What makes it hard to find the cause of a defect is the variety of influencing factors: apart from the process parameters, part and mold design, the material and its preparation, peripheral equipment or ambient conditions can cause a defect alone or together. Moreover, some defects look alike from the outside while their physical causes are completely different. The methods on this page turn guesswork into measurement.
1. First Questions in Defect Identification
- Does the appearance of the defect coincide with a change? (another machine, peripheral unit, batch change)
- Is it a systematic defect — how often and in which position does it occur?
- Does the appearance indicate the process phase in which the defect occurs (injection, holding, cooling, ejection)?
- Is the defect related to a process interruption? Was a parameter adjustment needed at the batch change?
- Does it also occur with an alternative material? (use natural material to eliminate the effect of glass fiber and additives)
- Does it also occur on another machine? Is the cooling time consistent with the wall thickness? Are there hot spots in the mold?
- To what extent can the defect be influenced by the process settings — or is a part/mold change required?
Critical distinction: The place where the defect appears and the place where it originates may not be the same. Air from an unvented rib can be carried along by the flow front and show up as a mark in a completely different area.
2. Short-Shot Study (Mold Filling Study)
This is the most powerful tool in defect analysis. Short shots make visible how the flow front progresses depending on the injection conditions and the part geometry; they reveal the location where the defect originates. In multi-cavity molds they also show unbalanced filling between cavities.
How to do it
- Keep the set shot volume constant; move the switchover point back and change it in reasonable steps. This way you do not need to change the position-dependent injection profile.
- Watch the screw movement to confirm visually that the switchover point is not overshot.
- No plastic should enter the cavity in the holding phase — if necessary, choose very low values for the holding pressure.
- Set a sufficiently long plasticizing delay so that no material enters the cavity during plasticizing.
- Keep the steps especially small in areas where there are changes in cross-section and where surface defects are seen.
- Label each sample with the switchover point or shot stroke value — so that the front position can be reconstructed later.
- Compare the advance of the front between two filling levels; detect melt hesitation or atypical progress in certain areas (e.g. ribs).
3. Keeping the Melt Front Speed Constant
For uniform surface quality, the melt front speed must be relatively constant — not the screw speed. At a constant screw speed the melt flows at a constant volume flow rate; in the narrow front cross-sections near the gate and at the end of the flow path the speed rises, and in the middle it falls. That is why the injection profile is usually set "slow – fast – slow": the screw speeds up as the front cross-section grows and slows down as the cross-section narrows.
What profiled injection delivers
- More precise control of the switchover point, a more stable transition from injection to holding
- Prevention of jetting and matte spots in the gate area
- Prevention of air streaks and air hooks in poorly vented areas
- Reduction of pressure peaks and burns at the end of the flow path
⚠️ When the injection speed is changed, the switchover point, injection time monitoring and pressure limitation must be corrected again.
4. Determining the Switchover Point
The switchover from injection to holding should take place when the part is about 98% full. A later switchover creates a risk of overpacking and mold damage; an earlier switchover leads to short shots.
The switchover point must be checked again when the following parameters change:
- Back pressure (changes the compressibility of the melt)
- Decompression (changes the volume of material in front of the screw)
- First injection speed step (changes the closing behavior of the non-return valve)
5. Effective Holding Time (Gate Freeze Point)
Determine the holding time not by guessing but by weighing: produce parts with a gradually increasing holding time and weigh each one (keep the holding pressure constant). When the weight gain stops significantly, the effective holding time has been reached — the gate has frozen. This point prevents undefined backflow of the melt from the part into the runner system or in front of the screw; this is the most common cause of weight and dimensional fluctuations.
⚠️ This method does not apply to TPEs — the soft component compresses strongly, creating a risk of overloading and surface defects. With valve gate systems, make sure that the gate does not freeze excessively (that the needle can still close).
6. Commissioning and Checklists
Material Preparation / Drying
- Clean the dryer, filter and machine hopper thoroughly every time the material changes
- Put a lid on the machine hopper; use a magnet in the hopper
- Protect open granulate bags against contamination
- Is the dryer volume suited to the shot volume and the resulting residence time?
- Are the drying temperature, time and residual moisture within the recommended range (measure)?
- Does the dryer type achieve the required residual moisture; is the molecular sieve working correctly?
- Does the material pick up moisture again in the hopper after drying?
- Adapt the hopper volume for small shot weights; check moisture pick-up in the feed pipes
Peripheral Units / Temperature Control
- Are the supply flow temperatures set to the correct values on the temperature control unit?
- Is the flow rate sufficient (is turbulent flow achieved)?
- Is the power of the temperature control unit sufficient for the mold mass?
- Are the temperature control circuits connected correctly (supply/return mix-up)?
- Does the mold wall temperature deviate during the process?
Observing the Screw Movements
- Injection: does the machine reach the specified flow rate/speed? Does it hit the pressure limit? Is there strong pressure fluctuation?
- Switchover point: is there fluctuation in the remaining cushion? Is the point overshot or not reached? Does the screw bounce back?
- Start of holding: does the screw move forward?
- End of holding: does the screw bounce? Is the cushion in the optimum range (2–5 mm)?
- Check the first injection speed step and the closing behavior of the check valve
- Inspect the check valve and the barrel for signs of wear
Mold Movement and Ejection
- Does the part stick on the ejector side?
- Does the part jam when the mold opens or during ejection?
- Do the gates eject properly and break off cleanly from the part?
- Is the clamping force set only as high as needed (too much is harmful for venting)?
- Is the clamping pressure at a reasonable value?
Plasticizing Conditions
- Is the shot volume suited to the screw diameter (stroke 1–3 screw diameters)?
- Check the flange (feed) temperature
- Set the back pressure correctly
- Is the measured melt temperature within the recommended processing range?
- The melt should be injected "into the air" with the set profile; for small volumes, collect it in an insulated container
- Is there fluctuation in the plasticizing time; does the time stay within the remaining cooling time?
- Very short plasticizing times create thermal and mechanical inhomogeneity — check the plasticizing performance