Balanced multi-cavity injection molding of ridged-wall plastic products

Balanced injection molding in a multi-cavity mold of a plurality of ridged-wall products having is accomplished by providing mold parts defining therebetween at least two cavities for forming the ridged-wall plastic products, wherein each of at least two cavities defines a plurality of flow channels in which injected plastic material flows within the cavity and in which flow channels the first side strips are formed; wherein the relative disposition of the mold parts is adjustable for adjusting the dimensions of the flow channels in at least two mold cavities separately so that when plastic material is injected approximately simultaneously into the at least two mold cavities the injected plastic material flows within the at least two cavities at respective flow rates that cause the at least two mold cavities to be completely filled approximately simultaneously with the injected plastic material.

BACKGROUND OF THE INVENTION 
The present invention generally pertains to injection molding of 
ridged-wall plastic products and is particularly directed to balanced 
multi-cavity injection molding of such products. 
One type of such a product includes a plurality of longitudinal folds of 
alternating ridges and furrows, each fold including a first side strip 
laterally extending from the top of a said ridge to the bottom of a said 
furrow; and a second side strip laterally extending from the bottom of the 
said furrow to the top of a said ridge next to the said ridge; wherein the 
first side strip has a different lateral orientation than the second side 
strip, and wherein the first side strip has a greater wall thickness than 
the second side strip Such a product is described in U.S. Pat. No. 
5,267,685 to Jens Ole Sorensen. 
For efficient manufacture, it is preferred to injection mold such products 
simultaneously with a multi-cavity mold. A problem incident to 
multi-cavity injection molding of such products is that all of the 
cavities of the mold in which the products are formed are not always 
completely filled simultaneously with the injected plastic material. 
Referring to FIG. 1, which illustrates a plurality of injection molded 
hollow ridged-wall plastic products 10, 10a, 10b remaining in one mold 
part 12 after the mold parts 12, 14 defining the respective cavities in 
which the products are formed have been separated, it is seen that failure 
to completely fill all of the mold cavities simultaneously results in some 
of the cavities being incompletely filled, such that a product 10a therein 
is not completely formed, and other cavities being overfilled, such that a 
flashing of excess plastic 16 is attached to a product 10b formed therein. 
The gates through which the plastic material is injected into the mold 
cavities are located at the center of the end of the respective cavities 
at which closed ends 18 of the hollow plastic products 10, 10a, 10b are 
formed. 
SUMMARY OF THE INVENTION 
The present invention provides a multi-cavity mold for injection molding a 
plurality of ridged-wall plastic products, wherein each of at least two 
mold cavities defines a product comprising a plurality of longitudinal 
folds of alternating ridges and furrows, each fold including a first side 
strip laterally extending from the top of a said ridge to the bottom of a 
said furrow; and a second side strip laterally extending from the bottom 
of the said furrow to the top of a said ridge next to the said ridge; 
wherein the first side strip has a different lateral orientation than the 
second side strip; and wherein the first side strip has a greater wall 
thickness than the second side strip, the mold comprising 
mold parts defining therebetween said at least two cavities for forming 
said products, wherein each cavity defines a plurality of flow channels in 
which injected plastic material flows within the cavity and in which flow 
channels the first side strips are formed; 
wherein the relative disposition of the mold parts is adjustable for 
adjusting the dimensions of the flow channels in said at least two mold 
cavities separately so that when plastic material is injected 
approximately simultaneously into said at least two mold cavities the 
injected plastic material flows within said at least two cavities at 
respective flow rates that cause said at least two mold cavities to be 
completely filled approximately simultaneously with the injected plastic 
material. 
