Guide to Plastic Molding Manufacturing : From Mold Design to Mass Production of Injection Molded Parts

Guide to Plastic Molding Manufacturing : From Mold Design to Mass Production of Injection Molded Parts

With the rise of mold-making technology , manufacturers who mastered injection molding, an advanced manufacturing technology, continued to rely on molds to produce most plastic products used in everyday life. Therefore, when you or your business needs to manufacture plastic products in large quantities, it is essential to understand the working principles, usage methods, and optimization techniques of mold design, manufacturing, and injection molding processes to meet specified production volume requirements.

This comprehensive guide provides detailed information on mold design, manufacturing, and high-volume injection molding production. It also delves into selecting the optimal cost, equipment for process optimization, and the role of 3D printing to ensure the success of your high-volume injection molding projects.

Injection molding requires a well-designed fixture, structure, or frame (called a mold) to shape a malleable raw material. Structurally, molds can be classified as either concave (female) or convex (male), depending on the final design the manufacturer wishes to create.

Therefore, molds are an indispensable part of the injection molding process, primarily used to replicate a given design and produce identical copies. The two main materials used in mold making are alloy steel and aluminum. After the design and development are completed using molding tools, the mold needs to be installed in the molding equipment. Then, raw materials such as thermoplastics are poured into the mold or shaped around it. Once the material solidifies, the mold imprints its design onto the material, forming the finished product.

Injection molds are the core tools in plastic product manufacturing, determining the product’s shape, size, and surface quality. During injection molding, plastic granules are heated and melted, then injected into the mold cavity, where they cool and solidify to form the desired product. A high-quality injection mold not only improves production efficiency but also ensures consistent product quality and reduces production costs.

Key considerations for mass production of plastic parts using injection molding.

Designing plastic parts is a complex task involving many factors that meet application requirements. “What is the purpose of the part?” “How will it fit with other parts during assembly?” “What loads will it withstand during use?” Besides functional and structural issues, processing considerations have a significant impact on the design of injection-molded plastic parts. The way molten plastic is injected, filled, and cooled within the cavity to form the part largely determines the necessary properties. Following some basic rules of injection-molded part design, in addition to making it easier to manufacture and assemble, often results in parts that are more robust in use. Dividing parts into basic groups helps you build them logically while minimizing molding problems. Throughout part development, always keep in mind how the part will be molded and how to minimize stress.

Development standards for mass production of plastic parts projects

A master manufacturing plan helps clarify detailed information such as the project’s expected goals, development timeline, and available budget. Whether you plan to manufacture custom parts or mass-produce hundreds of thousands of similar products will determine which options to choose in the next steps. Development criteria may vary from project to project, but generally include part design, dimensions, materials, quality, tolerance requirements, timelines, production volume requirements, and cost constraints. During product development, initial documents such as the Product Requirements Document (PRD) can often help you resolve questions, while the Bill of Materials (BOM), a comprehensive list, provides information on the parts, items, assemblies, and other materials required to manufacture the product.

Key considerations for mold design for mass production of plastic parts

Mold design is the soul of injection molding. A professional design team needs to create a 3D model based on product drawings and analyze key parameters such as wall thickness, draft angle, and gate location. A well-designed mold can effectively avoid common defects such as shrinkage, deformation, bubbles, and weld lines, thereby improving the yield rate of mass production.

Component Design

Simplify the design as much as possible and make it conform to the design rules of the specific manufacturing process in order to select the lowest cost technology. Simple designs may only require molds made by hand, while complex designs often require multi-piece molds and molds made using digital manufacturing tools such as 3D printers or CNC machining.

Yield

Mass production demands durable molds capable of accurately replicating the model without wearing out after a few uses. However, manufacturing such molds is costly and time-consuming. For disposable parts and small-batch production, choosing lower-cost and faster-to-produce molds may be more efficient, for example, by discarding sacrificial molds or soft molds when they begin to show signs of irreparable wear.

Structural elements to consider in the design of plastic molds for mass production of plastic parts

The structure of a mold generally varies depending on the type and properties of the plastic, the shape and structure of the plastic product, and the type of injection molding machine, but the basic structure remains consistent. A mold mainly consists of a gating system, a temperature control system, molding parts, and structural parts. Among these, the gating system and molding parts are the parts that directly contact the plastic and vary depending on the plastic and the product; they are the most complex, the most variable, and require the highest level of surface finish and precision in the mold. Therefore, the following structural design considerations should be taken into account when designing molds for mass production.

