Plastic Injection Moulding – Glossary of Terms

Abrasion Resistance The ability of a material to withstand mechanical actions such as rubbing, scraping or erosion that tend progressively to remove material from its surface.
Acceptable runner/cavity ratio Runner systems designed for high pressure drops to minimise material usage and increase frictional heating in the runner.
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”.
Additive A substance compounded into a resin to enhance or improve certain characteristics.
Adhesive Assembly The process of joining two or more plastic parts by means of an adhesive
Aging The process of, or the results of, exposure of plastics to natural or artificial environmental conditions for a prolonged period of time.
Air Burn/Burn Marks/Gas Burn/Dieseling A patch or streak of brown or black material on the component caused by air or gases that have not been properly vented from the mould and have caused the material to overheat and burn.
Ambient Temperature The temperature of a medium surrounding an object. The term is often used to denote prevailing room temperature.
Amorphous Devoid of crystallinity or stratification. Most plastics are amorphous at processing temperatures but most materials assumes semi-crystalline molecular structure when cooling and then when in a solid state.
Anisotropy The tendency of a material to react differently to stresses applied in different directions.
Annealing The process of relieving internal stresses of moulded plastic articles by heating to a predetermined temperature, maintaining this temperature for a predetermined length of time and slowly cooling the articles.
ANSI Abbreviation for American National Standards Institute.
Antiozonants These additives are used to prevent the negative effects of ozone on the resin materials.
Antistatic Agent Additive used to improve the electrical conductivity of the plastic part so that any charge can readily go to ground and not remain in the part. Artificially balanced runner system balancing a runner system by adjusting the pressure drop of a long large diameter runner against a short small diameter runner. Since pressure drop over the small diameter runner will be much more affected by heat loss than the large diameter runner, an artificially balanced runner system will work with a set range of moulding conditions. The width of this range of moulding conditions determines the stability of the moulding
Ash Content The solid residue remaining after a substance has been incinerated or heated to a temperature sufficient to drive off all combustible or volatile substances.
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. Sometines 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.
Aspect Ratio Ratio of total flow length to average wall thickness.
A-stage This is a early stage in the reaction of certain thermosetting resins where the molecular weight is low and the resin is still soluble in some liquids and still fusible.
ASTM Abbreviation for American Society for Testing and Materials.
Back Pressure The resistance of the molten plastic material to forward flow. In moulding, back pressure increases the temperature of the melt and contributes to better mixing of colours and homogeneity of the material. However, as back pressure increases, so does cycle time.
Backflow Molten resin flows back out of the mould returning to the runners.
Backing Plate A plate used as a support for the mould cavity block, guide pins, bushings etc.
Balanced Runner A runner system designed to place all cavities at the same distance from the sprue.
Barrel The part of the moulding equipment where the resin is melted.
Bead blasting using abrasives in a pressurised air blast to create a surface texture on the part.
Blister/Blistering Raised or layered zone on the surface of the part. This is caused by the tool or material being too hot, often by a lack of cooling around the tool or a faulty heater.
Blush Cosmetic blemish at the point of injection in the finished part.
Boss A cylindrical protusion within a part, often designed to accept fasteners.
Bridge tool A temporary or interim mould made for the purpose of making production parts until a high-volume production mould is ready.
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.
Bumpoff A feature in the mould with an undercut. To eject the part, it must bend or stretch around the undercut.
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 the core is where the resin solidifies and forms the part.
Clamp force The force required to hold the mould shut so resin cannot escape during injection. Measured in tonnes.
Colour Streaks Localised change in colour.
Cause: Masterbatch not mixing properly, or the material has run out and it is starting to come through as natural only. Previous coloured material may be “dragging” in the nozzle or check valve.
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 the core is where the resin solidifies and forms the part. Often the B-side of the mould is referred to as the core side, and in the case of a part like a drinking cuo, the entire B-side will be the 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 The direction the mould surface moves when they are moving away from the part surfaces, either when the mould opens or when the part ejects.
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 costemics and material characteristics.
Durometer A measure of hardness. It is measured on a numeric scale with numbers ranging from lower (softer) to higher (harder). This method is more commonly used for elastic materials where other methods, such as Rockwell, are not suitable.
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.
EDM Electric Discharge Machining. A mould making method which can ceate taller, thinner ribs than milling, text on the top of ribs and square outside edges on parts.
ESD Stands for “Electric 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 or reduce the flammability of a material.
Flash/Burr Excess material in thin layer exceeding normal part geometry.
