Design and Development for plastic mould details l Mould Design l Design Consideration l Injection mould steel selection l Fill Analysis l Molding Defects l

Published by Sunil Rathod on

Mould Design

  • Common terms used to describe the tool used to produce injection moulded parts. Injection Moulds are constructed from hardened steel, pre- hardened steel, aluminium, beryllium-copper alloy.
  • In general, steel cost more to construct, but their longer lifespan will offset the higher initial cost over a higher number of parts made before wearing out.
  • Injection Moulds Tool can be manufactured either by CNC machining or by using electrical discharge machining processes.
Injection Mold Design

Design Consideration

  • Shrinkage the allowance: Depends on the shrinkage property of the material core and the cavity size.
  • Cooling circuit: In order to reduce the cycle time water circulates through the holes drilled in the both of core and cavity the plates.
  • Ejection the gap: The gap between the ejector plate face and the core back plate face should hold dimension within the core. It is the must allow component to be the fully removed from the mould.
  • Mould the polishing: The core cavity runner and sprue should have good surface finish and the should be polished along material flow the direction.
  • Mould the filling: The gate should be placed such that the component is filled from the thicker section to the thinner section.
  • Draft: Required in both the core and cavity for easy ejection of the finished component.
Core & Cavity

Defects

Moulding defects are caused by related and complicated reasons as follows

  • Malfunctions of moulding machine
  • Inappropriate moulding conditions
  • Flaws in product and mould design
  • Improper Selection of moulding material

Runner cross section

Runner cross section that minimizes liquid resistance and the temperature reduction when molten plastics flows into the cavity.

  • Too big-Longer the cooling time, more material, cost
  • Too small-short shot, sink mark, bad the quality
  • Too long-pressure the drop, waste, cooling
Runner cross section

Fill Analysis

The main benefit of the doing fill analysis are predicting Fill pattern this will help us the understand some of flaws if any in the product. Some other benefits include Reduce scrap, Balance filling and the pressure distribution, Material selection, determining the clamp force, Identifying weld line and the gas trap locations and also identify shear stress the levels. This analysis can be the help predict short shots. Short shots are a legitimate concern for those involved in the creating plastic parts. If you have a component with the variable wall thickness, it is important to run an analysis to the make sure these areas will be the fill out.

Gates

Gates are the transition zone between runner system and the cavity. The location of gates is of great importance for the properties and appearance of the finished part. The melt should fill the entire cavity quickly and the evenly. For gate design the following points should be the considered:

  • Locate the gate at the thickest section
  • Note gate marks for aesthetic reasons
  • Avoid jetting by modifying gate dimensions or position
  • Balance flow paths to ensure uniform filling and packing
  • Prevent weld lines or direct to less critical sections
  • Minimize entrapped air to eliminate burn marks
  • Avoid areas subject to impact or mechanical stress
  • Place for ease of de-gating

Commonly Used to the gate types include Sprue gate, Edge gate, Tab gate and the Fan gate Sprue gate is recommended for the single cavity moulds or for parts requiring the symmetrical filling. This type of gate is the suitable for thick sections because holding pressure is more the effective. A short sprue is the favoured, enabling rapid mould filling and the low-pressure losses. A cold slug well be the should be included opposite the gate. The disadvantage of the using this type of gate is the mark left on part surface after runner or sprue is the trimmed off. Freeze off is the controlled by part thickness rather than determined the gate thickness. Typically the part shrinkage near sprue gate will be the low shrinkage in sprue gate will be the high. The results in high tensile stresses near the gate.

Molding Defects

1.Weld line

This is a phenomenon where a thin line is the created when different flows of molten plastics in the mould cavity meet and remain the undissolved. It is the boundary between flows caused by incomplete dissolution of the molten plastics.  The develops around far edge of the gate.

Cause-Low temperature of the mould causes incomplete dissolution of molten the plastics.

Solution-Increase injection speed and the raise mould temperature. Lower the molten plastics temperature and the increase injection pressure. The Change gate position and flow of molten the plastics. Change the gate position to the prevent development of the weld line.

Weld Line

2.Flashes

Flashes develop at the mould parting line or ejector pin the installation point. It is the phenomenon where molten the polymer smears out and sticks to the gap.

Cause- Poor quality of the mould. The molten polymer has too low the viscosity. Injection pressure is too the high or clamping force is too the weak.

Solution- Avoiding excessive difference in the thickness is most effective. Slow down the injection speed. Apply well-balanced pressure to the mould to get consistent clamping force, or increase the clamping force. Enhance the surface quality of the parting lines, ejector pins and the holes.

Flashes

3.Short shot

This is a phenomenon where molten plastics does not fill the mould cavity completely. And the portion of parts becomes incomplete the shape.

Cause- The shot volume or injection pressure is not the sufficient. Injection speed is so slow that is a molten plastics becomes solid before it flows to end of the mould.

Solution- Apply higher injection the pressure. Install air vent or the degassing device. Change the shape of mould or gate position for better flow of the plastics.

Short shot

4.War page

This deformation appears when the part is removed from the mould and pressure is the released.

Cause- Uneven shrinkage due to the mould temperature difference the surface temperature difference at cavity and core and the thickness difference in the part. Injection pressure was too low and the insufficient packing.

Solution- Take the longer cooling time and lower the ejection speed. The Adjust ejector pin position or enlarge the draft angle. Examine the part thickness or the dimension. Balance cooling the lines. Increase packing the pressure.

