Rss & SiteMap

塑料产业论坛-中国塑料技术论坛-专业专注-塑料技术人的分享家园 http://www.plas2006.com/

塑料产业论坛-中国塑料技术论坛-中国专业的塑料技术交流网站-塑料人自己的家-塑料聚合-塑料改性-塑料成型加工-塑料机械
共3 条记录, 每页显示 10 条, 页签: [1]
[浏览完整版]

标题:注射模塑周期的优化原理

1楼
上海北京顺德 发表于:2006/10/22 10:09:00
资料来源:SPECIALCHEM
Optimisation of the Injection Molding Cycle
(Oct 4, 2006)


Graham Webster

An optimised injection molding process is one that is only restricted by the physics of the process and not by the capability of either the mold or machine. There is an optimum filling time for any mold. The optimum filling speed is close to that which requires the least pressure. Fig 1 shows a typical relationship between filling time and pressure to fill. However faster filling speeds are often compromised by problems such as 'gassing' - the burning of the melt front due to entrapment of cavity air. Vents should be cut where the 'gassing' occurs. If vents do exist, and they are properly sized and clean, then reducing the locking force can help with venting and enable faster injection time. But the process should not be compromised because of the poor venting - the venting should be properly engineered to do the job required.

注射模塑工艺的优化只关注工艺的物理过程,而不管模具或者机器的容量。对于任何模具,都有一个最优充模时间。最优充模速度就是与最低注射压力接近时的速度。图1显示了充模时间和压力之间的关系。但是,较快的注射速度常常要与“排气不良”问题之间达成妥协,“排气不良”是指夹带空气的熔体前锋的烧焦现象。排气口要开在发生“排气不良”的地方。如果排气口的确存在,尺寸合适并且干净,那么减少锁模力会有助于排气,并使注塑更快。对于排气不良不能马虎,要恰当设计排气系统,以使工作达到要求。


Figure 1: Pressure to fill

It is common for filling times to be too long. Probably 95% of all molds should have a filling time between 1.5 and 3 seconds. If a mold takes longer than 3 seconds to fill it should be investigated. Small parts with short flow lengths will fill quicker but only very thick parts or those with flow lengths more than 500mm will need to be filled for more than 3 seconds.

通常充模时间太长。可能有95%的模具的充模时间在1.5-3秒之间。如果一个模具的充模时间超过3秒,就应该花点时间研究它了。具有短路径的小部件充模较快,只有那些很厚的部件或者路径长度超过500MM的部件才需要超过3秒的时间充模。

Optimising filling may only make a small time saving, but getting the filling time set at the optimum will enable savings to be made elsewhere - in the most misunderstood part of the injection molding process -the packing or holding phase. Here the semi-molten mass in the filled mold must be pressurised while it cools to allow additional melt into the mold to compensate for the shrinkage that occurs. With crystalline plastics this can be significant as volume shrinkage is high. The packing phase reduces sink marks, improves replication of the mold finish, consolidates welds and controls gloss level variation. The gate must be sized correctly to remain open long enough to enable a correct packing. A gate freeze check will determine the exact time available for packing. The cooling time for the part is determined by the wall thickness and the polymer type and is easily looked up from tables or determined accurately with CAE. The packing time is about 80% of the cooling time which itself is often less than that which is set. Cooling rate is an exponential relationship between plastic temperature and time. The rate drops continuously. Raising the temperature at which a molding can be ejected reduces the cooling time by a disproportionately large amount (see figure 2). Investigation will often find that the cooling time set on the molding machine was established by experimentation and was the time that either the part did not distort, or it was the minimum that prevented ejectors digging in or some other reason that was more related to issues with the mold than related to process physics. Thereafter the cycle time is constrained by a tooling fault - which if fixed would reduce the cycle time for ever.

