shrinkage problem explanation
发布日期：2013-12-03 17:12:33 点击量：1864 信息来源：create
Molding parts surface sag is called shrink/hole, is due to the shrinkage caused by the plastic molding, so the most difficult to eliminate.Injection molding process, is injecting molten plastic into cool inside the cavity, and because the plastic thermal conductivity is very poor, so cooling is very complex.Especially thick wall part than thin wall part is relatively slow cooling, so thick wall part in shrinkage.Another mold temperature slightly high condensation is slow, thus forming mould, local temperature difference with the mold itself the difference of heat conduction, the mold temperature on the high side and low conduction of parts will appear shrinkage cavity.Therefore, the design should consider the difficult to appear shrinkage cavity forming parts and die structure.Such as the reinforcement, highlights and add rounded corners and;Or the reinforcement design into a solid;The surface pattern to cover the defect.
1, the lack of compression flow of material (by the main sprue, runner and gate) than the molding parts to mold wall thickness (too small volume), due to the injection pressure is not fully function into the molten material within the cavity, make shrinkage increases, thus appeared larger shrinkage cavity.Especially the gate after hours, even if the holding time is enough, but the gate has been solidified, make the liquid inside the cavity pressure transmission is less than.Especially for solid melting inconsistent knot plastic character, it is easier to produce this kind of phenomenon, and the burr of the mould, due to the closure lax and not enough on the molding pressure, also appear shrinkage.Screw injection machine set a non-return ring to prevent the reverse flow of liquid along the screw, but the shrinkage than plunger type injection machine.At this point, it can be said that plunger type injection machine is better than screw stand type injection machine.As mentioned above, when the end of the molten material and cavity before melting solidification, plus the lack of enough pressure maintenance can prevent compression caused by the shrinkage cavity.So, increase the sprue, runner and gate, especially increasing gate diameter is very effective.In addition increasing injection pressure, or combined with enough pressure maintenance is also very important.And molten material insufficient also fuelling shrinkage cavity.Good plastic flow, if increase pressure, because produce burr can also cause shrinkage cavity, if necessary, reduce feed temperature or convert illiquid Jane plastic can prevent the shrinkage cavity.From gate far, flowing of molten material is prone to shrinkage cavity, at the end of the molten material flows to the end of the cause caused by the pressure loss, flow resistance so near a shrinkage of open gate, or increasing the thickness of the part is also effective.As a result, increase the number of point gate according to the circumstance, or change the gate location is more effective.
2, injection quantity, at the end of the improper adjustment screw injection molding machine injection must be between the screw head and the nozzle with adequate number of molten plastic (according to the size of the machine in 5 mm or so), and use it to buffer.If the buffer quantity is zero, and, at the end of the injection volume adjustment to the screw top to the end at the same time, when the pressure so the screw will not be able to move forward, and therefore cannot be pressure, plastic shrinkage becomes the shrinkage cavity and show it.The solution is to have a certain amount of buffer, the end of the injection screw stand still can count forward mm and 10 mm.Buffering capacity is zero) is up (at the end of the injection screw cap will shorten the life of the injection machine itself, must pay attention to.
3, shrinkage cavity appear some molding parts on the surface of the parts work even if the internal shrinkage cavity, sometimes also have no way.This kind of situation in the beginning, the mold temperature high side the shrinkage, the low temperature -- - it is hard to discern shrinkage cavity surface.So should not allow the shrinkage cavity surface cooled sufficiently, or vice versa will allow the shrinkage cavity (i.e., the relative surface does not allow the shrinkage cavity) high temperature molding is also very effective.
4, uneven cooling molding parts uneven thickness, thick wall part than slow cooling of the thin wall part, shrinkage cavity and thick wall parts.To eliminate the shrinkage cavity, due to uneven thickness of theoretically is difficult, so when designing parts should keep uniform wall thickness.That is to say, with a focus on the narrow wall thickness changes.When designing convex platform, for example, if you have any requirement on the diameter size, should be set up to eliminate shrinkage hole at the center;When the convex platform strength requirements, should not be bold convex platform itself, and should take advantage of reinforcement to increase strength.Shrinkage cavity under the gentle concave shrinkage cavity less conspicuous than sag down sharply, so does not require the precision parts, should be in the outer layer is solidified, the central part is soft can out of the mould, then slow cold or warm water in the air, which can make the shrinkage cavity is not obvious, do not affect use.
5, the thermal expansion coefficient of excessive shrinkage molding plastic itself is large, of course, a shrinkage (such as shrinkage rate 0.02 ^ 0.05 PE, PP shrinkage 0.01 0.02, 0.002 ^ ^ 0.02, PS shrinkage rate even as long as there is a little bit of stiffener, can produce dents).As a result, the plastic molding easy appear shrinkage cavity in low temperature.If improve the injection pressure can make more plastic injection mold cavity, so the higher shrinkage cavity pressure is reduced accordingly.But the temperature dropped to below the required minimum temperature, plastic even improve the injection pressure, it is hard to prevent "character plastic shrinkage cavity.Such as polypropylene, high density polyethylene, pom, etc., the density of its crystalline solid with molten state significant don't ask, so to prevent the shrinkage cavity is very difficult.If allowed to substitute non-crystalline sex co-polymer at this moment, can reduce shrinkage cavity.In addition, if filled with inorganic fillers, such as fiberglass, asbestos, etc. Also can make the smaller shrinkage cavity.
6, surface curing slow wall thickness of shrinkage cavity or luster, because surface layer did not form a solidified layer, hard when center of contraction will surface inward to form shrinkage cavity, even by the diffusion of heat outside introversion gloss surface layer will be melted and appear again.On the contrary, in the part of the wall thickness if the surface is strong enough, the contraction can form vacuum bubble to the center.Can reduce the mould temperature, reduce the temperature, reduce the speed of the molten material through wall thickness area, make the solidified layer thicker (but easy to appear the vacuum bubble);Adjust the wall thickness, such as reinforcement of thinning, thickness of graded;Using low shrinkage rate of plastic;Add the foaming agent in plastic.
7, mold aspects where produce dent repair upper edge.Materials in produce dent place side has a narrow place, upset this part.Should completely avoid the design product thickness difference.Prone to dent the stiffener, narrow shape should be short as far as possible.
