This is a formative manufacturing technology, i.e. material is formed from an amorphous shape into a fixed shape defined by a mould tool.
Almost every plastic part created today is by injection moulding as it allows identical parts to be created in huge numbers, in a short space of time, and at very low cost per part.
Injection molding also is used for manufacturing a wide variety of parts, from small components like AAA battery boxes to large components like truck body panels.
Nominal Wall Section: The general thickness of the main part walls
Parting line: Where parts of the tool meet
Injection point: Where plastic enters the mould cavity
Rib: Used to give a part stiffness
Fillet: Rounds a corner to help plastic flow
Gusset: Stiffens a part, usually across 90 degrees between vertical and horizontal
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 materials are fed into the hopper as pellets. Within the hopper these are mixed with additives such as pigment, color masterbatch and filler masterbatch etc to adjust the properties of the final part. Then the material is fed into the barrel, where a reciprocating screw rotates, moving the pellets towards the mould and compressing them.
The friction created in this process combined with heater units wrapped around the end of the barrel raises the temperature and the pellets are melted. Once there is enough melted plastic in front of the screw, the ram moves forward squeezing material through a nozzle into the mould cavity where it cools and hardens
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.
Melted plastic enters the part through a network of channels in the tool called the ‘runner system’
There are usually 3 main parts: The sprue, Runners, and 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 below diagrams show some of the common approaches:
High numbers of parts should be intended, and tooling cost factored in.
To optimise the costs, it should ensure that everything that is needed from the design is specified in detail before starting tooling because changes to steel tooling is costly and time consuming.
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, and help to specify material, and properties.
Times are quoted to samples, and there are often other time constraints, such as finishing, shipping, production sampling, assuming the design is completed
To do minimum of unexpected issues, this careful consideration and understanding of many parameters is needed including
Keep walls constant thickness, and avoid thick sections
It should be used as below recommendations
At the end of the moulding cycle, the cooling part needs to be ejected from the tool. Without draft, as the part shrinks it grips the vertical walls requiring greater force to push it off. This can result in a number of defects including ejector punch marks or drag marks where material rubs against the tool
For this reason taper is applied at an angle to the movement direction of the tool. With sufficient draft on all surfaces the part quality will improve and the cycle time will reduce.
Draft also helps with the CNC milling process to create the mould tools, allowing deeper features
For interior edges apply a radius >0.5x Wall thickness.
Exterior edges should have a radius of Interior radius + Wall thickness.
As with wall sections, ensuring that the plastic can flow easily round the part is essential to avoid warping. Sharp corners restrict the flow as they temporarily widen the flow path, and change direction quickly.
2.10 “Shell” Thick Areas
Avoid thick areas by evenly thinning outer walls creating a shell which follows the maximum wall thickness guidelines shown earlier in this document. Ribs can be added if more structure is needed.
These thick areas will cool inconsistently, leading in turn to contraction at different rates across the part, results in voids, warp, sink and stress points which lower the quality and performance of the part.
2.11. Use Ribs for Strength
Rib thickness should be 40 – 60 % of the primary wall thickness.
The height of a rib should be < 3x the thickness of the primary wall.
Ribs should be drafted >0.5 °.
There should be a radius of ¼ the primary wall thickness.
Ribs should be at least 2 x nominal thickness or ideally their height apart.
The requirement for keeping wall sections thin require consideration to introduce strength into a part. Ribs can be used to achieve strength and volume where needed.
If the area at the base of the rib becomes too thick the plastic will cool slower and cause a visible sink mark.
Shorter ribs minimise ejection problems and also make it easier to fill the part.
Multiple ribs should be place no closer than their height from each other.
2.12 . Screw Bosses
Boss diameter = 2.0 to 2.4 x Hole (Screw/insert) diameter
Bottom fillet = 1/4 Nominal wall section
Rib height= 1/2 Wall height
As with ribs, screw bosses require careful design to avoid thick sections at the base.
Ribs and supporting gussets are often necessary to give sufficient strength, whilst keeping the volume of material down.
Avoid merging bosses with side walls, as the sections become thick leading to sink marks
2.12 . Inserts
Commonly parts are held together with screws.
Thread cutting screws for plastic are available, however inserts offer longer part life where disassembly or servicing is required.
There are 3 common types of insert for thermoplastics:
The insert is vibrated using an ultrasonic transducer melting the plastic, allowing it to be pressed into place. This allows short cycle times and low residual stress.
The insert is heated until it melts the plastic, and then pushed into place. This is a low cost solution, less suitable for volume manufacture.
Inserts are placed into the injection mould tool, usually by hand unless with large volumes. The plastic completely surrounds the insert on injection, bonding it securely in place.
2.13. Undercuts – Sliding Cores– are moving tool parts which move through or between the main tools temporarily creating a volume for the plastic to flow around, and then removed as the tools open.
There is often the need to create features which require an under cut, such as snap fits or holes perpendicular to tool movement. A common way to make undercuts is to use sliding cores.
The sliding cores need to be drafted in line of movement:
1 degree or more will be needed where the core meets plastic.
3-5 degrees is recommended where two parts of the tool slide together
2.13. A Shut off – where two parts of the mould mate, preventing plastic from passing.
Avoiding an undercut all together is often the best option by changing the way the two parts of the mould meet.
Shut-offs offer low tooling cost approaches to creating features which would otherwise be achieved by sliding cores.
As the 2 parts move against each other, they will wear, shortening the tool life. Tool strength is also a consideration, and often features need to be adapted to keep enough tool material.
For this reason 3-5 ° draft is required between the mating tools.
2.13. Snap Fits and Lifters
Snap fits are often used to hold plastic parts together because they are quicker and lower cost to implement than screws.
The snap design will depend on a number of variables specific to your project including space available:
• Material choice
• Snap force
• Retention force
• Opening requirement
Lifters can also be used to create undercuts in order to be cost effective, but there are some some constraints on geometry to allow for the moving tool parts
2.14. Bump Offs
Small undercuts can be designed to be safely ejected from a straight pull mould without the need for a side action. This gives obvious cost savings.
The basic principle is to allow the plastic to deform as it is ejected. The plastic choice is important. e.g. unfilled polyethylene is flexible, glass filled nylon would be too stiff. The feature needs freedom to deform – e.g. once the cavity has moved away. The geometry should be designed to allow movement and flex.
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