Parting surface

This refers to the contact surface where the die and punch engage when the mold is closed. Its location and form are influenced by factors such as the shape and appearance of the product, wall thickness, molding method, post-processing techniques, mold type and structure, demolding method, and molding machine structure.

Structural components

This refers to components such as sliders, angled ejectors, and straight ejector blocks in complex molds. The design of structural components is crucial, affecting mold lifespan, processing cycle, cost, and product quality. Therefore, designing the core structure of complex molds requires a high level of comprehensive ability from the designer, striving for the simplest, most durable, and most economical design solutions possible.

mold precision

This includes components such as jam-avoiding devices, precision positioning, guide pillars, and positioning pins. The positioning system is related to the appearance quality of the product, the quality and lifespan of the mold. Different positioning methods are selected according to different mold structures. Positioning accuracy control mainly relies on machining, while internal mold positioning mainly requires designers to fully consider and design a more reasonable and easily adjustable positioning method.

Gating system

This refers to the material feeding channel from the injection molding machine nozzle to the mold cavity, including the main runner, branch runners, gate, and cold slug well. In particular, the selection of the gate location should facilitate the filling of the cavity with molten plastic in a good flow state, and the solid runner and gate cold slug attached to the product should be easily ejected from the mold and removed when the mold is opened (except for hot runner molds).

Injection mold processing and manufacturing

in the mass production of plastic products include tool steel, mold steel, high-speed steel, and superhard alloys with added carbon and chromium. In recent years, molds made of ceramic have also emerged. Most mold materials are difficult to machine, exhibiting properties that hinder cutting. Therefore, CNC machining centers are typically used for cutting, and post-processing steps such as grinding are usually performed to improve precision. For even finer machining, various electrical discharge machining (EDM) techniques are employed. This technique utilizes the sparks generated by electrical discharge to melt the surface of the workpiece, thereby performing machining. It not only achieves high-precision machining but is also suitable for machining complex three-dimensional shapes.

Injection molding cost

Injection molding molds are typically made of metal using CNC machining or EDM (Electrical Discharge Machining). These high-cost industrial methods require specialized equipment, advanced software, and highly trained personnel. Producing metal molds typically takes four to eight weeks and costs between $2,000 and over $100,000, depending on the shape and complexity of the part. However, there are alternative methods besides using metal molds. Compared to metal molds, using in-house metal 3D printing to create injection molding molds for prototyping and small-batch production can significantly reduce costs and time, while still producing high-quality, reusable parts.

Key Points for Cost Control in Mold Injection Molding

Mold life management

Proper mold maintenance can extend the lifespan of molds. Regularly cleaning molds, inspecting worn areas, and promptly replacing vulnerable parts can reduce the mold-related costs per unit of product.

Material utilization optimization

By optimizing gate design and recycling sprue material, material utilization can be improved. Sprue material from some materials can be crushed and reused in a certain proportion, reducing raw material costs.

Production efficiency improvement

Shortening the molding cycle is key to increasing production capacity. Optimizing the cooling system, adopting hot runner technology, and automating production can significantly improve efficiency.

Common accessories for injection molds

Punch, punch, guide post, guide sleeve, ejector rod, ejector pin, ejector sleeve, ball bearing sleeve, independent guide post, slider, date stamp, angled ejector slider, sprue sleeve (gluing sleeve), positioning block, guide fixing block, guide post auxiliary device (positioning post), guide post assembly, etc.

Basic information on injection molding process and manufacturing flow

This is a large-scale plastic mold production technology. Injection molding requires no human intervention; it is a fully automated process used to produce thermosetting and thermoplastic plastics. The method first defines the shape of the mold in a cavity. Next, pressurized plastic material is pumped into the mold at high temperature to maintain its amorphous state. Finally, the plastic is removed from the mold after it has hardened.

