Presentation on theme: "Mold Requirements Chapter 3 Professor Joseph Greene All rights reserved Copyright 2000."— Presentation transcript:
Mold Requirements Chapter 3 Professor Joseph Greene All rights reserved Copyright 2000
Mold Requirements Accuracy and Finish Productivity Physical Strength Wear Resistance Safety in Operation Maintenance and Interchangeability Ease of Installation Reasonable Mold Cost
Accuracy and Finish Requirement of mold- Parts meets customers expectation –Dimensional accuracy, tolerances, surface appearance, cost Accuracy –Control shrinkage Low shrink factor <0.6%: PS, PVC, ABS, Acrylic, PC, Glass-filled plastic High shrink factor >0.6%: PP, HDPE, LDPE, Acetal, Nylon 6/6 –Tolerances must be reasonable and achievable Finish –Surface appearance for part is on drawing. –Core is usually rougher than cavity to insure demold on core side –Texturing: Grained surface Real texturing with acid etching. Very expensive Texturing with EDM: Controlled with different current densities. Fairly expensive Sand Blasting: Cheapest method. Requires some masking to protect surfaces.
Productivity Productivity depends upon –Number of cavities; Quality of cooling; –Speed and timing of ejection; Strength and durability of the mold –Ease of installation and start-up Number of cavities –Mold cost is determined by cost of mold/total number of parts Example, –Container mold is built for $50,000 and is expected to produce 5 million parts. Machine costs are $100 per hour –Cycle time is 10 seconds per part in single cavity tool. »Mold cost is $50,000/5,000,000 = $0.0100 per part –Dual cavity cost $80K »Mold cost is $80,000/5,000,000= $0.0160 per part »Mold cost is $30,000 more expensive than single cavity mold –Machine cost is cost per number of parts per hour »Single = 1 part per 10 seconds or 360 parts per hour = $100/360 = $0.2778 per part »Dual cavity = 2 parts per 10 seconds or 720 parts per hour = $100/720 = $0.1289 per part. »Savings = $0.1289 * 5 million = $644,500 over life of tool
Productivity Multicavity molds require fewer molding machines and thus less floor space. Cycle time can increase with multicavities –Recovery time: time required for the extruder to prepare the plastic for the next cycle. More cavities = more time is required. Especially, for large shots –Injection time: time required to inject resin Volume of resin required is higher though not exceed machine rating –Pressure drop More cavities = longer flow length = higher pressure Other plant operations related to multicavities –Mold stoppages due to sloppy molding conditions –Housekeeping is needed for preventative maintenance –Power supply disruptions can cause mold stoppage
Productivity Quality of Cooling –Need for efficient heat transfer to remove heat from plastic and keep cooling time low and cycle time low. Speed and Timing of Ejection –Ejection time is lost time, similar to mold open and close –Try to eject during mold opening time Free fall ejection requires more stroke and time than Husky guide shutes, which are more expensive Strength and durability of mold –Productivity depends upon the quality of the mold Quality of the part starts with the quality of the mold Properly maintained and built mold should last 2 to 3 million parts (less with glass filled materials)
Ease of Installation and Start-up Some molds are in a machine for several months (years) Some molds are in a machine for a couple of days (weeks) –Productivity depends on how quick the molds can be changed –Use quick die change methods for standardization of mold components and attachment methods. Efficiency = (number of good pieces produced/number of pieces in total time) x 100 Efficiency = (Hours producing good pieces/hours machine is reserved) x 100 Efficiency = parts /8-hour shift Problem occurs if you take too many hours setting a mold. Best if tool change takes less than 30 minutes Idle machine time is very unproductive and should be minimized. Machine uptime should be > 75% Companies spend Millions of dollars reducing set-up time for mold change
Ease of Installation and Start-up –Example, Mold is to run for 40 hours, installation time is 6 hours.Includes –Mounting mold in the machine –Making all connections –Heating the mold –Running the mold to get on cycle fully automatic –Shutting down and disconnecting mold. Assuming all of the pieces are acceptable. Efficiency is: –E= (40 hours - 6hours)/40hours x 100 = 85% Assuming 90% of the pieces are acceptable. Efficiency is –E= (40 hours - 6hours)/40hours x 100 x 0.90 = 76.5% –Example 2, Reduce the installation time to 3 hours. Efficiency is: –E= (40 hours - 3hours)/40hours x 100 = 92.5% –Example 3, If production run is longer (2 weeks), then efficiency improves –E= (240 hours - 3hours)/240hours x 100 = 98.75%
Ease of Installation and Start-up Physical Strength –Mold strength depends on Mold material; Design of mold components; Allowing for fatigue strength Avoidance of stress risers; Proper heat treatment; Suitable direction of grain in steel; Direction in which mold parts are ground Important considerations for mold design –Tensile Strength Mold must be strong enough to withstand internal pressures during injection, where pressures can exceed 20,000 psi. –Compressive strength Mold must be strong enough to withstand compressive forces on core during injection, especially in areas with too thin cooling line walls. –Plate deflection Molds are often too weak in backing plates and ejector plates.
Important Considerations for Mold Design Tensile Strength Mold must be strong enough to withstand internal pressures during injection –Especially, with molds with large surface areas and deep sections that are under high internal pressures at right angles to the opening direction of the mold. Internal stresses can cause the following problems –Bursting strength: If the walls are too weak, the cavity will deform (stretch outward) permanently, or even burst. –Stretching (outwardly): All tool walls will stretch due to internal forces. No failure will occur as long as the stresses are less than the yield strength of the material. The cavity will return to the original state after pressure is released. –Problems can occur if: »the cavity walls are too thin,draft angles are < 3 degrees, and »wall thickness of the product is less than 1 mm –If mold opening force of the machine is not great enough to open the mold after the part has cooled, the part becomes trapped in the mold. The mold would have to be pried open causing damage to the mold, sometimes destroyed. –Fatigue is a potential problem for all molds. When mold is subjected to 2 million stress cycles without failure, it will last forever. –Safety factor of 10 is used to design for mold thickness. Use 10% of yield strength. For P-20 steel use a factor of 5 or 20% of yield strength.
