DOC PREVIEW
MIT 2 008 - Study Guide

This preview shows page 1-2-3 out of 9 pages.

Save
View full document
View full document
Premium Document
Do you want full access? Go Premium and unlock all 9 pages.
Access to all documents
Download any document
Ad free experience
View full document
Premium Document
Do you want full access? Go Premium and unlock all 9 pages.
Access to all documents
Download any document
Ad free experience
View full document
Premium Document
Do you want full access? Go Premium and unlock all 9 pages.
Access to all documents
Download any document
Ad free experience
Premium Document
Do you want full access? Go Premium and unlock all 9 pages.
Access to all documents
Download any document
Ad free experience

Unformatted text preview:

12.008-spring-2004 S. Kim12.008 Design & Manufacturing IISpring 2004Polymer Processing IIIThermoforming2.008-spring-2004 S. Kim 2Design for Manufacturing Moldable: flow path ratio, machine size Draft angle Shrinkage Reinforcements (ribs and bosses) Cycle time Appearance, defects Balance, balance, balance!!2.008-spring-2004 S. Kim 3Gate-Restricts the flow and the direction of molten plastics. -Quickly cools and solidifies to avoid backflow after molten plastics has filled up in the cavity.-Simplifies cutting of a runner and moldings to simple finishing of parts. Side gateSubmarine gate2.008-spring-2004 S. Kim 4Gate PositioningPoint 1: Set a gate position where molten plastics finish filling up in each cavity simultaneously. Same as multiple points gate. Point 2: Basically set a gate position to the thickest area of a part. This can avoid sink marks due to molding (part) shrinkage . Point 3: Set a gate position to an unexposed area of part or where finishing process can be easily done. Point 4: Consider degasing, weldline, molecular orientation.Point5: Fill up molten plastics using the wall surface in order not to generate jetting. Die swell > Thickness, tt2.008-spring-2004 S. Kim 5 2.008-spring-2004 S. Kim 6Molecular orientationGate 1Gate 2CCCCCCCCCCCCCσσ22.008-spring-2004 S. Kim 7DefectsMolding defects are caused by related and complicated reasons as follows:* Malfunctions of molding machine* Inappropriate molding conditions* Bad product and mold design* Improper Selection of molding material2.008-spring-2004 S. Kim 8Sink marks-Equal cooling from the surface-Secondary flow-Collapsed surfaceÆSink Marktstts < t2.008-spring-2004 S. Kim 9WeldlineIt is a boundary between flows caused by incomplete fusion of molten plastics. It often develops around the far edge of the gate.CauseLow temperature of the mold causes incomplete dissolution of the molten plastics.SolutionIncrease injection speed and raise the mold temperature. Increase the melt temperature and increase the injection pressure. Change the gate position to prevent development of weldline at high stress area.2.008-spring-2004 S. Kim 10JettingThis is the phenomenon where the part has a toothpaste flow pattern on the surface.CauseDue to inappropriate gate position, a flow of molten plastics into the cavity is cooled in a line shape and remains unfused with other plastics flow coming later.SolutionChange the gate position to make the molten plastics touch the facing side before making a line shape.Die swell > Thickness, tt2.008-spring-2004 S. Kim 11Die swell Exit zone- die die imparts shape on the material, e.g., rod, tube, sheet, channel exit material is calledextrudate extrudate swells at end of die due to normal forces from the polymer flow, called die swellDie Swell2.008-spring-2004 S. Kim 12Viscousity Shear thinning: paintsShear RateViscosityNewtonianShear ThinningShear Thickening32.008-spring-2004 S. Kim 13Flow markThis is a phenomenon where the initial flow of molten plastics which solidifies mixes with a later flow and remains undissolved. It develops distinctive patterns such as clouds, scales or tree rings.CauseInjection speed is too fast.Mold or molten plastics temperature is too low.SolutionEnlarge the gate area to decrease the speed of the molten plastics flowing through the gate.Increase the pressure retention time for better pressure quality.2.008-spring-2004 S. Kim 14Venting, degassing2.008-spring-2004 S. Kim 15Injection Molded Part Design Base feature + 2ndary feature (ribs, bosses, holes, etc.) Nominal wall : Keep part thickness as thin and uniform as possible. shorten the cycle time, improve dimensional stability, and eliminate surface defects.  For greater stiffness, reduce the spacing between ribs, or add more ribs.  Nominal wall thickness should be within +/- 10%chamfered corners2.008-spring-2004 S. Kim 16Uniform cooling Differential cooling, differences in shrinkage by different thickness is a leading cause of warpage, sinks, and voids.ÆSink Mark2.008-spring-2004 S. Kim 17Draft angle-for removing parts from the mold-1-2o, material, dimension, texture dependent- Cavity side smaller, core side larger.- Crystalline material has more shrinkage. - Amorphous material has smaller shrinkage. 2.008-spring-2004 S. Kim 18Reinforcement Thickness increase Stiffer grade material PP (unfilled), 4,400 psi tensile strength PP (20% glass filled), 7,700 psi Add secondary features, Ribs, bosses42.008-spring-2004 S. Kim 19Rib, Boss Design Parameters.2.008-spring-2004 S. Kim 20 Sink mark, Filling difficulty Ribs should be tapered (drafted) at one degree per side.  The draft will increase the rib thickness from the tip to the root. The typical root thickness ranges from 0.5 to 0.8 times the base thickness. Ribs aligned in the direction of the mold opening. A boss should not be placed next to a parallel wall; instead, offset the boss and use gussets to strengthen it.  Gussets can be used to support bosses that are away from the walls.  Ribs can take the shape of corrugations. (honeycomb)Design rules2.008-spring-2004 S. Kim 21Injection Molding Costs Total cost = Fixed cost + n x Variable cost Unit cost = Total cost/n + Variable cost Variable Cost Cost of resin and additivesAdditives cost, e.g., colorants, fillers, stabilizers, etc.  Material Cost = (resin cost)*(resin fraction) + (additives cost)* (additives fraction) Total Material cost=(part weight +scrap %) x $/lbs Scraps from runners, sprues, and part rejects Labor rate=labor cost ($/hr)/(part cycles x #of cavities) Variable cost=raw material+labor rate2.008-spring-2004 S. Kim 22Fixed cost = Engineering cost+Mold cost +Machine cost+ space Engineering cost: Man-hours X $/hr Space cost Mold costs Type of mold material Machining cost Number of mold sets for the parts needed Machine cost Original cost of machine/depreciation time (linear) Special equipment costs for particular jobs, e.g., special controllers or chillers2.008-spring-2004 S. Kim 23Reaction Injection Molding (RIM)Advantages-As this molding requires lower pressure than regular injection molding, an aluminum or fiber mold can be used. (RRIM)- Molding large sizes and complicated shapes is possible. (near 100% car bumpers)Disadvantages-A copolymerization generates gas, which compresses the air left in the mold and is likely to cause burns.-


View Full Document

MIT 2 008 - Study Guide

Download Study Guide
Our administrator received your request to download this document. We will send you the file to your email shortly.
Loading Unlocking...
Login

Join to view Study Guide and access 3M+ class-specific study document.

or
We will never post anything without your permission.
Don't have an account?
Sign Up

Join to view Study Guide 2 2 and access 3M+ class-specific study document.

or

By creating an account you agree to our Privacy Policy and Terms Of Use

Already a member?