Injection molding machines can be mounted horizontally or vertically. Most machines are horizontally mounted, while vertical machines are used for specific applications, such as insert molding, to utilize gravity. Furthermore, many vertical machines do not require a fixed mold. Traditional clamps are the most common method for securing the mold to the mold platen. However, hydraulic clamps and magnetic clamps are also used to secure the mold. Magnetic clamps and hydraulic clamps are typically used when quick mold changes are required.

Unlike other traditional molding methods, injection molding is well-suited for high-volume production and is cost-effective. It can also be used to manufacture materials of various sizes, such as plastic mechanical parts and other molded components. Furthermore, it can utilize either a cold runner system or a hot runner system to deliver plastic and filler from the injection molding machine to the cavity. A cold runner is a simple channel within the mold. The plastic filling the cold runner cools along with the workpiece and is extruded along with it. Hot runner systems are more complex and typically use a tube heater to maintain the temperature of the plastic within the runner as the workpiece cools. After the workpiece is extruded, the remaining plastic in the hot runner is injected into the next part.

Basic information on commonly used injection molding equipment

Injection molding machines, also known as presses, consist of a hopper, an injection plunger or screw piston, and a heating element. A mold is clamped onto the mold of the molding machine, into which plastic is injected through a sprue. The rating of a press is in tonnage, which is the amount of clamping force that the machine can apply. This force keeps the mold closed during injection molding. Tonnage can range from under 5 tons to 6000 tons, although higher tonnage presses are rarely used. The total clamping force required is determined by the projected area of the custom part being molded. There are 2 to 8 tons of clamping force per square inch of projected area. As a rule of thumb, most products can use 4 or 5 tons/inch. If the plastic material is very strong, more injection pressure is needed to fill the mold, thus requiring a higher clamping tonnage to keep the mold closed. The required force can also be determined by the material used and the size of the part; the larger the plastic part, the higher the clamping force required.

Common types of equipment for mass production of plastic parts

Plastic molding equipment is classified into three types. The mechanical and electrical systems of these machines form the basis of this classification. They are:

Hydraulic plastic mold injection equipment

Hydraulic injection molding machines are devices that use clamping force to inject plastic. Due to their powerful clamping force, they can achieve high injection speeds. These machines are exceptionally robust, durable, and efficient, with a lifespan far exceeding that of other types of machines. Furthermore, hydraulic injection molding machines can inject more plastic into the mold. They are highly efficient, but due to the use of clamping force, they are slower and cannot produce large quantities of molds. They are superior to most other types of machinery because of their consistent availability of spare parts and lower maintenance costs.

Electric injection molding equipment

As the name suggests, electric injection molding machines are low-energy-consumption devices driven by electricity. They are typically used in highly automated injection molding processes. These machines use moderate clamping forces and provide a cleaner molding process. Furthermore, they offer extremely high precision in mold making, thus reducing the likelihood of producing defective products.

Hybrid injection molding machine

These high-efficiency injection molding machines combine the powerful performance of hydraulic systems with the efficiency of electrical systems. Hybrid injection molding machines offer higher throughput because they consume less energy and produce less noise. They can potentially increase the output of both thick-walled and thin-walled parts. Plastic mold design is a straightforward process that depends on the method, machine, and material type. When considering long-term, high-volume production, choosing a cost-effective production method is crucial.

Two commonly used material types for injection molding

There are many choices of materials for manufacturing. However, a few materials dominate the market. Understanding the basic properties of these materials will help you make better decisions. Plastics are composed of polymer chains, which are made up of repeating clusters of atoms. The composition of these chains varies from type of plastic, resulting in differences in quality and performance. Understanding the general properties of plastics is crucial for design. Plastics are divided into two categories: thermosetting plastics and thermoplastic plastics.

thermosetting plastics

Thermosetting plastics undergo a chemical reaction during processing. During this process, new bonds are formed in the polymer chains. Because the chemical reaction is irreversible and can only occur once, recycled materials cannot be used. Their advantages include high strength and high-temperature resistance. Thermosetting plastics include epoxy resins, silicones, polyurethanes, and phenolic resins.

thermoplastics

Heating softens thermoplastics, making them easier to injection mold. Unlike thermoset plastics, they do not undergo chemical transformations. Therefore, they can be reprocessed after initial manufacturing. Thermoplastics exist in two forms: semi-crystalline thermoplastics and amorphous thermoplastics.

