How injection molding works

What is injection molding?

Injection molding is the process of injecting molten liquid plastic into a mold and clamping it until the liquid cools down and solidifies. Materials like plastic, ceramics, metal, and glass are melted at high heat to turn them into a liquid state. This hot liquid is injected into various molds, which are stamped shut and left to cool down. The molds are then cooled, and the material inside them solidifies, taking their shape. They are then removed from the molds and tested for quality before packaging and delivery.

Creating the mold is the first step of injection molding. Once you have the mold ready, it can be used multiple times to create a large batch of products. A mold is a hollowed-out block in the shape, size, and dimensions of the final product you want. Additives like color, sparkles, and more can also be added to the molds while injecting molten plastic into them.

Injection molding has become a popular technique due to its cost-efficiency and ease of use. It is a convenient technique to mass-produce large batches of final products in quick time. It also offers flexibility in designs. Additionally, manufacturers can choose different types of injection molding machines based on their requirements.

How injection molding works

Injection molding is a multi-stage method where every step is crucial. An error or lapse at any step can lead to defective products.

• Step 1. The raw material (plastic pellets) is added to a hopper and sent to a cylinder.
• Step 2. The cylinder is heated for the plastic pellets to melt and turn into a hot liquid.
• Step 3. The nozzle at the other end of the cylinder injects the molten plastic into the mold channel. It works much like a syringe.
• Step 4. The melt channel (also known as sprue) will be cooled for the material inside to harden and solidify in its shape.
• Step 5. The sprue is removed from the part after cooling down. Finishing touches will be added to the final parts before sending them to the quality check.

Common defects in injection molding

Even though injection molding has many advantages and is a reliable technique for mass production, it is not without a few issues. Not taking proper care during the production cycle can lead to the following defects and affect the overall product quality.

• Cracking. A highly common defect in injection molding where the final product develops cracks when removed from the mold. The usual cause is high residual stress in the part combined with force on ejection: the part is pushed off the core faster or harder than the stressed material can take. Lowering injection pressure and hold pressure reduces the stress; raising mold temperature lets the material relax before ejection; slowing ejection and checking draft and ejector layout removes the trigger.
• Weld lines. Weld lines are thin streaks on the final product, which affect their strength and lead to faster breakage. They are caused when the liquid cools too quickly in the mold. This can be prevented by increasing the temperature of the material, increasing the injection speed, or adjusting the pressure on the mold.
• Bubbles. Bubbles are also called voids, which are tiny air holes in the final product. They weaken the final product, resulting in breakage. Bubbles occur when the pressure on the mold is incorrect. The issue can be solved by adjusting the mold temperature and pressure, adjusting the gate position, and decreasing the thickness.
• Flashing. Flashes are also known as burrs. They occur when excess plastic seeps out of the mold. Check the quantity to ensure it is not high. Similarly, ensure the injection speed is not too slow or fast, and that the molds are not clamped tighter than necessary.
• Sinkage. Sinkage is when the surface of the final product is uneven or sinks due to incorrect pressure applied when molding. It happens when the pressure is inconsistent during the injection process. It can be avoided by checking the mold for leakage, clogging, contamination, and ensuring the quantity of liquid plastic should also be enough to fill the mold.

Plastics used in injection molding

Over the years, a plethora of plastics have been used by manufacturers to create components using the injection molding technique. The following plastics are more commonly used.

• Acrylonitrile Butadiene Styrene (ABS). A common plastic used to make household items, toys, and more.
• Polyethylene terephthalate (PET). A strong and shatter-resistant plastic used for food and beverage containers.
• Polyvinyl Chloride (PVC). A multipurpose plastic used in construction materials like plumbing, electrical wiring, flooring, and more.
• Polycarbonate (PC). A clear and shatter-resistant plastic used to make medical and safety equipment.
• Polypropylene (PP). A heat-resistant plastic used for manufacturing industrial and automotive components.

Benefits of injection molding

• Mass production. The biggest benefit of injection molding is its ability to produce the final products in bulk. A large number of items can be continuously created in batches without compromising quality. It is the best choice to deliver consistent results.
• Versatility. Just about any type of product can be created using this technique. Since the machines are compatible with more than one material, manufacturers find it easy to make products in varying sizes, colors, shapes, and designs.
• Cost efficiency. The injection molding technique is budget-friendly compared to other molding methods. The manufacturer can choose the type of machine that fits their budget and production needs. Runners and rejects can also be reground and blended back into virgin material, which reduces waste, though regrind is limited to a modest percentage because each heat history degrades the polymer, and medical and food-contact work usually forbids it entirely.
• Precision and repeatability. Precision and repetition are two variables to consider during bulk production. Injection molding offers both, and is still easy to use. Tool life is set by the steel: an aluminum prototype tool runs from a few hundred to a few thousand cycles, pre-hardened P20 runs into the hundreds of thousands, and hardened H13 runs into the millions.
• Appealing appearance. How the final products look is also an important factor. Fortunately, the items made through injection molding are ready for delivery. They need little to no finishing touches and can be packaged and shipped after quality control.

Applications

A variety of products can be made using injection molding. This technique is widely used in the following industries: automotive (spare parts and components for vehicles), toys, electrical components (plugs, switches, screw holders), houseware and kitchenware (plastic curtains, hangers, hooks, spatulas, bowls, glasses, cups), medical devices (stethoscopes, syringes), food and beverage containers (instant noodle cups, yogurt cups, bottles), tools (screwdrivers, hammers, wrenches), and sporting items (tennis balls, hockey pucks).

Conclusion

Injection molding is a versatile, precise, and reliable technique to mass-produce numerous products in different industries. What decides whether it works on your part is the tool, and the tool is decided before it is cut. Get the design reviewed against the process, choose the steel for the volume you actually expect, and price the tool separately from the part so you can see which one you are paying for.