Hey there! I'm a supplier of Plastic Forming Mould, and I've seen my fair share of plastic products coming out of these moulds. In this blog, I'm gonna talk about the common defects you might find in plastic products made by plastic forming moulds.
1. Shrinkage
Shrinkage is one of the most common defects in plastic products. When plastic cools down after being injected into the mould, it contracts. This contraction can lead to visible shrink marks on the surface of the product or internal voids.
The main reason for shrinkage is the difference in the density of the plastic in its molten and solid states. Different plastics have different shrinkage rates. For example, crystalline plastics like polyethylene and polypropylene generally have higher shrinkage rates compared to amorphous plastics like polystyrene.
To deal with shrinkage, we can adjust the injection pressure and holding time. Higher injection pressure can help pack more plastic into the mould cavity, reducing the space for shrinkage. Longer holding time allows the plastic to solidify under pressure, minimizing the shrinkage effect.
2. Warping
Warping occurs when the plastic product doesn't cool evenly. Uneven cooling can cause internal stresses in the product, which then lead to deformation. This is especially common in products with complex shapes or large surface areas.
There are several factors that can cause uneven cooling. The design of the mould, such as the location and size of the cooling channels, plays a crucial role. If the cooling channels are not properly designed, some parts of the plastic may cool faster than others, resulting in warping.
Another factor is the gate location. The gate is the point where the plastic enters the mould cavity. If the gate is placed in an inappropriate position, it can cause uneven flow of the plastic, leading to uneven cooling and warping.
To prevent warping, we need to optimize the mould design. This includes improving the cooling system and carefully selecting the gate location. We can also use annealing processes after the product is formed to relieve internal stresses.
3. Flash
Flash is the excess plastic that seeps out of the mould cavity during the injection process. It usually appears along the parting line of the mould or around the ejector pins.
The main cause of flash is excessive injection pressure or clamping force. If the injection pressure is too high, the plastic can force its way out of the small gaps in the mould. Similarly, if the clamping force is not sufficient to hold the two halves of the mould together tightly, flash can occur.
Worn-out or damaged mould components can also contribute to flash. For example, if the parting line of the mould is not smooth or has been damaged, plastic can leak through.
To solve the flash problem, we need to adjust the injection pressure and clamping force. We also need to regularly inspect and maintain the mould to ensure that all components are in good condition.
4. Burn Marks
Burn marks are dark spots or streaks on the surface of the plastic product. They are usually caused by overheating of the plastic during the injection process.
There are a few reasons for overheating. One is high injection speed. When the plastic is injected into the mould at a very high speed, it can generate a lot of friction, which in turn raises the temperature of the plastic. Another reason is poor ventilation in the mould. If the air cannot escape from the mould cavity properly, it can get compressed and heat up the plastic.
To avoid burn marks, we should reduce the injection speed and improve the ventilation system of the mould. This can be done by adding vents or using porous materials in the mould design.
5. Short Shots
Short shots happen when the plastic doesn't fill the entire mould cavity. As a result, the final product is incomplete.
The most common cause of short shots is insufficient plastic material. This can be due to incorrect setting of the injection volume or blockage in the plastic feeding system. Another reason could be too high viscosity of the plastic. If the plastic is too thick, it may not flow easily into all parts of the mould cavity.
To fix short shots, we need to check and adjust the injection volume. We also need to ensure that the plastic feeding system is clean and unobstructed. If the plastic viscosity is too high, we can increase the temperature of the plastic or use a plastic grade with lower viscosity.
6. Sink Marks
Sink marks are depressions on the surface of the plastic product, usually occurring near thick sections or ribs. They are caused by the shrinkage of the plastic in the interior of the product.
When the plastic in the thick sections cools and shrinks, the surface layer may not be able to follow the contraction, resulting in sink marks. To prevent sink marks, we can modify the product design to reduce the thickness variation. For example, we can add gussets or use a more uniform wall thickness.
7. Jetting
Jetting is the formation of long, thin streams of plastic on the surface of the product. It occurs when the plastic is injected into the mould cavity too quickly and at a high velocity.
The high - velocity plastic stream can break up and form these jetting patterns, which can affect the appearance and quality of the product. To prevent jetting, we can reduce the injection speed and adjust the gate design. A well - designed gate can help control the flow of the plastic and prevent jetting.
Conclusion
As a Plastic Forming Mould supplier, I understand how frustrating it can be to deal with these defects. But with the right knowledge and techniques, we can minimize their occurrence.
If you're in the market for high - quality plastic forming moulds, whether it's for making simple plastic parts or complex Robot Arm components, or even parts for Crusher equipment, I'm here to help. I can provide you with moulds that are designed to reduce these common defects and ensure the production of top - notch plastic products.
If you're interested in discussing your specific requirements, feel free to reach out. Let's work together to create the best plastic products for your business.
References
- "Plastic Injection Molding Handbook" by O. Olszewski
- "Mold Design for Plastic Injection Molding" by James F. Watts
