For previous reading, please refer to Mold Design Guide - Introduction to Properties of Commonly Used Plastics and Related Parameters and.
Product Structure
An unreasonable plastic part structure will cause difficulties in mold manufacturing and plastic part forming; mold engineers should propose improvement plans for plastic part structure and inform product designers for confirmation.
Upon receiving customer data, data should be processed as necessary.
Based on customer data, analysis of plastic part structure mainly includes following aspects: (1) Requirements of injection molding process for plastic part structure; (2) Requirements of mold for plastic part structure; (3) Requirements of product assembly for plastic part structure; (4) Surface requirements.

Process problems such as shrinkage, sunken areas, air entrapment, deformation, and scorching in plastic parts are related to factors such as local plastic thickness, gate setting, and cooling. Processability analysis of plastic part structure should be carried out from following aspects.
3.1.1 Wall Thickness
Wall thickness of plastic part should be uniform and consistent, avoiding abrupt changes and designs with significant differences in cross-sectional thickness; otherwise, uneven shrinkage will occur, causing defects on surface of plastic part. Wall thickness of plastic parts generally ranges from 1 to 6 mm, with the most common values being 1.8 to 3 mm, depending on type and size of part.
For a 3D model of a plastic part, cross-sectional analysis using Pro/E can reveal uneven wall thickness. Steps are: Analysis® Model Analysis® Thickness® [Given maximum and minimum plastic thicknesses, select start and end points of analysis, and determine corresponding parallel cross-sections]® Compute, as shown in Figure 3.1.1.

 

Furthermore, wall thickness of plastic part is closely related to melt flow path; flow path refers to distance molten material travels from gate to various points in mold cavity. Under normal process conditions, flow path length is directly proportional to wall thickness. The greater wall thickness, the longer maximum allowable flow path. Feasibility of forming plastic part can be checked using formulas or charts.
Wall thickness of plastic part is 2.5mm. Under normal molding conditions, flow of commonly used materials is as follows: ABS: 220mm flow; PC: 120mm flow; HDPE: 280mm flow; POM: 180mm flow.
Common problems caused by uneven wall thickness:
(1) Local thick areas, as shown in Figure 3.1.1, are prone to surface shrinkage and depressions.
(2) As shown in Figure 3.1.2, thin areas on both sides of plastic part are prone to molding stagnation.
(3) At stop position, as shown in Figure 3.1.3, a gradual thickness method is used to eliminate surface white marks; additionally, rounded corners are added to internal corners of plastic part to make wall thickness uniform.

 

(4) As shown in Figure 3.1.4, recess in the middle of plastic part is too deep, resulting in an arched deformation in actual molded plastic part; solution to this deformation is to reduce depth of recess to make wall thickness as uniform as possible.

 

(5) As shown in Figure 3.1.5, sharp corners are prone to heat marks. To avoid this, rounded corners are used for transition.

 

3.1.2 (Rib) Rib
Ribs of plastic parts serve to increase strength, fix bottom shell, provide support, and guide buttons. Because shrinkage and depressions are prone to occur at connection between rib and plastic part shell, rib thickness should be less than or equal to 0.5t (t is wall thickness of plastic part). Generally, rib thickness is in the range of 0.8–1.2 mm.
When rib depth is greater than 15 mm, it is easy to cause difficulties in material flow and air trapping. Inserts can be made on mold, which also facilitates mold saving and venting.
For rib depths less than 15 mm, draft angle should be greater than 0.5˚; for rib depths greater than 15 mm, thickness difference between root and top of rib should not be less than 0.2 mm, as shown in Figure 3.1.6.

 

To improve flow conditions in certain deep rib areas, flow guides are added to ribs; as shown in Figure 3.1.7, flow guides are added to flared ribs, and inserts are made in mold.

