Plastic injection molding is a process that transforms plastic pellets into complex shapes by melting them and injecting them into a mold cavity. The mold cavity is usually made of metal and has the shape of the desired part. The plastic solidifies in the mold and then the part is ejected. Plastic injection molding is widely used for producing plastic parts for various industries, such as automotive, medical, consumer electronics, toys, and more.
Plastic Injection Mold Fabrication Basics
Plastic injection mold fabrication is the process of designing and making the mold that will be used for plastic injection molding. The mold consists of two main parts: the core and the cavity. The core is the part that forms the inner surface of the molded part, while the cavity is the part that forms the outer surface. The core and the cavity are usually mounted on a base plate and aligned by guide pins. The mold also has other components, such as sprues, runners, gates, vents, ejector pins, and cooling channels.
The mold fabrication process involves several steps, such as:
- Mold design: This is the first and most important step in mold fabrication. The mold design determines the shape, size, features, and quality of the molded part. The mold design also affects the mold material, fabrication method, cycle time, and cost. The mold design should consider factors such as part geometry, material properties, molding conditions, dimensional accuracy, surface finish, and mold durability.
- Mold material selection: The mold material should be able to withstand high temperatures, pressures, abrasion, corrosion, and wear. The mold material should also have good machinability, weldability, polishability, and heat treatment properties. Some of the common materials used for plastic injection molds are steel (such as P20, H13, S7), aluminum (such as 6061-T6), copper alloys (such as beryllium copper), and plastics (such as epoxy resin).
- Mold fabrication method: The mold fabrication method depends on the complexity and precision of the mold design. Some of the common methods are machining (such as milling, turning, drilling), EDM (electrical discharge machining), CNC (computer numerical control), additive manufacturing (such as 3D printing), and casting.
- Mold assembly: This is the final step in mold fabrication. The mold components are assembled together according to the mold design specifications. The mold assembly should ensure proper alignment, fitment, sealing, lubrication, and testing of the mold.
Types of Plastic Injection Molds
There are different types of plastic injection molds depending on the shape, size, and function of the molded part. Some of the common types are:
- Single-cavity mold: This is a mold that produces one part per cycle. It is suitable for low-volume production or complex parts that require high precision and quality.
- Multi-cavity mold: This is a mold that produces multiple parts per cycle. It is suitable for high-volume production or simple parts that require low cost and fast cycle time.
- Family mold: This is a mold that produces different parts per cycle. It is suitable for producing parts that have similar shapes or sizes or belong to the same product family.
- Two-plate mold: This is a mold that consists of two plates: one for the core and one for the cavity. The two plates are separated by a parting line where the plastic flows into the mold. The two-plate mold is simple and economical but has limitations in terms of part complexity and gate location.
- Three-plate mold: This is a mold that consists of three plates: one for the core, one for the cavity, and one for the runner system. The three plates are separated by two parting lines where
This article will provide you with a comprehensive guide on plastic injection mold fabrication, a manufacturing process used for the mass production of identical plastic parts. You will learn about the basic principles, advantages, cost, materials, and design tips of injection molding, as well as how to prepare for your first injection molding order.
Table of Content
- What is Plastic Injection Mold Fabrication?
- What are the Advantages of Plastic Injection Mold Fabrication?
- What are the Cost Factors of Plastic Injection Mold Fabrication?
- What are the Common Materials Used in Plastic Injection Mold Fabrication?
- How to Design for Plastic Injection Mold Fabrication?
- How to Start Your First Plastic Injection Mold Fabrication Order?
What is Plastic Injection Mold Fabrication?
Plastic injection mold fabrication is a manufacturing process that involves injecting molten plastic material into a metal mold, where it cools and solidifies into the shape of the mold cavity. The mold is usually made of steel or aluminum and can have multiple cavities for producing multiple parts at once. The plastic material can be colored or filled with other additives to achieve different properties or aesthetics.
The plastic injection mold fabrication process consists of four main steps: clamping, injection, cooling, and ejection.
- Clamping: The two halves of the mold are held together by a clamping unit that applies a high pressure to prepare for the injection of the molten plastic.
- Injection: The plastic material is fed into a heated barrel where it is melted and then injected into the mold cavity through a nozzle. The amount and speed of injection are controlled by an injection unit that determines the pressure, temperature, and flow rate of the molten plastic.
- Cooling: The plastic part cools and solidifies inside the mold as heat is transferred from the plastic to the mold. The cooling time depends on the size, shape, and thickness of the part, as well as the type of plastic material used.
- Ejection: After the part is fully cooled and solidified, it is ejected from the mold by a mechanism that pushes it out of the cavity. The mold can then be closed again for the next cycle.
Plastic injection mold fabrication is widely used for producing a variety of plastic parts with complex geometries, high accuracy, and good surface finish. Some examples of injection molded parts include toys, automotive components, medical devices, electronic enclosures, and household appliances.
What are the Advantages of Plastic Injection Mold Fabrication?
Plastic injection mold fabrication offers several advantages over other manufacturing methods for plastic parts, such as:
- High efficiency: Plastic injection mold fabrication can produce thousands or even millions of identical parts in a short time span, making it suitable for mass production. The cycle time can range from a few seconds to minutes depending on the size and complexity of the part.
