Tutorial: Injection Molded Parts

Injection Molded Parts

Injection molding is a process in which custom molded component manufacturers heat a polymer until it reaches a highly plastic state and forces it to flow under high pressure into a mold’s cavity to solidify. Manufacturers then remove the molded part, or molding, from the cavity. The process produces discrete components that are almost always net shape, reducing the need for surface finishing. A mold may contain more than one cavity, allowing custom injection molding companies to produce multiple moldings each cycle. The production cycle time ranges between 10 and 30 seconds. Cycles of 60 seconds or longer are also common.

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Injection Molds and Molded Components

Injection molding is a widely used molding process for thermoplastics. Some thermosets and elastomers are injection molded when equipment is modified and operating parameters allow for the cross-linking of materials.

It’s possible to create complex, intricate shapes with injection molding. It requires designing and fabricating a mold whose cavity has the same geometry as the original part. It must also allow for part removal. Part sizes range from 50 grams (2 ounces) to 25 kilograms (55 pounds). Heavier items made using injection molding include automobile bumpers and refrigerator doors.

The mold determines the part’s shape and size. It’s also the special tooling in injection molding. When making large, complex parts, a mold may cost hundreds of thousands of dollars. When it comes to smaller parts, manufacturers can create molds with multiple cavities, adding to its cost. Incidentally, injection molding is most economical for large production quantities.

Injection Molding Products: Equipment and Process

Equipment

Injection molding equipment originates from metal die casting processes. An injection molding machine has two main components:

1. Plastic injection unit

2. Mold clamping unit

Plastic Injection Unit

In essence, an injection unit’s primary functions are to melt and homogenize a polymer before injecting it into a mold cavity.

The injection is similar to an extruder. It contains a barrel that a hopper with a supply of plastic pellets feeds from one end. Inside the barrel is a screw that turns to mix and heat the polymer. The screw also acts as a ram that quickly moves forward to inject molten plastic into the mold. A non-return valve near the screw’s tip prevents the melted material from flowing backward along the screw’s threads.

Later in the molding cycle, the screw, or ram, returns to its former position. Because of the screw’s dual action, it’s referred to as a reciprocating screw, which also identifies the machine type. Older injection molding machines use a simple ram without screw flights. However, the reciprocating screw design’s superiority has promoted its widespread adoption in today’s custom injection molding companies.

Mold Clamping Unit

The clamping unit’s primary functions include:

• Holding the two halves of the mold in proper alignment
• Keeping the mold closed during the injection process by applying a clamping force strong enough to resist the injection force
• Opening and closing the mold at the appropriate times during the molding cycle

Clamping units have two platens—a fixed and movable platen—and a mechanism for translating the movable platen. The mechanism is like a power press that a hydraulic piston or mechanical toggle device operates. Larger machines have clamping forces of several thousand tons.

Processes: Injection Molding Thermoplastic Polymers

When the mold is open and the machine is ready to start a new molding, the following occurs:

• The mold is closed and clamped.
• A shot of melt—which is the right temperature and viscosity thanks to heating and the screw’s mechanical workings—is injected under high pressure into the mold cavity.
• The plastic cools and solidifies when it contacts the mold’s cold surface. The equipment maintains ram pressure to pack additional melt into the cavity to compensate for retraction that occurs during cooling.
• The screw rotates and retracts with the non-return valve open so fresh polymer flows forward into the barrel’s forward portion.
• The polymer in the mold completely solidifies.
• The equipment opens and ejects the mold so it can be removed.

The Mold

A mold is a special injection molding tool. Custom molded component manufacturers design and fabricate the mold specifically for the part they’ll produce. When the production run for a part is finished, specialists replace the mold with a new mold for the next part they need to manufacture. Several types of molds exist for injection molding.

Two-Plate Mold

A conventional two-plate mold has two haves fastened to the molding machine clamping unit’s two platens. When you open the clamping unit, the two mold halves also open (see figure b). The mold’s prominent feature is the cavity, which is formed by removing metal from the two halves of the mating surfaces. Molds may have single or multiple cavities. The parting surfaces, or parting lines in a cross-sectional view of the mold, are where the mold opens to remove the part created.

