The Complete Guide To Injection Molding

Plastics have become part and parcel of our daily lives. Plastics are vital for manufacturing various consumer goods, packaging materials, pipes, prototypes, automotive, industrial, medical, aerospace, and household items. The flexibility and versatility of plastic materials make them dependable for manufacturing precision parts and can be molded into complex shapes and sizes.

Injection molded part with complex surface details

Converting plastic materials to consumer products is achieved through several processes. One popular method for manufacturing plastic products is injection molding. It is a simple yet critical production process that allows manufacturers, innovators and designers to create unique products and incorporate special features into the end products.

Why is injection molding a preferable manufacturing technique? How vital is it for rapid prototyping? This article explores how injection molding works, explores the benefits of this process and explains its relevance to the product development process.

 How Injection Molding Works

 Injection molding uses specialized machines to produce a variety of parts and products. The manufacturing equipment consists of an injection unit, clamping devices and molds. The manufacturing material is heated to a suitable temperature, making it molten and easy to flow. The pelletized material moves through a hot barrel. 

The injection molding machine has a helical screw that facilitates the mixing of manufacturing materials. The screw makes it possible to transport the molten material through the barrel. At the end of the barrel, there is a nozzle for injecting the molten material into a mold cavity. The cavity takes the shape of the desired end product. The molten material begins solidifying and hardening once it is injected into the mold cavity.

Usually, the injection molding equipment has a hopper that holds the manufacturing material. The size of the hopper depends on the size of the end product. Machines used for mass production have big hoppers to facilitate quick production. 

The injection molding manufacturing process uses different raw materials, predominantly using thermoplastics. Melting the thermoplastic pellets requires an external source of heat. Some molding equipment has electric heater elements, while others depend on friction to generate heat. Friction between the barrel and the screw is sufficient to melt small thermoplastic or rubber pellets.

After passing through the barrel, the molten material is deposited into the mold cavity at high pressure. The mold is designed to have two halves. The two halves of the mold must be clamped sufficiently to accommodate the high injection pressure.

The Injection Molding Process

Like other manufacturing processes, injection molding occurs in specific stages. These concurrent stages affect the final quality of the molded products. It is critical for individuals engaged in invention design to understand the steps of this mass manufacturing process. Here is a look into the typical injection molding process.

Clamping

As mentioned earlier, the mold or die is divided into two halves. One-half of the mold is fixed to the molding machine, while the other half is movable. The movable half can slide on a guideway. Before beginning the process, the mold should be clamped. 

There are two ways to clamp the mold.

  • Toggle clamping
  • Hydraulic clamping

The latter uses a hydraulically powered mechanism to close and open the die, making the clamping process faster. Clamping ensures that the molten material is secure during the high-pressure injection process.

Since injection machines vary in size and design, the clamping time for different processes or molding machines will vary. Larger molding machines require more time to set up compared to smaller ones for rapid prototyping or non-commercial use. One can estimate clamping time for different molding machines based on the designed dry cycle time of the molding equipment.

Injection

It is the critical stage of the manufacturing process. An industrial design company dealing with plastic products must optimize this process. It begins with selecting the right type and size of thermoplastic pellets. Since the pellets must melt before injection, they need to have closely similar sizes and shapes. The plastic pellets become molten due to friction and pressure between the screw and the barrel. The molten pellets accumulate at the front of the screw through a process known as metering. 

The material accumulates to the desired amount after which it is injected under pressure. The quantity of injected material, also called shot, has varying thicknesses depending on the nozzle size at the front end of the barrel.

Estimating the injection time for different grades of plastic material can be difficult. It is because the rate of flow of the molten material varies from time to time. The average injection time is a function of injection pressure and shot volume.

Cooling

Plastic takes little time to cool. Cooling begins immediately when the molten plastic comes into contact with the mold (die). As the molten material cools, it solidifies and takes the shape of the mold. Cooling can take a considerable amount of time. 

