Rss & SiteMap


共3 条记录, 每页显示 10 条, 页签: [1]


上海北京顺德 发表于:2006/10/22 10:09:00
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.


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.


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.


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.


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.


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 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」
法海 发表于:2006/10/24 9:36:00
jason 发表于:2006/10/24 10:25:00
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.