 

3.1.3 Gate
Selection of gate location and injection method directly affects molding quality of plastic part and smoothness of injection process. Gate location and form of plastic part should be analyzed and determined; even gates already determined in customer's plastic part data need to be analyzed, and suggestions should be made for any inappropriate aspects.
Principles for setting gate are as follows:
(1) Ensure that flow front of material can reach end of cavity simultaneously, and minimize its flow path, as shown in Figure 3.1.8;

 

(2) Gate should first enter from thicker wall section to facilitate pressure holding and reduce pressure loss;
(3) If there are small cores or inserts in cavity, gate should avoid direct impact to prevent deformation;
(4) Gate should be located in a place where material can be easily cleaned, making it easy to repair and not affecting appearance of material, as shown in Figure 3.1.9;

 

(5) Facilitate venting within cavity, allowing gas in cavity to be squeezed into vicinity of parting surface;
(6) Avoid "racetrack" effect of material flow, which can cause trapped air and weld lines in material;
(7) Avoid phenomena such as air drying and serpentine patterns at the gate, as shown in Figure 3.1.10, Figures 3.1.11 and 3.1.12;

 

(8) Direction of rubber material flow should be such that when it flows into cavity, it can flow evenly along parallel direction of cavity to avoid anisotropic flow of rubber material, which would cause warping deformation and stress cracking of rubber parts, as shown in Figures 3.1.13 and 3.1.14.

 

For some rubber parts with complex mold filling flow, and for molds with multiple cavities or multiple finished products, as shown in Figure 3.1.15, determination of gate position and size can be solved by applying for CAE (Moldflow software) analysis.

 

Analysis of whether structure of rubber parts meets requirements of mold forming and demolding can be carried out from following aspects: draft angle, rubbing position, slide, inclined ejector, thin steel position, demolding.
3.2.1 Draft angle
Rubber parts must have sufficient draft angle to avoid whitening, scratching and dragging phenomena. Draft angle is related to properties of plastic material, shape of part, and surface requirements.
Recommended minimum draft angle values for commonly used plastic materials are provided. For parts in the 3D part file that do not require a draft angle, refer to general draft angle requirements in technical specifications. Draft angle varies depending on whether part's surface is smooth or textured, as follows:
(1) Smooth surface: Draft angle /1˚ for small parts, Draft angle /3˚ for large parts;
(2) Textured surface: Ra < 6.3 Draft angle /3˚, Ra/6.3 Draft angle /4˚;
(3) EDM surface: Ra < 3.2 Draft angle /3˚, Ra/3.2 Draft angle /4˚.
For 3D model of plastic part, use Pro/E to check draft angle. Steps are as follows: Analysis® Surface Analysis® Draft Check® [Give maximum draft angle value, select analysis part or surface, and determine direction surface corresponding to analysis]®Compute. It should be noted that when modifying draft angle of plastic part, assembly relationship and appearance requirements of plastic part must also be guaranteed, as shown in Figure 3.2.1.

 

3.2.2 Scraping and Contact Surfaces
Scratching and contact surfaces of mold are shown in Figure 3.2.2. Scratching surface of mold should have a draft angle. Scratching surface draft angle has two functions: (1) Preventing overflow of glue, because vertical bonding surface cannot be preloaded; (2) Reducing wear. Scratching and contact surfaces can be analyzed from following aspects:
(1) Ensuring structural strength. As shown in Figure 3.2.3, in order to avoid deformation or breakage of protruding parts of mold, it is more reasonable to design B/H value to be greater than or equal to 1/3.

 

 

(2) Preventing formation of burrs. As shown in Figure 3.2.3, contact value is E/1.2mm. As shown in Figures 3.2.4 and 3.2.5, rubbing gap value is guaranteed to be e/0.25mm. If considering rubbing slope, when h ≤ 3mm, slope is α/5˚; when h > 3mm, slope is α/3˚; when certain plastic parts have specific requirements for slope, rubbing height is h/10mm, and allowable slope is α/2˚. Sealing position of rubbing contact tip should have a radius of R0.5 or more.

 

(3) Facilitates mold processing and maintenance. As shown in Figures 3.2.6 and 3.2.7, inserts are made on rotating shaft mold.

 

3.2.3 Sliding and Lifter 
When side wall of plastic part has concave and convex shapes, side holes, and snap-fit positions, side core must be pulled out before mold opens and ejects plastic part. This mechanism is called sliding. As shown in Figure 3.2.8, outer hole of plastic part requires sliding core to be pulled out by rear mold. As shown in Figure 3.2.9, for inner groove of plastic part, if a lifter is used for demolding, top opening distance is insufficient; therefore, an inner slide mechanism must be used.

