what is Injection process?
With injection moulding, granular plastic is fed by a forced ram from a hopper into a heated barrel. As the granules are slowly moved forward by a screw-type plunger, the plastic is forced into a heated chamber, where it is melted. As the plunger advances, the melted plastic is forced through a nozzle that rests against the mould, allowing it to enter the mould cavity through a gate and runner system. The mould remains cold so the plastic solidifies almost as soon as the mould is filled.
Injection moulding cycle
The sequence of events during the injection mould of a plastic part is called the injection moulding cycle. The cycle begins when the mould closes, followed by the injection of the polymer into the mould cavity. Once the cavity is filled, a holding pressure is maintained to compensate for material shrinkage. In the next step, the screw turns, feeding the next shot to the front screw. This causes the screw to retract as the next shot is prepared. Once the part is sufficiently cool, the mould opens and the part is ejected.:13
Scientific versus traditional moulding
Traditionally, the injection portion of the moulding process was done at one constant pressure to fill and pack the cavity. This method, however, allowed for a large variation in dimensions from cycle-to-cycle. More commonly used now is scientific or decoupled moulding, a method pioneered by RJG Inc. In this the injection of the plastic is "decoupled" into stages to allow better control of part dimensions and more cycle-to-cycle (commonly called shot-to-shot in the industry) consistency. First the cavity is filled to approximately 98% full using velocity (speed) control. Although the pressure should be sufficient to allow for the desired speed, pressure limitations during this stage are undesirable. Once the cavity is 98% full, the machine switches from velocity control to pressure control, where the cavity is "packed out" at a constant pressure, where sufficient velocity to reach desired pressures is required. This allows part dimensions to be controlled to within thousandths of an inch or better.
Different types of injection moulding processes
Although most injection moulding processes are covered by the conventional process description above, there are several important moulding variations including, but not limited to:
- Die casting
- Metal injection moulding
- Thin-wall injection moulding
- Injection moulding of liquid silicone rubber:17–18
- Reaction injection moulding
Like all industrial processes, injection moulding can produce flawed parts. In the field of injection moulding, troubleshooting is often performed by examining defective parts for specific defects and addressing these defects with the design of the mould or the characteristics of the process itself. Trials are often performed before full production runs in an effort to predict defects and determine the appropriate specifications to use in the injection process.:180
When filling a new or unfamiliar mould for the first time, where shot size for that mould is unknown, a technician/tool setter may perform a trial run before a full production run. They start with a small shot weight and fills gradually until the mould is 95 to 99% full. Once this is achieved, a small amount of holding pressure will be applied and holding time increased until gate freeze off (solidification time) has occurred. Gate freeze off time can be determined by increasing the hold time, and then weighing the part. When the weight of the part does not change, it is then known that the gate has frozen and no more material is injected into the part. Gate solidification time is important, as this determines cycle time and the quality and consistency of the product, which itself is an important issue in the economics of the production process.Holding pressure is increased until the parts are free of sinks and part weight has been achieved.
