July 2005 Design Tip

Rapid Injection Moulding 101

This month’s Design Tip consists of a collection of terms we use every day on the phone with customers, so we thought it might be useful to have them all in once place. Don’t worry, they won’t be on the Final Exam.

Rapid Injection Moulding Glossary

ACIS - A standard computer file format for exchanging CAD data, typically from AutoCAD programs. ACIS is an acronym that originally stood for "Andy, Charles and Ian's System"

A-Side - The mould half that mounts to the fixed side of the injection moulding press, through which resin is injected into the part cavity via the sprue. Sometimes referred to as the cavity side of the mould, the A-side does not have ejector pins and for this reason it often produces the outside or cosmetic side of the part.

B-Side - The mould half that mounts to the moving side of the injection moulding press. Sometimes referred to as the core side of the mould, the B-side has ejector pins to push the part out of the open mould. An analysis of the part geometry determines the optimal part orientation to ensure that it will remain on the B-side when the mould is opened.

Boss - A cylindrical protrusion within a part, often designed to accept fasteners.

Bead blasting - Using abrasives in a pressurized air blast to create a surface texture on the part.

Bevel - See “chamfer”.

Blush - Cosmetic blemish at the point of injection in the finished part.

Bridge tool - A temporary or interim mould made for the purpose of making production parts until a high-volume production mould is ready.

Barrel - The part of the moulding press where resin is melted.

CAD - Short for “computer aided design”.

Cam - See “side action”.

Cavity - A concave feature on either side of the mould into which an opposing core enters when the mould is closed. The void between the cavity and core is where the resin solidifies and forms the part. Often the A-side of a mould is referred to as the cavity side, and in the case of a part like a drinking cup, the entire A-side will be a cavity.

Chamfer - Also known as a bevel, it is a flat truncated corner.

Clamp force - The force required to hold the mould shut so resin cannot escape during injection.

Contoured pins - Ejector pins with the ends shaped to match a sloping surface on the part.

Core - A convex feature on either side of the mould that will enter an opposing cavity when the mould is closed. The void between the cavity and core is where the resin solidifies and forms the part. Often the B-side of a mould is referred to as the core side, and in the case of a part like a drinking cup, the entire B-side will be a core.

Core-cavity – The design of a mould where the A-side forms the outside of the part and the B-side forms the inside. The advantage to this approach is that the part will shrink onto the B-side so it can be ejected, and if the inside and outside are drafted with equal and opposite draft the wall thickness will be constant.

Cycle time - The time it takes to make one part including the closing of the mould, the injection of the resin, the solidification of the part, the opening of the mould and the ejection of the part.

Direction of pull - Refers to the motion of a part surface relative to a mould.

Draft - The taper of features in the direction of pull. It allows deeper features to be produced in three-axis milling machines and it also helps parts release from the mould during ejection.

Drying of plastics - Many plastics absorb water and must be dried prior to injection moulding to ensure good cosmetics and material characteristics.

Durometer - A measure of the hardness of a resin. It is measured on a numeric scale with numbers ranging from lower (i.e. softer) to higher (i.e. harder).

Edge gate - An injection method that uses a gate on the parting line of the mould. It typically leaves a vestige on the outside of the part and is sometimes referred to as a tab gate.

Ejection - The process of pushing a completed part out of a mould.

Ejector pins - Steel pins incorporated into the B-side of a mould that push out the plastic part.

End mill - A cutting tool that is used to machine a mould.

ESD - Stands for “electro static discharge”, an electrical effect that may necessitate shielding in some applications. Some special grades of plastic are electrically conductive or dissipative and help prevent ESD.

Family mould - A mould containing two or more different parts.

Flame retardant - A resin formulated to resist burning.

Flash - Excess plastic that flows into the parting line of the mould beyond the edges of a part and freezes to form a thin, sheet-like protrusion from the part.

Flow marks – Visible indications on the finished part that indicate the flow of plastic within the mould prior to solidification.

Food grade - Resins or mould release spray that are approved for use in the manufacture of parts that will contact food in their application.

Gate - The location where the plastic enters the part. There is typically a visible vestige when the gate is removed.

GF - Stands for “glass filled”, it refers to a resin with glass fibers mixed into it. Glass filled resins are much stronger and more rigid than the corresponding unfilled resin, but also more brittle. Resins can also be filled with carbon fiber, stainless steel, etc. In general, filled resins can be very susceptible to warp.

Gusset - A triangular rib that reinforces areas such as a wall to a floor or a boss to a floor.

Hot tip gate – An injection moulding method that uses a heated gate on the A-side of the part to eliminate the creation of any runner or sprue. The gate vestige will be a small sharp bump that can be trimmed if necessary.

IGES – Stands for “Initial Graphics Exchange Specification”. It is a common file format for exchanging CAD data. Protomold can use IGES solid or surface files to create moulded parts.

Injection - The process of forcing melted resin into a mould.

Jetting - Flow marks caused by the resin entering a mould at high speed, typically occurring near a gate.

Knit lines - Visible indications in a finished part, formed by the intersection of two hot plastic fronts. They are always formed downstream of through - holes and between multiple gates. They are also known as weld lines.

