Injection molds are tools used to produce plastic products. Before designing a mold, it is essential to understand and master the properties of plastics thoroughly. Only by being thoroughly familiar with the forming process of plastics can the dimensions of the mold and plastic products be accurately controlled, ensuring the design of high-quality molds.
1.1 Plastics and Their Advantages and Disadvantages
1.1.1 What is Plastic?
According to the national standard (GB/T2035-2008), plastic is defined as a polymeric, artificial material primarily composed of synthetic organic resins, with or without the addition of other compounding materials (additives). It can typically be molded into products with a specific shape under conditions of heat and pressure, and this shape remains unchanged at room temperature.
Some resins, such as polyethylene, polystyrene, and nylon, can be directly used as plastics. However, most resins, such as phenolic Resin, amino Resin, and polyvinyl chloride, require the addition of certain additives before they can be used as plastics.
Plastics have a wide range of applications, and their products exhibit great diversity. There are many types of plastics, each with different properties. Plastics can exist in glassy, elastomeric, and viscous flow. They are generally formed using molds, with standard molding methods including injection molding, extrusion molding, blow molding, compression molding, and transfer molding.
1.1.2 Advantages of Plastics
The advantages of plastics include:
1. Easy Processing and Molding, Suitable for Fully Automated Mass Production at Low Cost
Plastics can be molded into products even with complex geometric shapes, and their production efficiency far exceeds that of metal processing. Especially for injection-molded plastic products, even very complex items can be produced in a single step.
Due to the ease of processing and the capability for mass production with relatively low equipment costs, plastic products are cost-effective.
2. Customizable Colors and Transparent Products
Plastics can be easily colored to create vibrant, colorful, or transparent products. By blending different plastics, products with a pearlescent effect can also be made, significantly enhancing their added value and giving them a fresh, bright appearance.
3. Lightweight with High Specific Strength
Most plastics have a density comparable to water, approximately 1.0 g/cm3. Compared to metals and ceramics, they are lightweight. Although the mechanical strength of plastics is not as high as that of metals or ceramics, their specific strength (strength-to-density ratio) is relatively high, enabling the production of lightweight, high-strength products. Furthermore, filling plastics with glass fibers can significantly enhance their strength and wear resistance.
4. Rustproof and Corrosion-Resistant
Plastics do not rust like metals or corrode when exposed to chemicals. They are resistant to acids, alkalis, salts, oils, chemicals, humidity, and mold, making them highly durable in various environments.
5. Low Thermal Conductivity and Excellent Insulation
Plastics have a high specific heat capacity and low thermal conductivity, making them poor heat conductors. As a result, they provide excellent thermal insulation and heat retention.
6. Suitable for Conductive and Insulating Applications
Plastics are inherently good insulating materials. There is virtually no electrical component today that does not use plastics. By filling plastics with metal powders or fragments, conductive products with excellent performance can also be manufactured.
7. Superior Shock Absorption, Noise Reduction, and Light Transmission
Plastics have excellent shock absorption and noise reduction properties. Transparent plastics (e.g., PMMA, PS, PC) can produce precise products such as lenses, signage, and covers.
1.1.3 Disadvantages of Plastics
The disadvantages of plastics include:
1. Poor Heat Resistance and Flammability: plastics’ most significant drawbacks. Compared to metals and ceramics, their heat resistance is much lower; they deform at slightly higher temperatures and are prone to burning. Most plastics can produce heat, smoke, and toxic gases when burning. Even thermosetting resins will emit smoke and start to flake when exceeding 200°C.
2.Properties Change with Temperature VariationsNot only at high temperatures but even at low temperatures, various properties of plastics can change.
3. Low Mechanical Strength Compared to metals of the same volume, plastics have much lower mechanical strength. This difference is especially evident in thin-walled plastic products.
4. Susceptible to Corrosion by Specific Solvents and Chemicals Generally, chemicals do not easily corrode plastics. However, some plastics, such as PC (polycarbonate), ABS, and PS (polystyrene), are easily eroded by specific solvents and chemicals and have feeble resistance. In general, thermosetting resins perform better and are less prone to corrosion.