Important Considerations for Mold Design Compressive Strength –Injection pressures cause compressive stresses on the core, but do not generally cause problems EXCEPT when cooling channels or screw holes in the core are too close and can fail due to fatigue. –Clamping pressure on parting line can lead to early mold failure. Clamp pressure is total clamping force per surface area Hint: Try and mold with minimum clamping force Constant pounding of excessive clamp force can cause mold damage –Seen after a few weeks as small depressions in the parting plane of the mold. –Small vents can be closed causing vent problems is mold Plate Deflection –Molds are weak at the backing plates and ejector plates Plates are usually too thin or of a weaker material than needed. Deflection can be serious, especially with multicavities. Weak backing plates will cause core shift in the mold due to deflection in the center as the mold is clamped. Max stresses on plate should not exceed 20% of the yield strength of the plate steel.
Wear Resistance Wear of Mold Parts Moving under Pressure –Materials selection: Moving parts must have different grain and surface structure so that the grains of one piece will not interlock with the grains of other Select different steels to solve problem. NOT same steel with different hardness!! –Surface Finish: Special surface finish can avoid this problem, e.g. nitriding –Wear strips: Non-ferrous materials (bronze, plastics) can be used if pressures are low enough. –Surface conditions: Taper fits are usually ground surfaces. CAUTION- If grinding is done to create grooves at right angles to the motion, then it will act as a file and wear down the surface of matching part. –Lubrication: Can be used on sliding or rubbing surfaces provided Lubrication can easily be maintained during operation, either manually or tied in with the automatic machine lubrication. No danger that the lubricant contacts and contaminates the molded products. Fretting –Surface wear caused by moving two parts repeatedly over a short distance back and forth against each other. Could be fatigue of the metal surface –Reduced by design changes and not lubrication
Wear and Rust Wear from Abrasive and Corrosive Plastics –Abrasive wear can be controlled by materials selection Especially in the gate region for glass filled plastics –Typically the gate would be insert molded for easy replacement –Corrosive wear can be controlled by materials selection Stainless steel is frequently used Chrome plating is used though it is less preferable than stainless due to cost Rust –Rust is oxidizing of steel. –Rust can affect mold surfaces externally and internally –Molds must be properly maintained After each production run stoppage (weekends, holidays, storage), mold preserver needs to be applied to mold surface. –Stainless steel is an expensive option
Safety in Operation Mold must be constructed so as to: –Prevent damage to the mold itself –Prevent injury to persons operating the mold Damage to the mold –Slide cores or slides Fail to maintain position when actuating cam is disengaged Compression springs should help. Safest method is to have cams never leave engagement with slide –Return pins Must be provided when using ejector pins or sleeves to prevent a sticking ejector plate from causing ejector pin to damage cavity –Ejector pins Can collide with side cores if the pin is placed right under a hub in the product where a side core must come in. e.g., mold a hole. Motion of mold designed such that the side core is sufficiently withdrawn before ejector pin is allow to pass without collision.
Safety in Operation Damage to the mold –Returning stripper plate Should not be done with closing the mold. Use and independent method, ie, springs or air cylinders to return ejector or stripper plate. –Latches, links, and pivots in ejector plate Must be strong enough for the forces required and accurately aligned –Misalignment or vibration Can cause early fatigue failure Personal Injury –Mold designer must look for what is possible or foreseeable –Areas of risk Lack of communication (manuals, nameplates), Handling mold on hoist, Suspension for the mold in the machine, External springs breaking,
Safety in Operation Personal Injury –Areas of risk Lack of communication (manuals, nameplates), Handling mold on hoist, Suspension for the mold in the machine, External springs breaking, Hot plastic escaping, Compressed air escaping, Hydraulic pressure oil escaping, Screws snapping and heads flying off, Sharp edges, Heated exposed mold parts, Unprotected electric power distribution, Moving parts outside standard safety gate area, Motion which needs adjustment with safety gates open, Possible access to the open mold with gates closed
Maintenance and Interchangeability Maintenance –Mold be constructed so that maintenance is easily performed Cleaning of hot runner gates and the dirt Replacement of nozzle heaters Readjustment of taper fits Flashing Interchangeability –Hardware items are usually interchangeable heaters, nozzles, hozes, etc. screws and fastening devices. USE only 12.9 screws. Spare parts, e.g., multicavity molds.
Ease of Installation in Molding Machine Mold must fit the machine. Critical parameters are –Platen size: mold should cover at least 50% –Platen area (tie bar distance): Molds should fit between tie bars and not pull bars. –Tonnage: Rated machine for mold. Shot size rule is Min= 20% of total machine rating; Max = 80% of machine shot rating. Also, 1 to 3 tonnes per square inch of projected area. –Ejector hole pattern: Should fit and be standardized –Mounting hole patter: should be standardized –Force of hydraulic machine ejector: See machine specs –Injection and plasticizing capacity: Use 20%-80% rule –Quick mold change features: Should be used –Clearance to remove products: demold parts easily –Clearance for service: water lines clearance.
Reasonable Mold Cost Cost components –Mold design engineering –Production engineering- Sequence of machines –Each stack –Mold shoe –Assembling –Testing –Overhead –Profit Cost of multicavity Cost of stack molds Cost of piece –Tool cost + production cost +overhead + profit + shipping