Replacing conventional metal products with plastic has become standard procedure in the manufacturing of a variety of products. The process of forming plastic into its desired shape is a complicated procedure, as plastic is inherently flexible — it can be moulded and reshaped as needed.
However, when plastic is moving through its forming process, it is susceptible to several process defects. Unfortunately, these process defects are unavoidable and can occur at any phase of the process. Defects can range from minor flaws, to major defects that are deemed unacceptable and must be discarded.
Defects in the moulding process can be categorized as either scrap, or rework. Scrap is material that has been deemed unacceptable and is discarded, while rework is where the plastic is reheated and reformed to correct minor defects without discarding the whole piece.
Regardless of the type of defect, understanding of the root cause of each is imperative for process improvement. Unearthing the major causes of plastic moulding defects and the methods to reduce them will lead to better process control and efficient production.
The most common plastic moulding defects are classified as follows:
1. Flash: This is an excess of material that occurs when two halves of the mould cavity are misaligned. This creates a thin strip or layer that extends beyond the edge of the product.
2. Sink marks: These are shallow depressions in the surface of the plastic product caused by air or gas being trapped in the plastic.
3. Warp marks: An uneven cooling process can cause plastic to warp and form an incorrect shape that does not match the mould.
4. Short shot: This occurs when the plastic does not fill all areas of the mould properly, resulting in a product with features missing.
5. Void marks: Gaps in the plastic product are caused by air bubbles and are typically found between two or more layers of plastic.
6. Knit lines: These are thin, visible lines which form due to the flow of molten plastic along the parting line of a mould.
7. Blisters: Blisters form on the surface of a plastic product due to the presence of gas or air pockets trapped inside.
To reduce defects in plastic moulding, it is important to identify the causes and implement corrective measures to prevent them. Some of the common causes of plastic moulding defects are:
• Poor mould design
• Poor selection of mould materials
• Inadequate or non-existent process controls
• Poor injection parameters
• Poor resin quality
• Poor cooling systems
• Poor atmospheric conditions
• Improper cleaning of the mould
• Poor machine maintenance
• Poor gate and runner design
• Poor gate position
• Poor clamping force
• Poor ejection system
• Inadequate venting
By carefully considering each of the above factors and ensuring that they are all functioning as intended, you can reduce the likelihood of plastic moulding defects in your manufacturing process.
In conclusion, plastic moulding defects are an unavoidable part of the production process. While they cannot be completely eliminated, understanding the causes and implementing corrective measures can help reduce their occurrence. By improving the quality of mould design, mould materials, process controls, injection parameters, resin quality, cooling systems, atmospheric conditions, cleaning processes and machine maintenance, manufacturers can help ensure a higher standard of plastic products.