5. Poor Weather Resistance and Aging Plastics are not durable in strength, surface gloss, or transparency and exhibit creep under load. Additionally, all plastics are susceptible to exposure to ultraviolet and sunlight and quickly age under light, oxygen, heat, water, and atmospheric conditions.
6. Easily Damaged and Prone to Dust and Dirt Contamination Plastics have relatively low surface hardness and are easily damaged. Furthermore, being insulators, they carry static electricity, making them prone to attracting dust.
7 . Poor Dimensional Stability Compared to metals, plastics have a high shrinkage rate and are easily affected by injection molding process parameters. This results in significant fluctuations that are difficult to control, reducing the dimensional accuracy of molded plastic products. Additionally, plastic products can absorb moisture during use, and when temperature changes occur, their dimensions are prone to change over time.
1.2 Composition of Plastics
The main component of plastics is a variety of resins, which are polymers. However, plastics and polymers are not the same. Pure polymers often cannot meet processing, molding, and practical use requirements. They are generally not used alone but acquire industrial value only when additives are incorporated. Plastics combine synthetic ReResinith with various additives (auxiliary agents) as the primary component. Synthetic ReResinetermines are the fundamental properties of plastic products. Its role is to bind the different additives into a cohesive whole. Additives are incorporated to improve the plastic’s processing and molding performance, enhance the properties of the final product, or reduce production costs.
Plastic materials contain many additives, such as fillers, plasticizers, colorants, stabilizers, curing agents, and antioxidants. While RResin plays a decisive role in plastics’ performance, the contribution of additives cannot be overlooked. Together, they form the complete material system used in plastics.
1.2.1 Resinesin is a polymer that softens when heated and tends to flow under the influence of external forces. It is the most critical component of plastics and acts as a binder, also called an adhesive material. Resin determines the type and basic properties of the plastic, including thermal, physical, chemical, mechanical, and electrical properties.
1.2.2 Additives
(1) Fillers
Fillers, also known as reinforcing agents, are essential components in plastics, though not indispensable for every type. They are typically inert, powdery substances. Fillers are added to plastics to improve molding and processing performance, enhance specific product properties, impart new properties, and reduce costs.
For example, adding wood powder to phenolic ReResine reduces brittleness and costs. Calcium fillers added to polyethylene and polyvinyl chloride resins create rigid, heat-resistant, low-cost calcium-based plastics. Glass fiber as a filler dramatically enhances the mechanical properties of plastics. Certain fillers can even provide plastics with properties not present in the ReResin, such as conductivity, magnetism, or thermal conductivity.
Fillers can be classified as inorganic or organic and are available in three forms: powdery, fibrous, and flaky. Powdery fillers include wood powder, pulp, marble, talc, mica, asbestos, and graphite. Fibrous fillers include cotton, flax, glass fiber, asbestos fiber, carbon fiber, boron fiber, and metal whiskers. Flaky fillers include paper, cotton, linen, and glass cloth. The filler generally accounts for less than 40% of the total composition of plastics.
Fillers with spherical or cubic shapes typically enhance molding and processing performance but result in lower mechanical strength, whereas flake-shaped fillers have the opposite effect. The finer the filler particles, the more they improve the rigidity, impact resistance, tensile strength, stability, and appearance of plastic products.
(2) Plasticizers
Plasticizers are low-volatility, high-boiling organic compounds compatible with resins. They increase plasticity and flexibility, improve molding performance, and reduce rigidity and brittleness. They reduce the intermolecular forces within polymers, making resin molecules more mobile and allowing plastics to become more flexible and pliable at lower temperatures.
For example, adding dibutyl phthalate to polyvinyl chloride resin transforms it into a soft, rubber-like plastic. However, while plasticizers improve the molding and processing performance of plastics, they may also decrease specific properties such as stability, dielectric performance, and mechanical strength. Therefore, the amount of plasticizer added should be minimized whenever possible, and many plastics do not require plasticizers.
Requirements for Plasticizers:
- Good compatibility with the ReResinLow volatility, preventing it from leaching out of plastic products.
