225 改性尼龙与BMC团状模塑料怎么选
从一条卡壳的产线讲起
前年，温州一家做低压电器的厂遇到产能瓶颈：继电器外壳用 BMC 压塑，每模要人工修毛边，一个熟练工一天修八百件，旺季招工都招不到。厂长想整线转 PA-GF 注塑，把毛边工序和人工全省掉。
评估了两个月，最后的方案出乎意料：外壳的三个型号转 PA-GF30 注塑，产线周期从九十秒压到三十五秒，修毛边的工位直接撤了；但耐电弧要求最高的灭弧片，留在了 BMC 上——注塑 PA 的耐漏电起痕指标，在那里就是不够。
厂长总结了一句很精道的话：不是谁替代谁，是谁干哪一段更划算。 这一篇就把热固性的 BMC 和热塑性的 PA 这对搭档的分工讲清楚。
BMC 是什么
BMC（Bulk Molding Compound，团状模塑料）是不饱和聚酯树脂 + 玻璃纤维 + 矿物填料 + 固化剂的混合物。它属于热固性塑料——加热后发生交联固化，固化后不能再熔融。这个本质区别决定了BMC 和 PA 的所有性能差异。
热固性带来的优势
交联结构让 BMC 具有：耐热性好（长期 150-180℃）；尺寸稳定性极好（收缩率 0.05-0.1%，远低于 PA 的 1.5%）；抗蠕变优异（交联网络不流动）；
耐电弧和耐电痕化好；刚性高。这些优势让 BMC 在电机、电器、灯罩上长期占据稳固地位。
热固性带来的劣势
三点：一是不能回收——固化后无法再熔融，废料只能填埋或粉碎做填料，在环保要求日益严格的今天这是硬伤；二是成型周期长——要等固化反应完成，一般 1-3 min，比 PA 注射的几十秒慢；三是脆性大——韧性远不如 PA，不能做卡扣和薄壁。
与 PA 的应用分工
BMC 的主场：电机端盖、换向器、灯罩、电器外壳、绝缘子——要耐热、要尺寸、要电性能、不要韧性的场合。PA 的主场：齿轮、轴承、卡扣、结构件——要韧性、要效率、要可回收的场合。两者重叠的区域在电机和电器件。
成本对比
BMC 的原材料成本低于 PA66——不饱和聚酯和矿物填料都便宜。但综合成本要算三笔：成型周期长导致单件工时成本高；废品无法回收导致废料成本高；
模具和压机投资大（BMC 一般用压机模压）。算下来，BMC 在大批量简单件上成本优势明显，在复杂件上不如 PA 注射。
什么情况下从 BMC 换成 PA
四个理由：一是需要韧性——卡扣、薄壁、有冲击；二是需要回收——客户有环保要求；三是需要提高生产效率——PA 注射周期短；四是结构复杂——PA 注射能做出更复杂的形状。
换的时候要注意：PA 的收缩率和耐热要重新评估，尤其是耐热——BMC 的 180℃ 换成 PA66 的 120℃，差 60℃。
什么情况下保持 BMC
三个理由：一是耐热要求超过 150℃且不想用高温尼龙；二是尺寸精度要求极高——BMC 的 0.1% 收缩率是巨大优势；三是耐电弧要求高——BMC 的耐电弧性优于 PA。这三条中任何一条成立，BMC 都很难被替代。
工程实测：4 条强制测试
测试1：收缩率。BMC 0.05-0.1%，PA66 1.5%——BMC 尺寸优势巨大。
测试2：耐热。BMC 长期 150-180℃，PA66 120℃——差 60℃。
测试3：周期。BMC 模压 1-3 min，PA 注射 30-60 s——PA 效率高。
测试4：回收。BMC 热固性不能回收——环保要求下是硬伤。
边界声明
| 工况 | 推荐材料 |
|---|
| 耐热 150℃ 以上 | BMC 或高温尼龙 |
| 尺寸精度极高 | BMC |
| 需要韧性和卡扣 | PA |
| 需要可回收 | PA |
| 大批量简单件 | BMC 成本更低 |
工程备忘
BMC vs PA：BMC 赢在耐热、尺寸、耐电弧；PA 赢在韧性、效率、可回收。差 60℃ 是换料的关键门槛。
实战案例：常见踩坑与正解
踩坑一：尼龙与BMC只比强度就下结论。选材对比要看短板——PA 的短板是吸水和耐酸，PBT 的短板是耐热和冲击，金属的短板是重量和成本。正解：列一张短板对照表，看哪家的短板在这个工况下不致命。
踩坑二：以塑代金属时直接按金属件的形状做塑料件。正解：塑料和金属的设计逻辑不同，塑料靠加强筋和壁厚分布，金属靠截面惯性矩，必须重新设计。踩坑三：换了材料不重算成本。
材料便宜了但壁厚要加厚，或者后处理工序增加，总成本可能反而更高。正解：算整件成本，不算单公斤价格。