优化充模本身只是节省很少时间,但是优化充模时间使得其它地方可以节省时间------这是注塑工艺中最让人误解的地方------保压(PACKING)或保持压力(HOLDING)阶段。当已经充模的模具中的半熔融物质冷却到允许另外的熔体进入模具以弥补所出现的收缩时,这种半熔融物质必须受到压力。对于结晶塑料,这种压力是巨大的,因为它的体积收缩高。保压减少缩痕,改善模具仿形性,强化熔接并提高光泽水平。浇口必须尺寸合理,以保持足够长时间的对外开通,从而导致正确的补料。阻止浇口被冻结是决定正确保压时间的关键。塑料件的冷却时间由壁厚和聚合物类型决定,可以容易从表格中查到有关数据,或者用CAE精确确定。保压时间大约是冷却时间的80%,冷却时间常常小于其设定值。冷却速率是一种与温度和时间的指数关系。冷却速率连续下降。增加充模时的模具温度将大幅度减少冷却时间(图2)。研究表明,注塑机上设定的冷却时间是由实验建立的。它要么是制件不变形时的时间,要么是顶针不在制件上产生顶痕的最小时间,要么是与模具想光的更重要的问题而不是工艺物理条件。这样,循环时间由加工缺陷所确定,假如这些问题得到解决,我们就可以随意减少循环时间了。


Figure 2: De-mold temperature(脱模温度)

Frequently the cause is poor mold cooling and this is frequently caused by a lack of coolant rather than its temperature. Many myths and misconceptions exist about mold cooling but a fact is that all mold metal is more than 200 times more conductive than the plastic. The heat will readily transfer from the plastic to the mold but getting it from the mold and into the coolant at the same or greater rate is frequently the problem. It is important to know if the coolant is flowing in EACH channel. Circuits that are in parallel with others almost always mean that some channels get little coolant. Piping all of the channels in series is often the best solution. The concern that this will result in 'hot spots' is unfounded. The temperature rise in the coolant then becomes a function of coolant flow rate.

影响脱模的原因常常是差的模具冷却效果,而这又是由于缺乏冷却剂,而不是温度的设定所致。关于模具冷却,有很多神话和误解,而忽略了一个事实:所有的模具材料都比塑料传热快200倍。热量从塑料传输到模具,而从模具得到热量并同时或者以更大的速率传导到冷却剂则常常成为问题。重要的是要知道,冷却剂是否在每条通道中流动。彼此平行的电路总意味着一些通道得到的冷却剂较少。在所有的通道里布置上冷却管常常是最好的方案。担心由此导致“热点”是没有理由的。首先是冷却剂的温度升高,然后温度随冷却剂流动速率变化。

The time taken to remove the part from the mold is a major area for time wastage. No part, regardless of size should take more than 5 seconds to remove and no mold should take more than 8 seconds to open and close. If these times are not achieved then there is insufficient engineering being applied or poor machine setting. All parts of the mold movement should be accelerated and decelerated under control to minimise shock and to optimise time. Robot sequences should be efficient and grippers properly engineered.

从模具上脱掉制品所花的时间是占浪费时间的一大块。不管制品的尺寸大小如何,没有什么制品可以花5秒以上的时间去脱模,没有什么模具可以花8秒以上去开关。模具的所有动作都应该在受控下加速和减速,以优化震动和时间。机器人应该有效,机械手应该恰当设计。

The quest to optimise the molding cycle is rewarded with substantial cost reductions and greater productivity. The majority of production is far from optimum and the productivity of most molds can be improved by 10 % ~ 30% as the result of virtually no capital expenditure.

This document was provided by Omnexus' editor. If you want to share your technical expertise in a specific area related to plastics and elastomers or if you want to submit press releases, please send it at editor@omnexus.com. Omnexus reserves the right to refuse any article or news item.

「该帖子被 上海北京顺德 在 2006-10-23 17:05:53 编辑过」
2楼
法海 发表于:2006/10/24 9:36:00
学习了,顶!希望老贺多发点这方面资料,以提高论坛的整体水平!
3楼
jason 发表于:2006/10/24 10:25:00
好东西,学习一下哈.老贺多找点注塑方面的资料哈,论坛注塑和模具方面的技术比较薄弱些.
共3 条记录, 每页显示 10 条, 页签: [1]

Copyright 2005 - 2018Plas2006.Com 国家信息产业部ICP编号:京ICP备06020983号
Powered By Dvbbs Version 8.3.0
Processed in .09570 s, 2 queries.