- Non-toxic, colorless, and odorless.
- Stable under exposure to light and heat.
- Non-hygroscopic.
Commonly used plasticizers include dibutyl phthalate (DBP), dioctyl phthalate (DOP), and camphor.
(3) Colorants
Most synthetic resins are naturally white and semi-transparent or colorless and transparent. Colorants are often added during industrial production to produce plastic products with a variety of desired colors to alter the original color of the RResin and achieve vibrant, aesthetically pleasing products. Some colorants can also enhance the light and heat stability of plastics. For example, coloring natural polyoxymethylene plastic with carbon black can help prevent photoaging.
Colorants are mainly divided into pigments and dyes. Pigments are colorants that do not dissolve in common solvents and require mechanical methods to evenly disperse them into plastics to achieve the desired coloring effect. Pigments can be categorized into organic and inorganic types. Inorganic pigments have excellent thermal and light stability and are relatively inexpensive but have lower coloring strength and higher density (e.g., sodium crimson, sulfur-indoline yellow, pigment blue, carbon black).
Organic pigments have high coloring strength, bright colors, complete color ranges, and lower densities. However, their drawbacks include lower heat resistance, weather resistance, and opacity compared to inorganic pigments (e.g., chrome yellow, chromium oxide, lead powder). Dyes are organic compounds that dissolve in most solvents and can be used to color plastics. They offer advantages such as low density, high coloring strength, and good transparency, but their small molecular structures often result in migration during coloring (e.g., Solvent Blue).
Requirements for Colorants: Colorants should have strong coloring power, good compatibility with resins, and be chemically stable without reacting with other components in the plastic. They must resist degradation and discoloration during molding under varying temperatures and pressures and maintain stability throughout the long-term use of plastic products.
To achieve these requirements and ensure optimal coloration, color masterbatch machines play a crucial role in the plastic coloring process. These machines accurately measure and evenly disperse pigments and dyes, ensuring desired color effects in plastic products. For insoluble pigments, especially inorganic ones, the efficient mixing systems of color masterbatch machines overcome dispersion challenges, achieving uniform coloration. Additionally, their temperature control features protect organic pigments and dyes from degradation and discoloration during high-temperature processing, maintaining the stability and quality of the colored plastic products.
(4) Stabilizers
During processing and usage, resins can undergo aging, also known as degradation. Degradation refers to reactions where the polymer’s molecular chains break, or its chemical structure changes negatively under the influence of heat, force, oxygen, water, light, or radiation. Stabilizers are added to polymers to maintain their chemical properties and prevent degradation or crosslinking caused by external factors like heat, light, oxygen, and mold.
Stabilizers are classified based on their functions, including thermal stabilizers, light stabilizers, and antioxidants:
- Thermal Stabilize inhibits thermal degradation during plastic molding, ensuring smooth processing and high product quality. Organic tin compounds, which are non-toxic but expensive, are commonly used in PVC plastics.
- Light Stabilizers prevent degradation and performance loss in plastics caused by sunlight, artificial light, or high-energy radiation. Common types include ultraviolet absorbers (e.g., benzoic acid esters) and light-blocking agents (e.g., carbon black).
- Antioxidants: These prevent oxidation and degradation in plastics at high temperatures. Phenol-based and amine-based organic compounds are often used as antioxidants.
Most plastics require stabilizers, accounting for 0.3% to 0.5% of the total material. Requirements for Stabilizers: Stabilizers should be compatible with resins, effectively stabilize polymers, and resist water, oil, and chemical corrosion. They must also not degrade during molding, have low volatility, and not impact the plastic’s color.
(5) Curing Agents
Curing agents, hardeners, or crosslinkers are used to mold thermosetting plastics. Linear polymer structures in synthetic resins require crosslinking reactions to transform into a three-dimensional polymer network, and curing agents facilitate these reactions. For example, ethylenediamine and triethanolamine are commonly added to epoxy resins.
Other additives, such as foaming agents, flame retardants, antistatic agents, conductive agents, and magnetic agents, can be incorporated into plastics depending on the requirements. For example, flame retardants reduce the flammability of plastics, foaming agents produce foam plastics, and antistatic agents provide moderate conductivity to eliminate static electricity. The choice of additives and their types depends on the type of plastic and the specific application requirements of the plastic product.
