Polymers and Injection Moulding Flashcards
Properties
Very high tooling cost but depends on complexity and number of cavities
Very low unit cost
Typical Application
Automotive
Consumer electronics
Appliances
Industrial products
Household products
Suitability
High-volume mass production
Quality
Very high surface finish
Highly repeatable process
Related Processes
Reaction injection moulding
Thermoforming
Vacuum casting
Speed
Injection cycles between 30 and 60 secs
Process
Material Preparation:
Thermoplastic pellets or granules are chosen based on the desired properties of the final product. The selected material is loaded into the hopper of the injection molding machine.
Melting
The material is heated and melted inside the machine’s barrel, where a reciprocating screw helps in the melting process. The melted plastic forms a homogeneous liquid.
Injection
Once the molten plastic reaches the desired temperature and consistency, it is injected under high pressure into a mold cavity through a nozzle. The mold is a two-part tool made of metal with a cavity that represents the shape of the final product.
Cooling
The molten plastic takes the shape of the mold cavity and starts to cool rapidly, solidifying into the desired part shape. Cooling is a critical step in the process.
Ejection
After the plastic has solidified and the part has cooled sufficiently, the mold opens, and the newly formed part is ejected from the mold by ejector pins or other mechanisms
Cycle Repetition
The entire process cycle is repeated for the production of additional parts. The speed of the process depends on factors such as the material used, the complexity of the part, and the cooling time required
Faetures
Sprue
The sprue is the primary channel through which molten plastic is introduced into the mold cavity. It connects the nozzle of the injection molding machine to the entry point of the mold. The sprue is typically a larger-diameter channel that allows for the initial flow of plastic material. Once the plastic reaches the mold cavity, the sprue itself does not form part of the final product and is usually removed during post-processing.
Runner
The runner is the passageway that connects the sprue to the individual mold cavities. It channels the molten plastic from the sprue to the various parts of the mold where the final products are shaped. The runner system is an essential part of the mold layout, and its design influences factors such as the distribution of material, pressure, and cooling within the mold
Gates
Gates are the narrow openings through which molten plastic enters the mold cavity from the runner. These openings control the flow of plastic into the part, and their design has a significant impact on the final product’s quality.
Issues
Short Shot
A short shot occurs when the injection moulding process fails to completely fill the mould cavity, resulting in an incomplete or undersized part. This issue can be caused by insufficient material, inadequate injection pressure, or improper mold temperature.
Humidity and drying
High humidity can impact the quality of plastic materials used in injection moulding. Moisture absorption can lead to defects like bubbles or voids in the final product. Therefore, it is essential to dry hygroscopic plastics before processing to prevent these issues and maintain part quality.
Contamination
Contamination in plastic materials, often in the form of foreign particles or impurities, can lead to defects in the molded parts. It can result from inadequate material handling, poor storage conditions, or the presence of contaminants in the processing equipment.
Sink Marks
Sink marks are depressions or dimples on the surface of an injection-moulded part caused by uneven cooling. As the plastic cools and solidifies, thicker sections cool more slowly, leading to a visible indentation. Proper mould design, including gate placement and cooling strategies, is crucial in minimizing sink marks.
Design out by coring out
Knit Lines
Melt front approaches
weld line forms
Melt line forms
Quality
Visual inspection
Size inspection
Stress test
Pigments
Can recycle in process by melting unused or unwanted polymer
Mixed recycling can be coloured black
can separate colours
Colour pigments make up 3% of overall material
Important Specs
Clamping force: Pressure (Kn or t) to hold the mould together
Keep clamping force as low as possible
Too low clamping force means leakage
Generally flash means a low quality mould design or too high clamping force
Injection pressure: max force for the plastic to be pushed in the mould cavity
Shot size: volume of the part and plus runners and gates
Shot capacity: the full amount as a weight or volume of material injected during moulding from the screw
Volume of the part and plus runners, cold slug, sprue and gates
Closing force: force needed to hold the mould for packing and cooling
Other Techniques
Gas-assisted injection moulding: shut off valve closes to prevent the plastic material from seeping back into the injection head. The gas is injected into the core of the plastic, which at this stage is still molten. The gas progresses the molten plastic into the extremities of the cavity
Multishot Injection moulding: double injection moulding machine
Blow moulding
