structures vary. However, regardless of variations, following aspects should be considered when designing a mold:
(1) Technical specifications of injection molding machine (see Chapter 2)
(2) Processing properties of plastic (see Chapter 2)
(3) Gating system, including runners, gates, etc. (see Chapter 9)
(4) Molding components
(5) Commonly used structural components
(6) Sliding mechanism (see Chapter 7)
(7) Ejection mechanism (see Chapter 8)
(8) Mold temperature control (see Chapter 10)
(9) Venting (see Chapter 9)
(10) Mold material (see Chapter 4)
When designing a mold, various factors should be considered comprehensively to select a reasonable structural form that meets purpose of mold forming. Following explains some of the more important aspects not discussed in other chapters.
5.1 Part Layout
Part layout refers to arranging one or more required plastic parts according to customer requirements, based on a reasonable injection molding process and mold structure. Part layout is complementary to mold structure and plastic processability, directly affects subsequent injection molding processes. Mold structure must be considered during layout, and adjustments made to layout while meeting mold structure requirements.
From an injection molding process perspective, following points need to be considered:
(1) Flow length. Different plastic materials have different flow lengths. If flow length exceeds process requirements, plastic part will not be fully filled. (See Chapter 2 for details.)
(2) Runner waste. Under premise of ensuring each cavity is fully filled, runner length and cross-sectional dimensions should be as small as possible to minimize runner waste.
(3) Gate location. When gate location affects part layout, gate location must be determined first before layout. In the case of a multi-cavity part, gate location should be uniform.
(4) Injection balance. Injection balance means that plastic material fills each cavity simultaneously when they are essentially same. To achieve balanced injection, following methods are generally used:
A. Balanced arrangement (as shown in Figure 5.1.1), suitable for parts with roughly same size.

 

B. Arrange parts with larger parts closer to main runner and smaller parts further away, then adjust runner and gate dimensions to achieve balanced injection (see Chapter 9 for runner and gate design details). Note: When weight ratio of large and small parts is greater than 8, adjustments should be made in consultation with product designer. In this case, adjusting runner and gate dimensions is unlikely to meet balance requirements.
(5) Cavity pressure balance. Cavity pressure consists of two parts: axial pressure parallel to mold opening direction and lateral pressure perpendicular to mold opening direction. Arrangement should strive to balance axial and lateral pressures relative to mold center to prevent overflow and peak formation.
Methods to achieve pressure balance:
A. Uniform and symmetrical arrangement. Axial balance is shown in Figure 5.1.2; lateral balance is shown in Figure 5.1.3.

 

B. Using mold structure for balancing, as shown in Figure 5.1.4, is a commonly used method for balancing lateral pressure. For specific technical requirements, please refer to next section.

 

From perspective of mold structure, following points need to be considered:
(1) Meeting sealing requirements. Arrangement should ensure that runner and nozzle are a certain distance from edge of front mold cavity to meet sealing requirements. Generally, D1≥5.0mm and D2≥10.0mm are required, as shown in Figure 5.1.5. Distance between slide groove and sealing edge should be greater than 15 mm.

 

(2) Meet space requirements of mold structure. Layout should meet space requirements of mold structural components, such as ejectors, slides, and lifters. Following points should also be ensured:
A. Mold structural components have sufficient strength.
B. There is no interference with other mold base components.
C. When there are moving parts, stroke must meet ejection requirements. When there are multiple moving parts, there should be no mutual interference. (See Figure 5.1.6)
D. Location of ejector sleeve should avoid location of ejector pin hole.

 

(3) Fully consider screws, cooling water, and ejector devices. To achieve better cooling of mold, layout should consider influence of screws and ejectors on cooling water holes, reserve location of cooling water holes.
(4) Whether length-to-width ratio of mold is coordinated. Layout should be as compact as possible to reduce the overall size of mold, and length-to-width ratio should be appropriate. Installation requirements of injection molding machine should also be considered.
5.2 Determination of parting surface.
5.2.1 Principles for selecting parting surface.
Surface on which mold is opened to remove plastic parts or gating system is called parting surface. Besides being affected by arrangement of parts, parting surface is also influenced by various factors such as shape, appearance, precision, gate location, slide, ejection, and machining of plastic part. A reasonable parting surface is a prerequisite for successful molding of plastic part. Generally, following aspects should be considered comprehensively:
(1) It should meet basic requirements for demolding, allowing plastic part to be removed from mold. Parting surface should be located at edge of the largest projection in demolding direction.
(2) It should ensure plastic part remains on the rear mold side, facilitating ejection and preventing ejector pin marks from being visible on the exterior surface.
(3) Parting line should not affect appearance of plastic part. Parting surface should, as far as possible, not damage smooth outer surface of plastic part.
(4) It should ensure quality of plastic part. For example, placing parts requiring coaxiality on same side of parting surface.
(5) Parting surface should avoid forming side holes or recesses. If slide molding is required, slide structure should be simple, avoiding front mold slides as much as possible.
(6) Gating system should be arranged reasonably, especially gate location.
(7) To meet locking requirements of mold, place direction with larger projected area of plastic part on mold closing direction of front and rear molds, and use direction with smaller projected area as side parting surface; in addition, when parting surface is curved, a sloping surface should be added for locking.
(8) Facilitates mold processing.
5.2.2 Parting Surface Precautions and Requirements
(1) Stepped Parting Surface
Generally, horizontal distance D between top surface of step and root should be ≥0.25, as shown in Figure 5.2.1. To ensure requirement of D, included angle "A" is generally adjusted. When included angle affects product structure, it should be determined in consultation with relevant person in charge. When there are several stepped surfaces in parting surface, and H1≥H2≥H3, angle "A" should satisfy A1≤A2≤A3, and same angle should be used as much as possible for easy processing.

