3.1 the project. 5.1.2 DESIGN Injection mould

 

3.1    
METHODOLOGY

Upon receiving a request for a quote, at SMIIEL, I proposed
technical ideas to create a suitable part. This shows our attitude towards
manufacturing. The quality of plastic part is greatly affected by the mould, so
we collaborate with reliable mould manufacturers while enhancing in-house mould-building
ability as well. Following are the steps of Injection Mould design:

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1.    
Input from customer (Mould specification, 2D & 3D
drawing, etc.)

2.    
Design

3.    
Planning

4.    
Manufacturing

5.    
Assembling

6.    
Trial

5.1.1 INPUT & EVALUATION

 

 

 

 

 

Products
design department provides product information to injection mould design
department, injection mould designers complete cost analysis in two working
days, and provide to R&D manager. All negotiations are done during the
evaluation process and the time period is also discussed for dispatching the
mould. R&D department release “Mould Making Apply” to plastic
injection mould design department after confirming the project.

5.1.2 DESIGN

 

Injection
mould designers organize relevant personnel to review the project base on the
product information, including product structure, mould structure, cooling
system, runner, slag, venting, etc. Our design/engineering technologies
include:

1.     Pro/Engineer/UG
(3D)

2.     Auto Cad
(2D)

3.     Mould
flow analysis (Plastic flow/deform simulation)

4.     Master
Cam (CNC Programming)

Design
Example :

 

 

 

 

 

·     
STEP 1:
The Split Line or Parting Line

This is always a crucial stage in the design process. If we get this
wrong the repercussions will be severe in production. In-fact there are only
two possible places where this component can be split: at B or C. If split as
at A or D it would be undercut in the tool and it could not de-moulded in a
two-plate tool because it could not be ejected from the cavity.

 

 

 

 

 

 

Fig. Possible split line positions

If we were
to select position C for the split line, this wouldn’t work as the moulding
will shrink away from the cavity walls and on to the pin that forms the central
hole. Also, this is not the best method, from the point of view of accuracy and
tool making; because most of the cavity would be in the injection half of the
mould instead it should form in the ejection side of the tool. It is desirable
to have as much cavity form in the ejection half as possible because most of
the tool making work will be on this side of the tool.

From tool
making considerations, the greater the amount of component in the ejection
half; the more likely it will be that the component stays on this side of the
tool when it opens viz. the ejection side. Therefore, we will split the tool at
B.

 

·     
STEP 2: Gating System

 

 

 

If the
relationship between the hole and all the diameters were important and subject
to close concentricity tolerances, a two-plate tool might not be the best
choice. This is because gating this part from the side might lead to
differential shrinkage and warpage due to the unequal melt flow length. If this
were important, a three-plate tool or hot runner tool would be preferred as the
part could be gated at the top, providing more equal melt flow lengths. As we
require over half a million parts per year, sub gating is the obvious choice as
the parts will be automatically de-gated.

Fig. 
Sub Gating

 

·     
STEP 3: Ejection System

We could
eject this part with pins, with a stripper plate or with sleeve ejectors. It is
always preferable to avoid pin gating if the options of stripping or sleeve
ejection exist, for three reasons:

1.The ejection area of pins is
smaller than in the other methods and we would   achieve far greater ejection support with
stripping or sleeve ejection. This eliminates the tendency of pins to hob or
embeds them in to the part.

2. For an eight-impression tool we would need
32 rather slender pins for ejection, which entails more tool making work and
alignment.

3. The relatively slender pins may tend to
deflect in the tool during ejection, causing premature wear and breakage.

Basically,
the governing factor is the diameter of the part being stripped. Generally,
smaller diameters should be sleeve-ejected and larger diameters stripped. In
the opinion of the author, the cut-off point should be around 30 mm diameter.
This is around the maximum comfortable size for tool making and for working
with standard mould components.

In this case
the diameter of the base of the moulding is 15 mm and therefore we will opt for
sleeve ejection.

 

·     
STEP 4: Cavity Inserts

 

We can machine the impressions straight into
a plate, but this has two disadvantages: –

1.     A plate that has cavities sunk directly into
it may suffer from warping or distortion during the hardening process.

2.     If a cavity suffers any damage during
production, it can be very difficult to repair it.

It is
therefore it is a common practice to use cavity inserts also, it is easier for
the toolmaker to work on the inserts, as they are smaller and if any damage
occurs it is much easier to replace an insert instead of the cavity.