 

Additionally, an ejection mechanism that simultaneously ejects and pulls core using lifter is called a lifter. For parts of plastic part requiring core pulling, when slide space is insufficient, a lifter mechanism can be used. In lifter mechanism, lifter distance should be greater than core pulling distance (B > H), as shown in Figure 3.2.10, to prevent ejection interference.
As shown in Figure 3.2.11, inner and outer walls of plastic part have concave shapes. Due to obstruction from inner side by a rib and insufficient height, outer wall must be propelled by front mold slide, and inner wall by a lifter.

 

As shown in Figure 3.2.12, there should be no clamping lines around side holes of plastic part. Side holes must be pulled by front mold slide, then demolded by a snap-fit lifter.
3.2.4 Parting Surface
Regardless of whether parting surface is specified in plastic part documentation, mold designer must determine it specifically. Any unreasonable aspects of specified parting surface should be reported to relevant party.
When analyzing parting surface of plastic part, pay attention to following points:
(1) Determine position of surface clamping line according to appearance requirements, as shown in Figure 3.2.13.

 

(2) Place parts of plastic part with coaxiality requirements or prone to misalignment on same side of parting surface, as shown in Figures 3.2.14 and 3.2.15.

 

 

(3) Consider difference in size between large and small ends of plastic part caused by draft angle, as shown in Figure 3.2.16.

 

(4) Determine orientation of plastic part within mold so that parting surface formed should, as far as possible, prevent formation of side holes or side recesses to avoid using complex mold structures, as shown in Figures 3.3.16 and 3.3.17.

 

3.2.5 Sharp and Thin Steel Parts
Avoid sharp and thin steel parts that may affect strength and service life of mold. Sharp or thin steel marks are generally not easily visible on plastic parts. Analysis should be based on mold conditions of plastic part. There are two main reasons for sharp or thin steel marks on mold: structure of plastic part and mold structure.
(1) Sharp or thin steel marks caused by structure of plastic part. As shown in Figures 3.2.18 and 3.2.19, a double-forked plastic part produces sharp or thin steel marks on mold; this can be changed to a single-forked part or middle width can be increased to avoid sharp or thin steel marks on mold.

 

(2) Sharp or thin steel marks caused by mold structure. As shown in Figure 3.2.20, sharp steel marks easily appear on mold at rounded corners of plastic part; mold structure is shown in Figure 3.2.21, and this method of mold parting produces sharp steel marks; as shown in Figure 3.2.22, extending parting surface along arc normal direction can avoid sharp steel marks.

 

3.2.6 Molding of Plastic Parts
Molding parts are usually ejected using ejector pins, ejector sleeves, and push plates. If plastic part has special structures or surface finish requirements, other ejection methods must be used, such as ejector block ejection, angled ejection, threaded rotation ejection, or secondary ejection. For ejection of certain transparent plastic parts, care must be taken to ensure that ejection marks are not visible.

 

As shown in Figure 3.2.23, multi-cavity thin-shell small plastic parts are ejected using a push plate.
As shown in Figure 3.2.24, plastic part is a transparent sheet; to avoid ejection marks, an ejector block is used. Note that bottom edge of such plastic parts should not have rounded corners to prevent ejection marks from showing.

 

Assembly relationship of plastic parts in a product provides mold manufacturing with information about requirements of plastic parts, such as fit clearance and connection method with other plastic parts.
3.3.1 Assembly Interference Analysis
Mold engineers assemble 3D models based on connection method and fit clearance of each plastic part; they analyze whether there is interference between plastic parts. To analyze interference between individual components using Pro/E, follow these steps:
Analysis® Model Analysis® Pairs Clearance (Analyzes gap or interference between two parts in an assembly)® [Select two parts or surfaces to analyze]® Compute.
Another method for checking interference across the entire assembly:
Analysis® Model Analysis® Global Interference (Analyzes interference between individual parts in the entire assembly)® [Select the entire assembly]® Compute (Obtains interference information between individual parts in the entire assembly).
3.3.2 Assembly Clearance
Assembly clearance between each plastic part should be uniform. General clearance (one side) for plastic parts is as follows:
(1) Clearance between fasteners is 0-0.1mm, as shown in Figure 3.3.1;

