PVC, a commonly used thermoplastic in industrial fields, is widely used in pipes, fittings, and profiles due to its low cost and good moldability. However, it has an inherent characteristic of poor thermal stability—its molding temperature range (160-200℃) is close to its decomposition temperature. If not controlled properly during injection mold processing, decomposition can occur. This decomposition not only causes yellowing and scorch marks on the product surface but also produces HCl gas that corrodes the mold cavity, potentially leading to batch scrapping in severe cases. The spline testing mold, as a core tooling for predicting decomposition risks in advance, verifies material compatibility and mold performance before mass production, making it a crucial step in preventing such problems. The following analysis, based on actual production scenarios, provides a detailed explanation.
I. Core Function and Practical Points of the Spline Testing Mold
(I) Core Function Positioning
The spline testing mold is mainly used for testing the thermal stability and moldability of PVC materials. Following industry standards such as GB/T 1040, it prepares standard splines for tensile and impact testing to visually reflect the material's molding state within the mold. Compared to direct mass production, it can expose potential decomposition risks early—such as whether the material formulation is reasonable and whether the mold venting is sufficient—avoiding the cost waste caused by later mold adjustments or raw material changes. It is a necessary step before small- to medium-batch PVC product production.
(II) Key Structural Design
Mold Core Material Selection: Considering that the acidic gases produced by PVC decomposition can easily corrode the mold, two materials are preferred for the mold core of the sample test mold: Cr12MoV alloy tool steel is commonly used for manual test molds, with a surface hardness of HRC 58-62 after quenching; S136 stainless steel is used for automatic ejection molds, with higher hardness after cryogenic treatment, and the surface roughness needs to be controlled at Ra≤0.2μm to reduce local retention caused by material sticking to the mold.
Ventilation and Runner Design: Vent grooves are a key structure for preventing decomposition. The depth is usually set at 0.03-0.05mm and the width at 6mm. A cold slug well is also set at the gate to prevent insufficiently plasticized cold material from entering the cavity and forming hot spots. The runner uses a circular cross-section with a diameter of not less than 6mm to prevent the melt from staying in the runner for too long and causing overheating.
Ejection system configuration: The ejector pin of the automatic ejector mold is hydraulically driven, and the ejection speed can be adjusted between 5-50mm/s. An ejection delay of 0.5-2s is set to ensure that the sample is undamaged upon ejection and accurately reflects the surface condition after molding (e.g., whether there are fine scorch marks).
(III) Decomposition Early Warning Test Method
During the test, actual mass production process parameters must be simulated: the barrel temperature is set to "feed section 140-160℃, front section 170-190℃", the mold temperature is controlled at 30-60℃, and the screw speed is 50-80rpm. Decomposition risk is judged by observing the appearance of the sample—if light brown stripes appear on the sample 30mm from the gate, it is mostly due to excessive injection speed causing shear overheating; if there are irregular bubbles or small focal points on the surface, it may be due to poor mold venting, requiring timely cleaning of the venting grooves or adjustment of the process. II. Main Causes of PVC Injection Mold Decomposition
(I) Material and Formulation Issues
Insufficient Heat Stabilizer: Heat stabilizers are the core component for inhibiting PVC decomposition. If the amount added is lower than the standard ratio (usually 2-5 parts), or if low-quality stabilizers are used, the material's thermal stability will be significantly reduced. For example, when processing thick-walled PVC products, internal heat is difficult to dissipate, and insufficient stabilizer cannot neutralize the HCl produced during decomposition, quickly leading to scorch marks.
Substandard Raw Material Purity: Excessive impurities in the PVC resin, or a recycled material blending ratio exceeding 20%, will disrupt the thermal stability system. Furthermore, if the raw material moisture content exceeds 0.05%, the water vapor generated during heating mixes with the melt, creating localized high temperatures in the mold cavity and inducing decomposition.
Lubricant Imbalance: Insufficient internal lubricant increases friction between the melt and the mold wall, generating additional heat; excessive external lubricant may precipitate on the mold cavity surface, forming localized hot spots. An imbalance in both increases the probability of decomposition.
(II) Defects in Mold Design and Maintenance
**Ventilation System Failure:** Clogged venting channels are a common problem in production. After prolonged use, carbonaceous materials from PVC decomposition adhere to the venting channels. If not cleaned regularly, venting efficiency can decrease by more than 50%, preventing gas from escaping from the mold cavity and causing localized high temperatures due to the molten material encasing the gas.
**Abnormal Mold Cavity Surface:** Scratches or excessive roughness (Ra > 0.4μm) on the mold surface will hinder melt flow, creating stagnation zones. For ordinary steel cavities without hard chrome plating, prolonged contact with HCl gas will cause corrosion pits, further exacerbating material accumulation and creating a vicious cycle.