Unclamping the molding machine too soon will leave the final product with shrunk or defective sections. The molding process and machine setup allow the injection of additional molten material to cover the shrinkages on the surface.

It is advisable to leave the clamping unit intact until the desired cooling time is achieved. Cooling time for different grades of plastics can be estimated using the known thermodynamic qualities of the raw material. Plastic products with larger wall thicknesses require more time to cool and solidify.

Ejection

Once the cooling time elapses, finished products are removed from the mold to allow the production of other parts. Aside from the clamping mechanisms, the injection molding machine has a specially designed ejection system. The ejection system is fixed to one-half of the mold. Once the mold is open, the ejector pushes the finished product from the die. 

The ejection system applies sufficient force on the solidified part. Ejection utilizes a release agent, which is applied around the mold cavity and facilitates the smooth removal of the plastic part. Usually, the end product features uniform wall thicknesses.

Small scale injection molding machine

Ejection time includes the time taken by the part to detach and fall freely from the unclamped die. One can estimate ejection time using the machine’s dry cycle time values.

Benefits of Injection Molding

Any industrial design company venturing into plastic manufacturing will prioritize injection molding. The main advantage of this manufacturing process is its ability to support the mass production of different consumer products. 

It is cheaper to produce parts using injection molding. The prices drop as the number of molded parts to manufacture increases.

Aside from the relatively low cost of production, injection molding is associated with low raw material wastage. This manufacturing process becomes handy when the invention design process focuses on eliminating manufacturing waste and reinforcing sustainability principles. Injection molding has minimal scrap rates compared to subtractive manufacturing processes like CNC machining. It is only the advanced manufacturing processes like 3D printing that exhibit fewer scrap rates than injection molding. Since injection molding predominantly utilizes thermoplastic materials, scrap material is easy to recycle and re-introduce waste from the sprue, runners or gate locations to the manufacturing process. The recycled scrap also includes overflows and leakages from the mold cavities. 

However, it is vital to monitor the quality of the re-grind or rejected parts before recycling. Plastic material can degrade when exposed to heat and pressure severally. Re-grinding or recycling scrap plastic material proves vital for product development teams aiming to develop low-cost prototypes, demonstrational parts or materials with low-impact strengths.

Injection molding is a repeatable manufacturing process. It makes the molding process a preferred manufacturing method for producing parts requiring high precision standards. Production consistency relates to the replication of complex part designs over and over. The consistency of injection molded parts makes the process a preferable method for producing electronic components and other high-value products. The manufacturing process is fast and enables companies to achieve short lead times.

Shortcomings of Injection Molding

The advantages of injection molding outweigh its shortcomings. These are some of the disadvantages that manufacturers must face once they decide to use this manufacturing technique.

  • High initial investment cost
  • Requires precise handling to prevent surface deformation of the final product

Mold Design

Molds are manufactured from steel or aluminum material. Although the mold appears as a single unit, it consists of several components that facilitate proper operation. The mold tooling is designed so that one of the mold halves can slide to open or close the parting line. The mold consists of two parts:

  • Injection mold plate
  • Ejector plate

An example of the two halves of the mold with vents

Some molds have a single cavity, while others have multiple mold cavities and can be used to produce identical parts faster. Molten resin enters the cavity through a sprue or mold gate. The sprue has specially designed bushings that seal the nozzles to prevent flashing or overflow of material once the cavity is fu. The bushings also serve to direct the molten material to the mold cavity through precision-manufactured channels on the face of the mold and ejector plates. These channels are also called runners to allow timely accumulation of molten material to form desired product or part shapes.

The two halves of the mold have tiny vents around the parting lines. These vents are critical for expelling air trapped within the mold cavity. Some molding machines have vents located near the slides and ejector pins. These vents ensure that the quality of the final product is up to par. 