Living hinge - Very thin section of plastic used to connect two parts and keep them together while allowing them to open and close. They require careful design and gate placement. A typical application would be the top and bottom of a box.

Medical grade - Resin that may be suitable for use in certain medical applications.

Metal-safe - A change to the part design that requires only the removal of metal to produce the desired geometry. Typically most important when a part design is changed after the mould has been manufactured, because then the mould can be modified rather than entirely re-machined. It is also commonly called "steel safe".

Mould release spray - A liquid applied to the mould as a spray to facilitate the ejection of parts from the B-side. It is typically used when the parts are difficult to eject because they are sticking to the mould.

Multi-cavity mould - A mould with multiple copies of the same part, typically used to reduce piece-part pricing for higher volume runs.

Nozzle - The tapered fitting on the end of the barrel of the injection moulding press where the resin enters the sprue.

Packing - The practice of using increased pressure when injecting a part to force more plastic into the mould. This is often used to combat sink or fill problems, but also increases the likelihood of flash and may cause the part to stick in to the mould.

Parasolid - A file format for exchanging CAD data.

Parting line - The location where the pieces of a mould come together. Typically a thin line is created on the part here.

Post gate - An injection method that injects plastic through an ejector pin hole. This gating technique leaves a gate vestige on the B-side of the part where it can often be less visible for cosmetic purposes. There is often gate blush opposite a post gate.

Press - The injection moulding machine that makes the plastic parts. It holds the mould closed, melts the resin, injects it into the mould, opens the mould and ejects the part.

Process - The injection moulding environment consisting of input variables such as temperature, pressure, injection rates and time that are controlled to fill the mould while optimizing the tradeoffs between cosmetics and dimensional accuracy.

Radiused – An edge or vertex that has been rounded. Typically this occurs on part geometries as a natural result of the Protomold milling process.

Recess - An indentation in the plastic part caused by the impact of the ejector pins.

Reinforced resin - Refers to base resins with fillers added for strength. They are particularly susceptible to warp because the fiber orientation tends to follow flow lines, resulting in asymmetric stresses. Typically these resins are harder and stronger but also more brittle (i.e. less tough).

Resin – Synonymous with “plastic” as far as injection moulding is concerned.

Rib - A reinforcing member of a moulded part.

Runner - A channel machined into the mould that directs the resin from sprue to the gate.

Screw - The mechanical feature inside the barrel that forces the resin out the nozzle.

Shear - The force between layers of resin as they slide against each other or the surface of the mould. The resulting friction causes some heating of the resin.

Shrink - The change in size of the part during solidification, typically anticipated based on published material property data and built into the mould design prior to machining.

Shutoff - The surfaces where the A-side and B-side of the mould contact. The shutoff meets the part at the parting line.

Short shot - A part that wasn't completely filled with resin, causing short or missing features.

Side action - A sliding cam arrangement within the mould that allows for the moulding of parts with undercuts. The undercut-creating mould face is held in place during the injection process and then slides out of the way prior to ejection.

Sink – Undesired depressions in the surface of a part that are caused by the shrinking of resin as it solidifies. Sink is most common in thick sections of a part.

Splay - Discoloured visible streaks in the part, typically caused by moisture in the resin.

Sprue - The route the resin takes from the point where it enters the mould until it reaches the runner(s). When solidified, it remains attached to the part via one or more runners and is typically removed in finishing.

Steel safe - See “metal safe”.

Sticking - A problem during the ejection phase of moulding, where a part becomes lodged in one or the other half of the mould, making removal difficult. This is a common issue when the part is not designed with sufficient draft.

STEP - Stands for "Standard for the Exchange of Product Model Data”. It is a common format for exchanging CAD data.

STL - Originally stood for “STereoLithography”. It is a common format for transmitting CAD data to rapid prototyping machines and is not suitable for rapid injection moulding.

Straight pull mould - A mould without side actions. It is less expensive than a comparable mould with side actions.

Tab gate - See “edge gate”.

Telescoping shutoff – An area within a mould where metal slides along metal, usually creating a hole in the part. A three degree draft angle is required on any related part surfaces.

Texture - A surface treatment applied to the mould to create texture on the parts. Protomold offers polished, sanded and bead-blasted textures.

Tunnel gate - An injection method that uses a small gate located off the parting line in one of the mould halves. It leaves a vestige a short distance from the parting line of the part.

Undercut - A portion of the part geometry that would prevent the part from being ejected from a straight-pull mould without a portion of the mould passing through (and destroying) the part. The simplest example of an undercut feature on a part would be a through-hole aligned perpendicular to the direction of part ejection.

Vents – A very small (e.g. 0.001” – 0.005”) opening in the mould cavity, typically at the shutoff surface or via an ejector pin tunnel, that is used to let air escape from a mould while the resin is injected.

Vestige – A visible mark created by the manual process of finishing parts when the gate is trimmed. Ejector pins also may leave a vestige where they impact the part (also see “recess”).

Warp – The curving or bending of parts that typically occurs after ejection as the part cools. Warp is often caused by glass filled resins.

Weld lines – See “knit lines”.

Wireframe - A type of CAD model consisting only of lines and curves, in 2D or 3D. Wirefame models are not suitable for rapid injection moulding.