Design Process Guidelines for injection molding[after Douglas M. Bryce, Plastic injection molding : manufacturing process fundamentals, Society of Manufacturing Engineers, 1996] Parameter Change versus Property Effect
There are a number if different approaches that can be taken when developing a new product. Historically, new parts or products were developed using a Sequential Engineering approach to design, which begins with a new product idea generated by marketing groups and ends up with the manufacturing stage. The problem with this approach is that it may not result in an optimum design, and will no doubt take more time and money than would be required using a more Concurrent Engineering approach.
The concept of concurrent engineering can be illustrated by the following figure. This "Parallel" or "Concurrent Engineering" approach to product design reduces development time, improves quality, and minimizes the potential for unanticipated production or performance problems.
2. General Guidelines for injection
molding[Douglas M. Bryce]
(a). Determine required clamp force
clamp force=projected area * injection pressure
The projected area of the part can be found based on the geometry of the part. The thickness is important only if it is more than 1 inch. For every inch of thickness over 1 inch, the total clamp force must be increased by 10 percent.
The injection pressure must vary with the flow ability of the material. The typical value for injection pressure is between 2 to 8 tons per square inch (or 28,000~110,000 KPa). As a rule of thumb, 4 or 5 tons/inch^2 (55,000~69,000 KPa) can be used for most products. For example, if polycarbonate has been selected, then the injection pressure could be 5 tons per square inch.
Too low a clamping force can lead to flash, or non-filled parts; too high a clamping force can lead to mold damage.
(b). Determine the injection molding cost for a specific product
For calculating actual manufacturing cost, the following information is needed:
1. Material cost.
The material cost can be determined using the following three-step formula:
(1). Determine the total volume of the part (inch3).total volume = volume of the runner system + volume of the part.
(2). Determine the weight per unit volume (lb/inch3).
(3). Determine the cost per unit weight.
cost= (cost / lb) * (lb / inch3) * (inch3)
2. Machine cost.
The total machine cost is determined by the machine hourly rate (the hourly cost of machine and operator) and overall cycle time (so-called gate-to-gate cycle time).
The dominant effect in determining cycle time is the time it takes to cool the part, and cooling time depends on wall thickness. The cooling time for a plate-like part of thickness, h, can be estimated using this formula: [Tim .A. Osswald]
Td-average part temperature at ejection
The following table provides some information about the wall thickness and corresponding overall cycle time.
Wall thickness, in. (mm) Overall cycle time, seconds 0.060(1.5) 18 0.075(1.9) 22 0.100(2.5) 28 0.125(3.2) 36
Wall thikness, the melt temperature, and mold wall temperature as well as the final part temperature when it is ejected all have effects on the cooling time. The melt temperature is usually available from the manufacturer. The suggested melt temperature and mold wall temperature are listed in the tables below.
Material Melt Temperature(F) Acetal (copolymer) 400 Acetal (homopolymer) 425 Acrylic 425 ABS (medium-impact) 400 Cellulose acetate 385 Nylon (Type 6) 500 Polyallomer 485 Polyamide-imide 650 Polyarylate 700 Polybutylene 475 Polycarbonate 550 Polyethylene (Low density) 325 Polyethylene(High density) 400 Polypropylene 350 Polystyrene (general purpose) 350 Polystyrene (medium-impact) 380 Polystyrene (high-impact) 390 PVC (rigid) 350 PVC (flexible) 325
Material Mold Temperature(F) Acetal (copolymer) 200 Acetal (homopolymer) 210 Acrylic 180 ABS (medium-impact) 180 Cellulose acetate 150 Nylon (Type 6) 200 Polyallomer 200 Polyamide-imide 400 Polyarylate 275 Polybutylene 200 Polycarbonate 220 Polyethylene (Low density) 80 Polyethylene(High density) 110 Polypropylene 120 Polystyrene (general purpose) 140 Polystyrene (medium-impact) 160 Polystyrene (high-impact) 180 PVC (rigid) 140 PVC (flexible) 80
(c). Estimate the gate-to-gate cycle time
The following table shows the typical time estimates. The actual cycle time is less than the sum of these values, because there are overlaps between some operations. As shown, the cooling time is the most important part.
|Gate closing time||
|Mold closing time||
|Initial injection time||
|Injection hold time||
|Screw return time||
|Mold open time||
|Part removal time||
(d). Determine the wall thickness
(1). Design guidelines:
- All walls should be equal thickness if possible.
