1.1. Purpose
This document introduces mechanical properties of commonly used plastic materials and manufacturability process points that need to be reviewed before mold opening, such as parting lines, shrinkage, and draft angles. It also explains key points of mold design review and provides a method for mold price estimation.
1.2. Scope of Application
This guideline is suitable for design engineers and process engineers to understand performance, processing requirements, and design points of plastic parts in the early stages of design, to review manufacturability of plastic part structural design, control manufacturability of parts, stabilize quality of plastic parts during mass production, and ensure mass production of parts.
1.3. Referenced Standards
For example:
"Plastic Part Structural Design Guidelines"
"Plastic Part Unspecified Tolerance Standards"

Plastic process analysis mainly includes: material selection, dimensional accuracy, surface roughness, plastic structure/geometry, and manufacturability (such as parting lines, shrinkage, draft angles, etc.);
Characteristics of plastic process design: It should meet requirements of performance and molding process, striving for a reasonable structure, aesthetically pleasing shape, and ease of manufacturing. It should not only meet needs of mold opening but also ensure normal production.

1. Shrinkage
Characteristic of plastics to shrink in size after being removed from mold and cooled to room temperature is called shrinkage.
2. Fluidity
Ability of molten plastic to fill mold cavity under certain temperature and pressure is called fluidity of plastic.
3. Orientation Effect
Orientation behavior of plastics is phenomenon that polymer molecular chains tend to align parallel to direction of stress under action of stress.
4. Rheological Properties
Characteristic of molten plastic liquid to change molecular chain under force in a flow field is called "rheological properties"; due to influence of this characteristic, viscosity of molten plastic decreases with increase of shear rate (or tensile rate), which is different from general Newtonian fluids, and is also called "non-Newtonian fluid characteristics".

2.2.1. Wall Thickness of Plastic Parts
Wall thickness of plastic parts should be designed to be moderately thick and uniform;
△ Too thin – insufficient strength and rigidity, difficult plastic flow;
△ Too thick – waste of raw materials, long molding cycle, low efficiency;
△ Uneven wall thickness – prone to shrinkage defects or product deformation;
1. Design Principles:
△ Use the smallest possible wall thickness while meeting structural and performance requirements of plastic part;
△ Product wall thickness should be able to withstand impact and vibration of ejection mechanism, etc.;
△ Connecting points of product, areas where inserts are embedded, etc., should have sufficient thickness;
△ Ensure wall thickness required for strength during storage and handling;
△ Meet wall thickness required for melt to fill mold during molding;
2. Design Cases:

 

 

2.2.2. Reinforcing Ribs of Plastic Parts
Function: It can improve strength of part, prevent and avoid deformation and warping of plastic;
1. Key Design Point 1: Bottom of reinforcing rib should be connected to wall with a rounded transition to prevent stress concentration and damage under external force; direction of reinforcing rib should be parallel to or consistent with direction of plastic flow, as shown in figure below.

 

2. Design Point Two: Reinforcement rib thickness is less than wall thickness, there is a gap between reinforcement rib and support surface;
A = 0.6-0.75B (Design reference: "Plastic Part Design Guide")

 

2.2.3. Gate
1. Gate location: Consider shape of part, possible weld lines, and impact of gate removal on appearance;
2. Gate layout:
a) Avoid weld lines appearing on main appearance surface or affecting strength of plastic part
b) Prevent long rod-shaped plastic parts from deforming under injection pressure
c) Avoid affecting assembly between parts or leaving marks on exposed surfaces
d) To prevent streaking and burn marks, impact gates or bottom gates should be used
e) For ease of flow and pressure holding, gate should be located at thicker wall section of plastic part
f) Facilitates venting
g) Consider impact of orientation on quality of plastic part
h) For multi-cavity molds, prioritize setting gates according to a balanced runner layout
i) Consider efficiency of injection molding production and facilitate separation of runner system from plastic part
j) Consider ease of processing
3. Gate types
a. Direct gate; b. Pinpoint gate; c. Side gate; d. Fan gate; e. Submarine gate; f. Tunnel gate; g. Film gate; h. Ring gate; I. Claw gate;

 