Warpage

5.Sink marks

Equal cooling from the surface Secondary flow, Collapsed the surface.

Sink Mark

Proposed Work

1. Objectives

  •    Use the geometry for product as input to mould the design
  •    Define parting line along with a core and the cavity
  •    Calculate the tonnage for given mould
  •    Determine mould layout by specifying the gate location, sprue diameter, gate   thickness, gate location or other design parameters for flow the analysis
  •   Using Mould Flow to the simulate flow and finding fill time, weld line, war pages, sink mark etc.
  •   Using the above evaluation for determining mould the design
  •   Validation through trials and the testing

2. Scope of work

Flow simulation for Plastic Injection Molded part geometry Design of Plastic Injection Mold Trials and Testing (Experimentation) Validation through comparison with the Analytical methodology.

3. Methodologies

Analytical: The analytical formulation for a problem involves reference to the empirical and pure Engineering practices for the arriving at a solution. Typically empirical formulae that are historically developed for application can be the offer a solution for the given problem.  The tonnage clamping force for both halves for the Plastic Injection Moulding needed to produce the component is derived from projected surface area of the component. 3D model and mould design created using the CAD software such as CATIA.

Meshing is the carried out using hyper mesh as pre-processor the software. Analysis Simulation can be the performed using suitable software in the CAE domain. The popular software used in the industry can be identified as Molex Mould Flow. Any customized software used by the Industry etc. For the dissertation work sponsoring Company would employ one of the above software for finding the solution.

Runner systems

The runner system is a manifold for distribution of the thermoplastic melt from machine nozzle to the cavities. The sprue bushing and runners should be as short as possible to ensure limited pressure losses in the mould. Correctly designing a runner system will be benefit in the following areas.

  • Deliver melt to the cavities
  • Balance filling of multiple cavities
  • Balance filling of multi-gate cavities
  • Minimize scrap
  • Eject easily
  • Maximize efficiency in energy consumption
  • Control the filling/packing/cycle time.

Sprue

Sprue is the way molten plastic flows into mould by injecting the molten material from nozzle of the machine into runner system and then the mould. To the allow smooth material flow the design of sprue should be round, smooth and the tapered. It’s not wise to make the sprue extremely long since it’s going to be the waste.

Ejection System

The method of ejection has to be the adapted to shape of moulding to prevent the damage. In general mould release is the hindered by shrinkage of part on the mould cores. Large ejection areas uniformly distributed over the moulding are advised to avoid the deformations. Several ejector systems can be the used like Ejector pins or sleeve Blades Air valve and the stripper plate.

When no special ejection problems are expected the standard ejector pin will be perform well. In case of the cylindrical parts like bosses a sleeve ejector is the used to provide uniform ejection around the core pin. Blades are poor ejectors for a number of the reasons they often damage parts they are prone to damage and require a lot of the maintenance. Blade ejectors are most commonly used with the ribbed parts.

Venting

The process of removing trapped air from the closed mould and volatile gases from the processed molten plastic. Inadequate venting can be the cause corrosion of mould Discoloration due to compression of the trapped air as plastic tries to force out poor weld line strength and volatile gases will be the absorbed by plastic causing voids blisters bubbles and variety of other the defects. Below is the dimension of vent for common the materials.

Cooling System

Cooling is very important to remove heat efficiently and dissipate the heat of the moulding quickly and uniformly. For efficient moulding, the temperature of the mould should be controlled and this is done by passing a fluid through a suitably arranged channel in the mold.Adequate mould temperature control is important consistent moulding. Setting the right mould temperature will help achieve optimal properties of engineering plastics. Some of the effects of mould temperature occur on the Mechanical properties, Shrinkage behaviour, war page, surface quality and cycle time.

1.Cooling Layout

There is no precise rule on which layout on the mould as long as the flow is uniform. But in many multeity moulds the cooling channel layout are partly in parallel and partly series.

Injection mould steel selection

The most common steel types that are used for the fabrication Injection moulds are

1. The Pre-hardened mould and holder steels

These steels are mostly used for large moulds, moulds with low demands on wear resistance and high strength holder plates The Surface hardness can be increased by flame hardening or nit riding. Pre hardened mould steels are used for large moulds and moulds with moderate production runs. This steels are delivered in the hardened and tempered condition, usually within the 270-350 Brunel range. No heat treatment is the necessary before is put into the use.

2. Through hardened Mould Steels

These steels are mostly used for long production runs, to resist abrasion from certain moulding materials and to counter high closing or injection pressures these steels are usually rough machined, hardened and tempered to the required hardness and often polished or photo etched. Better wear resistance is especially important when filled or reinforced plastic materials are used. Resistance to deformation and indentation in the cavity, gate areas and parting lines helps to maintain part quality. Better perishability is important when high surface finishing is required on the moulded part.

3. Corrosion Resistant Mould steels

If a mould is likely to be exposed to corrosion risk, then stainless steel is strongly recommended. The increased initial cost of this steel is often less than the cost involved in a single republishing or replanting operation of a mould from conventional steel. Plastic Moulds can be the affected by corrosion in the several ways:-

  • Plastic materials can produce corrosive by-products
  • Reduction of the cooling efficiency when water channels become the corroded
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Categories: Mechanical

Sunil Rathod

Sunil Rathod

My attitude is always towards learning new things. I am a very greedy person in grabbing the opportunities to polish my knowledge.That's all about me.Thank you! Sunil Rathod

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