Tuesday, March 1, 2011

Glossary of Plastic Molding Terms

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 lan's System."

A-Side - The mold half that mounts to the fixed side of the injection molding press, through which resin is injected into the part cavity via the sprue. Sometimes referred to as the cavity side of the mold, 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 mold half that mounts to the moving side of the injection molding press. Sometimes referred to as the core side of the mold, the B-side has ejector pins to push the part out of the open mold. An analysis of the part geometry determines the optimal part orientation to ensure that it will remain on the B-side when the mold 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 mold made for the purpose of making production parts until a high-volume production mold is ready.

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

CAD - Short for 'computer aided design".

Cam - See "side action".

Cavity - A concave feature on either side of the mold into which an opposing core enters when the mold is closed. The void between the cavity and core is where the resin solidifies and forms the part. Often the A-side of a mold 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 mold 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 mold that will enter an opposing cavity when the mold is closed. The void between the cavity and core is where the resin solidifies and forms the part. Often the B-side of a mold 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 mold 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 on 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 mold, the injection of the resin, the solidification of the part, the opening of the mold and the ejection of the part.

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

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 mold during ejection.

Drying of plastics - Many plastics absorb water and must be dried prior to injection molding 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 mold. 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 mold.

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

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

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 mold - A mold 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 mold 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 mold prior to solidification.

Food grade - Resins or mold 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 molding 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 molded parts.

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

Jetting - Flow marks caused by the resin entering a mold 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 maybe 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 mold has been manufactured, because then the mold can be modified rather than entirely re-machined. It is also commonly called "steel safe".

Mold release spray - A liquid applied to the mold 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 mold.

Multi-cavity mold - A mold 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 molding press where the resin enters the sprue.

Packing - The practice of using increased pressure when injecting a part to force more plastic into the mold. 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 mold.

Parasolid - A file format for exchanging CAD data.

Parting line - The location where the pieces of a mold 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 molding machine that makes the plastic parts. It holds the mold closed, melts the resin, injects it into the mold, opens the mold and ejects the part.

Process - The injection molding environment consisting of input variables such as temperature, pressure, injection rates and time that are controlled to fill the mold 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 molding is concerned.

Rib - A reinforcing member of a molded part.

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

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

Shear - The force between layers of resin as they slide against each other or the surface of the mold. 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 mold design prior to machining.

Shutoff - The surfaces where the A-side and B-side of the mold 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 mold that allows for the molding of parts with undercuts. The undercut-creating mold 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 - Discolored 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 mold 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 molding, where a part becomes lodged in one or the other half of the mold, 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 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 molding.

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

Tab gate - See "edge gate".

Telescoping shutoff - An area within a mold 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 mold 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 mold 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 mold without a portion of the mold 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 mold cavity, typically at the shutoff surface or via an ejector pin tunnel, which is used to let air escape from a mold 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. Wireframe models are not suitable for rapid injection molding.

Davies Molding

Davies Molding
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