Good Plastic Injection Mold Guidelines

Table of Contents

1. Classification of Molds

2. Structure of Molds

3. Common Injection Molding Materials

4. Internal Structure Design of Molds

1. Classification of Molds

Molds can be broadly divided into the following four types:

Plastic Molds: Injection molds, blow molds, vacuum molds

Die Casting Molds: Aluminum alloy molds, zinc alloy molds, magnesium alloy molds

Casting Molds: Sand molds, lost-wax molds, gravity casting molds, rubber molds

Metal Molds: Stamping molds, stretching molds, cold heading molds.

2. Structure of Injection Molds

Components of Molds

Injection molds are further divided into: hot runner molds, cold runner molds, and overmolding molds (two-color molds).

Design of Hot Runners

The feeding method for cold runner molds can be thick gates and thin gates (also known as three-plate molds).

3. Common Injection Molding Materials

Classification of Plastics:

The term "plastic" commonly refers to various types of plastics, which are widely used. Therefore, classification methods vary. Generally, they can be divided into two main categories based on usage: general-purpose plastics and engineering plastics. General-purpose plastics, such as PE, PP, PS, modified polystyrene (e.g., SAN, HIPS), and PVC, are widely used materials with lower performance requirements and costs.

Engineering plastics refer to plastics with industrial quality for mechanical parts or structural materials. They exhibit superior mechanical properties, electrical properties, resistance to chemical environments, and tolerance to high and low temperatures, making them capable of replacing certain metals or other materials in engineering applications. Common types include ABS, PA (commonly known as nylon), PC, POM, PMMA, PET, PBT, etc. The first four are the fastest-growing and are recognized internationally as the four major engineering plastics.

Based on heating processing performance, plastics can also be divided into thermosetting plastics and thermoplastics. Thermosetting plastics harden upon heating, transforming their molecular structure into a network or solid form, and cannot be re-softened after hardening. Common examples include PF, EP, UP, etc. Thermoplastics soften and melt upon heating, retaining plasticity after cooling and can be reshaped multiple times; their processing involves physical changes. Common examples include PVC, PE, PP, PS and their modified varieties, ABS, PA, POM, PC, PMMA, etc. These plastics have relatively simple forming processes at specific plasticization temperatures and appropriate pressures, resulting in products with varying physical and mechanical properties.

Common Plastic Shrinkage Rates

ABS: Commonly known as super unbreakable glue, it is a high-strength modified PS.

ABS with a Three-Component Structure: Combines various inherent characteristics of its components:

1. Acrylonitrile provides high strength and surface hardness, enhancing chemical resistance and heat resistance.

2. Butadiene gives the polymer flexibility, allowing it to maintain toughness and elasticity at low temperatures and high impact strength without becoming brittle.

3. Styrene maintains molecular chain rigidity, resulting in hard, glossy material with good electrical properties and thermal flow, making it easy to process and color.

Properties of ABS: The natural color is light ivory, opaque, non-toxic, and odorless, classified as an amorphous plastic. Its viscosity is moderate, and its melt flowability is influenced by temperature and pressure, with pressure having a greater effect.

Combustion Properties: ABS resin is a slow-burning material, burning with a yellow flame, producing black smoke with a distinct odor, and does not melt or drip while burning.

Advantages of ABS

Excellent overall performance: High mechanical strength; strong impact resistance with minimal drop in performance at elevated temperatures; good notch sensitivity; good creep resistance; maintains strength at high temperatures; surface hardness; good abrasion resistance with low friction coefficient.

Good electrical properties that are minimally affected by temperature, humidity, and frequency variations.

Low-temperature resistance down to -40℃.

Resistant to acids, bases, salts, oils, and water.

Can be surface decorated via painting, printing, electroplating, etc.

Low shrinkage rate and wide processing range.

Disadvantages of ABS:

Not resistant to organic solvents, susceptible to swelling and dissolution by polar solvents.

Poor weather resistance, especially in UV resistance.

Insufficient heat resistance. The heat distortion temperature of standard ABS is only between 95℃ and 98℃.

4. Internal Structure Design of Molds

Design of the Gate Position

The wall thickness of the molded part should be uniform to avoid abrupt changes and significant variations in cross-section thickness, which could cause uneven shrinkage and surface defects. The typical wall thickness is in the range of 1-6mm, with the most common values being 1.8-3mm, depending on the type and size of the part.

Design of the Structural Ribs

Ribs enhance strength, support the base shell, and provide guiding for buttons. Since rib connections to the molded part's body can lead to surface shrinkage and dents, the rib thickness should generally be less than or equal to 0.5 times the wall thickness (t). Typical rib thickness is in the range of 0.8-1.2mm. If the rib depth exceeds 15mm, it may lead to difficulties in filling and venting.

Molds can include inserts to ease the molding process and venting. For rib depths below 15mm, a draft angle of at least 0.5° is required; for depths above 15mm, the thickness difference between the rib base and the top should be at least 0.2mm.

Design of the Gate

The flow path should ensure that the furthest gate position is in the middle, minimizing flow time to all parts of the molded part. The selection of the gate position and type directly affects the quality of the molded part and the injection process. The principles for gate design are as follows:

1. Ensure that the material flow front reaches the end of the cavity simultaneously and takes the shortest route.

2. Material should be injected first from thicker areas to maintain pressure and reduce pressure loss.

3. The gate should disconnect from the root of the gate section for easy cleanup.

4. Gates should avoid direct impacts on small cores or inserts to prevent deformation.

5. Gate positions should be easy to clean without affecting the appearance of the part.

6. Facilitate venting within the cavity to expel gas towards the parting line.

7. Avoid flow issues that could lead to defects like "runner" effects or weld lines.

8. Ensure that the flow direction allows for uniform filling along the cavity, avoiding uneven filling that could lead to warping or cracking.

Design of Draft Angles

Molds must have sufficient draft angles to prevent issues like sticking, white marks, or dragging. The draft angle is related to material properties, part shape, and surface finish requirements.

1. For small parts with smooth surfaces, a draft angle of 1° is recommended; for larger parts, 3°.

2. For parts with surface textures (Ra < 6.3), a draft angle of 3°; for Ra < 4, a draft angle of 4°.

3. For parts with spark eroded surfaces (Ra < 3.2), a draft angle of 3°; for Ra < 3.2, a draft angle of 4°.

Design of Ejectors and Slides

When molded parts have concave or convex shapes, side holes, or locking features, the mold must retract side cores before ejecting the part. This mechanism is referred to as the "side action."

For external holes, side actions are needed for retraction. For internal grooves, if the top clearance is insufficient, a side action can be used to achieve simultaneous ejection and core removal. In side action mechanisms, the side ejection distance should exceed the core pull distance (B > H) to prevent interference.

Ejection of Molds

Ejection of molded parts typically uses ejector pins, sleeves, and push plates. If a part has special structures or surface finish requirements, other methods may be required, such as blocks for ejection: angled ejection, threaded rotation, or secondary ejection. For certain transparent parts, care must be taken to ensure ejection marks are not visible.

Sean.chen?

Fortune?Plastic?&?Mold ??Co., Ltd

Mob: +86-13534226260.Tel: ?+86?755?89890045 Skype:igao.sean.chen.Email:[email protected];[email protected];Web:?www.fortunegroupcn.com