The document discusses design criteria for rotational and transfer molding processes, outlining key considerations such as venting, shrinkage, wall thickness, and mold design features. It provides specific guidelines for the design of molds, including recommendations on angles, flatness tolerances, vent design, and gate specifications to ensure effective molding. Additionally, it addresses the importance of material selection and design for durability and performance in molding applications.

1. DESIGN CRITERIA FOR
ROTATIONAL MOLDING PROCESS
What is rotational molding?
Rotational molding is used to produce hollow thermoplastic products such as drums, storage tanks and
litterbins.
A carefully calculated amount of plastic is placed in a closed mold that is heated in an oven and rotated
slowly around both a vertical and horizontal axes.
The plastic material fuses and sticks to the hot mold surface, building up the required thickness.
The mold is then gradually cooled by air or water while still rotating. The mold is opened, the finished
product removed and the mold reloaded and closed for the next cycle.

2. Design criteria
 Venting. Due to gasses that are expelled when the resins cures, a vent hole is required in all
molds and therefore products. The size of the vent is dependent upon the volume of the cavity.
Multiple vent holes may be required for larger and more complex parts.
 Shrinkage. Various resins have different shrinkage, .007, .025, .030, .035in/in. cast aluminum
molds shrink to 0.011in/in. The molding process can also vary these shrinkages. Restricted areas
will also vary the shrinkage to some degree.
The type of mold release or permanent coating will having varying shrinkage. If you are trying to
create around since the parting line will restrict the material shrinkage and force the part into an
oblong.

3.  Radiuses & angles.
 Due to nature of how the material flows and forms, sharp corners usually result in blow holes and porosity in
the corners. Adding a generous radius improves the molding of corners and distributes the stress over a
broader area adding to the product’s strength. The minimum recommended angle at the corner of a part for
polyethylene and PVC is 30°, nylon is 20°, polycarbonate is 45degrees.

4.  Wall thickness.
 It should be noted that because of material flow characteristics outside corners tend to be thicker than
nominal wall {usually an advantage since outside corners are frequently heavily loaded} whereas projections
into the molding tend to thin out. These are generally not regarded within the general wall tolerance.
A tolerance of +‐20% should be considered as a commercial tolerance whereas +‐10% would be
precision and be more expensive to maintain.

5.  flatness
 The flatness of product surface is subject to the design and cooling process. Typical
flatness tolerances for polyethylene would be 5% ideal with 2% as a commercial
tolerance and 1% as a precision tolerance.
 If at all possible, the design of parts to be rotationally molded should avoid large flat
areas. If absolutely necessary they should be broken up by reinforcing ribs or possibly
have a gentle curvature on them. Molded in detail, such as lettering, logo’s etc. can also
break a flat surface up visually so that lack of flatness is disguised.

6. Design criteria for transfer molding
 What is transfer molding?
Briefly, Thermoset charge is loaded into a chamber immediately ahead of mold cavity, where it is
heated; pressure is then applied to force soft polymer to flow into heated mold where it cures
transfer molding involves two variants i.e. pot transfer molding and plunger transfer molding.
A transfer mold is comprised of various components including, vents, sprues, gates and cores, cavity
and the material of the mold itself. This presentation tackles the design of these components,
material selection for the material of the transfer mold and criterion for choosing this type of
molding.

7. Criteria of design
 Vent design Typical vents design for a phenolic part should be 1⁄4" (6 mm) wide with a
recommended depth of 0.003”-0.0035” (0.08 mm - 0.09 mm) deep and vents for polyester parts
should be 1⁄4" (6 mm) wide and 0.002" - 0.0025" (0.05 mm - 0.06 mm) deep.
Along with location and depth of the vents is the vent length, which is the distance from the part to
the vent relief. The vent should be approximately 1” (25mm) long to allow pressure to build in the
cavity after the material in the vent cures. After this point, the vent can be relieved to a depth of
0.01" - 0.02" (0.25 mm - 0.50 mm).

8. Runner Design for transfer molds
When designing runners for transfer molds we can choose from the following
types; Standard full round with a centerline: This is the most efficient runner,
but in some cases it is necessary for the runner to be in only one half of the
mold.

9. Cont’d
 Standard trapezoid runner: This is often used in situations that require the
runner to be only in one mold half. The effective runner size is shown below;

10. Gates design for transfer molds
 The gates for thermoset molds are high wear areas of the mold and thus should be made using a
replaceable insert so when it becomes badly worn it can easily be replaced hence it should be
made of materials that do not wear easily.
 Materials commonly used for gate inserts are D-2, M-3 and Ferrotic steel. When designing an
edge gates for thermoset materials, the width of the gate can be as small as 1/16" (1.5 mm) but
the depth of the gate should not be less than 0.050" (1.3 mm).
 A gate should be large enough to allow the part to fill without using excessive transfer pressures
or requiring long transfer times. Transfer times of 3 - 8 seconds and transfer pressures of 1,100
psi (7.6 MPa) or less are desirable.

11. Cont’d
 Sub gate addition in transfer molding: A gate cutter can be used to mold a part with nearly all
signs of the gate gone. A gate cutter is a blade or a pin that is located in the mold directly below
the gate and its role is to seal off the gate once mold material has been injected into the cavity.

12. Cavities and core design in transfer
molds
The types of cavities and cores applied inserted in nature such that when an individual cavity or core
gets damaged, it can be removed and repaired while the rest of the mold is put into service. When the
parts are very small and there is a large number of cavities, individual cavity inserts might not be
feasible. In those situations we suggest using cavity blocks consisting of 3 or 4 cavities.
The materials most commonly used for cavity inserts are H-13 and S-7. Both of these materials will
harden to Rockwell 52 to 54 Rc. and can be polished to produce an excellent surface finish on the parts.

13. Transfer molding

14. Ejector Pin Location and Design
 Ejector pins are used in part removal after completion of molding by pushing
the part out of the mold without distorting it. Ejector pins should be located
in the deepest points of the cavity or core specifically on the deepest points
of ribs and bosses.
 Upon location determination, size of the ejector pin ought to be
accomplished and notably very small diameter ejector pins can be
problematic since they are easily prone to breaking. Due to this, ejector pins
smaller than 3/32" (2.4 mm) in diameter are not recommended.

16. Center Supports design
Molds built to run thermoset materials usually have little or no support in the middle and as a result
heavy flash around the sprue is a consequence and thus parts vary in thickness from the stationary
(sprue) side to the movable side. To resolve this problem we suggest installing substantial support
pillars down the center of the mold between the parallels 2” (50.8mm) diameter if possible.