◆Solutions for plastic molding damage
Scorching refers to the phenomenon in which, during the injection molding process, due to poor mold venting or injection too quickly, the air inside the mold cannot escape in time. Under instantaneous high pressure, the air heats up rapidly (in extreme cases, the temperature can reach 300℃), causing the melt to turn yellow or scorch in certain areas, as shown in the figure below.
Causes and solutions for burning：
| Cause Analysis |
Solution Methods |
|
① End sealing speed too fast |
① Lower the last few sealing speeds |
| ② Mold clamping not good | ② Increase or increase the mold clamping exhaust slot (increase vacuum degree) |
| ③ Injection pressure too high | ③ Reduce injection pressure (can reduce holding pressure) |
| ④ Melt temperature too high (low melt index) | ④ Lower melt temperature, lower flowability |
| ⑤ Gate or small position improper | ⑤ Change gate or modify its position (change to latent gate) |
| ⑥ Poor thermal stability of plastic material (easy decomposition) | ⑥ Use thermally stable good plastic material |
| ⑦ Excessive clamping force (very small venting) | ⑦ Reduce clamping force or edge lock type exhaust |
| ⑧ Air trapping or gas trapping in mold | ⑧ Clean the contamination or oil stain inside the mold cavity |
Black spots and solutions
Black spots often appear on transparent, white, or light-colored plastic parts during injection molding, as shown in the picture below. These black spots affect the appearance quality of the finished product, resulting in high scrap rates, significant waste, and high costs during production.
Black spot problems are a challenge in injection molding, requiring control at every stage, including the sprue (runner solidified material), scrap, batching, feeding, environment, downtime, and the entire production process, to reduce them.
The direct cause of black spots on plastic parts is the presence of contaminated material or the decomposition of the molten plastic at high temperatures, resulting in black spots on the product surface. Specific causes and solutions are shown in the table below.
Causes and solutions for black spots：
| Cause Analysis | Solution Methods | |
|---|---|---|
| 1 | Raw material is decomposed by excessive shearing inside the barrel | ① Lower the screw back pressure a. Reduce nozzle temperature b. Reduce barrel temperature c. Reduce screw RPM d. Reduce injection quantity |
| 2 | Raw material contains volatiles (black spots) or material is not sufficiently dried | ② Lower the screw back pressure a. Check if there are black spots in the raw material b. Dry the raw material thoroughly before use |
| 3 | Hot runner nozzle or gate size is too small, injection speed too fast | ③ Increase gate/runner size a. Enlarge gate dimensions b. Reduce injection speed |
| 4 | Dead corners inside the barrel cause raw material decomposition | ④ Clean the screw/barrel, reverse flush the barrel if necessary, or replace the screw/barrel assembly |
| 5 | Air is entrained into the melt when the mold is opened | ⑤ Adjust the mold opening stroke and wind direction (best to have side blowing), use the best injection molding machine |
| 6 | Poor pigment dispersion, causing streaking | ⑥ Add dispersant or replace with better-quality pigment |
| 7 | Powder pigment enters the barrel | ⑦ Pre-mix the pigment with pellets or use masterbatch |
| 8 | Sprue bushing or nozzle diameter is too small | ⑧ Enlarge the sprue bushing or nozzle diameter |
| 9 | Water-cooling nozzle missing or contaminated | ⑨ Control water-cooling nozzle (replace if used for the whole year) |
| 10 | Hopper dryer / material feeding system not cleaned properly | ⑩ Thoroughly clean the hopper dryer / material feeding system |
Whitening at the top (explosion at the top) and its solution
When a plastic part is demolded from the mold, if ejector pins are used, the ejector pins will often leave marks of varying depths on the plastic part. If these marks are too deep, a phenomenon known as "ejection whitening" will occur. In severe cases, the ejector pins may even penetrate the plastic part, a phenomenon known as "ejection bursting," as shown in the figure below.
The main reason for white marks on plastic parts is that the molded part has high adhesion force, while the ejector pin area is not strong enough, resulting in white marks at the ejector pin ejection point. Specific causes and solutions are shown in the table below.
