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Not guided or guided
Definition: mold. 1) A hallow form or matrix into which a plastic material is placed and which imparts to the material its final shape as a finished article. 2) To impart shape to a plastic mass by means of confining cavity or matrix. Ejection: The process which ejects the part/parts for a mold by use of pins, blades, plates etc., usually limited by means of stroke but at minimum longer than any protrusion on that side of mold which they are located at so as to eject part/parts. Guided: meaning that the ejector plate assembly is supported by leader pin/ bushing arrangement so as to allow ejector pins to move freely and not be in support of ejector plate assembly.
While the ejector mechanism can in some mold be very complex, what with unscrewing, collapsible and or slides, and what not, one aspect in simple molds is whether to guide the plates. I refer back to the ejector retainer and ejector plate. These shall be referred to as simply the ejector plate mechanism or EJM.
The EJM sits in what may be referred to as the ejector housing and from a mold base supplier is supplied with return pins. Please understand that dependent on supplier and needs this may or may not be the case. If it is there are normally 4 return pins with one offset so as to locate the plate in the same manner each time it is assembled. While these pushbacks do just that; push back the EJM they are sometimes also used by builders of the tool to align the EJM and ejector pins in the mold.
The pushbacks are now wearing in the mold base, and also supporting the EJM and pins which can in the long term cause wear of the ejector pins in the mold base, cavity area as the clearance for the push backs are not that tight. The other issue is that this issue can lead to higher ejector forces required due to misalignment and or wear. While in many cases with a short stroke this can be done, the push back are not there to guide.
Should the machine setup person not correctly set knockout bars to proper lengths and or they are not set evenly spaced to the EJM, especially if multiple bars are required the results can be an EJM that binds up.
A guided system is a set of bushings in the EJM which ride on pins which are supported by the ejector housing and in some cases the support plate. With a set of guide pins and bushing the EJM can smoothly move and support the pins in alignment with the cavity and support plates. This eliminates an issue with wear due to misalignment. This is a great concern with long ejector pins and movement within the EJM.
Figure 1
The red arrow above in the design shows force of weight the plates may have on the return pins. If guided than the weight of the plates is supported by the bushing and pin arrangement.
An issue arises as to which side the guide pin is located into. If in the support plate or is it the rear of the ejector housing with the use of an additional top clamp plate. The concern here is at what temperature the mold is to run, since if a large mold is built is the thermal expansion possibly going to have an issue in the alignment of the plate to guides and bushing. In many cases this is not an issue but as the mold becomes larger than thermal expansion may play a role.
If the tool is small than pin location is only on one side, while if large and heavy than support is on both sides.
Figure 2
The above illustrates the use of a guided pin and bushing from the ejector housing.
The EJM should be guided as this assures a better running tool. It does add cost and take up space in the ejector plates especially if support pillars must be used. If support pillars are used than consider them as the guided pins and bush the plates around these support pillars if possible.
SL SILVEY
18052014.01
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