- Low cost: Plastic injection mold fabrication has a low cost per unit when producing high volumes of parts, as the initial investment for the mold is amortized over many cycles. The material and labor costs are also relatively low compared to other methods.
- High quality: Plastic injection mold fabrication can produce parts with high dimensional accuracy, tight tolerances, and good surface finish. The parts can also have intricate features and details that are difficult or impossible to achieve with other methods.
- High flexibility: Plastic injection mold fabrication can use a wide range of plastic materials with different properties and colors to suit different applications and requirements. The parts can also be customized by adding inserts, logos, textures, or coatings during or after the molding process.
- High sustainability: Plastic injection mold fabrication can reduce waste and environmental impact by using recycled or biodegradable materials, minimizing scrap and excess material, and reusing or recycling the molds.
What are the Cost Factors of Plastic Injection Mold Fabrication?
The cost of plastic injection mold fabrication depends on several factors, such as:
- Part design: The design of the part affects the complexity and size of the mold, which in turn affects the material and machining costs. The design also influences the number of cavities in the mold, which determines how many parts can be produced per cycle. Additionally, the design affects the cooling time and ejection mechanism required for each part.
- Part volume: The volume of parts required affects the type and quality of the mold needed. For low-volume production (less than 10,000 units), prototype or aluminum molds can be used to reduce costs and lead times. For high-volume production (more than 10,000 units), steel molds are preferred for their durability and longevity.
- Part material: The material of the part affects the cost of the raw plastic material, as well as the injection and cooling parameters. Different materials have different melting points, viscosities, shrinkage rates, and thermal properties that affect the molding process and the quality of the part.
- Part finish: The finish of the part affects the cost of post-processing and secondary operations, such as trimming, polishing, painting, or coating. Some finishes can be achieved during the molding process by using textured or polished molds, while others require additional steps after the part is ejected.
The average cost of plastic injection mold fabrication can range from $1,000 to $100,000 or more depending on the factors mentioned above. The cost per part can vary from a few cents to a few dollars depending on the volume and complexity of the part.
What are the Common Materials Used in Plastic Injection Mold Fabrication?
Plastic injection mold fabrication can use a variety of thermoplastic materials that can be melted and reshaped multiple times. Some of the most common materials used in plastic injection mold fabrication are:
- Polyethylene (PE): A low-cost and widely used material that has good chemical resistance, impact strength, and flexibility. It is suitable for applications such as packaging, containers, bottles, and toys.
- Polypropylene (PP): A low-cost and versatile material that has good thermal stability, chemical resistance, and mechanical properties. It is suitable for applications such as automotive parts, medical devices, household appliances, and furniture.
- Polyvinyl chloride (PVC): A low-cost and durable material that has good flame retardance, electrical insulation, and weather resistance. It is suitable for applications such as pipes, fittings, cables, and flooring.
- Polystyrene (PS): A low-cost and easy-to-process material that has good optical clarity, rigidity, and dimensional stability. It is suitable for applications such as disposable cups, plates, cutlery, and packaging.
- Acrylonitrile butadiene styrene (ABS): A medium-cost and high-performance material that has good impact strength, toughness, and heat resistance. It is suitable for applications such as electronic enclosures, automotive parts, helmets, and toys.
- Polyamide (PA): A high-cost and high-performance material that has good mechanical strength, abrasion resistance, and thermal resistance. It is suitable for applications such as gears, bearings, bushings, and fasteners.
- Polycarbonate (PC): A high-cost and high-performance material that has good optical clarity, impact strength, and heat resistance. It is suitable for applications such as lenses, lighting fixtures, medical devices, and helmets.
How to Design for Plastic Injection Mold Fabrication?
Designing for plastic injection mold fabrication requires considering several aspects that affect the manufacturability and quality of the part. Some of the main design guidelines for plastic injection mold fabrication are:
- Minimize wall thickness: The wall thickness of the part should be as uniform and thin as possible to reduce material usage, cycle time, cooling time, warping, and sink marks. The recommended wall thickness ranges from 0.5 mm to 4 mm depending on the material and size of the part.
- Avoid undercuts: Undercuts are features that prevent the part from being ejected from the mold without damaging it or the mold. Undercuts should be avoided or minimized by using draft angles, side actions, or collapsible cores. The recommended draft angle ranges from 1° to 3° depending on the surface finish of the mold.
- Add ribs and gussets: Ribs and gussets are thin projections or reinforcements that increase the stiffness and strength of the part without increasing the wall thickness. Ribs and gussets should be designed with a thickness of 40% to 60% of the wall thickness to avoid sink marks or stress concentration.
- Add fillets and chamfers: Fillets and chamfers are rounded or angled edges that reduce stress concentration and improve the flow of molten plastic in the mold. Fillets and chamfers should be added to all sharp corners and transitions with a radius or angle of at least 0.5 mm.
- Avoid large flat areas: Large flat areas can cause warping or sink marks due to uneven cooling or shrinkage of the plastic material. Large flat areas should be avoided or broken up by adding textures, patterns, or ribs.
How to Start Your First Plastic Injection Mold Fabrication Order?
If you are ready to start your first plastic injection mold fabrication order, you can follow these steps:
- Prepare your 3D model: You need to have a 3D model of your part in a CAD software that meets the design guidelines mentioned above. You can also use online tools or services to check your model for errors or optimize it for injection molding.
- Choose your material: You need to select a suitable plastic material for your part based on its properties