 

Angle "A" should be selected according to following requirements:
When H≤3mm, draft angle a≥5; 3mm≤H≤10mm, draft angle a≥3°; H>10mm, draft angle a≥1.5°;
When certain plastic parts have special requirements for their draft angle, selection should be made according to product requirements.
(2) Curved Parting Surface
When selected parting surface has characteristics of a single curved surface (such as a cylindrical surface), as shown in Figure 5.2.2, it is required to construct parting surface according to type shown in Figure 5.2.2a, that is, by extending a certain distance along curvature direction of curved surface. Otherwise, an unreasonable structure as shown in Figure 5.2.3a will be formed, resulting in sharp steel and sharp-angled sealing surfaces. Sharp sealing positions are difficult to seal and are easily damaged.

 

When parting surface is a complex spatial curved surface, and it is impossible to extend it a certain distance along curvature direction, surface cannot be directly extended to a plane. This would result in steps and sharp sealing surfaces as shown in Figure 5.2.4a. Instead, a smoother sealing surface should be constructed along curvature direction, as shown in Figure 5.2.4b.

 

(3) Sealing Distance
In mold, it is important to ensure an effective sealing distance on same curved surface. As shown in Figures 5.2.3a and 5.2.3b, D > 3mm is generally required.

 

(4) Reference Plane
When constructing parting surface, if there are one or more parting surfaces with height differences, such as stepped or curved surfaces, a reference plane must be constructed, as shown in Figures 5.2.5a and 5.2.5b. Purpose of reference plane is to provide a placement plane and machining reference for subsequent machining.
(5) Parting Surface Turning Point
As shown in Figure 5.2.6, turning point here refers to stepped surface formed by parting surfaces at different heights to connect with reference plane.

 

Stepped surface should be as flat as possible. Dimension "A" in figure is generally required to be greater than 15°. This surface is allowed to be left unoccupied during mold closing. Turning angle R should be based on machining tool radius, generally R≥3.0mm.
(6) Balancing Lateral Pressure
Because lateral pressure generated by cavity cannot be balanced by itself, it easily causes misalignment of front and rear molds in direction of force. Generally, a locking bevel is added to utilize rigidity of front and rear molds to balance lateral pressure, as shown in Figure 5.2.7. Locking bevel must fit completely during mold closing. Angle A is generally 15°; the larger bevel, the worse balancing effect.

 

(7) Flattening parting surface of nozzle
When constructing parting surface, if there is a height difference between parting surfaces near nozzle, a flatter surface must be used for connection. Area of flat surface should be larger than nozzle diameter; generally, effective area should be greater than ∅18mm, as shown in Figure 5.2.6.

 

(8) Treatment of Parting Surfaces at Small Holes
Whether small hole is left as is or a pin is inserted, following methods are generally used to construct hole. For simplicity in mold making, it is recommended to insert a pin at hole, but this must be approved by designer.
A. Direct Through-hole. As shown in Figure 5.2.9, this is suitable for structures with relatively flat through-hole areas. However, for keyholes such as "keyboard" buttons (as shown in Figure 5.2.10a), to change direction of potential "burrs," an insert-type structure and size are often used, as shown in Figure 5.2.10b.

 

 

B. Intermediate Plane Penetration. As shown in Figure 5.2.11a, this is suitable for structures with steeper penetration points.
Using an intermediate plane penetration structure can effectively shorten height of steel part at penetration hole, improving stress distribution on steel part. To avoid misalignment of front and rear molds, it is recommended to use dimensions and structure shown in Figure 5.2.11a. In structure shown in Figure 5.2.11b, due to lateral force generated at penetration point, when penetration hole is small, steel part at penetration hole is prone to fracture under alternating stress, affecting mold life.

 

C. Insertion. This is generally not used, except in following situations:
(1) When height difference between point "a" and point "b" is less than 0.5mm, as shown in Figure 5.2.12a, an insertion structure is used.
(2) When point "a" is higher than point "b", as shown in Figure 5.2.12b, an insertion structure is used.

 

When using an insert structure, structure and dimensions shown in Figure 5.2.12c are commonly adopted. Minimum distance between sealing surfaces must be 1.0mm; guide angle A ≥ 5° and length H ≥ 2.5mm.
(9) Avoiding Sharp Edges
When parting line needs to divide a curved surface, to avoid sharp edges, direction of parting surface should be normal direction of any point on parting line. As shown in Figure 5.2.13.