1.3.Classification of plastics
There are many types of plastics, and currently, over ten thousand processable plastic materials (including modified plastics) have been developed worldwide, with more than 300 commonly used types. Plastics can be classified in various ways, with the following being the most common classification methods:
(1) Classification Based on Resin Molecular Structure and Thermal Behavior
According to the reResin’solecular structure and its heated behavior, plastics can be divided into two main categories: thermoplastics and thermosetting plastics.
① Thermoplastics
The molecular structure of thermoplastic resins is linear or branched, commonly called linear polymers. When heated, they can be molded into specific shapes, which retain their form upon cooling. They can soften and melt if reheated, allowing them to be reshaped into new products. This process can be repeated multiple times, exhibiting reversibility. There are generally no chemical changes during molding, only physical changes. Due to the reversible properties of thermoplastics, scrap and waste materials generated during plastic processing can be recycled by crushing them into granules and mixing them with raw materials for reuse.
Thermoplastics can be further divided into two types: crystalline plastics and amorphous plastics. Crystalline plastics have molecular chains arranged in an orderly, stable, and compact manner. Amorphous plastics have molecular chains arranged in a disordered and chaotic manner r. As a result, crystalline plastics are generally more heat-resistant, opaque, and have higher mechanical strength, while amorphous plastics exhibit opposite properties. Common crystalline plastics include polyethylene, polypropylene, and polyamide (nylon). Common amorphous plastics include polystyrene, polyvinyl chloride (PVC), and ABS.
From an appearance perspective, crystalline plastics are typically opaque or translucent, while amorphous plastics are transparent. However, there are exceptions, such as poly(4-methylpentene-1), a crystalline plastic with high transparency, and ABS, an opaque amorphous plastic.
② Thermosetting Plastics
Thermosetting plastics initially have a chain-like or branched molecular structure and exhibit both plasticity and fusibility, allowing them to be molded into specific shapes. When heating continues, chemical bonds form between the leading chains of these molecular structures, gradually creating a network structure through a process known as crosslinking.
As the temperature rises to a certain level, the crosslinking reaction progresses further, and the molecules ultimately form a three-dimensional structure—this process, known as curing, results in a material that neither melts nor dissolves. Upon reheating, the shape of the plastic product remains fixed due to the chemical reactions between the molecular chains. Thermosetting plastics lose their plasticity and cannot be reshaped, even at high temperatures, eventually charring or carbonizing at extremely high temperatures. This process is irreversible, and physical and chemical changes occur during molding. Thermosetting plastics cannot be recycled or reused.
Thermosetting plastics have better heat resistance compared to thermoplastics. Examples of thermosetting plastics include phenolic resins, melamine-formaldehyde resins, and unsaturated polyester resins.
Thermoplastics are commonly formed through injection molding, extrusion, or blow molding, while thermosetting plastics are typically used in compression molding and transfer molding, though injection molding can also be used.
Since plastics primarily consist of high-molecular-weight polymers, they are often named after the polymer used. However, the names of plastics are usually lengthy and inconvenient for both speaking and writing. Therefore, internationally recognized English abbreviations are commonly used. The abbreviations and names of thermosetting and thermoplastic plastics can be found in Appendix 1.
(2) Classification Based on Plastic Properties and Applications
Plastics can be classified into general-purpose, engineering, and specialty plastics based on their properties and applications.
① General-Purpose Plastics
General-purpose plastics are produced in large quantities, have broad applications, are low-cost, and perform well. They are typically used as non-structural materials. The six globally recognized general-purpose plastics are polyethylene, polypropylene, polyvinyl chloride, polystyrene, phenolic plastics, and amino plastics. These plastics account for more than 75% of the total global plastic production and form the backbone of the plastics industry.