延伸判断：最容易被漏掉的隐性变量
尼龙与BMC的量产事故里，有一半不是料选错了，是隐性变量没控住。
第一个变量是含水率。PA 系材料出厂含水率、干燥条件、注塑前的存放时间，三者共同决定实际含水率，含水率不对，强度和外观都会变。
第二个变量是模具温度。模温低 20℃，表面浮纤和熔接痕强度可能差一倍。
第三个变量是装配后的时间。装完 24 h 和装完 30 天的扭矩、尺寸、密封压缩量都不一样。
这三个变量都不写在物性表上，但都写在失效报告里。
把这三件事写成一张表发给供应商，比打十通电话有用——尼龙与BMC的选型沟通成本，基本都花在这几项反复确认上。
交联结构：BMC 的底气与枷锁
BMC 的一切优点和缺点，都来自同一个化学结构——交联。
固化之后，分子链之间架起了桥，成了一张三维的网。这张网让它受热不熔、不软不蠕变：两百多度的环境里，PA 早就开始变形，BMC 的尺寸纹丝不动。这张网还给了它两样 PA 短期追不上的本事：耐电弧和耐漏电起痕——电弧烧在表面，交联结构碳化得慢，绝缘不被击穿。开关、继电器、断路器里那些要直面电弧的部位，至今是 BMC 的地盘。
但网一旦织成就拆不掉了。热固性意味着不能回用——水口料、废品只能填埋或降级处理，PA 件粉碎回炉的循环在 BMC 这里走不通，环保压力和料耗成本都压在这一条上。
生产效率同样受制于网：压塑成型周期长，飞边毛边是热固工艺的伴生品，后处理人工是 BMC 成本表里最容易被低估的一行。
还有一条隐性约束：BMC 件的韧性差。掉在地上可能崩角，卡扣装配这种事它干不了，金属嵌件周围的应力也容易让它开裂。所以这一对的现实分工是：要绝缘耐弧耐热不变形的静态件，BMC；要韧性、量产效率、装配功能的结构件，PA。 两条线并行，各守各的段位，是行业跑了几十年验证过的最优解。
PA 与 BMC 的高频问答
问：BMC 转 PA 的模具能直接沿用吗？ 基本不能。压塑模和注塑模的流道、排气、顶出逻辑完全不同，嵌件定位方式也要重做。现实的做法是按 PA 的工艺重新开模，老模具留给还在产的老型号。
问：耐漏电起痕的指标怎么看？ 看行业要求：家用电器的带电件普遍要求 PTI 四百以上，工业断路器的灭弧部位要求更高。PA 系改性把 PTI 做到六百已经是配方极限，BMC 天生就能站上更高的档位——这一条的差距，是化工结构层面的，配方追不上。
问：BMC 也有注塑成型？ 有，用专用的螺杆和料筒，把团状料低温低速挤进模腔，纤维损伤比压塑小、周期比压塑短。但毛边和后处理依然存在，热固性的宿主逻辑没有变。
问：两条线并行的管理成本高不高？ 高，但可控。两套工艺、两套来料标准、两套品质判据，生产计划要把热固件的长周期提前排。换来的是每个部位都用在最合适的料上——管理成本买的是结构合理性，这笔账多数时候是划算的。
转产评估的五个核实点
BMC 转 PA 之前，把五个点核实清楚，转产才不会转出隐患。
耐电弧等级。件的部位直面电弧吗？灭弧、开关触点周边这些位置，PA 的配方极限也追不上 BMC，该留的留。
使用温度。长期温度一百三十度以上、有交变热源的，BMC 不软化不蠕变的天赋还在，PA 要按热变形温度重新核算。
结构韧性。件上有没有卡扣、搭扣、薄壁悬臂？BMC 崩角，PA 抗造，这一条上 PA 几乎全胜。
回收与环保要求。客户或行业有没有回收比例要求？BMC 的水口料不能回用，PA 可以闭环——这一条在欧盟市场越来越值钱。
嵌件与二次加工。预埋金属件多的件，压塑工艺反而成熟，转注塑要重新评估嵌件定位和应力。
五点核完，转哪款、留哪款，分界自然浮出来——转产不是整线推翻，是逐件裁决。
问：BMC 和 PA 的外观件能混线生产吗？ 能，但色差判定要分家——BMC 压塑面的光泽和 PA 注塑面天生不同，同一台设备上并排装的两个件，色板要分开签。混线的色差投诉多半出在拿 PA 的样板去验 BMC 的件上。
问：转 PA 之后，金属嵌件怎么办？ 两条路：注塑里预埋——模具加定位销，生产节拍慢一点；或者改成后植入——热压、超声、热熔都成熟，模具简化、嵌件位置精度反而更高。嵌件多的老产品，转产时逐件选方式，不搞一刀切。
修毛边的人工账
温州那家电器厂把转产前后的账摆在了全厂大会上。
