With development and change of society, pressure of cost competition between enterprises is increasing. How to reduce costs and enhance competitiveness of enterprises is focus of every enterprise. Same is true for mold industry, how to reduce cost of mold companies, its cost management becomes more and more important. Influence of plastic parts design, mold, injection production and other factors on cost management of mold companies is introduced.

Generally speaking, mold cost includes: mold design fee + mold material fee + processing fee + mold trial fee + tax and so on. From perspective of molds, this answer is correct, but not comprehensive. From perspective of enterprise management, mold cost management should include: plastic part design, mold, injection molding and other factors, as shown in Figure 1.

 

Figure 1 Diagram of mold cost

Whether plastic part design is reasonable or not is one of main factors affecting mold cost. At present, among domestic engineers and professional technicians engaged in development and design of plastic parts, few have experience in mold design and processing in mold industry, plastic parts designed do not know how to consider influence of mold factors. Most of plastic part designers consider realization of function of plastic part, feasibility of plastic part process and subsequent assembly process. Through following cases to explain to everyone.

Small card cover plastic part is shown in Figure 2. Material used is LCP. In mold trial process, after every dozens of mold samples are produced, position of buckle will break, causing mold trial to be interrupted. Mold assemblers often disassemble mold and clean broken buckle.
Main reasons for buckle fracture of plastic part are: stress concentration occurs at buckle part during injection molding of plastic part; side demolding angle of buckle is not enough. To solve problem only from mold, it is polishing and then polishing. For problem of plastic parts broken buckle, it is a phenomenon that treats symptoms rather than root cause. Repeated mold trials result in increased mold trial costs, mold polishing, assembly workloads, and higher manufacturing costs.
In mold processing process, fillet or corners need to be processed by electrical discharge machining to meet shape of plastic part. If plastic parts are designed with reasonable rounded corners at corners, electrical discharge machining process and related electrical discharge machining materials can be reduced, mold processing cost and processing time will be significantly saved.

 

Figure 2 Influence of rounded corner design

Wall thickness of plastic parts indirectly affects mold cost. It is mainly analyzed and discussed from two aspects: ① If wall thickness is too thin, plastic parts are prone to lack of glue, and at the same time, production of plastic parts is unstable, which increases trial cost in mold cost. ②If wall thickness is too thick, plastic parts will easily cause shrinkage, and plastic parts will be deformed to different degrees. In order to solve these problems, by increasing cooling time in injection molding cycle, above defects of plastic parts are reduced. Following two case analysis.

Figure 3 shows a circular thin-walled plastic part, wall thickness at the middle thread is 0.25mm,  wall thickness of upper circle is 0.6mm, wall thickness of lower circle is 0.4mm. Mold uses large nozzle side to feed glue from lower circular edge. Number of mold trials exceeds 10 times. The most difficult problem to solve is that there are weld marks at middle thread, insufficient filling due to trapped air, resulting in plastic parts waste.

 

Figure 3 Insufficient filling of thin-walled plastic parts
Because wall thickness of thread is too thin and cooling is fast, according to analysis of injection molding, end of flow glue is at thread position. From structural analysis of development department, there is no problem. From a production point of view, wall thickness of plastic part is designed to be too thin, which increases number of mold trials, and cannot solve problem at all. Finally, without changing size of drawing, analyze tolerance of size, add material to wall thickness of plastic part, adjust size of mold to solve problem of insufficient filling.

As shown in Figure 4, top wall thickness of plastic part is 2mm, while side wall thickness is only 1mm. Through mold flow analysis, deformation trend and amount of plastic part can be confirmed, and deformation occurs in direction of thickness of glue position. Therefore, when plastic part is still in development and design stage, it is necessary to consider uniformity of wall thickness of plastic part to reduce deformation of plastic part.

 

Figure 4 Plastic part deformation with uneven wall thickness
In terms of mold: Plastic part can be repeatedly verified by adjusting size or position of glue inlet, optimal deformation amount can be repeatedly verified or deformation jigs can be added, which increases cost of mold trial and auxiliary tools, labor of production operators strength.
In terms of injection molding production: by increasing cooling time of injection molding to reduce amount of deformation, this method for plastic parts is a symptom and not root cause, cannot completely solve deformation problem of plastic parts. This situation of leaving problem to next process virtually increases mold debugging times, prolongs injection molding cycle, reduces production efficiency, and increases injection molding production cost, which is not desirable.

