Every injection mold operator holds an opinion about how molds should be gated. Debate is vibrant about how many to use, where they can be placed, and what size or shape is best.
Most implementation of great mold designs emerges from experience, regardless if they are coming from a mold maker, a molder or material suppliers. Other heavy influences include aesthetic constraints of part designs such as gate vestige or the knit-line appearance - or by any number of tooling constrains like slides or lifters, for example.
Many have wished for a simple tool to predict how gating schemes are going to affect molded parts. Injection molding simulation requires engineers with years of experience. Careful analysis needs specialized software for study of the injection molds. The true power of simulation software lies in your engineer's ability to get the best possible results.
Determining Optimum Gate Locations
The considerations are many. Tradeoffs appear along the way as well. Simulating every different gating possibility provides the way to determine the best overall choice which helps every other stakeholder make sound decisions based on project priorities.
Gating schemes can't be determined without considering part aesthetics:
- Are you working to avoid a visible gate vestige?
- Are there areas where visible knit lines are inappropriate?
- Will sink marks appear in ribs or thick wall areas?
Part Aesthetics: Using Simulation for Locating and Sizing Gates
When considering gate positions--what is good for knit line placement may not be best for warpage, fiber orientation or pressure balance among other things. Flow balance and flow direction are the next questions to address.
Determining the importance and how to prioritize these are particularly important for glass fiber-filled materials. The primary issue is the irregular shrinking of the plastics with orientation of fibers or molecules within parts determining levels of shrinkage variability.
Molecular fiber orientation is dictated by flow direction as well as being determined by gate locations.
Material Flow Distances
The next question for determining best gating schemes is how far your material has to flow. For big automotive parts like a bumper fascia, it can be critically important.
A distance between gates often means the difference between having or not having appearance issues like tiger-stripping. Flow distance will also have an effect on the amount of pressure required to fill the mold. This in turn will affect the amount of clamp force required to hold your mold shut during the molding process.
The longer a flow length, the higher pressures and the greater clamp force requirements, especially with thin-walled applications. This means when the molding machine can't handle the pressure or the clamping requirements, it might be time to use more gates. Another option would be a sequential valve gate process.
Longer flow lengths will sometimes also lead to non-uniform packing of the part. If parts don't pack uniformly, they won't shrink uniformly. It is always the shrinkage within non-uniformity that causes war-page.
Proper Gate Sizing
Once the gate locations are established, it is then important to size them properly. Shearing and packing are the two most important reasons for paying close attention to gate sizing.
Laminar flow and shear rates are the two most common categories for injection molding plastic flow. Laminar flow creates layers throughout the runner system, gates and into the cavity. Shear rate determines which adjacent layers flow against each other, and at what force or velocity they will flow.
Every grade of plastic resin has a shear rate limit. This is the limit where the molecular chains are stretched too far. Once this happens the materials begin to degrade. This causes appearance problems like gate blush or a decrease in mechanical properties.
Balanced shear rates flowing through the gate are proportional to the flow rate of your material. They are also inversely proportional to its orifice size. Control shear rates by controlling flow rates or gate sizes. Flow rates are most commonly controlled by the injection molding speed of the machine.
Balance Through Simulation
Gate size is easily changed. Acceptable gate vestige size is also important. If de-gating is automated and becomes necessary, sometimes tunnel gates or cashew gates are needed.
Tunnel or sub gates and cashew gates have the highest shear rates. They remain small in order to de-gate cleanly. If they are kept small it can compromise part packing effectiveness and ability.
Simulation is the best way to strike the balance within several variables. This means evaluation through a series of pathways within the simulation software, which eventually provides the best overarching solution.
At Crescent Industries their engineers utilize 3D mold filling software to allow the mold and part designer to make these gate location decisions before the mold is manufactured. For more information on Crescent Industries design and development services, please click here.
Information for this article was obtained from Moldmaking Technology magazine.