BMC 压塑外壳，年产五百万件，每件修毛边三秒钟，一条产线养十一个修边工，旺季还要外协。转 PA-GF 注塑后，毛边工序归零，十个工位撤掉，一年省下的人工成本接近七十万；周期从九十秒压到三十五秒，同样的场地产能翻了近两倍。
但灭弧片没有动。BMC 的耐电弧等级在那里是保命的指标，注塑 PA 配到配方极限也够不着，这一部分的 BMC 产线原样保留，修边工位留了两个。
厂长最后算的总账是：六成产品转 PA，省人工省周期；四成留 BMC，保绝缘保安规——整厂成本降了三成，客诉零增加。这个案例后来被他们戏称为"半场革命"：转产转得动的地方大胆转，转不动的地方老老实实留着，比全盘推翻聪明得多。
交接期的双轨管理
转产不是切换开关，是几个月的双轨期，管理要跟得上。
温州那家厂的过渡方案有三条：图纸标注材料版本——老编号继续下 BMC 的单，新编号下 PA 的单，采购和生产各按版本走；品质双标准文件化——毛边判定、色差基准、耐压测试各有各的表，塞给产线前先培训；
客户端提前知会——外观件的光泽变化要拿到客户签样认可，免得量产后被当成品质异常。
双轨期最容易出的事故，是两套标准互相串门：拿 BMC 的毛边标准卡 PA 的件（PA 注塑根本没毛边可修），或者拿 PA 的光泽样板验 BMC（怎么调都不达标）。文件上写清"哪个版本用哪张表"，比车间里喊十遍管用。
双轨期跑完，转产才算真正落地。工艺切换的技术关好过，标准切换的管理关难缠——提前把两套体系分开立档，是过来人用返工换来的经验。
问：BMC 的原料储存有什么讲究？ 团状料里有增稠体系，稠度随存放时间一直在变——先进先出是铁律，超过有效期的料流动行为走样，压出来的件密度不匀。存放要阴凉密封，夏季高温仓库里堆两个月，料性就变了，这一条热固车间的老师傅都知道。
结语
我们交付的，不只是一包料——选料这件事，越早问越省事。
这类件的选料与试模，可以一起聊。
How to choose between 225 modified nylon and BMC bulk molding compound
Starting from a jammed production line
The year before last, a low-voltage electrical appliance factory in Wenzhou encountered a production capacity bottleneck: the relay housings were compression molded with BMC, and each mold required manual trimming of burrs. A skilled worker could trim 800 pieces a day, but even during peak season, they couldn't hire enough workers. The factory manager wanted to switch the entire line to PA-GF injection molding to eliminate the burr trimming process and labor entirely.
After a two-month evaluation, the final plan was unexpected: three models of the casing were switched to PA-GF30 injection molding, reducing the production line cycle from ninety seconds to thirty-five seconds, and the deburring workstation was directly removed; however, the arc-extinguishing piece with the highest arc resistance requirement remained on BMC—because the tracking resistance index of injection-molded PA just wasn’t enough there.
