When you start trialing a new injection mold, there are many defects that can occur. Short shots are one of the first defects that appear as they are related to material not completely filling the injection mold. The root cause for a short shot can be a multitude of things between the machine, the mold, and the material. If you have a new mold under construction, or are having difficulty diagnosing a short shot condition, check out these causes to find the fix.
The Machine Capacity May Just Be the Beginning
Machine capacity can be a subtle and overlooked check item that can cause a major problem later on. Your injection molding machine has a maximum and minimum rating for the amount of pressure it can generate, the amount of resin it can injection, and the heat the barrel heaters can achieve. Your injection mold requirements should safely be inside the capability and capacity of the machine. Too high of a temperature can cause the heaters to struggle over a long run. Too high or low of a pressure can strain the machine to keep the mold filling correctly. It’s a balancing act between machine capacity and the injection mold requirements to ensure short shots don’t occur among other defects and challenges to production.
Can the Product Design Cause Short Shots?
Production defects can often be traced back to the machine settings that need a small adjustment, but in some rare cases the product design is actually the true cause for defects. Resin must flow through the mold under pressure to completely fill every cavity. Anything that can restrict the flow or block the filling can cause a short shot to occur. In some cases the product design can be the reason. Most computer software applications now have the ability to estimate the filling process and predict a complete fill or identify where the mold will have trouble. Areas that change the wall thickness can often lead to short shots occurring. Adding large ribs for strength can also cause short shots to occur. If your estimation software identifies troublesome areas, the next step is to correct the product design to eliminate the potential for problems.
How Can a Mold Design Cause Short Shots?
The product design is just one aspect of design that can cause defects. The design of the mold to manufacture the product can equally cause as many problems. Many injection molds are designed to produce multiple parts with each injection shot, whether that is multiple copies of the same part or multiple components to a larger assembly. If short shots occur in either case, the gates to each cavity may not be balanced to allow for equal distribution of the molten resin.
If the gates are balanced, the path to fill each cavity may be too long for the resin chosen or the machine settings. Long flow paths generate more flow resistance and may require additional heat and/or injection pressure to overcome the resistance. A larger nozzle and larger sprues and runners can also positively affect the flow of the resin and cure a short shot condition.
The last item of note that contributes to short shots is the mold venting. Without proper gas venting inside the mold, trapped pressure may block a cavity from being filled completely. Typically a parting line adjustment or a separate vent added to a design can prevent further short shots.
Trust the Process
Next on the list of items that can contribute to short shots is the injection molding process itself. The process has many important variables, and one key item is temperature. The mold, nozzle, and barrel temperatures all need to be warm enough to sufficiently melt the resin to a molten state and keep it in that state until the mold is fully filled. If any of these temperatures are too low, they can contribute to the resin cooling before the mold cavities are filled. Injection speed, the time it takes to fill the mold, and sufficient backpressure must also be coordinated with the correct temperatures to ensure proper mold fill. If any of these are off the mark, a short shot can occur.
Backpressure is another key ingredient to successfully filling an injection mold. If too much plastic is sent into the barrel, injection pressure can be reduced. The resin pellets are compressed which will decrease the pressure that can applied. The solution is to adjust the amount of resin sent into the barrel to melt for the injection process.
The last note of the injection process is to keep it stable. The injection molding machine needs to keep consistent temperatures. Constantly interrupting the machine cycle can decrease or increase the time the resin pellets are melted. This can cause density and viscosity changes, burning of the resin, or inconsistent melting. Any of those can lead to short shots forming.
What Part Does Resin Play in Causing Short Shots?
The resin involved in the injection process doesn’t usually cause short shots, but never say never. Resin does have a ‘fluidity’ that indicates how well it can flow during the injection process. The mold in question may have long runner lengths that require a resin with great fluidity. That means it will flow well under the proper pressure. A resin with low fluidity properties won’t flow well, and it must be paired with a mold that has shorter runner lengths. Mixing the two will cause colder and slower resin flows leading to short shots in your finished products.
The end goal is to have molten resin reach the far points in the mold before it begins to cool into a solid form. That may require an adjustment to the injection speed and temperature, or changing the composition of the resin with the manufacturer. If those aren’t sufficient to achieve the results you want, a change to the gate location may be required to shorten the resin path.
Short shots are just one of many defects that can ruin production line efficiency. Temperature, pressure, material selection, mold design and employee training all play integral parts in a world-class operation. Finding a partner with elite levels of all factors is hard, but SEA-LECT Plastics prides itself at being at the top of the charts. SEA-LECT Plastics has an elite team that produces world-class prototypes and products. We have decades of experience with plastic injection molding operations and creating tooling for perfect parts. We can offer support to determine what type of mold you need, what resin to choose, and how to best invest in your future. Give us a call at (425) 339-0288 or email us at email@example.com. We’ll help to determine the best manufacturing for your next project.
Matthias Poischbeg was born and raised in Hamburg, Germany. Matt moved to Everett, Wash., after finishing his bachelor’s degree in business in 1995 to work for Sea-Dog Corporation, a manufacturer, and distributor of marine and rigging hardware established in 1923.
In 1999, Matt took over the reins at Sea-Lect Plastics Corporation, a sister company of Sea-Dog and a manufacturer of plastic injection molded products with an in-house tool & die shop. Matthias Poischbeg is also a contributor to Grit Daily.