Compressing the trapped air under the extreme injection pressure causes deformations on the surface of the end-products. Some sections of the final product will exhibit surface deformations like warping or bubbles. The result is a poorly formed part with inferior surface finish, strength and quality. The dangers of trapped air exceed surface deformation. In some cases, extreme compression can cause the air to ignite and cause fires that can damage manufacturing equipment.

We have mentioned that molten material requires cooling. It requires some sort of forced cooling to shorten manufacturing time. The mold contains tiny holes drilled into the half plates of the mold. Water pipes pass through these holes and absorb the heat from the molten plastic. Cooling should be gradual to allow plastic materials to cool and solidify evenly.

The mold plates require regular maintenance. The mold plates are divided into several sub-assemblies and contain detachable inserts or blocks. It allows manufacturers to utilize a single mold to produce different parts or create varying designs of the same product.

Manufacturing Injection Molds

Developing injection molds is a lengthy and perhaps tedious process. It begins with designing the desired mold using computer-aided design (CAD) software. In prototype design, innovators and manufacturers opt for custom mold designs. Custom molds cost more to produce. The innovator or manufacturer must select the right fabrication tools and choose the best material. 

Aluminum and steel remain the preferable materials to manufacturer injection molds. The main methods to manufacture injection molds are:

  • CNC milling
  • Electrical Discharge Machining (EDM)

CNC Milling is used to fabricate molds for injecting thermoplastics

CNC machining is a popular mold manufacturing process since it enables manufacturers to create complex mold designs and incorporate finer details to ensure excellent product qualities. The other manufacturing method, EDM, allows manufacturers to include complex part features like ribs which are problematic to design and produce using standard manufacturing processes.

Injection Molding Materials

Manufacturers use different grades of plastic materials to create consumer products and experimental prototypes. Each of these materials possesses unique properties affecting their performance in diverse application areas. Some of the features that an industrial design company should evaluate before settling on certain thermoplastic materials are:

  • Temperature resistance
  • Pressure endurance
  • Molecular structure

The most popular thermoplastic materials utilized for injection-molded products include:

  • ABS (Acrylonitrile Butadiene Styrene)– Has impressive tensile strength and is stable over a wide temperature range. The molded part often has a smooth surface finish.
  • Polypropylene (PP)– Plastic variant having impressive heat resistance. Products made from PP have better durability.
  • Polycarbonate (PC)– This grade of plastic is light and offers impeccable product strength and stability.
  • Nylon– Nylon material is suitable for manufacturing products with remarkable chemical resistance. It can be used for producing chemical packaging materials.

Other materials for injection molding include:

  • Polyethylene (PE)
  • Silicone rubber

Possible Manufacturing Defects

Manufacturing processes are susceptible to defects. Although injection molding is used for manufacturing precision parts, final products are sometimes prone to deformations and surface defects. Different types of defects accompany injection molded parts. These are the common defects and their causes.

Warping

Warped molded surfaces appear when the cooling rates are non-uniform. The coolant distribution is uneven, some cooling holes are blocked, and coolant circulation is poor.

Bubbles

Bubbles can appear on the surface of the finished product. These bubbles affect the quality of the surface finish and impact the strength of the products. Bubbles on injection molded products cause them to have a shorter lifespan. Bubbles appear on the product if:

  • The injection pressure exceeds the required level
  • Raw materials or pellets have a high moisture content
  • Sections of the molded material cool faster than the rest.

Flash

It is another visible product defect. Flashing occurs if:

  • The clamping force is low
  • The machine delivers excessive injection pressure than required

Unfilled products

After injection, one can easily spot unfilled sections on the end product. The emergence of voids on the final product indicates that the delivered shot volume is low or the flow rates are low.

Sunken surfaces

The molded piece may depict sunk surfaces. The sink marks are more visible when the injection pressure is low and in instances where the cooling rate is non-uniform. Some areas of the mold cool faster than others.

Ejection marks

The injection process can be smooth and facilitate fully formed pieces. However, the part may get damaged during the unclamping and ejection stage. The ejection marks appear if the ejection force is high or the product is removed before the requisite cooling time is achieved. 