The factory manager summed up a very insightful remark: It's not about who replaces whom, but about who does which part more efficiently. This article clarifies the division of labor between the thermosetting BMC and the thermoplastic PA duo.
What is BMC
BMC (Bulk Molding Compound) is a mixture of unsaturated polyester resin, glass fibers, mineral fillers, and curing agents. It belongs to thermosetting plastics—after heating, it crosslinks and solidifies, and once cured, it cannot be melted again. This fundamental difference determines all the performance differences between BMC and PA.
Advantages brought by thermosetting
The cross-linked structure gives BMC: good heat resistance (long-term 150-180°C); excellent dimensional stability (shrinkage 0.05-0.1%, far lower than PA's 1.5%); excellent creep resistance (the cross-linked network does not flow);
It has good arc resistance and tracking resistance, and high rigidity. These advantages have allowed BMC to maintain a strong position in motors, electrical appliances, and lampshades for a long time.
Disadvantages brought by thermosetting
Three points: First, it cannot be recycled — once cured, it cannot be remelted, and waste can only be landfilled or crushed for use as filler. This is a major drawback given the increasingly strict environmental requirements. Second, the molding cycle is long — it requires waiting for the curing reaction to complete, generally 1-3 minutes, which is slower than PA injection, which takes only a few tens of seconds. Third, it is very brittle — its toughness is far inferior to PA, so it cannot be used for clips or thin walls.
Division of Applications with PA
The stronghold of BMC: motor end covers, commutators, lampshades, electrical housings, insulators—applications that require heat resistance, dimensional stability, and electrical performance, but not toughness. The stronghold of PA: gears, bearings, clips, structural components—applications that require toughness, efficiency, and recyclability. The overlapping area for both is in motors and electrical devices.
Cost comparison
The raw material cost of BMC is lower than that of PA66—both unsaturated polyester and mineral fillers are cheap. But the overall cost has to be calculated in three parts: the long molding cycle leads to high labor cost per piece; scrap cannot be recycled, resulting in high scrap cost;
Mold and press machine investments are large (BMC is generally molded using a press). Calculations show that BMC has a clear cost advantage for high-volume simple parts, but is not as good as PA injection for complex parts.
In what situations should one switch from BMC to PA
Four reasons: First, resilience is needed — snaps, thin walls, impact; second, recycling is needed — customers have environmental requirements; third, production efficiency needs to be improved — PA injection has a short cycle; fourth, complex structure — PA injection can create more complex shapes.
When making the change, pay attention to: the shrinkage and heat resistance of PA need to be reassessed, especially the heat resistance — BMC's 180℃ being replaced with PA66's 120℃, a difference of 60℃.
Under what circumstances to maintain BMC
Three reasons: First, the heat resistance requirement exceeds 150℃ and high-temperature nylon is not desired; second, the dimensional accuracy requirement is extremely high—the 0.1% shrinkage rate of BMC is a huge advantage; third, a high arc resistance is required—BMC's arc resistance is better than PA. If any one of these three conditions is met, BMC is hard to replace.
Engineering Test: 4 Mandatory Tests
Test 1: Shrinkage rate. BMC 0.05-0.1%, PA66 1.5% —— BMC has a huge size advantage.
Test 2: Heat resistance. BMC long-term 150-180°C, PA66 120°C — difference 60°C.
Test 3: Cycle. BMC molding 1-3 min, PA injection 30-60 s——PA is highly efficient.
Test 4: Recycling. BMC thermosetting cannot be recycled — under environmental requirements, this is a serious drawback.
Boundary Declaration
| Operating condition | Recommended materials |
|---|
| Heat-resistant above 150℃ | BMC or high-temperature nylon |
| Extremely high dimensional accuracy | BMC |
| Requires toughness and a buckle | PA |
| Needs to be recyclable | PA |
| Large quantities of simple parts | BMC costs less |
Engineering Memo
BMC vs PA: BMC wins in heat resistance, size, and arc resistance; PA wins in toughness, efficiency, and recyclability. The 60℃ difference is the key threshold for material switching.
Practical Case Study: Common Pitfalls and Correct Solutions
Pitfall 1: Concluding by comparing only the strength of nylon and BMC. Material selection comparison should focus on weaknesses—PA's weaknesses are water absorption and acid resistance, PBT's weaknesses are heat resistance and impact resistance, and metals' weaknesses are weight and cost. Correct approach: Make a table of weaknesses to see which one's weaknesses are not fatal under this working condition.
