What factors are affecting your injection molding cycle time?

Time:2022-09-01 08:35:14 / Popularity: / Source:

What factors are affecting your injection molding cycle time 
Mold design requires a certain number of cooling circuits to achieve cooling purposes. A cooling loop is a combination of individual components between inlet and outlet manifolds on a temperature controller (thermolator), cooling tower or cooler.
So circuit will include fittings on manifold, fittings on hoses, hoses themselves, fittings on mold and cooling channels in mold, and components in cooling channels like baffles, hot pins, 90° elbows, etc. . In order to minimize cycle time, it is necessary to ensure maximum efficiency of entire cooling circuit and coolant flowing through it.
Check for correct cooling design
The first thing I recommend doing is to check that cooling design is set up correctly. Check for anything that might restrict flow of coolant in and out of mold or increase pressure required for coolant.
Are ball valves in manifold correct size? Is inner diameter of tubing sufficient? Is line too long? Are there sharp bends in pipeline? (Line has a minimum bend radius rating) Does fitting on line have an automatic shut-off valve? Most suppliers state that pressure and flow losses are "minimal" with automatic shut-off valves. But graph below shows that this is not case.
What factors are affecting your injection molding cycle time 
Flow of water through any component such as a hose, fitting or waterway depends on inside diameter, the overall length of line and pressure applied to coolant. Therefore, there is a need for an efficient circuit with the largest possible pipe diameter and shortest pipe length to minimize required pressure.
Contamination build-up in cooling channels will prolong cycle time
If you received an old mold with a long cooling line, it is recommended that you dismantle entire cooling water circuit and check cooling channel for impurities such as scale, rust, calcium oxide, lime, etc., as shown in the figure below.
What factors are affecting your injection molding cycle time 
These deposits thermally insulate the cooling channel walls and significantly reduce thermal conductivity of lines, reducing cooling efficiency and increasing cycle times.
Check connector bore
After removing fitting, compare inside diameter of fitting to inside diameter of hole in mold. Hole in mold should be at least equal to diameter of fitting, but usually needs to be larger. This is because cooling water piping is usually same size as that used for NPT fittings shown below.
What factors are affecting your injection molding cycle time 
However, due to part geometry, ejector pin location constraints, etc., mold designers often use smaller diameter holes. Don't assume mold maker used right size fittings, the smaller area the easier it is to overlook.
If a smaller ID management is used, pressure drop of cooling water can become quite large, especially if mold temperature controller uses a centrifugal pump. As pressure increases, pumping volume of centrifugal pump decreases rapidly, while positive displacement pump can maintain a relatively stable flow.
An increase in demand pressure can eventually exceed pump's maximum capacity (pressure and flow), which can result in longer cycle times or direct waste, as cooling effect is limited by cooling water pressure. Make sure that water flow in each loop is turbulent, not laminar (as shown in diagram below), and that temperature difference, or ΔT, between inlet and outlet of each loop is 4°F or less.
In laminar flow state, coolant flows in layers. As it passes through mold and begins to absorb heat from cavity, temperature of coolant layer near die steel increases. Since heat transfer rate is proportional to temperature difference between steel and coolant, an increase in temperature of coolant layer causes heat transfer rate to decrease. Variations in heat transfer rates can cause variations in part quality and affect product quality consistency.
In turbulent flow, coolant molecules do not move in layers, but are continuously mixed in flow channel, absorbing heat in die steel and distributing it evenly in coolant. This helps coolant maintain its temperature, so heat transfer rate remains same.
What factors are affecting your injection molding cycle time 
Difference between laminar and turbulent flow
To check water in each circuit for turbulence, at least one flow meter is required. It is recommended to buy two flow meters with built-in temperature and pressure gauges. This will save you from having to use a thermometer to check inlet-outlet temperature difference.
Measuring pressure loss through cooling circuit is not a critical requirement for scientific cooling. If a high pressure loss is measured, it could be that lines are fouled or flow to baffles or bubblers are not sized correctly, etc.
What should be flow rate of cooling channels in mold? It depends on flow area, shape of flow surface, coolant temperature, whether coolant contains glycol or not, and desired Reynolds coefficient. Reynolds number is a dimensionless group used to determine flow state of viscous fluids.
In 1883, Englishman Reynolds (O. Reynolds) observed flow of fluid in circular tube. He first pointed out that flow shape of fluid is not only related to flow velocity (ω), but also related to pipe diameter (d), viscosity of fluid (μ), density (ρ) of fluid is related to three factors.
Turbulence is a flow state of a fluid. When flow velocity is very small, fluid flows in layers and does not mix with each other, which is called laminar flow, also known as steady flow or sheet flow; gradually increasing flow velocity, streamline of fluid begins to wavy swing, frequency and amplitude of swing vary with flow velocity. This flow condition is called transition flow; when flow velocity increases to a great extent, streamlines are no longer clearly discernible, there are many small eddies in flow field, laminar flow is destroyed, adjacent flow layers not only slide, but also mix, forming turbulent flow, also known as turbulent flow, turbulent flow or turbulent flow. It is generally believed that for injection molding applications, when Reynolds number is greater than 5000, a turbulent state can be achieved, namely:
What factors are affecting your injection molding cycle time 
Once you have determined flow required for each loop, you need to add them all to see if your chiller, cooling tower or temperature control unit (TCU) has enough output (gallons per minute, gpm) to meet flow requirements for mold cooling.
water flowing down
If you have cooling channels with different inner diameters and flow areas in mold, and you don't have all loops connected in series, you may run into problems. Piping systems require a lot of pressure to push enough water through to get turbulent flow.
Problem is that water always follows path of least resistance, which is why water must be forced where it is needed most. Assuming that there are both 1/8 and 1/4 pipes in pipeline, most of water will pass through 1/4 pipe, because the larger pipe, the lower pressure loss. Problem is exacerbated if mold has baffles or bubblers in circuit, and a positive displacement pump is generally required to solve this problem.
To maximize heat transfer between part and steel mold, part needs to be adequately fed. When parts are under-fed, they shrink, creating gaps between surface of product and surface of die steel, reducing effect of heat transfer. This is especially prevalent in thick-walled parts, which is why deep gates are required. While deep gates increase gate sealing time, it reduces more important overall cooling time and reduces cycle time.
Most injection molding plants do not have air conditioning in their production halls. When dew point is high in summer months, mold often "sweats" - water droplets form on parting line, and these droplets appear on part. But what does this have to do with mold cooling? Many injection molding plants will use higher-than-recommended water temperatures in their molds to prevent mold sweating and thus extend cycle times.
Management may not want to spend a fortune to install air conditioners in production areas, which would also increase their monthly electricity bills. What they don't realize is that temperature of production facility doesn't need to be a comfortable 25 degrees, it just needs to be cold enough to lower dew point so that cycle times are at their lowest level all year round.

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