2.3.1. Structure (geometric shape) of plastic parts
1. Surface shape: Inner and outer surface shapes of plastic part should be designed to facilitate molding;

 

2. Draft angle
To facilitate demolding of plastic parts and prevent scratching during demolding, sufficient draft angle should be provided on inner and outer surfaces of plastic part in demolding direction. Draft angle depends on shape, wall thickness, and shrinkage rate of plastic part;
For external shapes, large end is used as reference, and draft angle is taken from shrinking direction;
For internal shapes, small end is used as the reference, and draft angle is taken from expanding direction. Mold Release Angle Design Considerations:
Ø△ Plastics containing lubricants use smaller mold release angles.
Ø△ Plastics requiring high dimensional accuracy use smaller mold release angles.
Ø△ For plastics with complex shapes that are difficult to demold, a larger angle should be used.
Ø△ Reinforced plastics use larger mold release angles.
Ø△ Larger shrinkage rates require larger angles.
Ø△ High-precision plastics use smaller mold release angles.
Ø△ Consider mold retention direction (to prevent material sticking to corresponding inner mold):
i. When retained on core (male mold), inner surface mold release angle < outer surface.
ii. When retained in cavity (female mold), outer surface mold release angle < inner surface.

 

2.3.2. Holes (Grooves) on Plastic Parts
There are three molding methods for holes in plastic parts: 1. Direct molding; 2. Molding a blind hole and then drilling a through hole; 3. Drilling hole after plastic part is molded;
Common hole design requirements:
Ø◆ When through-hole diameter < 1.5mm, it is not suitable for molding due to core being easily bent or broken;
Ø◆ Blind hole depth: h < (3~5)d, when d < 1.5mm, h < 3d
Ø◆ Holes for fastening and other load-bearing holes should be reinforced with bosses.

 

2.3.3. Determination of Parting Surface
Parting surface is set at point of the largest contour diameter of part. Mold release angle is designed based on parting surface. The simpler parting surface shape, the better. Besides considering mold cost, complex parting surfaces can easily lead to defects such as flashing due to processing accuracy issues.
Surface where mold is opened to remove plastic part or gating system is called parting surface. Parting surface is affected by various factors in addition to layout, including shape, appearance, accuracy, gate location, slide mechanisms, ejection, and processing of plastic part. A reasonable parting surface is a prerequisite for successful molding of plastic part. Generally, following aspects should be considered comprehensively:
(1) Meeting basic requirements for demolding plastic part, i.e., enabling plastic part to be removed from mold. Parting line should be located at the edge of the largest projected area of plastic part in demolding direction.
(2) Ensuring plastic part remains on rear mold side, facilitating ejection, and ensuring that ejector pin marks are not visible on external surface.
(3) Parting line should not affect appearance of plastic part. Parting surface should avoid damaging smooth outer surface of plastic part as much as possible.
(4) Ensuring quality of plastic part, for example, placing parts of plastic part with coaxiality requirements on same side of parting surface.
(5) Selection of parting surface should avoid forming side holes and undercuts as much as possible. If side core forming is necessary, side core structure should be kept simple, and front mold side cores should be avoided as much as possible.
(6) Reasonably arranging gating system, especially gate location.
(7) Satisfying mold locking requirements. Direction with the largest projected area of plastic part should be placed in clamping direction of front and rear molds, while direction with smaller projected area should be used as lateral parting surface; in addition, if parting surface is curved, a tapered lock should be added.
(8) Facilitating mold processing.
2.3.4. Sharp and Thin Steel Sections
Avoid sharp and thin steel sections that may affect mold strength and service life. Sharp and thin steel sections are generally not easily reflected in plastic part; their analysis should be combined with the mold design of plastic part. Reasons for sharp and thin steel sections in mold are twofold—plastic part structure and mold structure.
2.3.5. Ejector Pins
1. Layout of ejector pins needs to be considered;
2. Standard for selecting ejector pins needs to be confirmed;
2.3.6. Mold Inserts
Insert boundary line should be designed to form a step with plastic part body to reduce possible impact on appearance;
2.3.7. Inserts
Other parts are pressed into plastic part to form a non-removable connection; these pressed-in parts become inserts.
Inserts can be made of metal, glass, wood, or already molded plastic parts.