The injection parts stick to the mold, making it rather difficult for ejection. The causes thereof comprise:
1 Too much injection materials.
2 Overhigh injection pressure, overlarge dwell pressure and overlong dwell time.
3 Overhigh melt temperature.
4 Overlow or overhigh mold temperature.
5 Incorrect design of draft.
6 Improper design of ejector unit, incorrect position of extrusion, oversmall number of ejector pins,
and oversmall ejection force; no air intake device for the core, resulting in vacuum during ejection. 7 Rough molding surface of cavity.
8 Inadequate dosage of mold release agent or incorrect spraying.
Solutions:
1 Avoid excessive injection of melt.
2 Reduce injection pressure and dwell pressure, shorten dwell time.
3 Reduce injection rate and barrel temperature, optimize dimension of nozzle, runner and gate,
avoid overhigh shearing heating during the flow and filling of melt so that melt temperature can be properly reduced.
4 Properly adjust mold temperature, since under overhigh temperature the plastic parts will easily stick to the mold, whereas overlow temperature shall result in large molding shrinkage and high ejection resistance during ejection.
5 While ensuring proper retaining mode of plastic parts in the mold, increase draft.
6 Use appropriate ejection mode and ejection parts in accordance with the structural features of plastic parts; carefully analyze the ejection resistance and its distribution, properly arrange position and number of ejector pin, properly select section dimension of ejector pin; increase ejection force; as for plastic parts such as containers with deep cavity and thin walls, an air intake unit should be applied to the mold core to avoid vacuum during ejection.
7 Increase smoothness for the surface of cavity, and at the final stage of polishing for molding parts, the polishing should be done along the ejection direction of plastic parts.
8 Properly spray mold release agent and properly increase dosage of the agent at places with large ejection resistance.
For a processor, finished parts sticking in the mold can be an absolute headache. The parts may be picked by a robot, ejected off the mold to fall onto a conveyor system, or hand-picked by an operator after each cycle. Parts sticking in the mold can cause massive downtime not to mention mold safety alarms if the part remains in the mold for the next cycle. There are ways to fix this issue and we will cover them here under three categories; Machine, Mold, and Material.
1.) Machine: Within the process parameters a part sticking in the mold usually means that the part is over-packed. If this is the case the processor can try decreasing injection or hold pressures, reduce shot volumes, or decrease screw forward time.
2.) Mold: One cause for parts sticking relating to the mold could be scratched or marred surfaces in the cavity. For this you would want to remove the blemishes and polish the mold surface. Insufficient draft angles could also be an issue. You want a minimum draft angle of 0.5 degree per side. The greater the draft angle the easier the part will be to eject. The issue could also stem from a poor ejection mechanism. In order to solve this, options include installing new ejector pins or increasing the number of ejectors per cavity.
3. )Material: As far as material is concerned, there could be inadequate lubrication in the resin causing the parts to stick. In this case, you could incorporate using a mold release agent or mix a small percentage of lubricant to the resin itself before processing.
There are 2 reasons for this – the nozzle is stuck to the cavity at some point, or the release resistance of cavity side is larger than that of the Core side, so the part sticks to the cavity. The circumstances in which the part sticks in the cavity due to the resistance between the nozzle and the cavity include: The nozzle radius R is greater than the corresponding corner radius R in the mold, so when fixing the mold, the nozzle and the mold are not concentric, or there are plastics leaking from between the nozzle and the mold. Either of the cases will cause the part to stick in cavity side. To prevent this from happening, the mold should be installed properly. The release resistance of cavity side is too high because the degree of polish is too low or there are bumps on the sidewall. At this time, a Z-shaped pulling rod should be installed at the core side, so as to pull the molded part. Therefore, during mold design, sufficient consideration should be made to keep this from happening. It works even though there is a temperature difference between cavity side and the core side.
（1） The mold is overfilled with plastics – reduce injection molding pressure; Lower the too high temperature of the ejection cylinder
（2） The injection molding pressure is held for too long – reduce the time for the mold screw to move forward
（3） Mold surface scrapes, multi-holes or scratches – get rid of the stains and polish the mold surface
（4） Insufficient mold release angle – apply the minimum mold release angle of 0.5° on each side (the larger the angle, the more effortless the ejection and the faster the operation are)
（5） Improper undercut design – make sure there is no sharp angle in the undercut
（6） The injection molded part sticks to the highly polished mold surface – use discharge valve to empty the vacuum generated when the molded part is forced out through the highly polished mold surface
（7） Improper ejection mechanism – increase the number of ejector pins or change for a different system
（8） Insufficient plastic lubrication – use release agent whenever possible; Increase external lubricant, e.g. zinc stearate
This is true I have gone through this problem not in one company but in many, from the period when I started carrier from trainee engineer. Here are some techniques I used to solve this kind of issue through process. (By considering that injection mold design and mold maintenance is ok in all aspects)
Decrease the shot wt. Decrease the hold pressure at starting.
If cold water circulation required in injection mold, keep it off initially.
Increase the cooling time 4-5 secs for startup.
Ensure the purging of barrel for degrading material.
Dipper cavity mold, optimize injection mold initial opening speed some vacuum pull back component to cavity.
After repair of injection mold, ejector plate become much free in movement, (as all the contamination has cleaned, and if ejector rod is not coupling type). When injection mold clamping drops to zero, ejector plate move forward push the component to cavity.
Thanks alot for this explanation.
I have been experiencing this problems with mould purchased from China.
I will try to apply your solutions and see if they help.
Thanks for sharing your experience Sir.
thank you so much for your comments and recognition，if you have any question about purchasing mold from china, please contact, actually we can help you
This kind of problem is often encountered by injection molding engineers and mold engineers. The experience you Shared is good. I think you can write more