I can understand why the parts are manufactured on the sprue - it keeps them together and makes packaging easier. What I don’t get is what the little ‘nubs’ coming off the sides are for - the small bits that end in a circle. All that happens to them is that they get cut off. They wouldn’t be there for no reason, but does anyone know what it is?
I believe those are injection points where the plastic is injected into the mold and are in odd spots to be sure good “coverage” of the area to be injected.
I could be wrong, but that is what I always thought they were.
Just snip or file off and everything will be fine.
I could be wrong, I’ve been worng before and surely I will be wrong again.
That would make them gates and vents. When you inject liquid plastic into a mold, it gets forced into the gates under pressure. At the same time, you have to displace the air that is inside the mold. The air is forced out the vents. The vents would normally be the little pieces on the part that don’t seem to go anywhere. The plastic sprue that links all of the parts together is part of the gate system that the liquid plastic flows in.
I have worked in a plastics plant before. Usually on an injection molded process, the the injected hot plastic is injected in the center of the entire sprue. It is usually the largest and thickest portion of the sprue. What you are pointing to in the picture is where they have designed some access plastic to ensure the part is going to be good. I know that each company has different techniques and so forth. I am basing on what I learned when I worked at a plastic pipe fitting plant. We used a very similar process that had those extra pieces like that.
Thank you for listing the technical terms. I haven’t worked in the plastics industry for close to 5 years now. I only remember the terms of sprue, part, and tree.
OK , first a little background. I am a process engineer in a plant that has 18 plastic injection machines.
The “nubs” coming off of the sides that end in a circle are for the ejector pins that release the “shot” from the mold. If you look closely on any plastic molded part you will see these ejector pin marks. Some are in the part itself and some are attached to the sides of the part as in the picture Someone mentioned vents, and there are vents in the molds. a properly designed mold will not allow plastic into it. The vents in our molds are .0005 - .001" deep x .250-.300" wide. As for the injection point, it is not visible on the part you pictured. I also see where the spru was cut to remove what might have been another set of components. Generaly molds are made up to do multiple sets of components and the injection point is centered inbetween them.
Very interesting. And just to interject something from another Forum thread discussion about the late Art Curren’s kitbashing articles and projects, he saved every sprue and made clever use of them in his work.
Actually the mold shot is calculated to ensure a complete fill. when the plunger or screw, depending on what type of barrel, moves back it only moves far enough to take on the equivelent shot size to fill the mold. when it moves forward, it pushes the hot plastic under pressure into the cavities and holds there at what ever pressure is needed/calculated for the particular plastic/mold combination. This pressure is usually rated in tons. The hold time varies depending on what the freeze time is for the material being injected. The hold is also sometimes refered as pack time. This neccessary to keep plastic pushing into the cavities to overcome the shrinkage tendancies of the plastics. If we don’t pack the molds the part will shrink wrong and have sink marks and other deformities on the surface. Although we have not shot styrene here in a quite a few years, if memmory serves correctly, The total cycle time for a shot as shown in the original post might be in the area of 90 to 100 seconds Low budget molds such as this one will have the ejectors outside of the part/runners. Good quality ($$$$$) molds have the ejector pins in the runners and the parts which help minimize distortion when they are ejected out of the cavity