Cause: Mould is over packed or parting line on the tool is damaged; too much injection speed/material injection; clamping force is too low. Can also be caused by dirt and contamination around tooling surfaces.
Flow Marks/Lines Directionally “off tone” wavy lines or patterns.
Cause:Injection speeds are too slow (the plastic has cooled down too much during the injection; injection speeds must be set as fast as you can get away with at all times).
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 elminate the creation of any runner or sprue. The gate vestige will be a small sharp bump that can be trimmed if necessary.
Jetting Part deformed by turbulent flow of material.
Cause: Poor tool design, gate position or runner. Injection speed set too high.
Live 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.
Metal-safe/Steel-safe A change to the part design that requires only the removal of metal from the mould to produce the desired geometry. Typically most important when a part design is changed after the mould has been manufactured, as the mould can be modified through machining without the need to build up metal or entirely re-machining.
Mould release spray A liquid applied to the mould as a spray to facilitate the ejection of the 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.
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 increase the likelihood of flash and may cause the part to stick in the mould.
Parting line The location where the pieces of a mould come together. Typically a thin line is created on the part here.
Pickout A separate piece of metal that is inserted into the mould to create an undercut. It is ejected with the part then removed by the operator and re-inserted in the mould.
Polymer degradation Polymer breakdown from hysrolysis, oxidation etc.
Cause: Excess water in the granules; excessive temperatures in the barrel.
Finished parts are also suspectable to this, but to a far reduced rate.
Post gate An injection method that injects plastic through an ejector pin hole. This gating technique leaves a gate vetiage on the B-side of the part where is can often be less visible for cosmetic purposes. There is often gate blush opposite a post gate.
Press The part of the injection moulding machine that makes the plastic parts. It holds the mould closed, 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 optimising 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 milling the mould. When a radius is intentionally added on a part it is referred to as a fillet.
Recess An indentation in the plastic part caused by the impact of the ejecter pins.
Reinforced resin Refers to the 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.
Runner A channel machined into the mould that directs the resin from the 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 (NOTE: this definition is related to moulding).
Short Shot/Non-fill/Short Mould Partial Part.
Cause: Lack of material, injection speed or low pressure, mould is too cold and/or lack of gas vents.
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.
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 Marks Localised depression. This is generally in thicker zones.
Cause: Holding time/pressure is too low; cooling time is too short; with sprueless hot runners this can also be caused by the gate temperature being set too high. Excessive material or wall too thick.
Splay Marks/Splash Mark/Silver Streaks Circular Pattern around gate caused by hot gas.
Cause: Mositure in the material, usually when hygroscopic resins are not properly dried. Trapping of gas in ‘rib’ areas due to excessive injection velocity in these areas. Material too hot.
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 pin A cylindrical pin for formatting high-aspect-ratio small diameter holes in a part. A steel pin is strong enough to handle the stress of ejection and its surface is smooth enough to release cleanly from the part without draft.
STEP Stands for “Standard for the Exchange of Product model data. It is a common format for exchanging CAD data.
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.
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.
Stringiness/Stringing String like remnant from previous shot transfer in new shot.
Cause: Nozzle temperature too high or gate has frozen off.
Straight pull mould A mould without side actions. It is less expensive than a comparable mould with side actions.
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.
Tunnel gate An injection method that uses a small gate located off the parting line in one of the mould halves. It leaves a vestigate 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 or via an ejector pin tunnel, that is used to let air escape from a mould while the resin is injected.
Vestigate A visible mark created by the manual process of finishing parts when the gate is trimmed. Ejector pins also may leave a vestiagte where the impact the part.
Voids Empty space within the part.
Cause: Lack of holding pressure (holding pressure is used to pack out the part during the holding time). Filling too fast, not allowing the edges of the part to set up. Also mould may be out of registration (when the two halves don’t centre properly and the part walls are not the same thickness).
Warping/Twisting Distorted part.
Cause: Cooling is too short, material is too hot, lack of cooling around the tool, incorrect water temperature (the parts bow inwards towards the hot side of the tool). Uneven shrinking between areas of the part.
Weld or Knit Lines Small lines on the backside of core pins or window in parts that look like just lines.
Cause: The melt-front flowing around an object standing pround in a plastic part as well as at the end of the fill where the melt-front comes together again. Can be minimsed or eliminated with a mould-flow study when the mould is in the design phase. Once the mould is made and the gate is placed, one can minimse this flaw only by changing the melt and mould temperature.
Wireframe A type of CAD model consisting only of lines and curves, in 2D or 3D. Wireframe models are not suitable for rapid injection moulding.