Part Design

As compare to moulding, the following designing factors must be set.
1; gate and vent locations must allow or uniform filling of the cavity without melt jetting
or air entrapment.
2; the ejection area must be of sufficient size and location for uniform ejection of the
part without damage to it.
3; side sections must have ample draft for ease of ejection.
4; undercuts must be carefully designed with well rounded corners and draft angles to
allow uniform part remoulding without damage.
5; changes in wall thickness should be kept to a minimum or made gradually. Sharp
variations in wall thickness will result in variations in wall cooling rate, causing
war page.
6; ribs, bosses or fillets should be made thin to avoid sink marks on the surface of the
adjoining section, and to keep from increasing cycle time.
7; fillets should be placed at sharp corners to decrease stress concentration. Generally a
fillet radius to wall thickness ratio of - 0.6 is sufficient.
8; length of the flow paths must not exceed the flow ability of the resin selected for the
application.
Mechanical strength, appearance and function ability will also be affected by the design of
the various features of the part, for example :

Screw & Barrel Design

Screw & Barrel Design
Screw design varies widely depending upon the manufacturer. Almost all machines are
being made with single flighted standard type screws with one manufacturer using a
doublewave type of barrier screw.

1; flight land width : 0.1 (screw diameter)
2 flight/cylinder clearance : 0.001 (screw diameter)
3 screw length/diameter ratio : commonly 20-25 : 1
4 feed/transition/metering sections : typically the screw length will be divided 50%
feed, 25% transition and 25% metering.
5 materials : screws are frequently made from AISI 4140 heat treated steel. Flight
tips are usually hard surfaced with Stellite. Chrome plating of the screw is
common, for ease of cleaning. Barrels are normally bimetallic, with an Hastalloy
inner lining for wear resistance.
6;flight pitch : almost always square (0 = 17.7o)
7 mixers : some type of mixing device is frequently added to the screw to improve
melt homogeneity and pigment dispersion. Since pigments are commonly added
in the form of a masterbatch, a high-shear mixer is required to improve dispersion
(Maddock type).
8 compression ratio : C.R. is in the range of to 4 : 1; most commonly 2-3 : 1

9 check valve : screws are equipped with a one way valve at the tip to prevent melt
from flowing back through the screw during the ram injection stage. Two most
common types in use are the ring type and ball type.
10 injection nozzle : the function of the nozzle is to convey the melt from the
injection cylinder to the mould, with minimum pressure drop and heat loss.
Nozzles are generally kept short, with open streamlined flow passages to
minimize pressure drop and heat loss and to avoid material hang-up

Mould Cooling

Mould Cooling
It is necessary to remove heat from the part to freeze it and cool it below its softening
temperature before it can be remove. It is important to remove the heat rapidly, for fast
cycle time, and evenly to prevent warpage related to uneven crystallization. In multicavity
moulds, cooling must be uniform in all parts.
Mould temperature is controlled by circulating cooling water through conduits in the
mould. Cooling conduits must be carefully placed in the mould to ensure even cooling of
the part.
Hot spots may be encountered in areas around gates, due to shear heating, or in areas of
the mould which are difficult to reach with cooling conduits. Special metal inserts may
be used in these areas, made from material with high heat transfer rates, such as
beryllium copper.
Separate cooling channels can also be installed in these areas, operating with higher flow
rates or lower temperature cooling water than the rest of the mould.

Ejector Systems

Ejector Systems
Some force must be applied to the moulded parts to eject them from the mould.
Mechanical knock-out devices such as pins or sleeves can be used, driven by the
movement of the mould as it opens.
With mechanical ejectors, the area of the knock-out device in contact with the part must
be large enough to avoid damaging or stressing the part.
In designing the part and the mould, side wall taper should be included to assist part
ejection. For polyethylene, a draft angle of 0.5 - 1o is recommended.

Mould Venting

Mould Venting
At the start of the injection cycle, the mould cavity have an air which must be vented as
it is displaced by the incoming melt. Inadequate venting can result in considerable
compression and heating of the trapped air, resulting in slow filling, poor welds and
possible burning of the resin.
Vents are located at the end of flow paths, most often in the parting line but also around
ejector pins.
Vents must be large enough to allow free passage of air, but prevent the passage of melt

Gate Dimensions

Type of gate
Gate Dimensions
Small gates will result in high injection melt temperatures and pressures. Large gates
will increase the time required to seal the gate area, increasing cycle time, and can result
in non-uniform filling in multi-cavity moulds.
As a general rule. the gate area should not be less than 1.25 mm, and the maximum
diameter or thickness of the gate should not exceed one half the thickness of the part at
the gate point. Gate lands are generally about one half the gate diameter, for a circular
gate, and about 0.75 - 1.0-mm in the case of a rectangular gate.

Ring and Diaphragm Gates

Type of gate
Ring and Diaphragm Gates are used to produce cylindrical parts with good
concentricity and no weld lines. With a ring gate, the core can be registered in both
mould halves, preventing shifting.