Causes and solutions for cephalic streaks
| Cause Analysis | Solution Methods |
| Back pressure too low or too high | ① Adjust to appropriate back pressure |
| Ejection speed too fast | ② Reduce ejection speed |
| Air trapping in mold | ③ Inspect and repair mold (polish) |
| Uneven finished product (warped ejector pin plate) | ④ Inspect and repair mold (make ejector pins level) |
| Too many ejector pins or improper position | ⑤ Increase ejector pin quantity or change pin position |
| Vacuum phenomenon caused by mold closing | ⑥ Clean ejector pin holes for dirt and improve venting effect |
| Product sticking to core, pillar rough (undercut) | ⑦ Polish core and pillars |
| Injection pressure or holding pressure too large | ⑧ Appropriately reduce its pressure |
| Post-mold shrinkage too large | ⑨ Increase mold shrinkage |
| Side gate action time or position improper | ⑩ Inspect mold (make side gate action normal) |
| Pin-point gate or ejection speed too fast | ⑪ Increase pin-point gate area or slow down ejection speed |
| Uneven injection speed of multi-segment (burrs) | ⑫ Reduce the last segment of injection speed |
In actual production, even after the injection molding of a plastic part is completed, if the ejector pin does not eject, a bright spot with a very high gloss will still appear on the surface of the part at the head of the ejector pin, as shown in the figure. This phenomenon also occurs on the surface of parts molded by side-pulling mechanisms. This phenomenon is caused by the ejector pin or side-pulling mechanism being subjected to excessive force during molding, or the assembly clearance between the ejector pin and the side-pulling mechanism being too large, or the metal materials used for the ejector pin and the side-pulling mechanism being insufficiently hard and rigid. When the melt acts on the surface of the ejector pin and the side-pulling mechanism with a certain pressure, it causes them to vibrate. If this vibration is too large, it will cause a large amount of frictional heat between the surface and the melt, resulting in a local temperature rise in the melt at that location. As a result, the appearance quality of the plastic part is inconsistent with the surrounding surface, showing the bright spot characteristic. In severe cases, scorching can be seen at the bottom.
The above phenomenon is mainly caused by the high sticking force of the product to the mold and insufficient strength of the ejection area, resulting in white marks at the ejector pin ejection position. The causes and corresponding countermeasures for this type of defect are categorized as follows:
Factor analysis of the white spot phenomenon
| Category | Cause Analysis & Countermeasures |
| Injection Process | Uneven or excessive front-end pressure drop → Reduce the last segment of injection pressure and holding pressure |
| Excessive screw back pressure and high temperature | |
| Excessive holding time causes initial top pressure to drop too low (below 5%) | |
| Mold Design | Uneven or excessive shrinkage due to unreasonable wall thickness → Balance wall thickness, avoid sudden thick/thin changes |
| Poor gate position and quantity → Need to achieve balanced filling, adjust gate position and increase gate quantity if necessary | |
| Using the same gate method for products that are not fully symmetrical will inevitably cause uneven shrinkage. For long, flat products or products with asymmetrical holes, consider using multiple gates or film gates | |
| Too many ejector pins distributed unevenly → Affect shrinkage in different areas | |
| Too large or too small ejector pin head → Reduce local stress concentration | |
| Improper use of inserts in the mold | |
| Improper mold temperature control (core and cavity temperature difference too large, uneven heating/cooling) | |
| Part Design | Require guaranteed flatness under insufficient cooling time → Extend cooling time |
| Wall thickness too thin or too thick → Best thickness on the right side of 1/3 of the flow length | |
| Insufficient draft angle | |
| Material & Formulation | Poor compatibility between fillers and resin → Reduce filler content or improve compatibility |
| Excessive volatile content in material → Reduce volatiles, increase drying temperature/time | |
| For copper, aluminum and other soft metal inserts, the material has very low adhesion to them | |
| For crystalline plastics, the difference in shrinkage rate between parallel and perpendicular directions is large → Increase mold temperature or annealing treatment |