Which Molding Machine Should You Choose

After years of development, injection molding is now available for use across different sectors of the economy. They are versatile pieces of equipment that can produce any part imaginable for designers and innovators. The search for the ultimate injection molding machine is easier in the age of digitalization. 

Several manufacturers and vendors list their products in online shops. It includes injection molding machines of varying sizes, ready-to-manufacture pellets and replacement parts.

At the same time, companies are increasingly scaling up their operations. Several companies offer injection molding services at a fee. It means that an individual can design the desired product, share the design files and wait for the finished product to be delivered to their doorsteps. Such companies run websites for online quoting, allowing customers to compare pricing options and turnaround times based on part specifications.

When purchasing molding machines, individuals choose between hydraulic or electric molding machines. Each of these types of molding machines has its advantages. Recently, electric molding machines have gained popularity in the market. These machines operate faster than their hydraulic counterparts. However, hydraulic machines are cheaper than electric injection molding machines.

Manufacturers use tonnage to rate injection molding machines. The term tonnage refers to the effective clamping force the molding machine delivers. The tonnage of these machines ranges between 5 and 6000 tons.

Identifying The Feasibility of Injection Molding For Your Next Project

Injection molding is cheap for mass production. Even though it is an affordable manufacturing alternative, it does not imply that it is practical in every situation. There are a few questions that an innovator or product development team must ask before settling on this manufacturing method. It does not matter if you are working on prototype design or at the market penetration stage after a successful product launch. Let us look at every consideration in detail.

What is the available budget?

The first step is the financial muscle that an individual or company possesses. The allocated financial budget determines the number of parts for manufacturing. If the company or innovator intends to produce several consumer parts faster using a slim budget, then injection molding becomes the most viable option.

Production budgets cater to design, production, assembling, sorting and branding costs. It makes sense to outsource the manufacturing of products if it is a one-off project. However, if the company intends to manufacture the products for a long time, it must invest in the right set of molding equipment. It stretches the budget a bit. Remember, the machines cost several thousands of dollars.

Ensure that the investment rakes in substantial returns within the shortest possible time.

Which Product Designs is the Company Dealing With?

The injection molding machine handles a variety of product designs. For invention designs, companies focus on delivering unique products and incorporating specialized product features. When the company decides to follow the path of injection molding, it has to define the complexity limits of the desired end-products. It is simpler to manufacture parts with simple geometries.

Parts with complex features require specially designed molds. Given that molds are manufactured using other processes like CNC machining, laser sintering or EDM, it may force the company to spend more money fabricating the molds from aluminum or steel.

Eliminate complex designs at the design stage. It is better to optimize simple product designs to ensure few transfers of errors that will ultimately affect the quality of the final product or cause surface defects.

Monitoring the Process

The quality of the final product affects the company’s returns and the customer’s satisfaction. When choosing injection molding, it is essential to keep the final results in mind. It helps the company devise methods to monitor product quality and optimize processes to achieve the best quality of products.

Streamlining the manufacturing process from the word go enables the company to reduce cycle time systematically. It also enables them to identify production bottlenecks, predominant sources of scrap, overflows and overhangs. Eliminating bottlenecks and process waste is critical for maximizing savings from manufacturing activities. The small financial savings translate to millions of dollars if the injection molding process churns out thousands of products daily.

Summing Up

Injection molding is a suitable mass manufacturing process with several benefits. It has found relevance in different industries and enables invention design teams to develop prototypes cheaply and fast. It competes favorably against advanced manufacturing techniques like 3D printing, laser sintering and CNC machining. 

There are several types of injection molding machines bearing diverse physical characteristics and capabilities. Molding equipment utilizes special tooling to produce parts from a variety of materials. Ensure the machine is well clamped before use. Monitor the dry cycle, cooling time, and ejection time to ensure the final product meets the quality requirements and specifications.