Medical Plastic Injection Moulding – Procedure and Products

In 1868, John Wesley Hyatt developed the first plastic injection mold, and the plastic injection moulding process. He used the plunger method to successfully inject hot, liquid celluloid, often called the first thermoplastic, into a split-die mold. This process was little changed until James Hendry built the first screw plastic injection mold in 1946. Today, nearly all industrial and medical plastic injection moulding processes use the screw method.

Plastic moulding is a very methodological and technical process. Thus it needs experts in this type of manufacturing business for it to meet the safety terms and to be competitive in the market. A very scientific and systematic mechanical study is first made before going into this endeavor. Here I have discussed some known plastic injection moulding process that can help you to understand the various ways it is done for industry specific plastic products.

1. Injection Moulding.

Examples of applications: Medical plastic laryngeal mask components, laboratory products, extraction systems, toys, aircraft undercarriage components, kitchen utensils, bottle caps, and cell phone stands.

In Injection Moulding, melted plastic is forced into a mold cavity. Once cooled, the mold can be removed. This plastic injection moulding process is commonly used in mass-production or prototyping of a product. Typically this process is used to produce plastic mouldings where the relatively high tooling cost can be justified by low unit costs and tolerances which cannot be achieved by other moulding processes.

2. Blow Moulding.

Examples of applications: Automotive, Toys, Recreational, tubes and containers, Medical, Housework Appliances

The process is divided into three steps: injection, blowing and ejection. Blow moulding is like plastic injection moulding except that hot liquid plastic pours out of a barrel vertically in a molten tube. The mold closes on it and forces it outward to conform to the inside shape of the mold. When it is cooled, the hollow part is formed. Equipments needed in setting-up a blow moulding business are relatively higher than injection moulding.

3. Compression Moulding.

Examples of applications:  Automotive exterior panels especially for commercial vehicles, Radio & appliance knobs, ash trays & electrical parts.

In this type of plastic moulding, a slug of hard plastic is pressed between two heated mold halves. Compression moulding usually uses vertical presses instead of the horizontal presses used for injection and blow moulding. The parts formed are then air-cooled. Prices of equipments used for compression moulding are moderate.

4. Film Insert Moulding.

Examples of applications: FIM can be used to apply clear scratch-resistant hard coats, logos, text and graphics in any color or combination of colors to plastic parts prior to injection molding.

This plastic moulding technique imbeds an image beneath the surface of a molded part. A material like film or fabric is inserted into a mold. Plastic is then injected.

5. Gas Assist Moulding

Examples of applications:  Flat panels for office equipment, Computer enclosures, Furniture, i.e. tabletops, Automotive panels, Domestic appliances - e.g. fridges.

Also called gas injection moulding is used to create plastic parts with hollow interiors. Partial shot of plastic is then followed by high-pressure gas to fill the mold cavity with plastic.

6. Rotational Moulding.

Examples of applications:  All kinds of plastics mostly hollow Plastic Parts.

Hollow molds packed with powdered plastic are secured to pipe-like spokes that extend from a central hub. The molds rotate on separate axes at once. The hub swings the whole mold to a closed furnace room causing the powder to melt and stick to the insides of the tools. As the molds turn slowly, the tools move into a cooling room. Here, sprayed water causes the plastic to harden into a hollow part. In this type of plastic moulding, tooling costs are low and piece prices are high. Cycle time takes about 40-45 minutes.

7. Structural Foam Moulding.

Examples of applications: Typical products are large trash containers, freeway sand safety containers and in-ground housing for water systems.

Structural foam moulding is a process of plastic injection moulding usually used for parts that require thicker walls than standard injection moulding. Inserting a small amount of nitrogen or chemical blow agent into the plastic material makes the walls thicker. Foaming happens as the melted plastic material enters the mold cavity. A thin plastic skin forms and solidifies in the mold wall. This type of plastic moulding can be used with any thermoplastic that can be injection molded.

8. Thermoforming.

Examples of applications: Clamshells, Trays, Blisters, Displays, Guards, Covers, Totes, Shields, medical plastics

In this plastic moulding process, sheets of pre-extruded rigid plastics are horizontally heated and sucked down into hollow one-piece tools. When the hot plastic solidifies, its shape conforms to that of the mold. Tooling costs are usually low and piece prices vary on the machinery.