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How To Avoid Short Shot In Your Plastic Production

Jun 8, 2018 8:00:00 PM

Creating plastic casing using the injection molding process is more and more common during the development of a new product. However, a lot of quality issues might appear during the production process and might result in a loss of time and money. When dealing with plastic production, it's exceptionally important to understand what aspect of the production needs your attention and, most of all, how to deal with it. Today we will examine the short shot issues.


If you are interested in Plastic Molding, you might want to have a look to our post about Adding Plastic Molding to your Project!


We call this phenominon a "short shot" due the fact that a molding shot falls short of filling the mold. In other words, the melted plastic does not manage to occupy all the space designed by the mold and create an unfinished and deficient product like the following pictures:

 short shoe

Maybe you have experienced this issue before and have some questions like : "Why are they like this?" or "How could I avoid it?" This can be due to several reasons that we will outline here. 

 

Reason 1: the equipment is not suitable

Each plastic injection molding machine has its own injection rate. It might happen that your machine's injection rate is too low regarding the mass of plastic needed.

What can you do?

In this case, the only solution is to change the equipment. It is highly recommended to choose an injection equipment that, at least, proposes an injection rate 15% more than the injection volume needed (for both plastic and condensed material).

 

Reason 2: the material Exhibits BAD "FLOWABILITY"

In injection molding, the flowability of the used material might impact the result and the quality of your product. More flowable your material is, easier it will be injected. If your material is not flowable enough, you might experience the Short Shot

What can you do?

To solve this issue, you can either act on the material or on the injector. You can improve the placement of the runner, enlarge the gate or the size of the nozzles as a few examples. For the materials, you can also put some additional ingredients in the raw material to make its flowability better.

 

Reason 3: excess of lubricant

If lubricants are needed, an excess of it might make the changes of a short shot higher.

What can you do?

You can either reduce the amount of lubricant or make some adjustment spaces between the cylinder and the screw.

 

Reason 4: the runner is blocked by a cold material

After some uses, some residual materials might obstruct the runner and contribute to bad circulation of the material (as shown on the picture below).

Short Shot: blocked runner due to cold material 

What can you do?

In order to resolve this issue, you need to disassemble to mold and injectors to clean the nozzles and cold slog well. A larger size runner may also be needed.

 

Reason 5: bad mould's exhaust

 Another cause for a short shot is when there isn't enough space for the gases in the mould to escape quickly enough to prevent bubbles or other abnormalities in the moulded part. 

Short Shot: bad mould's exhaust 

What can you do?

Reposition the exhaust groove to a suitable place and potentially increase the size of exhaust groove. You can design exhaust groove (deep 0.02-0.04mm, wide 5-10mm) on the mould joint.

 

REASON 6: THE MELT'S TEMPERATURE IS TOO LOW

If the melt's temperature is insufficient, the material will not be able to go through all the injection process and will definitely cause a Short Shot. 

What can you do?

The temperature of the melt must be suitable and you can increase time of injection to improve this defect too. Make sure to consult the manufacturers datasheet on the minimum melt point of the plastic. 

 

Reason 7: the mould's temperature is too low

More than the temperature of the material, you should also care about the mould temperature. Indeed, that permits to favorise the flowability of the material 

What can you do?
You can Pre-heat the mould to the temperature of process requirement before the injection machine starts; if the temperature of the mould can’t reach the target you should check in the cooling system.

 

Reason 8: The injection speed is too Slow

The mold filling speed is related to injection speed, if the mold filling speed is too low the temperature of the melt is reduced so that the flowability is affected where the product will suffer from this effect.

What can you do?

You have to increase the injection speed appropriately

 

Reason 9: the structure of the plastic design is unreasonable

Like the picture below, if the scale between width and thickness of the plastic is too large and the shape of plastic is complex, the flowability of melt will be suffocated at the thin-wall of plastic so the short shot might be the result.

Short Shot: design structure 

What can you do?

According to our experience, the best thickness of plastic is around 1-3mm and the large plastic parts is 3-6mm. The thickness of plastic becomes challenged if the thicknesses are more than 8mm or less than 0.5mm, so it's best to stay within these tolerances.

 

Need Help with any of this?

We have an experienced team of mechanical engineers on the NexPCB staff that are ready to help you get your plastics project off-the-ground and on the way to mass production. If you have a project you'd like to have us help you with, just hit up our project request form, give us a little information on your project and off we go!  You'll be well on your way to your project