Detailed Explanation of Blasting in Injection Molding
In the injection Molding Industry, flash (also known as burrs, flash, or rough edges) is a common and troublesome problem. It not only affects the appearance quality of the product but also increases subsequent repair costs and improper repair can even lead to product scrap. We will start with the flash value of plastic materials, a key parameter and explore in depth how to effectively control injection molding quality.
What is overflow value?
The flash value refers to the maximum clearance size in the mold at which no flash occurs during the injection molding process of plastic material. This value is an important technical indicator for measuring material flowability and mold design, and it is directly related to the design of the mold venting system and the determination of mold closing accuracy.
Different plastic materials have different overflow values due to their varying molecular structures and viscosity properties. Based on material viscosity, plastics are generally classified into three categories:
Low viscosity materials: overflow value range is 0.01-0.03mm (such as PA, PE, PP, PPS, etc.)
Medium viscosity materials: overflow value range is 0.03-0.05mm (such as ABS, PS, POM, etc.)
High viscosity materials: overflow value range is 0.05-0.08mm (such as PC, rigid PVC, PMMA, etc.)
Examples of flash values for common plastic materials
- Polypropylene (PP) and polyethylene (PE)
PP and PE are crystalline materials with low hygroscopicity but excellent flowability. The overflow value of PP is about 0.03mm, while that of PE is even lower, at only about 0.02mm.
These materials are sensitive to pressure changes, have slow cooling rates, high shrinkage rates, and significant directionality. Strict mold temperature control is necessary during processing; it is generally recommended to keep the mold temperature below 50°C to avoid dullness or poor weld lines in the plastic parts. However, temperatures above 80°C can easily cause warping and deformation.
- Polyamide (Nylon, PA)
Nylon is a typical high-flow-rate material, with a flash value generally ranging from 0.015 to 0.02 mm. Its relatively small flash value necessitates high precision in the mold. It is highly hygroscopic and must be preheated and dried before molding, with a moisture content not exceeding 0.3%. Nylon exhibits poor thermal stability in its molten state; temperatures exceeding 300°C and residence times exceeding 30 minutes can easily cause material decomposition.
- Polycarbonate (PC)
PC is a high-viscosity material with an overflow value of approximately 0.06 mm. Although it has good thermal stability and a wide molding temperature range, it is highly water-sensitive, and its moisture content must not exceed 0.2%. It must be dried before processing.
PC has poor fluidity, is sensitive to temperature changes, and cools down quickly. The mold gating system should be thick and short, with a cold slug well, and the gate should preferably be a direct inlet.
- ABS and PS materials
ABS has an overflow value of approximately 0.04mm, classifying it as a medium-flow material. PS (polystyrene) is an amorphous material with an overflow value of approximately 0.05mm. It has medium flowability but is brittle and prone to cracking, has a high coefficient of thermal expansion, and is susceptible to internal stress.
When processing PS, it is advisable to use high material temperature, high mold temperature, and low injection pressure. Extending the injection time helps to reduce internal stress.
Key Factors Affecting Overflow Value
- Material flowability
Material flowability is the core factor determining flash value. Materials such as PP, PA, and PE have low viscosity and strong penetration, making them prone to flash even with very small mold gaps. This means that higher precision is required in mold manufacturing .
- Effects of temperature and humidity
For hygroscopic materials such as PA, ABS, and PC, moisture significantly reduces flow viscosity at high temperatures, increasing the likelihood of flash. This is why these materials must be thoroughly dried.
- Process parameter settings
Excessive injection pressure, excessive speed, and excessively high material temperature will all increase the risk of flash. Once the mold cavity is full, the injection pressure should be immediately reduced to a lower holding pressure to allow the plastic at the front of the melt to cool and solidify.
Practical Solutions to the Overflow Problem
- Mold Design and Maintenance
The accuracy of the mold parting surface is the first line of defense against flash. Old molds may experience fatigue collapse of the parting surface due to long-term use. When the machine is stopped, the parting surface must be kept clean, and the parting surface should be regularly maintained by reflowing the mold.
A reasonable venting system design is crucial. When designing and machining venting grooves, the venting gap should be set reasonably according to the material overflow value.
- Process optimization
Multi-stage injection technology is used, with slow molding in areas prone to flash, while the size of the cushioning pad is precisely controlled to avoid excessive material addition.
- Equipment Selection
Ensure the clamping force of the injection molding machine is sufficient and uniform. Uneven mold platen and uneven deformation of the Green Pillars can both lead to localized flash. The product of the molding pressure (cavity pressure) and the total projected area of the part must be less than the clamping force.
Understanding the flash characteristics of different plastic materials is fundamental to achieving precision injection molding. From material selection and mold design to process parameter settings and equipment maintenance, every step requires scientific control. Only by deeply understanding material properties can the most reasonable production process be developed, improving production efficiency while ensuring quality.