When demolding plastic parts, if the mold cavity has deep grooves on its sides, but the draft angle of the mold cavity is insufficient, the grooves will become blurred after the plastic part leaves the cavity. This phenomenon is called scratching or dragging, as shown in the left figure.
The main causes of scratching in plastic parts are excessive injection pressure or holding pressure, and excessively deep grooves on the inner side of the mold cavity. Specific causes and solutions are shown in the table below.
Causes and solutions for muscle strain：
| Cause Analysis | Solution/Treatment | |
|---|---|---|
| 1 | There is hair inside the mold (steel) | Clean the hair inside the mold (steel) |
| 2 | Injection pressure or holding pressure is too high | Reduce injection pressure or holding pressure |
| 3 | Mold release slope is not enough | Increase the mold release slope |
| 4 | Excessive wear on the mold surface | Polish rough areas or re-texture/plate the mold cavity |
| 5 | Uneven clamping force (large mold deformation) | Reduce clamping force, assist mold deformation |
| 6 | Melt temperature or mold temperature is too high | Reduce overall temperature and extend cooling time |
| 7 | Mold opening is too fast | Slow down mold opening speed |
| 8 | End position of ejector pins is too fast (ejector pin marks too high) | Slow down ejector pin speed, prevent excessive ejection height |
Color difference and solutions
When a plastic part exhibits inconsistent color or uniform gloss on the same surface after molding, it is called color difference or gloss difference, as shown in the figure below.
Color difference is caused by uneven color distribution in plastic parts, or by the colorant and melt flowing in different directions, leading to thermal damage and severe deformation of the plastic parts. Additionally, excessive demolding force can also cause uneven color and color difference.
During injection molding, color difference can occur due to changes in raw materials or colorants, inadequate control of sprue recycling, changes in injection molding processes (material temperature, back pressure, residual weight, injection speed, and screw speed, etc.), changes in the injection molding machine, different mixing times, excessively long raw material drying times, separate molds for products requiring matching colors (multiple molds), color changes in samples, and inconsistencies in the color of inventory products, among other factors.
Causes and solutions for color difference:
| # | Cause Analysis | Solution/Treatment |
|---|---|---|
| 1 | Masterbatch lot number / batch different | Use the same batch of masterbatch for the same order/product |
| 2 | Unstable colorant quality (different batches) | Change to a stable-quality toner/masterbatch |
| 3 | Large change in melt temperature (causing color difference) | Reasonably set melt temperature to keep it stable |
| 4 | Inconsistent number of times raw material is recycled | Strictly control the number of times material is recycled |
| 5 | Too much toner/masterbatch residue in the barrel (cross-contamination) | Reduce toner residue in the barrel |
| 6 | Screw cleaned too fast or back pressure too low | Reduce screw cleaning speed or increase back pressure |
| 7 | Products with high-gloss color sets are not in the same mold cavity | Mold design should place high-gloss color products in the same cavity |
| 8 | Injection molding machine sizes are not identical | Use injection molding machines of the same tonnage/model as much as possible |
| 9 | Dosing time and screw speed inconsistent (uncontrolled) | Control screw speed and dosing time (use the same settings) |
| 10 | Product inventory time too long | Reduce inventory time, ensure products are shipped as new stock |
| 11 | Feeding time test inconsistent | Control feeding time, do not change or extend it |
| 12 | Poor color fastness (same bag sealing) | Ensure good color fastness (use sealed bags properly) |
| 13 | Poor colorant stability (fades a lot, low hiding power) | Use pigments/colorants with good stability |
It is particularly important to note that color difference in plastic parts is a frequent and one of the most difficult problems to control in injection molding. Solving this problem is a systematic project that requires control at every stage of the injection molding process to achieve effective improvement.
Color mixing and solutions
If a noticeable color deviation occurs in localized areas on the surface of a plastic part or at locations where the melt flow direction changes, this phenomenon is called color mixing, as shown in the image below.
Color mixing can be caused by many factors, such as uneven diffusion of color powder during injection molding (poor compatibility), unclean barrel, the presence of sprue of other colors in the raw materials, unstable recycled material ratios, and poor melt plasticization. Specific causes and solutions are shown in the table below.
Causes and solutions for color mixing
| # | Cause Analysis | Solution/Treatment |
|---|---|---|
| 1 | Poor melt plastication | Improve plastication condition, increase back pressure and screw speed |
| 2 | Insufficient drying of colored masterbatch | Re-dry the masterbatch (follow color masterbatch drying instructions) |
| 3 | Melt temperature too low | Increase barrel temperature, screw temperature, and nozzle temperature |
| 4 | Material not cleaned thoroughly (contains other pigments) | Clean the barrel/screw thoroughly (use cleaning material or original resin) |
| 5 | Material trapped during screw retraction, causing burning | Repair or replace the check ring (non-return valve), fit the screw and barrel properly |
| 6 | Too little toner/masterbatch added | Increase the amount of toner/masterbatch appropriately, or replace the screw |
| 7 | Poor compatibility between toner and raw material | Replace masterbatch/toner (choose one suitable for the base resin) |
| 8 | Toner contains too much water or volatile components | Inspect and replace masterbatch/toner |
| 9 | Scratches, burrs, or dead spots on screw/barrel surface | Clean the outer surface of screw/barrel |