In addition to the cavity, there are other essential features in a mold that serve critical functions during the molding cycle:

• Distribution channel: A mold has a distribution channel through which the polymer melt flows from the injection barrel’s nozzle into the mold cavity. The distribution channel contains a:
  • Sprue leading from the nozzle into the mold
  • Runners that lead from the sprue to the cavity
  • Gates constricting the flow of plastic into the cavity; there may be one or more gates for each cavity in the mold
• Ejection system: An ejection system ejects the molded part from the cavity at the end of the molding cycle. Ejector pins in the mold’s moving half typically complete the ejecting function. The cavity is divided in between the two halves of the mold so the mold’s natural shrinkage causes the part to stick to the moving half. When the mold opens, the ejector pin pushes the part out of the mold cavity.
• Cooling system: Cooling systems are required for molds. They have an external pump connected to passageways in the mold where water circulates to remove heat from the hot plastic. Custom molded component manufacturers must also remove air from the mold cavity as the polymer enters. Much of the air passes through small ejector pin clearances in the mold. Narrow air vents that are about 0.03 millimeters (0.001 inches) deep and 12 to 25 millimeters (0.5 to 1.0 inches) wide are often machined into the parting surface. The channels allow air to escape outside but are too small for the viscous polymer melt to exit.

Three-Plate Mold

While two-plate molds are the most common in injection molding products, there are instances in which three-plate molds are better suited for a project. Three-plate molds have the following advantages:

• Gate position: The flow of molten plastic is through a gate located at the cup-shaped part’s base instead of inside at the side. This allows for a more even distribution of melt onto the sides of the cup. In the side-gate, two-plate design, the plastic must flow around the core and join on the opposite side, which could create a weakness at the weld line.
• More automatic molding machine operation: Three-plate molds allow for a more automatic operation of the molding machine. As the mold opens, it divides into three plates with two openings between them. This forces a disconnection of the runner and parts, which drop by gravity into different containers beneath the mold. Blown air or a robotic arm may assist the gravity-related drop.

Reusing the Sprue and Runner in Two- and Three-Plate Molds

The sprue and runner in conventional two- or three-plate molds are waste materials. In many instances, custom injection molding companies may grind and reuse these materials. In some injection mold and molded components, a product must be made of virgin plastics that were not previously molded.

A hot-runner mold eliminates the solidification of the sprue and runner by locating heaters around the corresponding runner channels. As the plastic in the mold cavity solidifies, the material in the sprue and runner channels remains molten and ready to be injected into the cavity in the next cycle.

Injection Molding Machines

Injection molding machines are usually identified by the type of injection used. They may be injection or clamping units.

Injection Units

There are two modern types of injection units:

• Reciprocating screw machine: The most common injection unit; reciprocating-screw machines have a design that uses the same barrel for melting and injecting the plastic.
• Screw-preplasticizer machine, or two-stage machine: A screw-preplasticizer machine uses separate barrels for plasticizing and injecting a polymer. In this design, plastic pellets enter a hopper in the first stage. A screw drives the polymer forward to melt it. This barrel feeds a second barrel that uses a plunger to inject the melt into the mold. Older plunger-type injection molding machines use one plunger-driven barrel to melt and inject the plastic.

Alternative injection systems to the reciprocating screw machine: (a) screw-plasticizer, and (b) plunger type

Clamping Units

Clamping units have three designs:

• Toggle clamps: Toggle clamps have various designs. They have an actuator that moves the crosshead forward, extending the toggle link so it pushes the moving platen toward a closed position. At the beginning of a movement, the mechanical advantage is low and the speed is high. Near the end of the stroke, the opposite is true. As a result, toggle clamps provide high speed and high force at different points in the cycle when they’re desirable. Hydraulic cylinders or ball screws driven by electric motors actuate them. Toggle-clamp units are generally best suited for low-tonnage machines.

  Two clamping designs: (a) one possible toggle clamp design in which (1) is open and (2) is closed; (b) a hydraulic clamping design in which (1) is open and (2) is closed. The tie rods used to guide a moving platen is not shown.

• Hydraulic clamps: Hydraulic clamps are used on higher-tonnage injection molding machines. They typically range between 1,300 and 8,900 kilonewtons (150 and 1,000 tons). The units are more flexible than toggle clamps in regard to setting the tonnage at given positions during a stroke.
• Hydromechanical clamps: Designed for large tonnages above 8,900 kilonewtons (1,000 tons), hydromechanical clamps operate by:
  • Using hydraulic cylinders to move a mold toward the closing position rapidly
  • Locking the position mechanically
  • Using high-pressure hydraulic cylinder to close the mold and build tonnage

Sinotech is a custom injection molding company with competitive rates and more than 12 years of experience auditing, qualifying and working with QS-9000- and ISO-certified rubber molding factories in China, Taiwan and Korea. As a U.S.-based company operating under U.S. laws, we’re dedicated to managing your project on-site and delivering your parts at prices that beat the competition without sacrificing the quality, service and terms you expect from a domestic supplier.

Get started on your injection molding project today by contacting Sinotech.