Injection Molding Overview Introduction Aim The aim of

Injection Molding Overview

Introduction § Aim – The aim of this chapter is to review § The injection molding process § Flow behavior of thermoplastics in injection molds § Why do it – Understanding injection molding and flow behavior is critical for proper use of MPI 2

Injection Molding Machine Hopper Tool Barrel Screw (Ram) 3 Hydraulic Unit

Injection Molding Terminology Mold Components Polymer Entrance Point Sprue Primary Runner Part Cavity Gate Cold Slug Well 4 Secondary Runner

Injection Molding Process § Filling – Mold closes – Screw forward – Frozen polymer skin forms at mold walls § Packing Time – Cavity filled – Pressure applied to polymer – Cooling occurs – Gate freezes 5 Hopper Barrel Screw Mold Screw is applying a specified pressure to the polymer melt in order to pack more plastic into the cavity.

Injection Molding Process § Cooling – Part continues to cool until rigid enough to withstand ejection – Screw moves back plasticating resin for next shot § Mold Open – Part is ejected 6

Injection Molding Cycle Time: Fill Time: Pack Time: Cooling Time: Mold Open Time: 7 22 Sec. 1 9 10 2

The Injection Mold Stationary Half 8 Moving Half

Injection Pressure § Pressure is required to push the plastic into the § mold cavity Limited by machine capability – Hydraulic pump limitations – Usually around 140 – 180 MPa – Modern machines can go up to 300 MPa 9

Variables Affecting Injection Pressure § § Part Design Mold Design Processing Conditions Material Selection § Each area is affected by other areas § Some are easily changed, while others are not 10

Pressure - Drives Flow § Flow is driven by pressure – Overcomes the melt's resistance to flow § Plastics flow from high to low pressure areas § Pressure decreases along the flow length Advancing Pressure Flow Front 11 Gate Sprue Runner Part Flow Length

Part Design Affecting Injection Pressure § Part Thickness Thin Part § Surface Area More surface area to be cooled Higher Pressure Thick Part Less surface area to be cooled Lower Pressure 12 Lower Pressure

Mold Design Affecting Injection Pressure § Gate size § Flow length (gate location) Restrictive Gate Higher Pressure Long Flow Length Higher Pressure Generous Gate Lower Pressure Short Flow Length Lower Pressure 13

Processing Conditions Affecting Injection Pressure § Fill time Injection Pressure [MPa] Injection Pressure vs Time Optimum time Range Injection time [Sec. ] 14

Processing Conditions Affecting Injection Pressure § Melt temperature 15 § Mold temperature Colder Melt Higher Pressure Colder Coolant Temperature Higher Pressure Hotter Melt Lower Pressure Hotter Coolant Temperature Lower Pressure

Material Selection Affecting Injection Pressure § Different grades of the same material can have widely different pressure requirements 16

Material Selection Affecting Injection Pressure § Material selection affects injection pressure – Different materials have different required pressures § Resin Flow Properties – Low melt index g/10 min = higher pressure – High melt index g/10 min = lower pressure 17

Factors Affecting Injection Pressure Requires Higher Injection Pressure Requires Lower Injection Pressure 18

Flow Behavior What Does a Plastic Molecule Do in an Injection Mold?

Phases of Molding § Filling – Volumetrically fill the cavity § Pressurization – Build up pressure in the cavity § Compensation – Add extra material to reduce shrinkage Filling Phase Pressurization Phase Compensation Phase 20

Fountain Flow § § 21 Fastest flow rate is in the center of the cross section First material in forms frozen skin by the gate Melt Last material in is the center of the cross-section Has direct influence on molecular and fiber orientation

Cross-Sectional Flow & Molecular Orientation § Molecular orientation is caused by shear flow § The highest amount of shear is inside the frozen layer – Produces the highest orientation Shear rate - min 22 max

Cross-Sectional Flow § 3 D Flow analysis – Left rib is filled – Right rib is the only thing in the part not filled yet Scaled so velocities higher than 50 are red 23

Cross-Sectional Heat Transfer § Should be a balance between – Heat input from shear – Heat loss to the tool Hot Plastic Melt High Shear Rate Plastic Flow Heat Input Cold Mold Heat Loss into the Tool Frozen Layer Slower Injection Rate 24 VS. Faster Injection Rate

Pressure and Temperature vs Time § Pressure will always be a “U” shaped curve § Temperature will always fall with as injection time increases § Optimum molding window has flow front temp near melt temp 25

Specific Volume –pv. T Diagram § Displays relationship of a range processing melt temperatures and pressures over the specific volume 26

Shrinkage § Normally – Unfilled materials, shrink most in flow direction – Filled materials, shrink most perpendicular to flow direction Same material and processing for both parts. Top part not considering glass fibers. Bottom part calculated with fiber orientation. 27 Parallel Perpendicular