A Beginner's Guide to Injection Molding

How Does Injection Molding Work?

Tooling fabrication: Once an injection molding design is finalized the first step in the manufacturing process is to mill the tooling, which is typically fabricated from steel or aluminum. In most cases, the metal block of material is placed in a CNC mill, which then carves out a negative of the final plastic part. Additional treatments like polishing or laser etching can then be applied to the tooling to achieve specific surface finishes.

If you want to learn more, please visit our website.

Part production: The actual production of plastic parts begins by loading resin pellets into a barrel. The temperature of the barrel is raised until the resin pellets reach a molten state and are then compressed. Next, the molten plastic is injected into the metal tool through a runner system, which then feed into the mold cavity through gates. The part then cools down, solidifies, and is ejected from the tool with ejector pins.

Types of Injection Molding

The term injection molding encompasses a handful of processes that inject liquid resin into a tool to form plastic parts. Here are four common types:

Thermoplastic injection molding: Thermoplastic injection molding is the most common type of molding. It injects thermoplastic resin into the mold where the material cools to form the final part.

Liquid silicone rubber molding: Liquid silicone rubber uses thermoset materials and a chemical reaction creates the plastic part.

Overmolding: Overmolding is a process used to manufacturing plastic parts with two or more materials. You&#;ll often find this on parts to improve grip by adding rubber to the handle.

Insert Molding: Insert molding is process that begins with an insert component placed into the mold before resin enters. The material is then injected and flows around the insert, typically metal, to form the final part. This is frequently used for parts that require metal threads.

Basic Design Principles for Injection Molding

Tolerances
With our injection molding process, we can hold about ±0.003 in. machining accuracy. Shrink tolerance depends mainly on part design and resin choice. It varies from 0.002 in./in. for stable resins like ABS and polycarbonate to 0.025 in./in. for unstable resins like TPE.

Wall Thickness
Wall thickness is important because it can lead to defects such as sink and warp. It is best practice to maintain a uniform thickness throughout an injection-molded part. We recommend walls to be no less than 40 to 60 percent of adjacent wall thickness, and all should fit within recommended thickness ranges for the selected resin.

Core Geometry
Core out parts to eliminate thick walls. You get the same functionality in a good molded part. Unnecessary thickness can alter part dimensions, reduce strength, and necessitate post-process machining.

Draft
Applying draft to molded parts is critical to ensure parts do not warp during the cool down process and it helps the part easily eject from the mold. Applying 1 to 2 degrees works well in most scenarios. If there are vertical faces, we advise incorporating .5 degrees of draft.

Side Actions
A portion of the mold that is pushed into place as the mold closes, using a cam-actuated slide. Typically, side-actions are used to resolve an undercut, or sometimes to allow an undrafted outside wall. As the mold opens, the side action pulls away from the part, allowing the part to be ejected. Also called a &#;cam.&#;

Link to Wingtat

Undercuts
A portion of the part that shadows another portion of the part, creating an interlock between the part and one or both of the mold halves. An example is a hole perpendicular to the mold opening direction bored into the side of a part. An undercut prevents the part from being ejected, or the mold from opening, or both.

Bosses
A raised stud feature that is used to engage fasteners or support features of other parts passing through them. There can be a tendency to design thick bosses which will increase the likelihood of sink and voids in a part. Consider reinforcing bosses with ribs or gussets for extra strength

Gates
A gate is an opening in the injection mold tool that allows resin to enter and fill the cavity. There are three common types of injection molding gates.

• Tab gates are the most common type of gate since it works well with additives and is the most cost effective option.
• Hot Tip gate is best for parts that cosmetic appearance is a priority. These gates can also reduce wear on tooling and flash.
• Pin, Post, or Tunnel gates  are ideal for cosmetic parts that don&#;t require a vestige. Sometimes not an option depending on material and geometry.

Ribs
A rib is a thin, wall-like feature parallel to the mold opening direction, its used to add strength and support to features like bosses and walls. To prevent sink, ribs should be no more than 60% of the wall&#;s thickness.

Ejector Pins
Ejector pins are installed in the B-side of the mold and help to release the plastic part from the tool after the part has cooled sufficiently. Designing in sufficient draft can help reduce the need for ejector pins on a part.

Logos and Text
Sans serif fonts will be the easiest to mill into a mold with text. We recommend font larger than 20 pt. and no deeper than 0.010 in to 0.015 in.

The injection molding process works by adding plastic pellets or metal powder to the injection molding machine&#;s hopper. Next, the hopper pours the plastic into the barrel, where it is heated into a liquid form. The molten plastic or metal enters the mold from the barrel through the nozzle.

A nozzle in the extruder presses tightly into a recess in the stationary half of the mold. The two halves of the mold, with a vertical parting surface, are made from heavy steel and mounted on thick steel tie-bars. Within the injection mold is a system of runners that distributes melt from the sprue to the mold cavities that determine the dimensions of the molded product. Using clamp plates, the mold halves clamp together with high pressure from a hydraulic piston.

Cooling channels passing near the cavities keep the mold cold, well below the solidification range of the plastic or other material. Next, the screw moves forward, forcing the melt through the runners and into the cavities, and maintains melt pressure (5,000-15,000 psi, 34-103 MPa) while the newly injection molded part cools and solidifies.

Then, the far half of the injection mold is withdrawn, activating ejector pins that push the molded part and runners out of the mold halves. At the same time, the extruder retracts and generates more hot melt.

Typically, the process is mostly automated with computer controls. Injection machines are rated by their melting capacity, shot size, and clamping force. They range from machines shooting two ounces of melt into a family mold that makes the parts for a hobbyist&#;s model airplane to machines that can mold 40-gallon trash cans in injection molds with two cavities&#;one for the can, the other for its lid. Injection-molded parts come in many shapes and sizes, meaning mold components come in various types.

If you are looking for more details, kindly visit Electronics Component Molding.