In injection molding industry, flash (also known as burr, burr, or burr) is a common and frustrating problem. It not only affects product's appearance and quality, but also increases subsequent refinishing costs. Improper refinishing can even lead to product scrap. Today, we will delve into how to effectively control injection molding quality, focusing on key parameter of plastic material flash value.

Flash value refers to maximum mold gap dimension at which a plastic material can be injected without flashing. This value is a crucial technical indicator for measuring material flowability and mold design, is directly related to design of mold venting system and determination of mold closing accuracy. Different plastic materials have varying flash values due to their molecular structure and viscosity characteristics. Plastics are generally categorized into three categories based on their viscosity:
Low-viscosity materials: Flash values range from 0.01-0.03mm (e.g., PA, PE, PP, PPS, etc.)
Medium-viscosity materials: Flash values range from 0.03-0.05mm (e.g., ABS, PS, POM, etc.)
High-viscosity materials: Flash values range from 0.05-0.08mm (e.g., PC, rigid PVC, PMMA, etc.)
Examples of flash values for common plastic materials
1. Polypropylene (PP) and Polyethylene (PE)
PP and PE are crystalline materials with low hygroscopicity but excellent fluidity. PP has a flash value of approximately 0.03mm, while PE has an even lower value of approximately 0.02mm.
These materials are sensitive to pressure changes, cool slowly, and shrink significantly with a pronounced directional pattern. During processing, mold temperature must be strictly controlled. It's generally recommended to stay below 50℃ to avoid dulling molded part or causing poor welding. However, temperatures above 80℃ are prone to warping and deformation.
2. Polyamide (Nylon, PA)
Nylon is a typical high-flow material, with a flash value generally ranging from 0.015 to 0.02mm. This small flash value requires high mold precision. It is highly hygroscopic and must be preheated and dried before molding. Moisture content must not exceed 0.3%. Nylon has poor thermal stability in its molten state, temperatures exceeding 300℃ and holding times exceeding 30 minutes can easily cause material decomposition.
3. Polycarbonate (PC)
PC is a high-viscosity material with a flash value of approximately 0.06mm. While it has good thermal stability and a wide molding temperature range, it is highly water-sensitive, with a moisture content of no more than 0.2%. Drying is essential before processing.
PC has poor flowability, is sensitive to temperature changes, and cools quickly. Mold gating system should be thick and short, with a cold slug well and a direct feed gate.
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4. ABS and PS Materials
ABS has a flash value of approximately 0.04mm, making it a medium-flowable material. PS (polystyrene) is an amorphous material with a flash value of approximately 0.05mm. It has medium flowability but is brittle and prone to cracking. Its high thermal expansion coefficient is prone to internal stress.
When processing PS, high material and mold temperatures, low injection pressure, and extended injection times are recommended to reduce internal stress.

Material Flowability: Material flowability is core factor determining flash value. Materials such as PP, PA, PE have low viscosity and strong permeability, making them prone to flash even with minimal mold clearance. This requires higher mold manufacturing precision.
Influence of Temperature and Humidity: For hygroscopic materials such as PA, ABS, and PC, moisture at high temperatures significantly reduces flow viscosity, increasing likelihood of flash. This is why these materials must be thoroughly dried.
Process Parameter Settings: Excessive injection pressure, high injection speed, and high material temperature all increase risk of flash. Once mold cavity is filled, injection pressure should be immediately reduced to a lower holding pressure to allow plastic at the front of melt an opportunity to cool and solidify.

Mold Design and Maintenance: Mold parting surface accuracy is the first line of defense against flash. Old molds may experience fatigue and collapse due to long-term use. During downtime, parting surface must be cleaned and regularly maintained.
Proper venting system design is crucial. When designing and machining venting grooves, venting gap should be appropriately set based on material flash value.

 

Process Optimization: Use multi-stage injection technology, employ slow molding speeds in areas prone to flash, and precisely control size of cushioning material to avoid overfilling.

 

Equipment Selection: Ensure that injection molding machine clamping force is sufficient and uniform. Unbalanced mold plates and uneven deformation of Green column can cause localized flash. Molding pressure (product of cavity pressure and the total projected area of part) must be less than clamping force.

Understanding flash characteristics of different plastic materials is fundamental to achieving precision injection molding. From material selection to mold design, from process parameter setting to equipment maintenance, every step requires scientific control. Only with a deep understanding of material properties can we develop the most optimal production process, ensuring quality while improving production efficiency.