The present invention further provides a method of balancing a multi-cavity 
mold for injection molding a plurality of ridged-wall plastic products, 
wherein each of at least two mold cavities defines a product comprising a 
plurality of longitudinal folds of alternating ridges and furrows, each 
fold including a first side strip laterally extending from the top of a 
said ridge to the bottom of a said furrow; and a second side strip 
laterally extending from the bottom of the said furrow to the top of a 
said ridge next to the said ridge; wherein the first side strip has a 
different lateral orientation than the second side strip; and wherein the 
first side strip has a greater wall thickness than the second side strip, 
the method comprising the steps of: 
(a) providing mold parts defining therebetween said at least two cavities 
for forming said ridged-wall plastic products, wherein each cavity defines 
a plurality of flow channels in which injected plastic material flows 
within the cavity and in which flow channels the first side strips are 
formed; 
(b) injecting plastic material at a given injection pressure approximately 
simultaneously into said at least two mold cavities in order to form the 
products with the injected plastic material; 
(c) measuring the relative extent to which said at least two mold cavities 
are filled with the injected plastic material; and 
(d) when said at least two mold cavities are not filled to the same degree 
approximately simultaneously, adjusting the relative dimensions of the 
flow channels for forming the first side strips in said at least two mold 
cavities to be such that when plastic material is injected in accordance 
with step (b) injected plastic material flows within said at least two 
cavities at respective flow rates that cause said at least two mold 
cavities to be filled to the same degree approximately simultaneously with 
the injected plastic material. 
Additional features of the present invention are described with reference 
to the detailed description of the preferred embodiments.

DETAILED DESCRIPTION 
Referring to FIGS. 2 through 9A, a preferred embodiment of the mold of the 
present invention includes a first mold part 20 and a second mold part 
including at least two segments 22 which when combined with the first mold 
part 20 define therebetween at least two cavities 24 for respectively 
forming ridged-wall plastic products, such as described in the 
above-referenced U.S. Pat. No. 5,267,685. Each product includes a 
plurality of longitudinal folds of alternating ridges and furrows. Each 
fold includes a first side strip laterally extending from the top of one 
ridge to the bottom of one furrow; and a second side strip laterally 
extending from the bottom of the one furrow to the top of another ridge 
next to the one ridge. The first side strip has a different lateral 
orientation than the second side strip and the first side strip has a 
greater wall thickness than the second side strip. Each mold cavity 24 
defines a plurality of flow channels 26 in which injected plastic material 
flows within the cavity 24. The first side strips are formed in the flow 
channels 26. 
The respective disposition of the at least two separate segments 22 of the 
second mold part in relation to the first mold part 20 are separately 
adjustable for adjusting the dimensions of the flow channels 26 in the at 
least two mold cavities 24 separately so that when plastic material is 
injected approximately simultaneously into the at least two mold cavities 
24 the injected plastic material flows within the at least two cavities 24 
at respective flow rates that cause the at least two mold cavities 24 to 
be completely filled approximately simultaneously with the injected 
plastic material. 
When the product is a hollow product, the separate segments 22 of the 
second mold part are separately rotatable in relation to the first mold 
part 20 for adjusting the dimensions of the flow channels 26 of the 
respective cavities 24. 
When the first mold part 20 is joined at a parting line P.L. to the 
separate segments 22 of the second mold part to define the respective 
cavities 24 therebetween, a plurality of adjustment spaces 28 are also 
defined about the parting line P.L. between portions of the first mold 
part 20 and portions of the respective segments 22 of the second mold 
part. 
Bolts 30 that are anchored to the first mold part 20 extend into each of 
the adjustment spaces. Adjustment blocks 32 are screwed onto the bolts 30 
for disposition in the adjustment spaces 28 in order to lock the 
respective segments 22 of the second mold part in a fixed position in 
relation to the first mold part 20 when the first mold part 20 and the 
respective segments 22 of the second mold part are joined at the parting 
line P.L. The blocks 32 each have a first section 34 for disposition in at 
least a region of the adjustment space 28 on the same side of the parting 
line P.L. as the first mold part 20 and a second section 36 for 
disposition in at least a region of the adjustment space 28 on the same 
side of the parting line as the segment 22 of the second mold part. There 
are a plurality of sets of blocks 32 for enabling different degrees of 
adjustment between the first mold part 20 and the respective segments 22 
of the second mold part; and the disposition of the first section 34 in 
relation to the second section 36 is different for the different sets of 
blocks 32. With reference to the disposition of a segment 22 of the second 
mold part in relation to the first mold part 20, as shown in FIG. 7, that 
is provided when the relative disposition between the first section 34 and 
the second section 36 of the block 32 is as shown in FIG. 7A, a set of 
blocks 32 having the relative disposition between the first section 34 and 
the second section 36 of the block 32 shown in FIG. 8A is used for 
rotating a segment 22 of a second mold part counterclockwise in relation 
to the first mold part 20 to provide the disposition of the segment 22 of 
the second mold part in relation to the first mold part 20 that is shown 
in FIG. 8; and a set of blocks 32 having the relative disposition between 
the first section 34 and the second section 36 of the block 32 shown in 
FIG. 9A is used for rotating a segment 22 of a second mold part clockwise 
in relation to the first mold part 20 to provide the disposition of the 
segment 22 of the second mold part in relation to the first mold part 20 
that is shown in FIG. 9. 
