Medical Injection Molding Rapid Guide

Introduction to Injection Molding

Injection molding is a manufacturing technique where molten material is injected into a mold cavity under high pressure. After the material cools and solidifies, the mold opens to eject the molded part. This process is suitable for producing parts with complex shapes and high precision requirements, ideal for low-cost mass production.

The process is as follows

Mold cavity defines the shape of the part: The mold is a cavity or space with the desired shape of the part.

Material (molten plastic) injected into the mold cavity: Molten plastic is injected into the mold cavity through a nozzle and under pressure.

Plastic cools and solidifies: Once the plastic enters the mold cavity, it rapidly cools and solidifies using the mold's cooling system, maintaining the required shape and structure.

Part ejected: After the plastic has fully cooled and solidified, the mold opens, allowing operators to remove or eject the molded part.

Process repeats: The entire process starts again with the injection of new molten plastic, continuing until the desired production quantity is achieved.

Material Selection

Mold Materials

The choice of mold materials depends on production volume, cost, and precision requirements.

• Plastic Mold Steel

• High-Speed Tool Steel

• Hot Work Tool Steel

• Cold Work Tool Steel

Injection Molding Materials

All thermoplastics (polymers that soften when heated and harden when cooled) can be molded. Some silicones and thermosetting resins can also be molded.

• Polypropylene (PP)

• Polycarbonate (PC)

• Polyetheretherketone (PEEK)

• Polyethylene Terephthalate (PET) 

Advantages and disadvantages of injection molding

• Suitable for high-volume production

• Extremely fast part manufacturing speed

• Can provide a variety of materials with different properties

• Very low part cost in mass production

• Excellent repeatability

• High precision

• Outstanding appearance

• Capable of creating very complex geometries

• Minimal or no additional surface finishing required

• Low scrap rate

Disadvantages of injection molding

• High tooling installation cost

• Longer lead time compared to other processes

• High cost of design modifications

Injection molding principle

Part analysis

• Nominal wall thickness: General thickness of the main body part walls.

• Parting line: Junction where different sections of the tool meet.

• Gate: Point where plastic enters the mold cavity.

• Ribs: Used to increase the stiffness of the part.

• Fillet: Rounds corners to aid plastic flow.

• Gusset: Reinforcement for a section, typically spanning a 90-degree angle between vertical and horizontal parts.

Part Fundamentals

A part is created by two or more tools moving together to create a closed volume, into which plastic is injected under pressure.

In a 2 part tool the cavity (A) creates the quality outer surfaces and the core (B) creates inner details.

There are often extra “cores” sliding into the cavity to allow more complex parts.

Raw plastic feeding

Raw plastic pellets are fed into the hopper. Within the hopper, these pellets are mixed with additives such as pigments and fillers like glass fibers to adjust the performance characteristics of the final part.

Plastic heating and melting

The material is then conveyed into the barrel of the injection molding machine. A rotating screw pushes the pellets towards the mold while compressing them. Friction generated by the rotation of the screw and heating units around the end of the barrel raises the temperature and melts the pellets.

Injection molding

Once there is sufficient melted plastic in front of the screw, the machine's ram moves forward, forcing the plastic through a nozzle into the mold cavity. Inside the mold cavity, the plastic cools and hardens, forming the final shape of the part.

Ejection

Throughout the injection process the tools are clamped tightly together. Once the plastic has cooled sufficiently to maintain its shape the tool opens, usually by the core and part moving backwards away from the cavity.

Ejector pins are then pushed through the core against the part, releasing it to drop free from the tool.

This process does result in a few witness marks on the part

• Parting lines: where the two halves of the mould meet.

• Ejector marks: where the pins meet the part. Normally these are hidden on the rear.

The Runner System

Melted plastic enters the part through a network of channels in the tool called the ‘runner system’

There are usually 3 parts to this

• The sprue: The primary channel where material flows into the tool.

• Runners: Smaller channels which guide the plastic to the gates.

• Gates: The entry point to the part.

Gates

Gates are the point where the melted plastic enters the cavity. The position and geometry of a gate control the flow into the part.

The diagrams to the right show some of the common approaches

• Edge gates are the most common, injecting at the part line where the two halves of the mould meet. The runner system is removed manually after ejection which leaves a small witness mark.

• Direct gates connect from the spur to the top of the part to reduce material wastage on runner systems. This is ideal for very large parts but does leave a visible mark on the part.

• Tunnel/Sub gates inject below the parting line. There is no need for manual removal of runners as they are snapped off on ejection. This is ideal for very large volumes.

• Pin gates inject material from the inside of the part in the Core or “B tool”, away from the visual surfaces making them favoured for appearance.

Before Tooling

As with all mass manufacturing processes there are a few planning steps before committing to tool.

Is moulding the right process at this stage?

High numbers of parts should be intended, and tooling cost factored in.

Optimising

Ensure that everything that is needed from the design is specified in detail before starting tooling. Changes to steel tooling is costly and time consuming.

Specify Material, and Properties 

Every polymer has different flow rates and shrinkage, it always helps to know the planned material before tooling so the injection point can be designed appropriately.

Lead Times

Time to part with injection moulding is long. Times are quoted to samples, and there are often other time constraints, such as finishing, shipping, production sampling, assuming the design is complete. 

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