- If a thicker wall is needed, a gentle transition should be specified.
- To avoid sink marks, ribs should be 2/3 the wall thickness, gussets should be 1/2 the wall thickness.
- Sharp corners should be eliminated by using radii.
(2). The wall thickness is mainly determined by the flow ability of the plastic. The ability to flow also determines how far a plastic can be injected for a specific wall thickness of product.
The approximate maximum flow-path-to-thickness ratio of some common thermoplastics are listed here:
|Polyvinyl Chloride, rigid||100:1|
The following table is a listing of common materials and the wall thickness they can flow through.
This table shows that an easy-flow material (such as crystalline nylon) allows thinner wall.
|Material||Minimum (in./mm)||Maximum (in./mm)|
(e). Determine the runner system
(1). General guidelines for runner and gate design:
- The runner cross section diameter also depends upon the type of plastic being molded. High-viscosity(very stiff) materials require larger-diameter runners than low-viscosity materials.
- The longer the flow path the plastic must travel along, the larger the runner diameter must be at the start.
- Right-angled turn in a runner system requires an additional 20% increase in the diameter to compensate for pressure drops.
- A part should be gated into its thickest section, from thick to thin, never the reverse.
- Cavity sets should be located as close to the sprue as possible to minimize travel time and distance.
(2). Runner diameters for some common materials
|Cellulose acetate butyrate||0.093/2.4||0.109/2.8||0.125/3.1|
3. Parameter Change versus Property Effect [Douglas M. Bryce]The purpose for hot runner system is to reduce the overall cycle times. The advantage of the hot runner system is that the runner does not have to be included in the calculation of cycle times. The cooling portion of the molding cycle only applies to the molded part and the overall cycle can be much shorter than if runners were included.
What is the best setting for the injection pressure, back pressure, melt temperature and mold temperature, etc.?
-It all depends on the material being molded and the type of mold being used, as well as the status of the injection machine and environmental conditions. Generally, the effect of parameters on the product properties would be:
Parameter Property Effect Injection Pressure(+) Less shrinkage, higher gloss, less warp, harder to eject Injection Pressure(-) More shrinkage, less gloss, more warp, easier to eject Back Pressure(+) Higher density,more degradation, fewer voids Back Pressure(-) Lower density, less degradation, more voids Melt Temperature(+) Faster flow, more degradation, more brittle, flashing Melt Temperature(-) Slower flow, less degradation, less brittle, less flashing Mold Temperature(+) Longer cycle, higher gloss, less warp, less shrinkage Mold Temperature(-) Faster cycle, lower gloss, greater warp, higher shrinkage
- C-MOLD Reference Manual, CAE, Ithaca, New York.
- Tim A. Osswald, Polymer Processing Fundamentals, Hanser, Munich, 1998.
- Robert A. Malloy, Plastic Part Design for Injection Molding, Hanser, Munich, 1994.
- Douglas M. Bryce, Plastic Injection Molding: Manufacturing Process Fundamentals, Society of Manufacturing Engineers, Dearborn, 1996.
- Douglas M. Bryce, Plastic Injection Molding: Material Selection And Product Design Fundamentals, Society of Manufacturing Engineers, Dearborn, 1997.
- Douglas M. Bryce, Plastic Injection Molding: Mold Design and Construction Fundamentals, Society of Manufacturing Engineers, Dearborn, 1998.
- Dominick V. Rosato, Donald V. Rosato and M. G. Rosato, Injection Molding Handbook, Kluwer Academic Publishers, Amsterdam, 2000