In balancing the mold, the dimensions of the flow channels 26 initially are 
larger than required for forming first side strips of a preferred 
dimension, as shown in FIG. 9A, and plastic material is injected at a 
given injection pressure approximately simultaneously into the at least 
two mold cavities 24 in order to form the products with the injected 
plastic material. The relative extent to which the different cavities 24 
are filled with the injected plastic material is measured. When the at 
least two mold cavities 24 are not filled to the same degree approximately 
simultaneously, the relative dimensions of the flow channels 26 are 
adjusted so that when plastic material is injected into the mold cavities 
24, the injected plastic material flows within the at least two cavities 
at respective flow rates that cause the at least two mold cavities 24 to 
be filled to the same degree approximately simultaneously with the 
injected plastic material. 
The relative dispositions between a segments 22 of the second mold part and 
the second mold part 20 are adjusted by inserting respective sets of 
blocks 32 into the adjustment spaces 28 that result in 
Preferably, the dimensions of the flow channels 26 of the respective 
cavities 24 are adjusted such that the flow channels 26 for at least one 
of the mold cavities 24 are of a dimension required for forming first side 
strips of the preferred dimension, as shown in FIG. 7A. Sometimes, the 
flow channels 26 for at least another of the mold cavities 24 are either 
of a dimension larger than required for forming first side strips of the 
preferred dimension, as shown in FIG. 9A, or of a dimension smaller than 
required for forming first side strips of the preferred dimension, as 
shown in FIG. 8A. The relative differences between the dimensions of the 
flow channels shown in FIGS. 7A, 8A and 9A are greatly exaggerated in the 
Drawing. 
However, if such an adjustment does not result in the injected plastic 
material flowing within the at least two cavities 24 at respective flow 
rates that cause the at least two mold cavities 24 to be filled to the 
same degree approximately simultaneously with the injected plastic 
material, the dimensions of the flow channels 26 of the respective 
cavities 24 are adjusted such that the flow channels 26 for at least one 
of the mold cavities 24 are of a dimension larger than required for 
forming first side strips of the preferred dimension, as shown in FIG. 9A, 
and the flow channels 26 for at least another of the mold cavities 24 are 
of a dimension smaller than required for forming first side strips of the 
preferred dimension, as shown in FIG. 8A. 
When such adjustments result in the injected plastic material flowing 
within the at least two cavities 24 at respective flow rates that cause 
the at least two mold cavities 24 to be filled to the same degree 
approximately simultaneously with the injected plastic material, the 
multi-cavity mold is provided for balancing injection molding of a 
plurality of ridged-wall products and plastic material is injected 
approximately simultaneously into the at least two mold cavities 24 to 
form such products. 
In alternative embodiments (not shown), either or both of the first mold 
part and the second mold part includes segments of which the disposition 
may be adjusted in relation to the other mold part in order to cause the 
injected plastic material to flow within the at least two cavities at 
respective flow rates that cause the at least two mold cavities to be 
filled to the same degree approximately simultaneously with the injected 
plastic material. 
The advantages specifically stated herein do not necessarily apply to every 
conceivable embodiment of the present invention. Further, such stated 
advantages of the present invention are only examples and should not be 
construed as the only advantages of the present invention. 
While the above description contains many specificities, these should not 
be construed as limitations on the scope of the present invention, but 
rather as exemplifications of the preferred embodiments described herein. 
Other variations are possible and the scope of the present invention 
should be determined not by the embodiments described herein but rather by 
the claims and their legal equivalents.