Injection moulding is a complex technology with possible production problems. They can be caused either by defects in the moulds, or more often by the moulding process itself.:47–85
|Moulding defects||Alternative name||Descriptions||Causes|
|Blister||Blistering||Raised or layered zone on surface of the part||Tool or material is too hot, often caused by a lack of cooling around the tool or a faulty heater|
|Burn marks||Air burn/gas burn/dieseling/gas marks/Blow marks||Black or brown burnt areas on the part located at furthest points from gate or where air is trapped||Tool lacks venting, injection speed is too high|
|Color streaks (US)||Colour streaks (UK)||Localised change of colour||Masterbatch isn't mixing properly, or the material has run out and it's starting to come through as natural only. Previous coloured material "dragging" in nozzle or check valve.|
|Contamination||Unwanted or foreign material||Different colour matter seen in product, weakening the product||Poor material introduced by bad recycling or regrind policy; may include floor sweepings, dust and debris|
|Delamination||Thin mica like layers formed in part wall||Contamination of the material e.g. PP mixed with ABS, very dangerous if the part is being used for a safety critical application as the material has very little strength when delaminated as the materials cannot bond|
|Flash||Excess material in thin layer exceeding normal part geometry||Mould is over packed or parting line on the tool is damaged, too much injection speed/material injected, clamping force too low. Can also be caused by dirt and contaminants around tooling surfaces.|
|Embedded contaminates||Embedded particulates||Foreign particle (burnt material or other) embedded in the part||Particles on the tool surface, contaminated material or foreign debris in the barrel, or too much shear heat burning the material prior to injection|
|Flow marks||Flow lines||Directionally "off tone" wavy lines or patterns||Injection speeds too slow (the plastic has cooled down too much during injection, injection speeds should be set as fast as is appropriate for the process and material used)|
|Gate Blush||Halo or Blush Marks||Circular pattern around gate, normally only an issue on hot runner molds||Injection speed is too fast, gate/sprue/runner size is too small, or the melt/mold temp is too low.|
|Jetting||Part deformed by turbulent flow of material.||Poor tool design, gate position or runner. Injection speed set too high. Poor design of gates which cause too little die swell and result jetting.|
|Knit lines||Weld lines||Small lines on the backside of core pins or windows in parts that look like just lines.||Caused by the melt-front flowing around an object standing proud in a plastic part as well as at the end of fill where the melt-front comes together again. Can be minimised or eliminated with a mould-flow study when the mould is in design phase. Once the mould is made and the gate is placed, one can minimise this flaw only by changing the melt and the mould temperature.|
|Polymer degradation||Polymer breakdown from hydrolysis, oxidation etc.||Excess water in the granules, excessive temperatures in barrel, excessive screw speeds causing high shear heat, material being allowed to sit in the barrel for too long, too much regrind being used.|
|Sink marks||[sinks]||Localised depression (In thicker zones)||Holding time/pressure too low, cooling time too short, with sprueless hot runners this can also be caused by the gate temperature being set too high. Excessive material or walls too thick.|
|Short shot||Short fill or short mould||Partial part||Lack of material, injection speed or pressure too low, mould too cold, lack of gas vents|
|Splay marks||Splash mark or silver streaks||Usually appears as silver streaks along the flow pattern, however depending on the type and colour of material it may represent as small bubbles caused by trapped moisture.||Moisture in the material, usually when hygroscopic resins are dried improperly. Trapping of gas in "rib" areas due to excessive injection velocity in these areas. Material too hot, or is being sheared too much.|
|Stringiness||Stringing or long-gate||String like remnant from previous shot transfer in new shot||Nozzle temperature too high. Gate hasn't frozen off, no decompression of the screw, no sprue break, poor placement of the heater bands inside the tool.|
|Voids||Empty space within part (air pocket is commonly used)||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 part walls are not the same thickness). The provided information is the common understanding, Correction: The Lack of pack (not holding) pressure (pack pressure is used to pack out even though is the part during the holding time). Filling too fast does not cause this condition, as a void is a sink that did not have a place to happen. In other words, as the part shrinks the resin separated from itself as there was not sufficient resin in the cavity. The void could happen at any area or the part is not limited by the thickness but by the resin flow and thermal conductivity, but it is more likely to happen at thicker areas like ribs or bosses. Additional root causes for voids are un-melt on the melt pool.|
|Weld line||Knit line / Meld line / Transfer line||Discoloured line where two flow fronts meet||Mould or material temperatures set too low (the material is cold when they meet, so they don't bond). Time for transition between injection and transfer (to packing and holding) is too early.|
|Warping||Twisting||Distorted part||Cooling is too short, material is too hot, lack of cooling around the tool, incorrect water temperatures (the parts bow inwards towards the hot side of the tool) Uneven shrinking between areas of the part|
|Cracks||Crazing||Improper fusion of two fluid flows, a state before weld line.||Threadline gap in between part due to improper gate location in complex design parts including excess of holes (multipoint gates to be provided), process optimization, proper air venting|
Methods such as industrial CT scanning can help with finding these defects externally as well as internally.