Pitfall 2: When replacing metal with plastic, directly making the plastic part in the shape of the metal part. Correct approach: The design logic for plastic and metal is different; plastic relies on ribs and wall thickness distribution, while metal relies on section moment of inertia, so a redesign is necessary. Pitfall 3: Changing materials without recalculating costs.
The material is cheaper, but the wall thickness needs to be increased, or additional post-processing steps are added, which may actually result in higher total costs. The correct approach: calculate the cost for the entire piece, not the price per kilogram.
Extended Judgment: The Hidden Variable Most Likely to Be Overlooked
In the mass production incidents of nylon and BMC, half were not due to wrong material selection, but because hidden variables were not controlled.
The first variable is moisture content. The factory moisture content of PA series materials, drying conditions, and storage time before injection molding together determine the actual moisture content. If the moisture content is incorrect, both strength and appearance will be affected.
The second variable is the mold temperature. When the mold temperature is 20°C lower, the surface fiber floating and weld line strength may differ by a factor of two.
The third variable is the time after assembly. The torque, dimensions, and seal compression amount are different at 24 hours after assembly and 30 days after assembly.
These three variables are not listed on the material properties table, but they are all included in the failure report.
Writing these three things into a single sheet and sending it to suppliers is more effective than making ten phone calls—the cost of nylon and BMC model selection communication is basically spent on these repeated confirmations.
Cross-linked Structure: The Confidence and Shackles of BMC
All the advantages and disadvantages of BMC come from the same chemical structure—cross-linking.
bridges were built between molecular chains, forming a three-dimensional mesh. This mesh made it resistant to heat, neither soft nor creepy: in an environment over 200 degrees, PA had already begun to deform, and the BMC's dimensions remained completely unchanged. This mesh also gave it two abilities that PA could not catch up with in the short term: arc resistance and leakage resistance — when the arc burns on the surface, crosslinked structures carbonize slowly, and insulation is not broken down. The parts in switches, relays, and circuit breakers that face arcs directly are BMC's territory to this day.
But once the mesh is woven, it cannot be removed. Thermosetting means it cannot be reused—sprue material and scrap can only be landfilled or downgraded; the cycle of crushing and reproducing PA parts does not work at BMC, and environmental pressure and material costs all fall on this issue.
Production efficiency is also constrained by the internet: long compression molding cycles, flash and burr are byproducts of thermosetting processes, and post-processing labor is the most easily underestimated part of BMC cost sheets.
There is also an implicit constraint: BMC parts have poor toughness. If dropped on the ground, they may chip, and it cannot do clasp assembly, and the stress around metal inserts can easily cause cracking. So the practical division of labor between these two is: static parts with insulation, arc resistance, heat resistance, and no deformation, BMC; structural parts with toughness, mass production efficiency, and assembly functions, PA. These two lines run parallel, each maintaining its own level, which is the optimal solution proven by the industry over decades.
PA High-frequency Q&A with BMC
Q: Can molds for BMC to PA be directly reused? Basically not. The flow channels, venting, and ejection logic of compression molds and injection molds are completely different, and the insert positioning method must be redone. The practical approach is to remold according to PA processes, leaving old molds for models still in production.
Q: How should I assess the indicators for leakage resistance and scratching? Industry requirements: Live parts in household appliances generally require a PTI of above 400, while industrial circuit breakers require even higher arc extinguishing parts. PA system modifications to PTI of 600 are already the formula's limit, while BMC naturally stands at a higher level—this gap is at the chemical structure level, and the formula cannot catch up.
Question: BMC also has injection molding? Yes, using dedicated screws and barrels to extrude clumped material into the mold cavity at low temperature and speed, causing less fiber damage and a shorter cycle than compression molding. However, burrs and post-processing still exist, and the host logic of thermosetting remains unchanged.
Question: Is the management cost of running two parallel lines high? High, but controllable. Two processes, two incoming material standards, two sets of quality criteria; production planning must prioritize the long cycle of thermal fasteners in advance. The trade-off is that every part is used for the most suitable material—management costs are about structural rationality, which is usually cost-effective.