 

(2) Clearance between front and back shell stops is 0.05-0.1mm, as shown in Figure 3.3.2;
(3) Clearance (one side) for regular buttons (diameter Ø>15) is 0.1-0.2mm; clearance (one side) for regular buttons (diameter Ø>15) is 0.15-0.25mm; clearance for irregular buttons is 0.3-0.35mm, as shown in Figure 3.3.3;

 

3.3.3 Column and Fastener Connections
Analyze column and fastener connections connecting each plastic part, as shown in Figures 3.3.4 and 3.3.5. Check column and fastener dimensions in assembled 3D model and 2D files of each plastic part; their positional dimensions must be consistent. When dimensions of pillars or snap-fit parts of a plastic component are changed, dimensions of mating plastic components should also be changed.

 

Because plastic wall at connection between pillar root and plastic shell will suddenly thicken, and some plastic component specifications do not include instructions for reducing plastic thickness, steel reinforcement (a volcano vent) must be added to pillar root of mold to prevent shrinkage marks on plastic component surface.
Common pillar dimensions with volcano vents are shown in table below:

 

Notes: Average plastic thickness of above data is 2.5mm, as shown in Figure 3.3.6; For screw pillars smaller than M2.6, volcano vents are generally not required, but plastic thickness at the bottom of pin should be between 1.2 and 1.4mm; For screw pillars with volcano vents, rocket feet should generally be provided to improve strength and facilitate plastic flow.

 

This refers to the condition of exposed parts of each plastic component after assembly; text, patterns, textures, shapes, and safety standards required for surface of plastic components.
3.4.1 Text, Patterns, and Reliefs
Text and patterns directly molded onto plastic parts can be convex if customer has no specific requirements. When text and patterns on plastic parts are concave, mold will be convex, making mold making relatively complex.
There are generally three methods for making text and patterns on molds: Photolithography (also known as chemical etching); Electrode machining mold, engraving electrodes or CNC machining electrodes; Engraving or CNC machining mold.
If electrode machining is used for text and patterns, process requirements for text and patterns on plastic parts are as follows:
(1) For convex text and patterns on plastic parts, protrusion height should be 0.2–0.4 mm, line width should not be less than 0.3 mm, and distance between two lines should not be less than 0.4 mm, as shown in Figure 3.4.1.

 

 

(2) For recessed text or patterns on plastic parts, recess depth should be 0.2–0.5 mm, with 0.3 mm being the most suitable. Line width should be no less than 0.3 mm, and distance between two lines should be no less than 0.4 mm, as shown in Figure 3.4.2.
Creation of reliefs on the surface of plastic parts commonly uses engraving methods to process molds. Since 3D files of plastic parts do not contain relief shapes, and size of relief in 2D files is also inaccurate, shape of relief is based on a template. Therefore, mold designers and manufacturers should understand engraving mold making process; fit and positioning of engraving mold should be determined during analysis.
3.4.2 Plastic Part Shape
Shape of plastic parts should meet safety standards for each type of product. Sharp edges and points should not appear on plastic parts; for inner and outer surfaces at corners, rounded corners can be added to avoid stress concentration, improve strength of plastic parts, and improve flow of plastic parts, as shown in Figure 3.4.3. When 3D modeling plastic parts, if wrinkles or small, fragmented surfaces appear, a surface improvement plan should be determined; alternatively, electrodes can be adjusted during manufacturing to meet requirements for a smooth curved surface, as shown in Figure 3.4.4.

 

3.4.3 Surface Texture
Surface texture requirements for plastic parts are often smooth or textured; textured surfaces include two types: etching (also known as chemical etching) and spark etch. Draft angle is specified in Section 3.2.1.
When surface of plastic part requires painting or screen printing, surface should be smooth or finely textured (Ra<6.3). Overly coarse textures are prone to oil overflow. Screen printing is best done on protruding or flat parts of plastic part; surface after painting will amplify surface marks generated during molding.