Trouble-shooting guide for thermoplastic part defects
In most cases ,the surface quality of thermoplastic injeciton molded parts is the main criterion for their quality. Due to the complex inter-relationship between the molded part and the mold ,the molding compund and the processing , it is often very hard to recognize the origin of occurring problems and thus to take immediate action.
The trouble-shooting guide of thermoplastic part defectsould help you analyze surface defects at the injeciton mold and to provide nts on avoiding and or reducing defects.
1, sink marks
Sink marks appear for example near material accumulations as depressions on the surface of the molded part , if the thermal contraction cannot be compensated.
Physical cause: Sink marks occur during the cooling process, if the thermal contraction of the plastic cannnot be compensated in certain areas. If the outside walls of the molded part are no stable enough , due to insufficient cooling, the outer layer is drawn inside by cooling stresses. There are three fundamental causes as below.
@ Solidification too slow
@ Effective holding pressure time too short
@Not enough holding pressure transfer, because flow resistances in the mold are too high.
Trouble-shooting and remedy:
For product design –change wall thickness of the product and design the rib ratio and prominent parts of the product again.
For mold design-enlarge ventilation(air vent) and change the place and dimensions of sprue and gate.
For injection conditions- increase cooling time,optimize holding pressure time and injection pressure, check non-return valve and adapt mold temperature control and injection rate.
2， Burnt streak/brown marks/silver streaks
If the melt was damaged thermally by too high temperatures and /or too long residence times,gaseous decomposition products are created ,which are visible on the surface as brownish or slivery discoloration.
Physical cause: burnt streaks are caused by thermal damage to the melt. The result can be a decrease of the length of the molecule chain (silvery discoloration) or a change of the macro molecules (brownish discoloration). Possible causes for the thermal damage are shown as below:
Temperature too high or residence time too long during pre-drying.
Melt temperature too high.
Shearing in the plasticizing unit too high( e.g. screw speed too high)
Residence time in the plasticizing too long.
Trouble-shooting and remedy.
For mold design- check ventilation system, enlarge sprue ,runner and gate.
For use of material- use molding compound or coloring agents with higher thermal stability ,reduce reclaim part and check pre-drying of material.
For injection conditions- lower injection pressure ,injection rate and screw speed decrease the barrel temperature.
3.weld line/ meld lines
The weld line on plastic parts in most cases represents an optical and mechanical weakness . a notch and /or a color change can appear. Notches are particularly visible on dark or transparent parts with smooth, high-polished surfaces .color changes are particularly visible on parts with metal-effect pigments.
Physical cause:
Weld lines are created when two or more melt flows meet. The rounded flow fronts of the melt streams are flattened and bonded when touching each other. This are not high enough, the corners of the flow fronts will not completely develop. Creating a notch. Furthermore , the flow fronts no longer melt together homegeneously , possibly producing a weak spot, if molding compounds containing additives (e.g. color pigments) are used, strong orientations of these additives near the weld line are possible. This can also lend to color changes near the weld line.
Trouble-shooting and remedy:
For product design- increase thickness of product, move gate place and change gate dimension ,use small thickness ratio.
For mold design-increase sprue and runner dimensions and a ventilation on weld line.
For injection conditions-increase mold wall temperature ,injection rate or pressure.
4, jetting
A serpentine-rough or mat-line is visible on the surface of the molded part. Often jetting causes differences in gloss and color. In some cases there are similarties to the record grooves effect.
Physical cause: jetting is caused by an undeveloped frontal flow of melt in the cavity . A melt strand is developed which. Starting at the gate, enters the cavity with uncontrolled movements. During this phase the melt strand has cooled down to such a degree that it cannot be fused homogeneously with the rest of the molding compound.
This often happens with discontinuously increasing cross-sections of the molding part in conjunction with high injection speeds.
Jetting can also be influenced by the position of the mold. In order to avoid defects. The cavity should not be filled from top to bottom.
Troubling shooting and remedy:
For mold design-use overlapped gates to prevent material going into cavity directly.
For forming conditions- decrease injection rate and melt temperature.
5. Air streaks/air hooks
In most cases air streaks appear as mat, silvery or white streaks and can be found near domes,ribs,and wall thickness variations. Near the sprure, starting at the gate, lamellar streaks can appear. Air streaks also appear near letters or depressions.
Physical cause: air which cannot escape in time during the mold filling,is drawn to the surface and streatched in the direction of the flow . Especially near writing ribs, domes and depressions, the air can be rolled over and thus entrapped by the melt. The result in the formation of air streaks or air hooks. If air is sucked into the area in front of the screw during decompression , air streaks will appear near the gate. Here , the air is transported into the cavity during the injection , is then pushed towards the mold wall where it freezes.
Trouble-shooting and remedy:
For mold design- adapt gate place and enlarge ventilation
For product design-change thickness ratio of product
For forming conditions-decrease injection rate