How to reduce the cost and risk of mass injection molding?

In high-volume plastic product manufacturing projects, related errors directly lead to additional costs and longer-than-expected delivery times. Therefore, finding a clear Design for Manufacturing (DFM) methodology, controlling it, and ultimately potentially eliminating these problems entirely can significantly reduce injection molding costs while simultaneously improving operational efficiency. The following are relevant lessons learned by Elimold’s mold design and injection molding manufacturing engineering team.

Establish basic design rules to minimize iterations.

We adhere to a core set of DFM parameters: standard wall thickness (1.5-2.5mm), minimum draft angle of 1°, and elimination of complex undercuts. By providing guidance in the form of clear injection molding guidelines, we systematically reduce mold rework, thereby reducing the typical number of sampling iterations from 3-5 to 1-2. This directly addresses major cost and schedule issues.

Apply advanced analytics to prevent warping and subsidence

In addition to fundamental laws, we also use simulation to address the integrity issues of specific parts. Therefore, in electronic housing projects, a warpage of 0.8 mm can result in an 8% scrap rate. Our investigation identified rib design as a factor, and we set the rib bottom-to-height ratio to ≤3:1. This refined design reduced warpage to as low as 0.2 mm, achieving a scrap rate of 0.5%, while maintaining product visual and dimensional stability.

Integrate gate and cooling design early in the process.

Cost reduction is not just a matter of part shape. Often, during the first DFM review of a client’s project, we consider the integration of the gate location and the design of conformal cooling channels. In this way, we can avoid problems such as jetting, sink marks, and unnecessary long cycle times, thus producing high-quality injection molding from the very first trial, ensuring reliable project economics.

Scientific and standardized DFM methodology

By shifting from general recommendations to a prescriptive, analytical, and supported DFM approach, our methodology effectively addresses the complexities of cost and quality. This technology transforms initial part designs into manufacturable, off-the-shelf blueprints, thus enabling customers using our injection molding services for high-volume production of plastic parts to gain faster time-to-market and cost control, resulting in a measurable and significant competitive advantage.

Injection molding using metal 3D printed mold inserts

The Elimold team also utilizes metal 3D printing services to manufacture mold inserts and mass-produce plastic parts. Typically, we use both aluminum and steel to create inserts, manufacturing simpler parts in-house and outsourcing the production of more complex molds. However, this increases costs and lead times, especially for one-off or small-batch prototyping required before mass production of plastic parts. Therefore, the Elimold team has helped clients reduce project costs and lead times to less than a third in various projects requiring mass production of plastic parts by using our in-house metal 3D printing services.

This service is ideal for prototyping plastic parts before mass production, as our team can quickly test different iterations by easily changing the 3D-printed inserts. During the prototyping phase, we only need to assemble a few components with others to test the symmetry of all components. Using 20 or 50 components is sufficient to prove the effectiveness of the customer’s design. However, if fiber-reinforced plastic is used, it causes greater wear and tear on the molds than with traditional materials. Therefore, the inserts have a shorter lifespan. One insert might be able to produce 20 components, but if production continues, the insert needs to be replaced.

Contact Elimold to develop your company’s latest great project.

The Elimold team is committed to innovation, constantly exploring ways to push the boundaries of existing plastic injection molding in future projects. We are your preferred high-volume injection molding partner. We effectively combine our experience, expertise, and robust facilities to bring your plastic parts from production to market in a short time. Our rapid quoting capabilities, utilizing the latest analytical methods, can provide you with a quote within hours. Furthermore, we pride ourselves on our ability to achieve rapid turnaround for small-batch plastic and prototype testing before mass production by incorporating metal 3D printing technology, without compromising the quality of the final product (this service is available after you place a large order). Choosing Elimold means you’ll receive high-quality prototypes, short lead times, and affordable prices. Send us your design files and let us provide the best service.

Conclusion

Injection molding is a cost-effective solution for large-scale production, but due to the small volume of plastic molding, small and medium-sized enterprises (SME) can leverage Elimold’s “injection molding + metal 3D printing” service to create rapid molds at low cost for producing small quantities of plastic parts for testing. This leads to faster market entry, better product quality control, and shorter lead times. This article discusses everything you need to know about mold design and high-volume injection molding so you can make a more informed decision.

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