Mold companies often encounter such a situation. In a certain project, mold has just been designed, mold base has just been ordered, customer's design change notification will come. Structure of plastic part has changed a lot, such as size of outer shape has become larger, buckle has been added, etc., mold needs to increase mechanism to meet structure of plastic part, and even mold base has to be reordered, which will increase cost.
Structure of plastic part has not changed much, mold designer needs to repeatedly modify mold drawing files, increase error rate of mold drawing, make mold design time longer, disrupt early mold processing plan, cause mold enterprise's early manpower and material resources to make useless efforts and no waste of cost.
In order to reduce design changes of plastic parts, current mature 3D printing technology can be used. Before mold is opened, prototype is made according to 3D, structure assembly verification is performed, and structure of plastic part is modified in time. After modifying structure of plastic part, review it with mold engineers and production department technicians, which can minimize cost increase caused by design change of plastic part.
Summary of experience: Plastic parts designers should learn as much as possible about mold knowledge, so that plastic parts designed can meet feasibility of mold processing, reduce difficulty and cost of mold manufacturing. Avoid a misunderstanding: cost of plastic parts is a matter of plastic part development department, cost of molds is a matter of mold department, and cost of injection molding is a matter of injection molding department. If product development, mold, and injection departments are integrated into one company, it will be a relatively easy task to reduce cost of plastic parts.

Mold designers have different working experiences and different design levels. In order to solve problems in design process, the most effective method is to conduct plastic parts and mold reviews.
As shown in Figure 5, since plastic part has several slots, slots are composed of four types of inserts A, B, C, and D. The smallest insert is A, size specification is: 45×0.6×65mm. It can be found from design dimensions of inserts that these 4 inserts are all thin inserts. After mold test, it was found that flash of insert part, no matter how to adjust molding process parameters, flash of inserting part could not be controlled. Regarding problem of plastic parts flashing, repeated mold repairs, mold disassembly, repeated mold trials have virtually increased mold assembly labor costs and mold trial costs, but problem has not been resolved. By combining 4 inserts and designing 2 inserts, mold tolerance has been significantly improved.
Another situation is design waste. Some newly developed plastic parts that customers use to develop market or experimental needs only need 2,000 plastic parts, as long as production life of mold is within 5,000 plastic parts. If mold is designed according to mold life of 100,000 molds, cost will be large, which is a waste of design.

 

Figure 5 Illustration of mold insert design

Implementation of standard mold parts is mainly to shorten mold manufacturing time, reduce mold processing costs, achieve high quality and low price. At present, there are many suppliers of mold standard parts. If you are greedy for cheap and hasty purchases, causing mold problems, it is not suppliers who suffer the most, but mold companies and mold users.
In order to reduce cost of mold, in mold design, standard parts can be used as much as possible. It is best to meet specifications of multiple standard parts suppliers. Purchase can be compared with multiple suppliers, and there is more room for selection, which is conducive to reducing mold costs.

Mold company’s calculation of mold processing fee is based on number of days from when mold is ordered and mold leaves factory. If mold cannot be delivered after multiple trials, in addition to a fine according to contract, the more days mold is in factory, the more processing costs, and the higher trial cost. Therefore, multiple or uncontrollable mold trials are a big blind zone in mold cost control.
Reasons for high number of mold trials: ①Mold design did not go through a sufficient collective review, and mold structure was unreasonable, which made mold undergo many improvements after mold trial; ②Mold design was very good, but mold processing is not in place, which makes number of trial molds increase for no reason, increases cost of mold; ③The best molding process and injection molding machine model were not selected when mold was tested. Mold was done well, but injection molded plastic parts were not ideal; ④Physical properties of plastics are not fully understood, and shrinkage rate is not selected accurately, resulting in repeated mold repairs.

Molds for large-scale air-conditioning appearance parts or transparent parts require a lot of mold materials, and mold material costs account for a large proportion of mold price. Choosing stainless steel mold materials will cause high material costs, high mold unit prices, and no competitive advantage. Choosing poor mold steel, polished surface of mold does not meet requirements, is not wear-resistant, and plastic parts cannot meet customer requirements. In order to reduce cost of mold, to meet customer's requirements for high gloss on the surface of plastic part at the same time, it is necessary to select poor materials for processing, perform surface coating treatment on the surface of mold cavity to improve high gloss, rust and wear resistance of mold.

Mold cost pays little attention to information about injection molding production. In fact, for a company that produces products, it is not that mold makes money, company makes money, but it is concerned about final profitability of product. In injection molding production process, mold is the most important link, stability of mold production is reflected in injection molding production.
A pair of molds are often removed for maintenance due to broken ejector rods, mold collapse, mold oil stains, mold rust, ejection failure, broken inserts, slider burnout, deformation, etc., which will increase mold maintenance costs. And most of hidden costs such as raw materials for injection molding, downtime of injection molding machine, and so on, these cost increases will eventually be passed on to product.

Managing cost of molds is not only work of mold department, but also requires cooperation of product development department and injection molding production department to minimize mold costs, design more reasonable and high-quality molds. Reasonable plastic part design, few plastic part design changes, advanced mold design, and reasonable material selection can reduce mold costs. In order to make a good product, product designers, mold engineers, and injection molding production departments all need to maintain good communication.