       The diameter of the
inserts is large enough to ensure that the whole of the sub gate lies inside
the insert. This makes the spark machining of the sub gate easier and
eliminates join lines that may prevent the gate exiting cleanly during
ejection. As a rule of thumb, the following guidelines are suggested, further
refinements come with experience.

1.     The length of the lower insert L1 should be
the depth of the part below the split line + 1.5–2 times the length of the
sleeve ejector diameter for adequate sliding location L3.

2.     The length of the upper insert L2 should be
1.5–2 times the height of the form in the insert L4.

Note that the lengths L1 and L2
automatically determine the plate thickness for the fixed half and ejection
half of the tool. However, plates are only available in standard thicknesses
from suppliers such as DME, DMS and HASCO. Therefore, the nearest standard
plate sizes should be selected to determine final cavity depths.

 

 

Fig. Designing the cavity insert

 

·     
STEP 5: Venting System

The material is initially directed down
towards the bottom of the part and will then fill the cavity upwards from this
point.

Any air in the cavity will be forced upwards
and escape via the split line of the tool until the melt reaches the split
line. Once the melt continues beyond this point, it seems that there is no exit
path by which the air can escape. This means that the air may become trapped in
the fixed half cavity and result in burning of the moulding.

The actual fill pattern depends on the gate
size, speed of injection, injection pressure, tool temperature, and so on. To
accurately simulate the most likely fill pattern a computer simulation is
preferable e.g. Mould-flow.

In this case, however, we have identified the
possibility of air entrapment and, if this possibility exists, we should do
something about it before the event, and to counter this it is necessary to
provide for a route for the air to escape.

Fig. Providing venting

 

If we extend the core pin upwards into the fixed half cavity insert, we
will give extra support to the pin, preventing any tendency for it to deflect
because of nonsymmetrical melt pressures. By extending the locating hole for
the pin upwards to the top of the insert we also provide an escape route for
the air. This ensures the air will exhaust along the top of the fixed half
cavity and the cavity retaining plate.

This method works well for moderate injection speeds, but extra provisions
will have to be made if high injection speeds are used. This can be achieved by
grinding a flat channel approximately 0.03–0.05 mm deep along the cavity
retaining plate. The width of the channel is not critical and can be any width
within reason. As soon as possible this vent should be opened up to allow the
air to exhaust and expand into a larger space, because the air and gases being
forced out of the cavity are very hot and can reach very high temperatures. By
opening up the vent, the gases will be allowed to expand rapidly and thereby
cool rapidly.

With high injection speeds it may also be necessary to grind small flats
on the core pin where it locates in the top insert to allow the gases to escape
more easily.

 

 

·      STEP 6: Water Cooling

Fig. General cooling layout of a mould.

 

Temperature control is essential for all mould tools. We need to cool the
moulding as soon as possible so that we keep moulding cycles within acceptable
limits.

Desirable attributes of the mold cooling
design include:

1.    
Constant mold
temperature for uniform quality

2.    
Reduced cycle time
for productivity

3.    
Improved surface
finish without defects

4.    
Avoiding warpage by
uniform mold surface temperature (warpage caused by non-uniform cooling)

5.    
Long mold life

In this case we have two options: incorporating cooling channels into the
cavity inserts if possible, or putting water channels through the mould plates
next to the cavity inserts. Since locating cooling channels into the cavity
inserts would be difficult in this case, we will use cooling channels through
the mould plates.

 

Fig Adding water cooling

 

In this case we have two options: incorporating cooling channels into the
cavity inserts if possible, or putting water channels through the mould plates
next to the cavity inserts. Since locating cooling channels into the cavity
inserts would be difficult in this case, we will use cooling channels through
the mould plates

 

·      STEP 7: Impression Centres

We are now in a position to start looking at the impression centres. The
first stage is to establish the type of sprue bush we will be using. One is
selected from a standard parts catalogue and from this we can establish the
centre distance of the impressions. The minimum distance between the cavity
insert and the sprue bush is around 10 mm.

Fig.
Establishing impression centres

·      STEP 8: Mould Layout

This stage determines that the rest of the mould layout in plan view. The
first stage is to lay out the impressions which enables us to draw in the sprue
bush and then arrange the cavity inserts around it.