 

(10) Comprehensive Consideration of Product Appearance Requirements
When there are multiple options for parting surface for a single product, product appearance requirements should be comprehensively considered, and a more concealed parting surface should be selected. For finished products with slatted parting lines, slatted parting line must take into account structure of adjacent finished products. If adjacent finished products also require slatted parting lines, then slatted parting lines should be adjusted and aligned; as shown in Figures 5.2.14a; 5.2.14b; 5.2.14c; if adjacent finished products do not require slatted parting lines, slatted parting line should be shortened as much as possible while meeting structural requirements, as shown in Figure 5.2.7d.

 

5.3.1 Strength Verification
Mold strength calculation is relatively complex. A simplified calculation method is generally used, employing a conservative approach. Principle is to select the most unfavorable stress structure, choose a large safety factor, and then optimize mold structure to maximize mold strength.
To ensure mold functions properly, it is necessary to verify not only the overall strength of mold but also strength of its local structures.
Overall strength mainly addresses thickness of cavity sidewalls, thickness of cavity bottom plate, and pressure that parting surface can withstand. Calculations should be performed according to Appendix D, "Mold Design Verification," in MQP702. Actual selected dimensions should be larger than calculated dimensions and rounded down.
For strength of other components, such as inserts, ejectors, slides, spades, and even guide pillars, verification should be performed using simplified calculations below. Verification should consider both strength and bending aspects, selecting larger dimension.
Simplified model is shown in Figure 5.3.1.

 

Strength calculation: [δ] = M_Max/W_z; Bending calculation: f_y = FL³/3El_z
F - Force on member; fy - Elastic deformation; M_max - Maximum bending moment
W_z - Bending modulus, W_z = ab²/6 for square members; = πD³/32 for circular members
z - Section modulus, W_z = ab³/12 for square members; = πD³/6⁴ for circular members
D - Diameter of circle
[δ] - Maximum allowable bending stress, [δ] = 200 MPa for premium steel; [δ] = 300~350 Pa for pre-hardened mold steel; E = 2.1 x 10⁵ MPa
5.3.2 Improve overall strength
(1) Avoid sharp corners in cavity as much as possible. As shown in Figure 5.3.2, adding rounded corners significantly helps to enhance rigidity of side walls. In addition, it can also reduce stress fatigue and extend service life of mold. Therefore, four corners of front and rear mold frames must be made into rounded corners, inserts in the front and rear molds should also try to avoid appearance of sharp corners.

 

(2) Add locking blocks to reduce elastic deformation, as shown in Figure 5.3.3.

 

y---Virtual elastic deformation; w---Cavity wall thickness
For deep cavity molds, in order to reduce amount of elastic deformation, a sloping locking block is added between front and rear molds to utilize rigidity of mold plate to strengthen constraint on cavity wall.
(3) Reduce spacing between square iron pieces, as shown in Figure 5.3.4.
To reduce elastic deformation y, minimize spacing L between square iron pieces while still meeting ejection requirements. Simultaneously, shift cavity pressure towards square iron pieces to ensure requirements shown in figure are met as much as possible.

 

(4) Pay attention to direction of mold core assembly and select a reasonable assembly structure, as shown in Figure 5.3.5.

 

(5) Add supports as shown in Figure 5.3.6.
Arrangement of supports should be determined according to actual situation, with as many supports as possible. During assembly, both end faces must be flat, and all supports must be of same height.
Common support specifications: ∅38mm; ∅45mm; Support material: High-quality steel
5.3.3 Enhancing Component Strength
For molds, component strength is as important as overall strength. Component stress conditions are complex. Besides simple calculations for verification, a fundamental principle must be followed: maximize strength, meaning maximizing component structure within permissible structural space.
Below are some methods to improve component strength.
(1) Modify plastic component structure to avoid sharp or thin steel bars.
If an unreasonable plastic component structure leads to sharp or thin steel bars in mold, a solution should be discussed with product design team.

 

(2) Add locking blocks to improve mold structure and increase strength of component (shovel), as shown in Figure 5.3.8.

 

(3) Utilize rigidity of mold blank to increase strength of component (shovel), as shown in Figure 5.3.9.

 

(4) Improve component structure, increase component size, and enhance component strength.
As shown in Figure 5.3.10, left component "W1" is smaller and more prone to deformation; right component not only has an improved structure but also an increased component size "W2," which is beneficial for improving strength. In this structure, to reduce deformation, fillet at "R" should be increased, and size of "H" should be reduced. "H" is generally taken as 8.0~10.0mm.

 

(5) Positioning ends of high or long cores to improve strength and reduce core deformation.
In mold structures with high or long cores (as shown in Figure 5.3.11a), design should fully utilize through holes at the ends for core positioning, as shown in structure of Figure 5.3.11b. If through holes are not permitted at ends, a solution should be sought in consultation with mold design manager.

 

(6) Utilizing interlocking structures to improve local strength.
In fine structures of plastic parts, if there are thin steel sections or stress concentration points (as shown in Figure 5.3.12a), these areas should be designed as interlocking structures to eliminate stress concentration points, reduce fatigue damage, and facilitate heat treatment of inserts to increase strength, as shown in Figure 5.3.12b.