Five verification points for conversion evaluation
BMC Before switching to PA, verify these five points clearly to avoid potential risks during transfer.
Arc resistance grade. Does the part face the arc directly? Around arc extinguishing and switch contacts, PA's formula limits can't keep up with BMC, so keep what should be retained.
Operating temperature. For long-term temperatures above 130°C and alternating heat sources, BMC's ability to resist softening and creep is still present. PA must be recalculated based on thermal deformation temperature.
Structural toughness. Are there clips, buckles, or thin-walled cantilever on the part? BMC chipping and PA resistance — PA is almost always the winner in this area.
Recycling and environmental requirements. Does the customer or industry have recycling ratio requirements? BMC sprue material cannot be reused, PA can be closed-loop—this is becoming increasingly valuable in the EU market.
Inserts and secondary processing. For parts with many embedded metal parts, the compression molding process is actually mature, so transfer injection molding requires re-evaluating insert positioning and stress.
After five points of review, which model to transfer or keep naturally becomes the boundary—switching production is not about overturning the entire line, but about adjudicating each piece.
Question: Can BMC and PA appearance parts be produced on a mixed line? Yes, but color difference determination is different—the gloss of BMC press-molded surfaces and PA injection-molded surfaces are inherently different. Two pieces mounted side by side on the same machine have separate color plates. Most complaints about color difference in mixed lines come from using PA samples to inspect BMC parts.
Question: After switching to PA, what should be done about metal inserts? Two paths: embedded in injection molding—add molds and positioning pins, and slow down production cycles; Or switch to post-implantation—hot pressing, ultrasonic, and hot melt are all mature, molds are simplified, and insert positioning accuracy is actually higher. For older products with many inserts, choose each part individually during production transfer, avoiding a one-size-fits-all approach.
Manual for Trimming Rough Corners
That Appliance Factory in Wenzhou put the pre- and post-production schedule at the factory-wide meeting.
BMC Compression-molded housings, annual output of 5 million pieces, each deburring in three seconds, one production line employs eleven trimmers, and during peak seasons, outsourcing is required. After switching to PA-GF injection molding, the burr process was reset to zero, ten stations were removed, and annual labor cost savings were nearly 700,000 yuan; The cycle was reduced from 90 seconds to 35 seconds, nearly doubling the capacity of the same site.
But the arc extinguishing plates didn't move. BMC's arc resistance rating is a lifesaver there, and injection-molded PA can't meet the formula's limits. This part of the BMC production line remains intact, but two trimming stations are retained.
The factory director finally calculated the total amount: 60% of products converted to PA, saving labor and reducing cycles; Forty percent kept BMC, insulation and safety regulations were enforced—the entire factory cost dropped by 30%, and customer complaints increased by zero. This case was later jokingly called a "half-field revolution": boldly rotate where production can be transferred, keep the parts that can't be turned honestly, much smarter than completely overturning it.
Dual-track management during handover period
Transfer isn't a switch switch, it's a two-track period of several months, and management must keep up.
That Wenzhou factory had three transitional plans: drawing labeling material versions—the old number continues with BMC orders, new numbers under PA orders, and procurement and production follow each version; Quality dual standards are documented—burr judgment, color difference benchmark, and pressure resistance testing each have their own forms, and are trained before being handed to the production line;
Client should notify in advance—gloss changes in appearance parts must be approved by the customer's sample to avoid being labeled as abnormal quality after mass production.
The most common accident during the dual-track period is when two sets of standards visit each other: taking BMC rough edges to card PA parts (PA injection molding has no burrs to fix), or using PA gloss samples to inspect BMC (no matter how you adjust them, it doesn't meet standards). Clearly stating "which version uses which sheet" is more effective than shouting ten times in the workshop.
Only after the dual-track period is complete does the transition truly be implemented. Technical checks for process switching are well managed, but management for standard switching is tricky—preemptively setting up files for the two systems is the experience gained through rework.
Q: What are the key considerations for BMC raw material storage? Aggregate materials have a thickening system, and their consistency changes with storage time—FIFO is the rule: if the material exceeds its shelf life, its flow behavior changes, and the compressed parts have uneven density. Storage should be kept cool, sealed. After two months of storage in a high-temperature warehouse in summer, the material properties change—every experienced technician in the thermosetting workshop knows this.
Conclusion
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