② Engineering Plastics
Engineering plastics generally refer to industrial-grade plastics that manufacture mechanical parts or structural materials. In addition to their high mechanical strength, these plastics exhibit superior wear resistance, corrosion resistance, heat resistance, self-lubrication, and dimensional stability compared to general-purpose plastics. They possess specific metallic characteristics and are widely used in machinery manufacturing, light industry, electronics, daily-use products, aerospace, missiles, and nuclear energy, increasingly replacing metals in specific mechanical components.
Currently, commonly used engineering plastics include polyamides (PA), polyoxymethylene (POM), polycarbonate (PC), ABS, polysulfone (PSU), polyphenylene oxide (PPO), and polytetrafluoroethylene (PTFE). The first four are the fastest-growing and are internationally recognized as the four primary engineering plastics.
③ Specialty Plastics (Functional Plastics)
Specialty plastics have specific functions and are suitable for special applications. These include medical plastics, photosensitive plastics, magnetic plastics, superconductive plastics, radiation-resistant plastics, and high-temperature-resistant plastics. Their main components are resins, which can be specially synthesized or obtained by treating or modifying general-purpose or engineering plastics to gain unique properties. These plastics are produced in small quantities, have excellent performance, and are expensive.
As the scope of plastic applications expands, the boundary between engineering and general-purpose plastics has become increasingly blurred. For example, general-purpose polyvinyl chloride (PVC) is extensively used in chemical machinery as a corrosion-resistant material.
(3) Classification Based on Crystalline Morphology
Plastics can generally be divided into crystalline and non-crystalline plastics (amorphous plastics) based on their crystalline morphology.
Crystalline plastics refer to plastics whose molecules can form a geometric structure under suitable conditions (e.g., PE, PP, PA, POM, PET, PBT). Most crystalline plastics are partially crystalline.
Non-crystalline plastics refer to those whose molecular shapes and arrangements do not form a crystalline structure but remain disordered (e.g., ABS, PC, PVC, PS, PMMA, EVA, AS). Non-crystalline plastics are also known as amorphous plastics. Amorphous plastics exhibit isotropy, meaning their mechanical properties are uniform in all directions.
Requirements for Injection Molding Machines and Molds for Crystalline Plastics:
- Higher energy input: Crystalline plastics require more energy to disrupt their crystalline lattice during melting. Consequently, the injection molding machine must have higher plasticizing capability, and the rated injection volume should be increased accordingly.
- Narrow melting point range: To prevent crystallization and nozzle blockage as the temperature drops, the nozzle diameter should be appropriately enlarged, and a heating element with independent temperature control should be installed.
- Uniform mold temperature: Since mold temperature significantly affects the degree of crystallinity, the mold cooling system should include as many water channels as possible to ensure uniform mold temperature during forming.
- High shrinkage rate: Crystalline plastics undergo significant volume shrinkage during crystallization, resulting in a higher molding shrinkage rate. This must be carefully considered during mold design.
- Anisotropy and internal stress: Due to the pronounced anisotropy of crystalline plastics, they generate high internal stress. Mold design must consider the location and size of gates and ribs to avoid warping or deformation, which can be difficult to correct later through process adjustments.
- Wall thickness and crystallinity: Thicker plastic parts cool more slowly, leading to higher crystallinity, more significant shrinkage, and an increased likelihood of sink marks and voids. Mold design should carefully control the wall thickness of plastic parts.
Molding characteristics of crystalline plastics:
1. Because crystalline plastics release heat, we should fully cool and carefully control the cooling time during high-temperature molding.
2. The density difference is enormous; the molding shrinkage is significant, too, and it is easy to produce sink marks and voids, so the holding press should be set reasonably.
3 . The mold temperature will affect the crystallinity. If the mold temperature is low, the toy’s cooling is fast, the crystallinity is low, shrinkage is slight, and transparency is good. But the toy’s cooling is slow for thick-walled toys, with high crystallinity, considerable shrinkage, and good physical properties. So, mold temperature should be controlled as required.
4. Crystallization characteristics after molding. The uncrystallized molecules tend to crystallize gradually after demolding. This phenomenon causes energy imbalance and leads to deformation and warping. To solve this problem, material and mold temperatures should be appropriately and moderately increased. Injection pressure and speed should be used.