**Inadequate Cooling System:** Excessive spacing or blockage of cooling water channels can cause mold temperature fluctuations exceeding ±5℃, resulting in slow melt cooling and prolonged residence time. This is especially problematic in the core areas of thick-walled products, where poor heat dissipation can easily lead to decomposition.
(III) Improper Process Parameter Settings
Temperature Control Exceeding Standards: Nozzle temperatures exceeding 190℃ or barrel tip temperatures exceeding 200℃ will directly breach the thermal stability threshold of PVC. If the screw and barrel are eccentric, frictional heat generated during operation will cause a sudden rise in localized temperature, potentially leading to localized decomposition even if the overall temperature remains normal.
Excessive Residence Time: When the actual injection volume is less than 20% of the theoretical injection volume of the barrel, the material's residence time in the barrel is likely to exceed 5 minutes. Under high-temperature conditions, this will rapidly degrade, forming charred material that enters the mold cavity with the melt.
Excessive Shear Stress: Screw speeds exceeding 80 rpm or back pressures exceeding 8 MPa will generate excessive shear heat, disrupting the thermal stability system. This manifests as radial charring on the sample surface, and this decomposition is often accompanied by a decrease in melt fluidity.
III. Systematic Prevention Strategies for Decomposition
(I) Raw Material and Formulation Control
Optimize the stabilization system: Adjust the amount of heat stabilizer according to the product thickness—for thick-walled products (wall thickness > 5mm), increase the stabilizer dosage by 1-2 parts. Prioritize calcium-zinc composite stabilizers (environmentally friendly and with long-lasting heat stabilization), and avoid using low-quality lead salt stabilizers that easily precipitate.
Strictly control raw material purity: When purchasing PVC resin, prioritize Grade 1 materials with impurity content < 0.1%. The proportion of recycled materials should not exceed 20%, and they must be dried before use (moisture content controlled below 0.05%) to prevent water vapor generation during heating.
Balance lubricant ratio: The ratio of internal lubricant (e.g., stearic acid) to external lubricant (e.g., paraffin wax) should be 1:1.2. Avoid excessive use of any single lubricant. This can be verified through sample testing—if the sample surface is smooth without precipitation and demolding is smooth, the ratio is reasonable.
(II) Mold Maintenance and Optimization
Regular Cleaning and Inspection: Perform a comprehensive mold maintenance weekly – clean carbides from the venting grooves (using a fine copper wire brush), check the cavity surface for corrosion or scratches; if slight corrosion is found, polish with fine sandpaper to Ra≤0.2μm; unclog the cooling water channels quarterly to ensure smooth water flow.
Targeted Structural Optimization: If poor venting frequently occurs during sample testing, add 1-2 auxiliary venting grooves (0.03mm depth) at the end of the cavity; for areas prone to stagnation (such as corners of the sample), appropriately enlarge the runner diameter to reduce melt residence time.
Corrosion Prevention: Before using a new mold, apply a hard chrome plating (5-10μm thickness) to the cavity surface to enhance corrosion resistance; for molds that will be out of use for a long time, apply anti-rust oil to the cavity surface to prevent oxidation caused by high humidity.
(III) Process Parameter Optimization
**Precise Temperature Control:** Adjust the temperature based on the sample test results. If scorch marks appear on the sample, first reduce the temperature of the barrel front section by 5-10℃, and control the nozzle temperature at 180-190℃ to avoid dry injection (dry injection will cause the material at the nozzle to remain at a high temperature for a long time).
**Controlling Dwell Time and Shear:** Select a suitable injection molding machine according to the product weight (the actual injection volume should not be less than 30% of the theoretical injection volume of the barrel) to reduce material retention. Set the screw speed to 50-80 rpm and control the back pressure at 3-5 MPa. Judge by the appearance of the sample—if the surface has no scorch marks and the color is uniform, it indicates that the shear stress is moderate.
**Optimizing Cooling Conditions:** Control the cooling water temperature at 25-35℃. For thick-walled products, the cooling time can be appropriately extended (increase the cooling time by 2-3 seconds for every 1mm increase in wall thickness) to avoid overheating of the product due to insufficient cooling, indirectly reducing the risk of decomposition.
The decomposition phenomenon in PVC material injection molding is essentially a result of a mismatch between "material thermal stability," "mold structure," and "process parameters." Slip testing of the mold, as a core tool for early verification, can effectively shorten the problem-solving cycle and reduce mass production losses. In actual production, it is necessary to optimize the formula based on raw material characteristics, regularly maintain the mold, and precisely control the process to fundamentally prevent decomposition. With the promotion of environmentally friendly PVC materials (such as lead-free stabilizer formulas), it is still necessary to continuously adjust the mold and process through slip testing to adapt to the characteristics of the new material, ensuring stable production and product quality.
HUIWEN Professional Injection Plastic Parts,like Household appliances plastic parts, Automotive Plastic parts, agricultural tools plastic parts, medical equipment plastic parts, toys plastic parts, food bottles, boxes, buckets and so on
